Griesinger's teachings show up in Klippel, Linkwitz, Toole, and Geddes

Duke LeJeune

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David Griesinger is an acoustician and psychoacoustican who is not particularly well known in home audio circles. In this post we will examine how some of the concepts he articulates are present within the works of other researchers.

To lay a foundation, here are three quotes from Griesinger (which you may have seen before):

“The earlier a reflection arrives the more it contributes to masking the direct sound.”

“Envelopment is perceived when the ear and brain can detect TWO separate streams: A foreground stream of direct sound, and a background stream of reverberation. Both streams must be present if sound is perceived as enveloping.” (Implied is that there must be a time gap in between the two streams to differentiate them.)

“Envelopment is the holy grail of concert hall design. When reproducing sound in small spaces [such as home listening rooms], envelopment is often absent.”

Starting on page 185 of his book “Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms”, Floyd Toole presents some interesting observations about sound fields:

“A serious examination of listener reactions to complex sound fields in stereo reproducion was underaken by [Wolfgang] Klippel. The investigation attempted to relate listener descriptions of what they heard to measured quantities... According to Klippel the relevant loudspeaker measurements are the anechoic on-axis frequency response and the sound power response – or at least a sufficient collection of off-axis measurements to describe the reflected sounds arriving at the listening location.”

So Klippel was trying to uncover the relationships between listener descriptions and loudspeaker measurements.

“Of special interest was his finding that what he called a “feeling of space” figured prominently into listener responses... responses were solicited for two broad categories, “naturalness” and “pleasantness”, one relating to realism and accuracy, and the other to general satisfaction or preference, without regard to realism.”

Klippel found that “naturalness” (realism and accuracy) was 30 % related to sound quality (coloration, or the lack thereof); 20% related to tonal balance; and 50% related to the “feeling of space”.

“Pleasantness” (general satisfaction or preference) was 30% related to sound quality and 70% related to the “feeling of space”.

Wow! I would not have expected the “feeling of space” to make a 50% contribution to "realism", and a 70% contribution to "preference"!!

Note that Klippel's “feeling of space” sounds an awful lot like Griesinger's “envelopment”.

Toole continues: “Therefore, whether one is a picky purist or a relaxed recreational listener, the impression of space is a significant factor... Klippel chose as his measure of the “feeling of space” the difference between the sound levels of the multidirectional reflected sound and the direct sound at the listening location.”

In other words, Klippel found that the reflected-to-direct sound ratio was related to this highly desirable “feeling of space”. BUT (quoting Toole again; emphasis mine):

“There is an optimum amount of reflected sound; there can be too much or too little.” This has been my experience as well. There definitely seems to be such a thing as too much reverberant energy.

Tool then writes in conclusion: “A good loudspeaker for this purpose would therefore be one that has two qualities: wide dispersion, thereby promoting some amount of reflected sound, and a relatively constant directivity index, so that the direct sounds and reflected sounds have similar spectra.”

Now this is a place where I diverge somewhat from what Toole recommends: I propose taking Toole's “wide dispersion” radiation pattern and chopping it into TWO narrower patterns. One of these narrower patterns is aimed at the listening area, and the other is aimed in a different direction, such that it takes considerably longer to bounce its way to the listening area. One way to do this would be to trade Toole's wide monopolar pattern for the twin (and ideally somewhat narrower) lobes of a dipolar pattern, and then position the dipole speaker well out into the room to get that nice long reflection path length for the backwave.

And it just so happens that the very next section of Toole's book looks at findings of dipole advocate Siegfried Linkwitz.
 
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Duke LeJeune

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Seigfried Linkwitz was a brilliant man who experimented enthusiastically and while his findings were seldom the result of extensive controlled blind testing, Toole finds his comments to “lack obvious bias”.

Linkwitz built two loudspeakers which, after equalization, both had flat on-axis response. One was a very wide-pattern monopole and the other was a dipole. Linkwitz wrote that “in terms of timbre or perceived frequency response, both loudspeakers sound almost identical on program material. The main differences are in their spatial presentation.” [emphasis mine]

Recall from Klippel that the “feeling of space” made a 50% contribution to realism, and a 70% contribution to preference. Recall that “envelopment is the holy grail of concert hall design”, and is often absent when reproducing sound in small spaces. So the perceptual difference between dipoles and wide-pattern monopoles is arguably in an area that matters a lot!

Linkwitz goes on to say that “the different room reflection patterns due to the different polar responses becomes perceptually fused with the direct sound of the loudspeakers, if the reflections are sufficiently delayed and if their spectral content is coherent with the direct sound.” He later clarifies what he means by “sufficiently delayed”: “Reflections generated by the two loudspeakers should be delayed copies of the direct sound to the listener. The delay should be greater than 6 ms.”

So we have Linkwitz advocating what is basically a “Two Streams” approach, something Griesinger described: First we have the foreground stream of direct sound, and then at least 6 milliseconds later we have the background stream of reflections.
 
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Duke LeJeune

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The next section of Floyd Toole's book is somewhat autobiographical. In 1975 Toole had a house built with home audio in mind. In the smaller of his two listening rooms he used an unorthodox diagonal arrangement. Toole explains:

“This was deliberate. I think many people are unaware of the advantages of a diagonal arrangement. There are essentially no sidewall reflections.”

His other listening room was much larger, with a more conventional configuration and apparently it was his primary classical music listening room. Regarding loudspeaker choice for that room, Toole writes:

“Over the years [presumably starting in 1975], a parade of loudspeakers went through that room, and all disappointed... Then in 1989, a new loudspeaker came on the scene: the almost omnidirectional, bidirectional-in-phase “bipolar” Mirage M1. They performed well in double-blind listening tests in the small NRCC room, and also in this large one. They simply “became” the orchestra.”

That last sentence sounds like the unrestrained ravings of one of us!!

Toole later moved and now uses more conventional wide-pattern Revel speakers, and judging by the photo in his book early sidewall reflections are again largely absent (the room seems to be part of an “open floorplan” to the right-hand side, and extends at least another eight or ten feet beyond the speakers to the left). Toole reports that “stereo reproduction is very satisfying, but I still employ tasteful upmixing for many recordings to embellish the sense of space.” Note that the system's “sense of space” plays a prominent role in Toole's preference, as predicted by Klippel back at the beginning of this post. And could an “embellished sense of space” be something akin to “envelopment”?

The reverberant field needs to have enough, but not too much, energy (and by “reverberant field”, I mean “all of the reflections in the room”). Ideally the reflections should arrive no earlier than 6 milliseconds according to Linkwitz, though obviously the floor and ceiling reflections (which fortunately are perceptually fairly benign) typically arrive earlier than that.

Let me here restate two of the principles articulated by Griesinger, and then we will glance back at Toole's three rooms through that lens:

"The earlier a reflection arrives the more it contributes to masking the direct sound.”

“Envelopment is perceived when the ear and brain can detect TWO separate streams: A foreground stream of direct sound, and a background stream of reverberation. Both streams must be present if sound is perceived as enveloping."

In the diagonal room, the unorthodox configuration has “essentially no sidewall reflections”. So ignoring the floor and ceiling bounces (which are perceptually fairly benign), Griesinger's criteria are met: No early reflections, and a fairly long time delay between the direct sound and the onset of reflections at the listening position.

In the large classical music room early sidewall reflections are inherently present, and it took Toole fourteen years to find speakers which worked well in there. (Note that during this time he was director of the Canadian National Research Center which put him in a unique position to choose from among many high quality, professionally evaluated loudspeakers.) When he finally did find speakers which worked well in that room, they were bipolars, whose backwave energy arrives long enough behind the direct sound to meet Griesinger's “two streams” criterion.

In his current open-floorplan room early sidewall reflections are precluded by room geometry, and Toole adds a bit of late-onset reverberant energy via multichannel “tasteful upmixing for many recordings to embellish the sense of space.” Again all of Griesingers criteria are met: No significant early reflections, and an enhanced amount of fairly late-onset reverberant energy.
 
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Duke LeJeune

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Finally, let's turn to my mentor, Earl Geddes. These quotes come from an interview conducted by Larry Borden for Dagogo:

“Anechoic chambers are very poor listening rooms. They have no feeling of spaciousness since there are no reflections. What is generally viewed as the ideal in a playback system is one that has good imaging – locations of instruments – as well as good spaciousness – the feeling of being in an acoustic space.”

Again we see something akin to Griesinger's “envelopment” in Earl's “spaciousness”.

“Image perception is dominated by the very earliest sound from the speaker... Basically what one wants for good imaging is a pseudo point source response, i.e. a single direct sound free from any diffractions or reflections for at least 5, but hopefully a full 10 milliseconds... To get spaciousness you need a lot of reflections (preferably lateral) from places other than the direct sound for times greater than 10 ms.”

Griesinger said that early reflections tended to mask (degrade the clarity of) the direct sound, and now we have Geddes saying that early reflections also degrade imaging. So early reflections not getting much love from these guys. Earl further informs us that “spaciousness (aka “envelopment”?) requires a lot of reflections arriving later than 10 milliseconds. This is all consistent with Griesinger's Two Streams paradigm.

In conclusion, while Griesinger's ideas may seem quite unorthodox at first glance, we find them showing up in the works of Klippel, Linkwitz, and Geddes, as well as in Toole's personal listening rooms.

The reason I am drawn to Griesinger is because I find his “Two Streams” paradigm to be elegant, concise, and illuminating.

In a future post (or perhaps a future thread) I'll briefly touch on my own little journey as a speaker designer, including lessons learned the hard way, as seen through the lens of Griesinger's ideas.
 
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Blackmorec

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Well I have to say that I pretty much disagree with quite a bit of the above.:rolleyes:,

Firstly, one needs to differentiate first arriving reflections on the recording (which you want to hear) from those of the listening room (which you don’t). When they talk about first reflections, which do they mean? Those on the recording or those in the listening room. 2 quite different things....one good, one potentially bad.

Second, from a psychoacoustics standpoint, an early reflection in a concert hall would be a late arrival in a small listening room due simply to the time delays caused by distance. So, while early reflections could mask direct sound in a concert hall, a small room‘s early reflections are going to fall under the listener’s ‘echo threshold’ and only be heard as direct sound (Law of First Wavefront). In a small room, only those soundwaves that bounce more than once are going to have an arrival time outside the ‘echo threshold’ and they will certainly mask direct sound.
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Third, a stereo soundscape can be enveloping without detecting 2 separate ‘direct sound’ and ‘reflected sound’ or ambience. If this was true all you’d need are recordings with a lot of reverb. Of course those exist, and some do sound enveloping, while others don’t. By the same token, there are fully enveloping recordings without a great deal, if any reflected sound. The direct soundscape is enveloping on its own. What they do have is lots of air with lots of texture and Loooong decays that move across the soundscape, . ..allowing you to hear the decay dying away way in the distance. That’s not a reflection, its just a massive soundscape created by the recording engineer. Now it could be that the the same phase effects are in play to create envelopment, but its no longer true to say that a recording needs 2 separate fields of direct and reflected sound. That maybe the old way but modern recordings can create fully enveloping soundscapes without needing ’ambience’ and reflections.

The whole article feels like there’s a lack of separation between the actual music venue and the listening room. If you think about it, the recording should have everything it takes to make an enveloping recording....it should have the space, the air and the reverb (ambience). All the listening room needs to do is get the hell out of the way. if the listening room starts adding and contributing stuff, its going to add it to every single recording in the same way.....not at all what you want. :)

I think its also worth a quick word about dipolar speakers. You mention in one of your posts that moving dipolars away from walls and boundaries sounds better because it delays reflections, therefore proving that early reflections are detrimental, and late reflections are better. But on the whole, that’s not what’s going on.
Dipole speakers, Magnepans for example radiate bass in a figure 8 pattern, the sound pressure waves being generated equally front and back and phase cancelling at the sides. When the speakers are close to the wall and especially if their alignment is close to parallel with the wall, the backward wave is launched by the speaker diaphragm, travels a millisecond or 2 then crashes right back into the speaker membrane 180 degrees out of phase. When you consider the size of that membrane and the undamped and almost undecayed pressure wave that hits it, that’s what’s happening with the sound. Its a physical disturbance of the speaker membrane. Effectively the back radiating speaker is playing into an acoustic impedance similar to a horn, and the reflected wave is going to hit the membrane and cancel a part of the membrane’s energy and movement. Not good.
Moving dipoles away from walls and especially toeing them in simply gives the back wave some time, distance and angle to dissipate and lowers its acoustic impedance
 
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Duke LeJeune

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Well I have to say that I pretty much disagree with quite a bit of the above.
Thank you for disagreeing without being disagreeable! Even if you are convinced that I'm obviously an idiot, you responded with well thought-out analysis instead of taking the the easy path and just stating the obvious!

Firstly, one needs to differentiate first arriving reflections on the recording (which you want to hear) from those of the listening room (which you don’t). When they talk about first reflections, which do they mean? Those on the recording or those in the listening room. 2 quite different things....one good, one potentially bad.
All of the reflections that I'm talking about in my four posts above are listening room reflections, not the reflections which are already on the recording. I use the terms "First Venue" for the sound space captured on the recording (whether it's real or engineered or both), and "Second Venue" for the acoustic signature of the playback room.

I totally agree that we want to clearly hear the First Venue, and we do not want to hear (or be aware of hearing) the Second Venue.

I believe that applying Griesinger's Two Streams paradigm gives us the BEST chance of hearing the First Venue clearly, without the Second Venue spoiling the show for us, and I hope to explain that in this post. I was going to do a separate thread on the subject but it looks like now is the time and this is the place.

Second, from a psychoacoustics standpoint, an early reflection in a concert hall would be a late arrival in a small listening room due simply to the time delays caused by distance.
The ear determines the size of a room by the earliest reflections, so an early reflection in a small room is an early reflection in a small room, and a late reflection in a small room is a late reflection in a small room.

All of the information about the First Venue's acoustics is already on the recording. We're not trying to synthesize that by adding a few milliseconds of path length here and there; we are trying to effectively present the First Venue cues while degrading the Second Venue cues. This probably doesn't make sense yet but I think it will in a few minutes.

So, while early reflections could mask direct sound in a concert hall, a small room‘s early reflections are going to fall under the listener’s ‘echo threshold’ and only be heard as direct sound (Law of First Wavefront). In a small room, only those soundwaves that bounce more than once are going to have an arrival time outside the ‘echo threshold’ and they will certainly mask direct sound.
The Law of First Wavefront only applies to localization, not to loudness and perceived spectral balance. And even the localization cues are not locked in - sound images can be pulled towards an early reflection.

Also, don't get too caught up on the word "mask" as it pertains to the early reflections... perhaps "blur" or "smear" would be better words in this context.

In a small room if you have an early sidewall reflections, several things happen: The reflection can pull the apparent location of hard-panned images, such that you now get images appearing to the outside of the speakers. Toole reports that listeners generally like this broadening of the "apparent source width", as he calls it. This is one example of how early reflections blur the direct sound. Recording engineers do not like it because it blurs image location, and they want to hear exactly where the sound images are. Some audiophiles also do not like it for the same reason.

That early sidewall reflection can also cause coloration, particularly if its spectral balance is significantly different from that of the first-arrival sound. Even if the spectral balance is identical, there will inevitably be some comb filtering.

The early sidewall reflection can degrade clarity, because now you have multiple arrival times. This is another reason why recording engineers do not like early reflections. (Back in the 80's Live End/Dead End became a fairly common practice in high-end recording studios, the Dead end being where the speakers were located. The advantages claimed were improvements in clarity and image localization. The technique resulted in an overly dead space however, because when reflections inevitably bounced back into the Dead end they were absorbed.)

My point being, early reflections fuse with the direct sound such that what we perceive spectrally, and to a lesser extent spatially, is a weighted average.

Third, a stereo soundscape can be enveloping without detecting 2 separate ‘direct sound’ and ‘reflected sound’ ... its no longer true to say that a recording needs 2 separate fields of direct and reflected sound.
Envelopment calls for two separate streams, the direct sound and the relected sound, and the ones that we want to hear are already on the recording. The ones we don't want to hear - or more precisely, don't want to be aware of - are the ones caused by the playback room.

...modern recordings can create fully enveloping soundscapes without needing ’ambience’ and reflections.
Again, the reflections required for "envelopment" are hopefully already on the recording. What needs to happen is, those cues need to be PRESENTED effectively. And that means they need to arrive in abundance from many different directions. So the Second Venue reflections are in effect the CARRIERS for the highly desirable First Venue reflections, which is why we don't want to kill off all of the Second Venue reflections with too much absorption.

To be continued...
 
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Duke LeJeune

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The whole article feels like there’s a lack of separation between the actual music venue and the listening room.
That's because I didn't differentiate between the First Venue and the Second Venue in my first four posts. You are absolutely correct, the picture is incomplete without unpacking that topic. There is a lot involved in all of this, and I have a long ways to go in figuring out how best to present it.

If you think about it, the recording should have everything it takes to make an enveloping recording....it should have the space, the air and the reverb (ambience). All the listening room needs to do is get the hell out of the way. if the listening room starts adding and contributing stuff, its going to add it to every single recording in the same way.....not at all what you want
YESSS!!!

We are FINALLY getting around to the heart of the matter. I'm going to tell you how a Two Streams-friendly setup GETS THE LISTENING ROOM OUT OF THE WAY!

Let me start by borrowing a quote from a paper written by KlausR which he attached to a post in another thread:

“ The delay time of the early reflections determines the impression of the size of the room (Kuhl 1978).”

This is a basic principle and it applies to the First Venue cues, as well as to the Second Venue cues.

So IF "All the listening room needs to do is get the hell out of the way", then its WORST reflections are the early reflections! THOSE are the ones which superimpose a "small room signature" atop every recording.

Remember it is the DELAY TIME of those early reflections which determines the apparent room size. So, what would happen if we significantly INCREASED that delay time? Well, doing so would give the impression we're in a larger room! In other words, we can significantly reduce the WORST thing about a small room (from a spatial standpoint) by increasing the delay times of the earliest reflections!

The "Two Streams" approach does this. The time gap in between the direct sound stream and the reverberant sound stream - THAT is the KEY to suppressing "small room signature". THAT is the 6 milliseconds minimum that Linkwitz recommends, or imo better yet, the 10 milliseconds Geddes recommends.

One thing to be aware of - we don't want to rely too much on absorption, because we NEED a lot of Second Venue reflections coming from many directions as carriers, to effectively present the First Venue spatial cues to the ears. And absorption is far more effective on short wavelengths than on long ones, so unless we're careful (i.e. spend the big bucks on the right room treatment products), we are likely to skew the spectral balance of the reflections which is not a good idea (see the Linkwitz quote in Post #2 above).

So IF we can significantly delay most of the early reflections, the ear interprets this as "we're in a big room", and therefore LESS "small room signature" is superimposed atop the First Venue cues on the recording. In practice the ear gets mixed messages - SOME of the early reflection energy will not be significantly delayed. I think this may actually work in our favor, and somewhat "blur" the Second Venue cues. With the Second Venue cues fuzzily indicating a considerably larger room, the First Venue cues are no longer overshadowed and dominated by the Second Venue cues. So we can hear them more clearly.

In other words, a good Two Streams setup effectively "unmasks" the spatial cues on the recording by significantly weakening the most detrimental the First Venue cues, but WITHOUT killing off the later-arriving Second Venue reflections which have the essential role of delivering the First Venue spatial cues from many different directions.

In a future post I'll get into the specifics of one or two ways to accomplish this. It's much easier to do if you have a big room, but we'll look at a way to do it in a small room too.

I think its also worth a quick word about dipolar speakers....
Moving dipoles away from walls and especially toeing them in simply gives the back wave some time, distance and angle to dissipate and lowers its acoustic impedance
True, but that's not the only thing going on. The backwave has just as much energy as the frontwave, which is a significant amount to be adding to the reverberant field, and it can be our enemy or our friend depending on how long its arrival is delayed relative to the direct sound.
 
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Blackmorec

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Quick stream of consciousness:
a listening room’s reflections can’t be the carrier for the venues reflections because the timing is all wrong....they’ll arrive first, followed by the larger recording venue’s reflections. The time difference is directly proportional to the size of the rooms so if the room‘s bigger, the time’s longer....smaller, shorter...so the reflections appear at altogether different times

Then the two streams...direct sounds and reflections.
In a highly spacial and sound scape resolving system those 2 streams become one......The direct sound, its decay into the distance and its reverberation out of the distance...1 stream, because those sounds are actually connected....just the same wave at different times.

Then you say that The Law of First wavefront isn’t amplitude and frequency sensitive but i beg to differ. The brain examines the arriving first reflection and checks that it has a very close relationship to the direct sound. If the frequencies match and the amplitude is in line with the time delay (spls drop at a defined rate with distance so the brain can equate distance (by time delay) with amplitude drop for the round trip. If the frequencies don’t match up very closely and the amplitude drop doesn’t correspond to the time delay (distance) then you’ll hear it as a separate sound, otherwise as long as its under the echo threshold it’s summed with the original wave and heard as a single wave with the original wave‘s location.
 
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Duke LeJeune

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a listening room’s reflections can’t be the carrier for the venue's reflections...
Sure they can and yes they are! The listening room's reflection paths just add a few milliseconds.

If the listening room's reflections DON'T carry the venue's reflections to your ears, then HOW do they get there? Do you think they exist ONLY in the direct sound? They are in the direct sound as well, but that is the WORST possible direction for a reflection to come from (Toole).

Try listening in an anechoic chamber and see how spacious the recording sounds. Or if you don't have an anechoic chamber handy, read what someone experienced has to say:

"Anechoic chambers are very poor listening rooms. They have no feeling of spaciousness since there are no reflections." - Earl Geddes


Then the two streams...direct sounds and reflections.
In a highly spacial and sound scape resolving system those 2 streams become one......The direct sound, its decay into the distance and its reverberation out of the distance...1 stream, because those sounds are actually connected....just the same wave at different times.
Once the direct sound has traveled past your ears it becomes reverberant sound. Is that what you mean?

Then you say that The Law of First wavefront isn’t amplitude and frequency sensitive...
That's not really what I said; sorry for not being clear.

I understand that the ear looks at the spectrum and amplitude of a sound to determine whether or not it is a reflection. If it IS a reflection, then the DIRECTIONAL cues from that reflection are suppressed, so that they don't scramble the localization. But the ear is still picking up loudness and spatial/ambience cues from reflections whose directional cues are being suppressed. Those loudness cues translate into timbral cues (I can explain if you would like).

...as long as its under the echo threshold it’s summed with the original wave and heard as a single wave with the original wave‘s location.
Yes but the mechanism isn't 100% effective. A strong early sidewall reflection (occurring well within the echo suppression interval) can still shift the apparent location of a sound image.
 
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Mike Lavigne

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i did read the first 3 or 4 posts, and scanned the rest. you scientist types get too deep for me. however; as the owner of a purpose built listening room i'll share my views on the two separate streams concept.

“Envelopment is perceived when the ear and brain can detect TWO separate streams: A foreground stream of direct sound, and a background stream of reverberation. Both streams must be present if sound is perceived as enveloping.” (Implied is that there must be a time gap in between the two streams to differentiate them.)

my room resembles a concert hall in many ways. you can observe it with the Audiogon link in my signature. it's oval shaped, with hardwood cabinet covering all the walls and ceiling, lots of diffusion built in. including the ceiling. very little absorption other than carpet over the rear two thirds of the floor. the rear third has media shelves built in. my room is very live sounding, which was the idea. that i could always tame it but adding energy is very difficult.

i moved into my room in 2004, but it took me 10 years to get it to work right. and it had almost everything to do with controlling early reflections while retaining longer time late reflections. in 2014 i visited a room with the same speakers and electronics that i have and heard better imagining, although in other ways preferred my room. this room had cloth covering on the front walls.

a couple of months later i tried adding cloth to a point on my drop ceiling that was a natural first reflection from my speakers. wow! improvement. next more cloth to more parts of the ceiling. better. then cloth behind the speakers. better. and so on, and so on and so on. for 9 months i painstakingly applied cloth to flat wood cabinet surfaces and listened. i used thumb tacks and a hammer. i worked so much on the ceiling i felt like Michelangelo. what surprised me was places i would have never imagined early reflections came from. i had to just ignore geometry and try everything.

this was not always a linear effort.

what the cloth did was knock down the reflection without changing tonality. i had to be careful to retain energy. there were tricky parts especially next to my massive bass towers where a distortion reflection occurred. i had to use Aurial T-Fusors there to break it up.

now my imaging is much better, decay is much better, my room scales without limit. i have direct sound from my twin tower speakers, and then no early reflection, but lots of room energy and late reflections. i listen to lots of big music and it's glorious. i cannot overdrive the room acoustically; and i have 2000 watts on each bass tower, and 550 watts into 8 ohms on my 97db, 7 ohm passive towers. so lots of headroom in amplification.

it is the definition of enveloping.

as a bonus at controlling early reflections, i can sit up close, in the near-field, to my 7 foot tall 750 pound each twin towers and the sound is completely natural. to me that choice requires control of early reflections. those drove me back until i did this.
 
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(...) a couple of months later i tried adding cloth to a point on my drop ceiling that was a natural first reflection from my speakers. wow! improvement. next more cloth to more parts of the ceiling. better. then cloth behind the speakers. better. and so on, and so on and so on. for 9 months i painstakingly applied cloth to flat wood cabinet surfaces and listened. this was not always a linear effort.

what the cloth did was knock down the reflection without changing tonality. i had to be careful to retain energy. there were tricky parts especially next to my massive bass towers where a distortion reflection occurred. i had to use Aurial T-Fusors there to break it up.

(...) .
Mike,

Cloth will not knock down the reflections - it will just attenuate some limited bands, changing their spectra. Possibly it is what was needed in your room. In order to suppress first reflections we need thick absorbers - two inches of high density fiber glass are considered a minimum to suppress them, although just one inch thick panels suppresses them above 1 kHz.
 

Mike Lavigne

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Mike,

Cloth will not knock down the reflections - it will just attenuate some limited bands, changing their spectra. Possibly it is what was needed in your room. In order to suppress first reflections we need thick absorbers - two inches of high density fiber glass are considered a minimum to suppress them, although just one inch thick panels suppresses them above 1 kHz.
Micro,

of course, i won't argue the technical aspect of cloth over hardwood and controlling reflections.

but......my guess is that what is called upon in controlling reflections has much to do with proximity. my room is quite large, 21' wide, 29' long, and 11' tall, and the distance from my tweeters and mid range drivers (the passive main towers are 115" wide tweeter to tweeter in a 21' wide room) involved in the reflection frequencies is considerable. and since there are bass towers outside the main towers blocking any near wall reflections, the side reflections are only to the opposite wall. the rear wall behind the speakers is 9 feet from the tweeter plane.

the reflections are already relatively later due to the longer distances compared to most rooms, so the need to be controlled reduced. and i'm not looking for maximum reflection control, i'm wanting to do the least amount needed to retain overall musical energy.

my point being that there is not much energy to dissipate at any one spot. so the cloth is sufficiently capable in this particular situation.
 
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Duke LeJeune

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#14
i did read the first 3 or 4 posts, and scanned the rest...
Thank you VERY MUCH Mike for dropping in and posting about your room in the context of this thread. I appreciate your willingness to spend time sharing your knowledge and experience when you could be listening!!

my room resembles a concert hall in many ways...

it took me 10 years to get it to work right. and it had almost everything to do with controlling early reflections while retaining longer time late reflections.
Sounds familiar, though I've never been involved with a room like yours.

now my imaging is much better, decay is much better, my room scales without limit. i have direct sound from my twin tower speakers, and then no early reflection, but lots of room energy and late reflections. i listen to lots of big music and it's glorious. i cannot overdrive the room acoustically...

it is the definition of enveloping.

as a bonus at controlling early reflections, i can sit up close, in the near-field, to my 7 foot tall 750 pound each twin towers and the sound is completely natural. to me that choice requires control of early reflections. those drove me back until i did this.
Beautiful!!
 
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KlausR.

Well-Known Member
Dec 13, 2010
284
23
93
#15
Hi Duke,

I started reading the thead and I join Blackmorec in not agreeing with your POV.

One issue in passing:

The ear determines the size of a room by the earliest reflections ...
The delay of reflections depends on locations of source and listener. You move one (or both) and the delays change. Does the perceived room size change as well? What Kuhl stated in his 1978 paper apparently is not true, as I have pointed out with reference to more recent psychoacoustic research, where reverb. time was identified as main suspect.

Klaus
 
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Blackmorec

Well-Known Member
Feb 1, 2019
296
370
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67
#16
Sure they can and yes they are! The listening room's reflection paths just add a few milliseconds.

If the listening room's reflections DON'T carry the venue's reflections to your ears, then HOW do they get there? Do you think they exist ONLY in the direct sound? They are in the direct sound as well, but that is the WORST possible direction for a reflection to come from (Toole).
When you’re playing a piece of recorded music, the loudspeakers launch both the venue’s recorded original sound AND the venue‘s recorded reflections....you hear both....you hear the direct sound and at a time appropriate to the venues size, you hear the reflections....with no help whatsoever from the listening room.

But, if the listening room is contributing its reflections then what you get is the direct venue sound from the loudspeakers, followed by the listening room’s early reflection at a time appropriate to the listening room’s size, followed by the recorded venue’s reflections at a time appropriate to the venue‘s size. In other words, 2 sets of early reflections....one early because the listening room is small and one later, because the recording venue is much larger. Not what you want.
What you want is for the listening rooms early reflections to be either combined with the direct sounds via Law of Fist Wavefront’ or in larger rooms absorbed. Either way, you don’t want them because they don’t belong in the sonic picture of Venue’s direct sounds plus reflections.

Try listening in an anechoic chamber and see how spacious the recording sounds. Or if you don't have an anechoic chamber handy, read what someone experienced has to say:
"Anechoic chambers make very poor listening rooms. They have no feeling of spaciousness since there are no reflections." - Earl Geddes

Listening to music in an anechoic chamber is like listening to the starved version....it sounds thin and lacking


Once the direct sound has traveled past your ears it becomes reverberant sound. Is that what you mean?
No.....essentially you only hear sound that’s moving past your ear drums. What I’m saying is that you hear sound, its decay and then its reflection. When you hear its decay, you hear it decaying in a direction...e.g the sound starts strong on right and dies away to the left, from where the reverberation emerges, much quieter but stronger from the left and decaying to the right. This is all one event....the original sound wave passing through the venue, gradually dying away, then bouncing back with much reduced amplitude from the wall of the venue. The space and immersion comes from the dying away into the distance, as well as from the reverberation...its all just the same wave....not two events. If its two events, there a piece of the soundscape missing. The piece that comprises venue ’air’, ‘space’, and ‘texture’....the piece you can still hear on highly resolving systems....not total silence....the presence of ambient space that the sound wave is passing through. That’s true immersion, when the listener is immersed in air and space that‘s alive and belongs to the recording venue.


I understand that the ear looks at the spectrum and amplitude of a sound to determine whether or not it is a reflection. If it IS a reflection, then the DIRECTIONAL cues from that reflection are suppressed, so that they don't scramble the localization. But the ear is still picking up loudness and spatial/ambience cues from reflections whose directional cues are being suppressed. Those loudness cues translate into timbral cues (I can explain if you would like).
When a sound wave is identified as a reflection that falls within the listener’s ’echo’ threshold, those reflection sound waves are summed together with the original wavefront so what you hear is a single event with the original direction and the amplitude and frequency of the original soundwave plus the reflection.....so you don’t hear the reflection and if you can’t hear it, it can’t have ‘loudness’ and ‘spatial/ambient clues’, since those elements have become part of the original sound wave in terms of hearing.



Yes but the mechanism isn't 100% effective. A strong early sidewall reflection (occurring well within the echo suppression interval) can still shift the apparent location of a sound image.
 
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KlausR.

Well-Known Member
Dec 13, 2010
284
23
93
#17
Hello Duke,

Finally, let's turn to my mentor, Earl Geddes. These quotes come from an interview conducted by Larry Borden for Dagogo:

“Image perception is dominated by the very earliest sound from the speaker... Basically what one wants for good imaging is a pseudo point source response, i.e. a single direct sound free from any diffractions or reflections for at least 5, but hopefully a full 10 milliseconds... To get spaciousness you need a lot of reflections (preferably lateral) from places other than the direct sound for times greater than 10 ms.”
There is a much more interesting passage you did not quote:

"Image perception is dominated by the very earliest sound from the speaker, i.e. the direct sound (first arrival), and the sound that arrives in the first 5-10 milliseconds. The ear simply integrates this all into one lumped sum. This includes the speakers’ anechoic response along the listening axis, cabinet diffractions, and diffractions and reflections from nearby objects like equipment cabinets or televisions. Basically what one wants for good imaging is a pseudo point source response, i.e. a single direct sound free from any diffractions or reflections for at least 5, but hopefully a full 10 milliseconds. Let me call these Very Early Reflections VER (but they will also include the early diffractions as well)."

As Earl describes it these VER would be from the same direction as the direct sound. In his AES paper 6888 (Audibility of Linear Distortion with Variations in Sound Pressure Level and Group Delay) he finds that sound (in this case linear distortion) delayed by 0-1 ms is more audible when delay increases. 0-1 ms is the range where cabinet edge diffraction might play a role. Ando finds that reflections arriving within 2-3 ms, such as from surfaces very close to loudspeakers, produce high interaural cross-correlation and are the least beneficial.

At 5 m listening distance 10 ms delay means the reflection is down by 4.5 dB, at 6 ms delay it is down by 3 dB. All first reflections coming later are still well above perception threshold, so what is the reasoning behind these figures? Why are late first reflections good and early first reflections bad, when both are above perception thresholds?

Klaus
 
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Duke LeJeune

[Industry Expert]/Member Sponsor
Jul 22, 2013
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Princeton, Texas
#18
a couple of months later i tried adding cloth... what the cloth did was knock down the reflection without changing tonality. i had to be careful to retain energy...

now my imaging is much better, decay is much better, my room scales without limit.
Cloth will not knock down the reflections - it will just attenuate some limited bands, changing their spectra.
Micro, I wondered about that too, and here is my guess as to what MIGHT be happening:

Mike's speakers have wide dispersion all the way up the spectrum because their ribbon tweeter is very narrow. My understanding is that high frequencies carry a lot of our imaging localization cues, so assuming Toole is right about early reflections affecting sound images, having a LOT of high frequencies in those early reflections might result in correspondingly greater effect on sound images.

Cloth would only be an effective absorber at very short wavelengths (very high frequencies); its effect on tonality would probably be minimal and might well be beneficial. Let me explain:

I'm going to pick a nit with Linkwitz, who I quoted as saying: "Reflections generated by the two loudspeakers should be delayed copies of the direct sound to the listener."

Imo, reflections which are "delayed copies" may not actually be ideal. I read an article by Robert E. Greene in which he talks about the attenuation of high frequency energy in the reflections in a concert hall due to the sheer path lengths involved, the implication being that we want the reverberant field to have a little bit of top-end rolloff relative to the direct sound. I think Mike's cloth may be doing exactly that, in addition to improving the imaging.

Also, bringing down the highs in the reverberant field just a little bit would probably contribute to the system being able to scale to "very loud" without becoming fatiguing.

(After reading Greene's article I experimented with reducing the top-end energy in the reverberant field independent of the direct sound, and in my opinion that was beneficial.)

In order to suppress first reflections we need thick absorbers - two inches of high density fiber glass are considered a minimum to suppress them, although just one inch thick panels suppresses them above 1 kHz.
One argument against relying heavily on absorption of early reflections is that, unless the material is very thick, it doesn't attenuate longer wavelengths very much. So on the one hand absoprtion significantly reduces the amount of energy in the early reflections, but on the other hand it does so by absorbing short wavelengths much moreso than longer ones. Thus the spectral balance of the reflections changes, which can be undesirable. So absorption can be a two-edged sword.

Also as Mike noted, he "had to be careful to retain energy...", and absorption works against that. Whatever energy strikes the absoptive material, be it an early or a late reflection, wavelengths where the material is most effective are essentially removed from the reflection.
 
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Duke LeJeune

[Industry Expert]/Member Sponsor
Jul 22, 2013
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Princeton, Texas
#19
The delay of reflections depends on locations of source and listener. You move one (or both) and the delays change. Does the perceived room size change as well?
In my experience, it can!

Or to be more precise, the Second Venue cues can be reduced relative to the First Venue cues, such that we become less effected by the former and moreso by the latter.

What Kuhl stated in his 1978 paper apparently is not true, as I have pointed out with reference to more recent psychoacoustic research, where reverb. time was identified as main suspect.
BOTH play a role.

So from the perspective of loudspeaker/room interaction, imo it is desirable to push the onset of strong early reflections back in time, AND to provide a well energized reverberant field so that it does not die away into inaudibility too quickly.
 

Duke LeJeune

[Industry Expert]/Member Sponsor
Jul 22, 2013
518
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Princeton, Texas
#20
What you want is for the listening rooms early reflections to be either combined with the direct sounds via Law of First Wavefront’ or in larger rooms absorbed. Either way, you don’t want them because they don’t belong in the sonic picture of Venue’s direct sounds plus reflections.
I might quibble about the best way(s) to address the listening room's (Second Venue's) early reflections, but definitely agree that "they don't t belong in the sonic picture of Venue’s direct sounds plus reflections."

This is all one event....the original sound wave passing through the venue, gradually dying away, then bouncing back with much reduced amplitude from the wall of the venue.
I think we are disagreeing over definitions of terms. How about this:

We perceive one "event", which is composed of two "streams". I'll quote your words:

"the original sound wave passing through the venue" = the direct sound = the FIRST stream.

"gradually dying away, then bouncing back with much reduced amplitude from the wall of the venue" = the reverberant sound = the SECOND stream.

When a sound wave is identified as a reflection that falls within the listener’s ’echo’ threshold, those reflection sound waves are summed together with the original wavefront so what you hear is a single event with the original direction and the amplitude and frequency of the original soundwave plus the reflection.....so you don’t hear the reflection and if you can’t hear it, it can’t have ‘loudness’ and ‘spatial/ambient clues’, since those elements have become part of the original sound wave in terms of hearing.
I think we have been using the word "hearing" differently. I have been using it to mean "the sound goes into your ear and has an effect", and I THINK you have been using "hearing" it to mean "the integrated sonic event that you perceive, regardless of how many sounds go into creating it."

Your way is probably the correct use of the term.

I think that we agree on this: The ear integrates the direct sound, the early reflections, and the later reflections into a single perception event. We are not aware of the reflections as separate sounds (unless they arrive late enough and strong enough to be perceived as echoes).

As you put it, "those elements [contributed by the reflections] have become part of the original sound in terms of hearing."

So I THINK we are on the same page... and maybe I've just been holding the book upside-down??
 

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