Bass Trap and general room treatment advice please

Bighairybloke

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Apr 3, 2017
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Hi,

I am building a new home cinema/critical listening room. The dimensions are 5.2m wide by 5.6m long by 3m high. The walls are 100year old 10"+ solid rock/lime mortar mix, so pretty solid. The floor is concrete with a small amount of insulation underneath. The ceiling will be 3/4" ply.

The plan is to line the walls with paper backed rockwool panels with the soft side facing the room at the front sides and rear and with the paperside facing towards the room at the rear side walls and on the bass traps...

For the bass traps, I plan on making wood frames for two foot square traps on the side and rear ceiling corners and all vertical corners. Rather than use cut up panels stacked on top of each other, my builder has suggested lining the frames with geotextile membrane and filling the resulting bags with blown-in fibreglass fibres from the loft space.

This sounds a good solution to me, and because the fibre is blown in, the light fluffyness of the fibres is maximised and, from what I have read, light and fluffy beats firmer rockwool or fiberglass wool panels when the bass traps are quite substantial.

The ceiling, between the basstraps will be treated with 3" acoustic wedge panels. When I am happy with the sound of the room, everything will be covered in fabric to make it all look smart.

I would welcome any comments on my plan, before I set-to:)

Many thanks,

Steve
 

microstrip

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Hi,

I am building a new home cinema/critical listening room. The dimensions are 5.2m wide by 5.6m long by 3m high. The walls are 100year old 10"+ solid rock/lime mortar mix, so pretty solid. The floor is concrete with a small amount of insulation underneath. The ceiling will be 3/4" ply.

The plan is to line the walls with paper backed rockwool panels with the soft side facing the room at the front sides and rear and with the paperside facing towards the room at the rear side walls and on the bass traps...

For the bass traps, I plan on making wood frames for two foot square traps on the side and rear ceiling corners and all vertical corners. Rather than use cut up panels stacked on top of each other, my builder has suggested lining the frames with geotextile membrane and filling the resulting bags with blown-in fibreglass fibres from the loft space.

This sounds a good solution to me, and because the fibre is blown in, the light fluffyness of the fibres is maximised and, from what I have read, light and fluffy beats firmer rockwool or fiberglass wool panels when the bass traps are quite substantial.

The ceiling, between the basstraps will be treated with 3" acoustic wedge panels. When I am happy with the sound of the room, everything will be covered in fabric to make it all look smart.

I would welcome any comments on my plan, before I set-to:)

Many thanks,

Steve

IMHO you will have a difficult room to deal with. It is almost a square, and very solid walls. Solid walls are a no in such cases - they do not absorb bass and reflect the bass energy, creating peaks and nulls in the frequency response. IMHO you will need a good experienced acoustician to help designing the bass traps of your room, as just using rockwood will not be enough. My room has two feet thick massive stone walls and the only way to manage bass quality was adding very large membrane bass traps.
 

EccentricDyslexic

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Jul 8, 2012
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Thanks microstrip, I recognise its not ideal, hence going so heavy with basstraps. Also, the infinite baffle subs in the loft space placed at the 1/4-1/3 wavelength area should ensure a decent bass response. Time will tell though:) Heres a sketchup model I made - https://youtu.be/NA2yIPEaltI

Cheers,

Steve
 

Bighairybloke

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Apr 3, 2017
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I have had some thoughts on the ceiling, I was going to have a plywood ceiling with foam wedges glued on, but now I am thinking of doing away with the ceiling and wedges and stapling some acoustic transparent fabric across in its place and then filling the super chunks, soffits and space between the joists from above in one go with blown-in light and fluffy rockwool/fibreglass and then boarding over the top of the joists. Here's a video I have done using sketchup.

https://youtu.be/9qaqpgumWzU

I have checked that there are no first reflection points on proposed corner traps and soffits.

https://youtu.be/20Y_ZuAns58

Cheers,

Steve
 

flyer

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The fact that you are paying attention to the bass from the start is excellent. The fact you do that in the corners and rear ceiling corners is very good. But the fact you intend to do this with real bass traps, i.e. filled with insulation , well... not so good in my opinion.

You intend to kill/attenuate the bass energy. By doing so, you will have to put the volume harder to compensate for that loss of energy. By putting the volume harder, you will be faced with a load of other problems in the mids and highs which, to solve them as well, you will be adding in even more absorbers. This may turn into a never ending story because each time you absorb excessive energy (or rather energy that is not good enough diffused), you will want to compensate that by addding power in your system and each time other phenomenons pop up, until all your walls will be covered with absorbers and you are frustrated you do not get hall ambiance you look for in the recording, nor the stage width, etc.

I know I paint a pretty grim image here, sorry for that. I am just not a fan of bass trapping ... bass.

Microstrip refers to membrane traps of which are already much better suited. But given your (unfinished) room condition, the fact you have space for it, the fact you intend to fill corners up with traps anyway, I would firmly recommend to go with adjustable helmholtz resonators: no loss of energy, tunable till the lowest frequencies (and automatically their harmonics) and flattening out the bass response.
 

KlausR.

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Dec 13, 2010
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Steve,

Bighairybloke said:
I am building a new home cinema/critical listening room. The dimensions are 5.2m wide by 5.6m long by 3m high. The walls are 100year old 10"+ solid rock/lime mortar mix, so pretty solid. The floor is concrete with a small amount of insulation underneath. The ceiling will be 3/4" ply.

The only issue I would address at this stage is reverberation time. With the walls and floor you describe it must be rather high(ish), just a guess. Since you have ceiling height an acoustic ceiling could be an option. In our living/listening room (8x5x3 m, walls of brick 'n mortar, concrete floor with tiles) I have a stretched synthetic fabric at 2.5 m with the space above loosely filled with rock wool, WAF = 1. Acoustics is dry but not too dry. That ceiling also treats the room modes, both vertical and horizontal.

Should room modes be an issue active treatment such as source-to-sink subwoofers could be a option. Some of these subwoofer based approaches have been tested in an AES paper, which I could send via email if you are interested:

http://www.aes.org/e-lib/browse.cfm?elib=16324

As far as early reflections are concerned, there is no evidence that these are detrimental as a matter of principle. I had a good look at the available scientific literature and have prepared a write-up.

Klaus
 

KlausR.

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Dec 13, 2010
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flyer said:
You intend to kill/attenuate the bass energy. By doing so, you will have to put the volume harder to compensate for that loss of energy.

The only substantial contributors to sound pressure level at listening position I know of are direct sound and first reflections. Corner traps don't affect any of these so why would the level be lowered?

Klaus
 

Bighairybloke

New Member
Apr 3, 2017
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Steve,



The only issue I would address at this stage is reverberation time. With the walls and floor you describe it must be rather high(ish), just a guess. Since you have ceiling height an acoustic ceiling could be an option. In our living/listening room (8x5x3 m, walls of brick 'n mortar, concrete floor with tiles) I have a stretched synthetic fabric at 2.5 m with the space above loosely filled with rock wool, WAF = 1. Acoustics is dry but not too dry. That ceiling also treats the room modes, both vertical and horizontal.

Should room modes be an issue active treatment such as source-to-sink subwoofers could be a option. Some of these subwoofer based approaches have been tested in an AES paper, which I could send via email if you are interested:

http://www.aes.org/e-lib/browse.cfm?elib=16324

As far as early reflections are concerned, there is no evidence that these are detrimental as a matter of principle. I had a good look at the available scientific literature and have prepared a write-up.

Klaus

Thanks Klaus, I'd appreciate any info you have, can you pm me? I have not heard of source-to-sync subwoofers. That will be an interesting read. Due to the positioning of the IB subwoofers at the anticipated room mode nulls, I am hopeful room modes will be kept in check. The copious bass traps are there to control bass decay times as much as possible.

Do you have any pictures of your cieling? I am envisioning using a fabric for holding the loose fill in place and another layer (black Devore velvet) for the WAF layer.

I'd appreciate your thoughts on the proposed treatment of the wall, i.e. Having the insulation bats with alu foil side facing towards the wall at the first reflection points and rear wall and facing the room elsewhere to add a bit of life back in to the room. I.e. The back of the room at the sides and also on all sofit and corner bass traps too.

Regards,

Steve
 

Bighairybloke

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Apr 3, 2017
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The fact that you are paying attention to the bass from the start is excellent. The fact you do that in the corners and rear ceiling corners is very good. But the fact you intend to do this with real bass traps, i.e. filled with insulation , well... not so good in my opinion.

You intend to kill/attenuate the bass energy. By doing so, you will have to put the volume harder to compensate for that loss of energy. By putting the volume harder, you will be faced with a load of other problems in the mids and highs which, to solve them as well, you will be adding in even more absorbers. This may turn into a never ending story because each time you absorb excessive energy (or rather energy that is not good enough diffused), you will want to compensate that by addding power in your system and each time other phenomenons pop up, until all your walls will be covered with absorbers and you are frustrated you do not get hall ambiance you look for in the recording, nor the stage width, etc.

I know I paint a pretty grim image here, sorry for that. I am just not a fan of bass trapping ... bass.

Microstrip refers to membrane traps of which are already much better suited. But given your (unfinished) room condition, the fact you have space for it, the fact you intend to fill corners up with traps anyway, I would firmly recommend to go with adjustable helmholtz resonators: no loss of energy, tunable till the lowest frequencies (and automatically their harmonics) and flattening out the bass response.

Thank you flyer, I was under the impression that by trapping the bass, or rather reducing its unwelcome hanging-around is a good thing. The loss of energy is not so much of a concern as the IB subwoofers in the cieling will be pretty powerful and have meaty amp.

Ps:love some of that high end gear you sell:)

Regards,

Steve
 

flyer

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Dec 16, 2012
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The only substantial contributors to sound pressure level at listening position I know of are direct sound and first reflections. Corner traps don't affect any of these so why would the level be lowered?

Klaus

Hi Klaus, I agreed with your previous post regarding the ceiling solution which, if well worked out, effectively will address both the bass issue as the reverb time. To do so, ideally the ceiling would have to be a mix of diffusing and absorbant effect and along the wall/ceiling corners air tight resonators tuned to the problematic frequencies. Again, that is ideally... what you describe is certainly better than nothing, albeit there is a danger to deaden the sound too much as well. Proper execution...

Regarding your question above. SPL is a result of all the energy arriving at one's ears, direct, 1st, 2nd, 3rd, ..., 100th etc reflections... You are right that the direct sound and first reflection will have highest amplitude.

Corner traps will have effect on bass (depending of course how they are conceived and executed), but let's assume just 'bass' as a generic. Speakers are designed to be as linear as possible, not taking room modes into account.
If you, through bass treatment, want to handle those room modes then there are two ways:

either you absorb the bass energy (converted to heat (well, think of millidegrees) by the absorbing material) => bass trapping
or you store the excess energy of the bass (think milliseconds as well) and release it again when the null follows after that excess => (helmholtz) resonator

In the case of bass trapping, you are not really dealing with the room modes, you only combat the effects of it. With a resonator, you are root solving the modes since you are simulating then a larger room than it really is.

Beware, that is all quite theoretical and none of the two above statements is ever going to be 100% accurate, far from it.

What is not theoretical is the following: Room modes will reinforce or null bass frequencies (and their harmonics till well in the medium range). If you absorb (without releasing again like a HH resonator does) the reinforced frequencies, then only the nulls and the lesser excesses are remaining to hear... so, you will want to regain a certain pressure in the bass, resulting in increasing the volume to compensate for that. But now you start to have too much energy in the mids and/or highs (depending on room etc), so you want to attenuate those as well...


Anyway, my theory is the following:
First make sure the bass room modes are taken care of (and that is not so easy, reason why a tunable resonators is the best available).
Then make sure there is enough diffusion at the first reflection points (if you put absorbers there, you will not get a reflection from that point and hence you kill again sound and above all reduce sound stage and density)
And once the two above are done, only then evaluate the room reverb time + its value in the frequency range. You must then foresee absorbing material to get the reverb time down to an acceptable level. You should also take possible flutter echo into account whilst treating the reverb, if properly placed you might kill two phenomenons at once.


I struggled for 10 years with absorbers etc before I started to take classes in acoustics for 6 months.

PS: sorry but I am not into subs or DSP solutions... I think these should be added at the end to finetune, not at the beginning to counter root causes, but that's me :)
 
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flyer

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Thank you flyer, I was under the impression that by trapping the bass, or rather reducing its unwelcome hanging-around is a good thing. The loss of energy is not so much of a concern as the IB subwoofers in the cieling will be pretty powerful and have meaty amp.

Ps:love some of that high end gear you sell:)

Regards,

Steve

If you counter the room modes by placing different subs, you have a solution as well, albeit they also take place, current and money... I could then regard a sub as an 'active helmhotz resonator', so we are in sync you might say :)

Thanks for the compliment on the gear.

Best,
Michel
 

KlausR.

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Dec 13, 2010
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Hi Steve,

Bighairybloke said:
Due to the positioning of the IB subwoofers at the anticipated room mode nulls, I am hopeful room modes will be kept in check. The copious bass traps are there to control bass decay times as much as possible.

PM is sent. Source-to-sink subs are placed in room mode peaks, that is against walls, and drive the modes in opposite phase, which results in cancelling them. For generating a flat wave front, which you'd need for optimum results, two subs work better than one. IMO subs are less intrusive than huge bass absorbers.

Do you have any pictures of your cieling? I am envisioning using a fabric for holding the loose fill in place and another layer (black Devore velvet) for the WAF layer.

For this ceiling mounts (metal rails) have to be fixed to the walls at the desired height, then the room has to be heated to about 50 degrees C, the fabric is inserted into the rails and clamped, when cooling down the fabric shrinks and is put under tension. Pics can be found here:

https://www.plameco.co.uk/

Look under "assortment".


I'd appreciate your thoughts on the proposed treatment of the wall, i.e. Having the insulation bats with alu foil side facing towards the wall at the first reflection points and rear wall and facing the room elsewhere to add a bit of life back in to the room. I.e. The back of the room at the sides and also on all sofit and corner bass traps too.

I never felt the need for acoustic treatment other than the ceiling for the purpose of getting reverberation time down. Radiation pattern of my speakers in combination with placement on the long wall gets side wall reflection levels down to almost perception thresholds, should that ever be of concern to me, which it is not. In the 15 years that we have this room I've found only a handfull of tracks where a mode is audibly excited, which is what I can live with. This said I did not explore absorbers and related physics, so I can't say anything about that. In any case absorbers for bass and first reflections can always be installed later, so I would first put only what is needed for getting reverb time down to acceptable levels, put furniture and so forth, and listen to how that sounds.

Klaus
 

KlausR.

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Dec 13, 2010
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Hi Michael,

What is not theoretical is the following: Room modes will reinforce or null bass frequencies (and their harmonics till well in the medium range). If you absorb (without releasing again like a HH resonator does) the reinforced frequencies, then only the nulls and the lesser excesses are remaining to hear... so, you will want to regain a certain pressure in the bass, resulting in increasing the volume to compensate for that. But now you start to have too much energy in the mids and/or highs (depending on room etc.), so you want to attenuate those as well…

Some theory: a room mode is a particular case of a standing wave. A standing wave is the superposition of a first wave travelling towards a boundary, say a wall, and a second wave, which is the reflection, in a direction opposite to the first wave train. During reflection the incident wave changes phase by 180 degrees, so the reflected wave is in opposite phase. The amplitudes of the two waves are summed up and, depending on the phase of the two waves, the resulting amplitude is equal to or larger or smaller than the amplitude of the first wave, with points where the amplitude is maximum and points where the amplitude is minimum. The position of these points within the room is fixed hence the term standing wave. Standing waves exist at all frequencies, and at some frequencies, the so called eigenfreqencies, the situation may occur that the resulting standing wave has the maximum possible peak amplitude and lowest possible dip amplitude. This depends only of the position of the loudspeaker with respect to the wall.

If you now place an absorber against the wall, the first wave travelling towards the wall has to run through that absorber and will lose energy. The reflected wave, which already at the outset has less energy, runs through the absorber and, again, loses energy. It is still in opposite phase and still interferes with the first wave and will generate a standing wave, but because of its lower energy level, and hence amplitude, the standing wave has less energy, hence less amplitude, which means that the peak amplitude is lower and the dip amplitude is higher.

Only when there is no reflected wave at all, like in the anechoic chamber, there is no standing wave. As I understand it, a sink subwoofer would do the same, virtually eliminate the reflection.

There are no frequencies where you have reinforcement and others where you have nulls, every standing wave, hence every mode, has locations in the room with reinforcement and others with nulls. As a consequence, an absorber does not absorb the reinforced frequencies and leaves the nulls untouched, it lowers the energy level of the reflected wave, at all frequencies.

Not familiar with how Helmholtz resonators work, hence no comments.

With corner traps there are still reflections from the walls, floor and ceiling, which, on top of the direct sound, are the major contributors to the SPL at listening position, so I don’t think that they have an effect which has to be compensated. But I could be wrong, are there any measurements showing how these traps change SPL at listening position, if at all?



And once the two above are done, only then evaluate the room reverb time + its value in the frequency range.

This is what is commonly said, but I found no evidence that the frequency response of reverb times typical for small rooms has an audible effect.

You must then foresee absorbing material to get the reverb time down to an acceptable level.

The question is: what is an acceptable level. Again, there is no evidence that a typical reverb time for domestic rooms of say 0.6 seconds is bad.


You should also take possible flutter echo into account whilst treating the reverb, if properly placed you might kill two phenomenons at once.

Flutter echoes are generated with impulsive sounds and only when the following condition is met:

t << l/c
with

t= impulse length
l= distance between walls
c= speed of sound

For a typical room of 8 x 5 m the necessary impulse lengths would have to be much smaller than 23,5 and 14,7 ms, respectively.


Klaus
 
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Bighairybloke

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Thanks Klaus for the documents regarding early reflections, I hadn't realised there was doubt in the. General acceptance that first reflections were bad. For my room, I won't instal WAF velour until after listening tests and REW sweeps satisfy me. I am curious about the first reflections, as intended, I can experiment with the aluminium foil covered insualtaion panels facing in or out so will do just that in the side walls. For the rear I will face the foil away from the room, and maybe devise some refraction panels for hanging.
For the front wall, I had assumed I was going to face the foil towards the room, now I'm not so sure!

Regards,

Steve
 

KlausR.

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Dec 13, 2010
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Steve,

Bighairybloke said:
Thanks Klaus for the documents regarding early reflections, I hadn't realised there was doubt in the general acceptance that first reflections were bad.

Don't get me wrong: I'm not saying that reflections are good, always, whatever the circumstances are. However, the general idea is that reflections are bad, always, whatever the circumstances are, and precisely for that there is not the slightest shred of evidence. Louspeakers with irregular frequency response may cause trouble, acoustic left-right asymmetry may cause trouble, too low a reverberation time may cause trouble. There is no general approach, one has to look at every case individually. And no, measurements won't tell you anything unless they are correlated to human hearing.

And don't forget:
J. Gordon Holt said:
The listener is the heart of the high-fidelity system, and is noted for having high distortion, poor frequency response, marginal stability, and arbitrarily variable performance characteristics. Listener instability is the most common defect in a high-fidelity system, which is why manufacturers recommend that the ears be checked periodically by a qualified service agency to ensure that they are meeting their specifications.

Klaus
 
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microstrip

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(...)

And don't forget:

Klaus

Thanks for reminding us of this 1959 great article. I can't resist quoting an extra sentence, my preferred since long:

GordonHolt said:
The final link in the high-fidelity sausage is the listening room. The room itself does not listen, but is called a Listening room because it contains a listener.

WBF readers can find the whole text here https://archive.org/stream/am-1959-10/Audio-1959-Oct_djvu.txt - it is filled with delicious sentences such as

GordonHolt said:

Every turntable has a motor, futuristic styling, and lower rumble than all competing brands tested by an Independent Consumer Investigations Laboratory.
 

flyer

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Hi Michael,



Some theory: a room mode is a particular case of a standing wave. A standing wave is the superposition of a first wave travelling towards a boundary, say a wall, and a second wave, which is the reflection, in a direction opposite to the first wave train. During reflection the incident wave changes phase by 180 degrees, so the reflected wave is in opposite phase. The amplitudes of the two waves are summed up and, depending on the phase of the two waves, the resulting amplitude is equal to or larger or smaller than the amplitude of the first wave, with points where the amplitude is maximum and points where the amplitude is minimum. The position of these points within the room is fixed hence the term standing wave. Standing waves exist at all frequencies, and at some frequencies, the so called eigenfreqencies, the situation may occur that the resulting standing wave has the maximum possible peak amplitude and lowest possible dip amplitude. This depends only of the position of the loudspeaker with respect to the wall.

If you now place an absorber against the wall, the first wave travelling towards the wall has to run through that absorber and will lose energy. The reflected wave, which already at the outset has less energy, runs through the absorber and, again, loses energy. It is still in opposite phase and still interferes with the first wave and will generate a standing wave, but because of its lower energy level, and hence amplitude, the standing wave has less energy, hence less amplitude, which means that the peak amplitude is lower and the dip amplitude is higher.

Only when there is no reflected wave at all, like in the anechoic chamber, there is no standing wave. As I understand it, a sink subwoofer would do the same, virtually eliminate the reflection.

There are no frequencies where you have reinforcement and others where you have nulls, every standing wave, hence every mode, has locations in the room with reinforcement and others with nulls. As a consequence, an absorber does not absorb the reinforced frequencies and leaves the nulls untouched, it lowers the energy level of the reflected wave, at all frequencies.

Not familiar with how Helmholtz resonators work, hence no comments.

With corner traps there are still reflections from the walls, floor and ceiling, which, on top of the direct sound, are the major contributors to the SPL at listening position, so I don’t think that they have an effect which has to be compensated. But I could be wrong, are there any measurements showing how these traps change SPL at listening position, if at all?





This is what is commonly said, but I found no evidence that the frequency response of reverb times typical for small rooms has an audible effect.



The question is: what is an acceptable level. Again, there is no evidence that a typical reverb time for domestic rooms of say 0.6 seconds is bad.




Flutter echoes are generated with impulsive sounds and only when the following condition is met:

t << l/c
with

t= impulse length
l= distance between walls
c= speed of sound

For a typical room of 8 x 5 m the necessary impulse lengths would have to be much smaller than 23,5 and 14,7 ms, respectively.


Klaus

I will answer in reverse order...

Flutter echo will depend on the finish of the walls, floor, ceiling. Even in a room of +100 m² you can find flutter echo. Flutter echo is effectively mostly audible in higher frequencies.

I wrote intentionally an 'acceptable reverb time' because everyone has his taste and the reverb time is dependent on room size though for 90% of listening rooms the 0.4s is effectively the measure. But I had a customer who used to have 0.9 sec and that was, obviously, not good. By just adding diffusors in the room, it came down to 0,6 sec and he (and I) found that very good although in theory for that room it should have been 0,4 sec. i don't say the 0,4 would not be better, but it was 'acceptable' for the customer at 0,6 as otherwise he would have had to make extra aesthetical sacrifices he didn't want to make.

The science of psycho-acoustics has determined that 0.4 seconds in a normal room for listening to music is experienced as most neutral. Personal tastes may and will vary.

I had another customer who had a medium-sized room with huge windows (single layer glazing). The reverb time in the lowest bass was 9 seconds!! (not a typo) and I can't recall the rest of the FR but it was not 9 seconds (obviously). So yes, FR is relevant in reverb measurement. (even in a 0.4 sec room the bass is always going to be going till double or triple that value)

Regarding the absorbers, you are going in a lot more detail than I wanted to go, for the sake of loosing the readers...

I know how the standing waves function and am aware that they can be heard at varying intensity according to where you are in the room (sometimes you don't even hear anything at all...). You are correct that for those resonant frequencies (not always the same thing as a standing wave though!) you can solve this by moving not only the speakers, but also the listening position. But you will not handle the standing waves by it or you have to be sitting very near the wall (distance depends on the frequency that is problematic).

As you write yourself, absorbers do lower the amplitude, so you loose sound pressure, so you will try to recover that. So globally, I agree with what you write in the first part except the following phrase: "As a consequence, an absorber does not absorb the reinforced frequencies and leaves the nulls untouched, it lowers the energy level of the reflected wave, at all frequencies."

I don't think I wrote that an absorber absorbs only reinforced frequencies. I did write that it lowers the energy level of the reflected wave (I recall having spoken about the fact you cannot achieve 100% on this). At all frequencies is only correct in sofar that an absorber (depending on placement, material, thickness etc ) will have maximum effect to a certain frequency only and a rapidly decreasing effect on the surrounding frequency and no effect on the rest of the spectrum.
Maybe its me but the way you write "... at all frequencies" let me understand that an absorber will always have effect on all frequencies and I am very much disagreeing on that.
 

flyer

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Thanks Klaus for the documents regarding early reflections, I hadn't realised there was doubt in the. General acceptance that first reflections were bad. For my room, I won't instal WAF velour until after listening tests and REW sweeps satisfy me. I am curious about the first reflections, as intended, I can experiment with the aluminium foil covered insualtaion panels facing in or out so will do just that in the side walls. For the rear I will face the foil away from the room, and maybe devise some refraction panels for hanging.
For the front wall, I had assumed I was going to face the foil towards the room, now I'm not so sure!

Regards,

Steve

Many acoustic material manufacturers try to push absorbers at first reflection. Why? Because it is quite audible (<> better!!) so has a wow effect for the customer, absorbers are immensely cheap to make (i.e. margins are phenomenal) and continue to build on concepts developed in the middle of the 20th century, lasting till the beginning of the current century. With concepts like dead end, live end etc. which were developed when psycho-acoustics was not yet a science.

Now, the latter has fully matured, and found its way in most or all modern concert halls, it is only very slowly finding its way to the domestic arena.

Reflection is effectively, as Klaus says, not bad at all. I would say, on the contrary but it needs to be well controlled. I would suggest you put diffusors on the reflection points, they will lower the room's reverb time, increase soundstage and add density of tone. Klaus is again right when he says that it depends on the speaker's power range (horizontal and vertical distribution or, in other words, the speaker's degree of directivity).
 
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KlausR.

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Hi Michael,

Flutter echo will depend on the finish of the walls, floor, ceiling. Even in a room of +100 m² you can find flutter echo. Flutter echo is effectively mostly audible in higher frequencies.

Flutter echoes are generated by impulses of very short duration. A periodic sequence of single echoes is generated by reflection of the impulse at opposite parallel walls. These echoes return to the observer after a time which depends on the distance of both the source and the observer to the walls. The observer perceives a series of sound pulses having frequency and duration of the original impulse. The time between these pulses, or pulse frequency f, depends on distance between source and observer, and is determined by

f = c/l

with

l = distance between walls
c = speed of sound

The smaller that distance the higher the pitch or frequency of the resulting sound: for 3.4 m that would be 100 Hz. For very small distances the flutter echo has a metallic sound, above 2 m a rattling sound, above 16 m the single echoes are audible as such. There is hence no such thing as “flutter echo in the higher frequencies” since the pitch of the echo is determined exclusively by the room dimensions.

Maa, “The flutter echoes”, J. of Acoustical Society of America 1941, vol. 13, p.170

I have doubts that normal music program material contains impulses of durations short enough to trigger flutter echoes in domestic rooms.

The science of psycho-acoustics has determined that 0.4 seconds in a normal room for listening to music is experienced as most neutral. Personal tastes may and will vary.

There is not much research on reverb time, here is I what I could find, if you know of other sources, please let me know.

The BBC made some experiments in the late 70ies to see how reverb times in their control rooms should be. These tests were done with speech and it was found that reverb time should be not greater than 0.4 s.

Gilford, “The acoustic design of talk studios and listening rooms”, J. of the Audio Engineering Society 1979, p.17

Dutton has made experiments to see whether or not the reverb time has an effect on phantom source localization of speech recordings. Tests were done in 8 different rooms with reverb times between 0.1 and 1.4 s. The effect was judged to be insignificant.
Localization in the anechoic chamber was no better than in a normal room with reverb time of 0.3 (10 kHz) and 0.7 (100 Hz). The room with 1.4 was judged to be very disturbing.

Dutton, “The assessment of two-channel stereophonic reproduction performance in studio monitor rooms, living rooms and small theatres”, J. of Audio Engineering Society 1962, p.98


Leonard made experiments to see if the reverb time of a studio control room had an effect on the level of reverberation that was to be added to the mix. Reverb time of the room was increased from 0.2 to 0.4 s by adding reflective panels. For the condition of higher reverb time less reverberation was added to the mix.

Leonard et al., “The effect of acoustic environment on reverberation level preference”, Audio Engineering Society preprint 8742 (2012)

Weisser made experiments in 8 different rooms with reverb times between 0.24 and 1.6 s, using speech and music. With speech the lower reverb times around 0.3 s were preferred, with music the range 0.6 s – 1s.

Weisser et al., “Evaluation of sound quality, boominess and boxiness in small rooms”, J. of the Audio Engineering Society 2006, p.495

I had another customer who had a medium-sized room with huge windows (single layer glazing). The reverb time in the lowest bass was 9 seconds!! (not a typo) and I can't recall the rest of the FR but it was not 9 seconds (obviously). So yes, FR is relevant in reverb measurement. (even in a 0.4 sec room the bass is always going to be going till double or triple that value)

Higher reverb times in the bass are no problem, even the recommendations such as EBU 3276 allow values up to 0.7 s at 63 Hz. 9 seconds measured in the lowest bass more likely is mode decay time rather than true reverb time. I don’t know of any research re: FR of reverb times in the range of 0.3 – 1 s, so any statement in this regard is pure speculation.

I don't think I wrote that an absorber absorbs only reinforced frequencies. I did write that it lowers the energy level of the reflected wave (I recall having spoken about the fact you cannot achieve 100% on this). At all frequencies is only correct insofar that an absorber (depending on placement, material, thickness etc.) will have maximum effect to a certain frequency only and a rapidly decreasing effect on the surrounding frequency and no effect on the rest of the spectrum.

Maybe it’s me but the way you write "... at all frequencies" let me understand that an absorber will always have effect on all frequencies and I am very much disagreeing on that.

Porous absorbers have maximum effect when the velocity of the air molecules is greatest, which is the case when pressure is zero, hence at a null of the standing wave, which is at a distance of a quarter wavelength from the wall. All frequencies for which the quarter wavelength of their respective standing wave is within the thickness of the absorber are subjected to maximum possible absorption. All other frequencies are absorbed to a smaller degree, but they are still absorbed. Porous absorbers hence are not frequency specific as you seem to imply. For perpendicular incident an absorber of 1 m thickness will have maximum effect on 85 Hz and all higher frequencies, lesser effect on frequencies below 85 Hz, an absorber of 2.85 m will have maximum effect on all frequencies above 30 Hz.

A corner trap will have an effect on standing waves, but no effect on direct sound and first reflections.

As you write yourself, absorbers do lower the amplitude, so you loose sound pressure, so you will try to recover that.

This might be true but where’s the problem, you just crank up the volume a bit. However, this is not what you initially said. What you said is
You intend to kill/attenuate the bass energy. By doing so, you will have to put the volume harder to compensate for that loss of energy. By putting the volume harder, you will be faced with a load of other problems in the mids and highs which, to solve them as well, you will be adding in even more absorbers.

This seems to imply than only bass is attenuated and mids/highs remain untouched, so that turning the volume knob higher leads to problems with mids/highs.

Porous absorbers are not absorbing selectively, so they won’t attenuate bass alone. If anything, they will attenuate bass and everything above, and the latter even stronger than the bass frequencies, so what you would get get is over-attenuation of mids and highs, because absorption coefficients are much higher for mids and highs than for the lows.

With concepts like dead end, live end etc. which were developed when psycho-acoustics was not yet a science.

Psychoacoustics took off when Charles Lindbergh was welcomed back in New York after his famous flight across the Atlantic. By the time that the Davis brothers published their paper about LEDE, which was in 1980, psychoacoustics was a well established scientific discipline with thousands of publications: in 1950 scientists from the Bell labs designed the ABX test for psychoacoustic experiments, the paper describing the Haas-effect was published in 1972, perception thresholds of reflections were known for both speech and music, German physicists had made numerous experiments in synthetic sound fields.

Klaus
 

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Klaus,
Your answers are very theoretical and I actually mostly agree with them, as they give meat on the bone of what I explained earlier.
Regarding flutter echo, you are spot on in your theory explanation. In practice, listen to music in a domestic room with or without the presence of flutter echo. The difference is well audible as, like you write, the highs are sounding more shrill bringing listening fatigue because your brain needs to actively eliminate the echo effects.

Regarding the 0.4 seconds reverb time. You are writing things that either seem to confirm the 0.4 seconds claim, things that contradict it or things that have nothing or little to do with it. And beware, I am talking about smaller listening rooms. Not rehearsal nor mixing rooms.
If you think a 1 second reverb time is no problem, then good for you but to others I would say, would you prefer to listen to music in a living room or in a bath room?

When I said the reverb time in the bass is always going to be double or triple the rest of the spectrum, I did not mean this was not desirable. Quite on the contrary, it is desirable and normal. But not 9 seconds!

Regarding the absorption, you are right and does not contradict what I wrote. With the 2.45 m depth you are almost describing the installation of an anechoic chamber… My point is that one commercially available absorptive panel will have very limited effect on the whole spectrum.

Corner absorbers effectively do not affect direct sound nor first reflections. But those two are making up for only part of the SPL that we hear and the ambiance acoustic experience.

It puzzles me what Charles Lindbergh’s arrival in NY has to do with psychoacoustics… anyway, depending the angle you take on it, you can say the Greek were already into that art (with their advanced acoustic theatres, but I call that acoustic science not psycho-acoustics) or that this only became a true science in itself as the instruments that have enough precision to measure what we hear started to become available in the 80s.
The coming to maturity of the science of psychoacoustics (e.g. why does a brain perceive sound A as a rich sound and sound B not as such?) in the late 20th century and early 21st, explains why concert halls that are built today are so more often good halls whereas until till late in the 20th century it was very difficult to build a good concert hall. There had been some lucky shots or outright geniuses at work, explaining those world famous ancient concert halls. But there is a much bigger number of other halls built, before and after that sound bad altogether.
Nowadays, with the newly gained insight, this is not true anymore.

Finally, the thing that started this chain of posts is that I think absorption (or at least too much of it) lowers SPL. Now, this statement is confirmed when you write: This might be true but where’s the problem, you just crank up the volume a bit.
That is my whole point. Cranking up the volume a bit is not going to be 1 or 2 dB, it is likely going to be 3 or well up. Knowing that 3 dB requires to double the output power (i.e. double the load on your amp) and 6 dB quadruples the need for power. And once you do that, you have a fair risk that other parts of the spectrum start to give issues; either acoustically or due to system limitations.

Bottomline, the addition of diffusors can solve most of the acoustic problems for the medium and high frequencies and does not have the perverse effect just described. Putting in absorbers should only come as a last remedy, not as the first.

PS: I will look up on the studies that put 0.4 seconds forward as a kind of norm. I do not have that information at hand now.
 

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