Audibility of single reflections

Jose Almagro

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I'm designing and measuring some rooms and most of all I use EBU 3276 recommendations. An important part is that about reflections and the criterion is they should be under -10 dB compared to direct sound. I use octave filtered ETC in the frequencies that allow such analysis, I think that's what they meant but I don't think that's enough.

I'd like to go one step beyond and get a "warning curve" so I can somehow have an idea of the audibility of reflections, maybe not too reliable but quick and useful. I thought using the first 7 ms of pink noise thresholds and the clicks after that. Which curve do you think I should use? Envelope? filtered?

umbrales.jpg
I took this graph from: SOURCE TO BRAIN: MODELLING SOUND PROPAGATION AND LOCALISATION IN ROOMS (Jonathan Sheaffer's Thesis).

Thanks for your attention.
 

amirm

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Audibility of reflections depends on its direction. Side reflections for example are very different in that the reflection reaches the two ears differently. They generally are pleasing and no warning should be associated with them. They also differ based on level of reflectivity in the room.

Also note that ETC measurements are inherently unreliable because they are devoid of spectrum. You could have two reflections with the same magnitude but be completely different in spectrum and hence audibility.

The references 1989 Toole and Olive paper are the bibles here if you like to read more. A more understandable version is in Dr. Toole's book (Chapter 6).
 

Jose Almagro

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I've usually used Calibrated Acoustic String (TM):
I can't believe they sell that. I use a measuring tape provided that 1 m equals 2,93 ms but nowadays you can use REW with an ECM8000 to get the ETC and use the method to identify the reflections. Also with CAD it's very easy to do that using the mirror image of the source.
 

Jose Almagro

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Side reflections for example are very different in that the reflection reaches the two ears differently. They generally are pleasing and no warning should be associated with them.
I think we should clarify this because I found several people that misunderstood Toole's book and said "Toole says lateral reflections are good and then he places lateral absorbers". Lateral reflections are very important in big halls and sometimes Toole quotes Barron, a nice and easy to understand explanation of this is/could-be in this paper:
http://www.europhysicsnews.org/articles/epn/pdf/2015/01/epn2015461p15.pdf
A control room in a studio is a very different issue and a Hi-Fi room mmmm... quoting Oasis: Definitely Maybe

Audibility of reflections depends on its direction. Side reflections for example are very different in that the reflection reaches the two ears differently.
I'm aware of that and I add that we have temporal masking that should be taken into account, and frequency masking (this one is a clue to explain why RT growing with frequency are usually so upsetting) and there are some papers demonstrating that the previous listening in a room improves speech transmission (It enables your brain to discard detrimental reflections!). Hearing is a very complex cognitive process we still don't completely understand so I know I can't be accurate, that's why I said I wanted to have a "warning curve": a simple Excel file to tell me: "Hey dude, you should take a closer look at this measurement". That's why Sheldon Cooper hates engineers.;)

Windowing is critical as well, so graphs like this one can be of help (or not):
audibilidad de reflexiones individuales.jpg

Also note that ETC measurements are inherently unreliable because they are devoid of spectrum. You could have two reflections with the same magnitude but be completely different in spectrum and hence audibility.
Please note that I said I use octave bandpass filtered impulses for the first 15 ms according to EBU 3276. Also note that the shape of the wave strongly depends on the phase so it's more complex than Energy-Time-Frequency, that's why a time-aligned tweeter make a louspeaker sound tighter or a Reflection Phase Grating overcomes a curved surface.

A simple conclusion I get from the graphic above (dark green and blue lines) is that a single reflection in the first 10 ms is perceived as a spectral change but if it arrives later you perceive it as a time related artifact (I guess calling it echo is too much).

EBU says something like the decays should be even to avoid echoes. But how much? There are echo criteria for big rooms like Dietsch & Kraak's but they don't work for small ones because they require a long delay.

Quoting you: "It is not simple":b
Thanks Amir, it's a pleasure to hear your comments.
 
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amirm

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Are you trying to build a home listening space or a recording studio? The context in this forum is always the former unless you say otherwise.

Here is an article I wrote on side reflections (for home listening use): http://www.madronadigital.com/Library/RoomReflections.html

One has to be very careful of following industry standards for acoustics. Most are aimed for repeatability of experiments than ultimate fidelity. One such example is FHG institut (same people who brought us MP3) as they tried to build their reference listening room while complying with ITU standards:

Vision and Technique behind the New Studios and Listening Rooms of the Fraunhofer IIS Audio Laboratory
Andreas Silzle, Stefan Geyersberger, Gerd Brohasga1, Dieter Weninger and Michael Leistner
Fraunhofer Institute for Integrated Circuits IIS, Am Wolfsmantel 33, D-91058 Erlangen, Germany
Innovationszentrum für Telekommunikationstechnik GmbH IZT, Am Weichselgarten 5, D-91058 Erlangen, Germany
Fraunhofer Institute for Building Physics IBP, Nobelstraße 12, D-70569 Stuttgart, Germany


Figure 11 shows sections of the first 30 ms of two impulse response measurements, band pass filtered from 1 kHz to 8 kHz, both from the center loudspeaker to the reference listening position. The graphs are almost identical with each other, except around 13 ms. The first 10 ms with zero signals represent the direct sound travel time, the strong peaks at 10 ms originate from the direct sound. In the measurement of the untreated room with the regular carpet covered floor (black graph), a second peak at 13 ms with 0.6 amplitude relative to the direct sound, which is equivalent to 4.5 dB attenuation, represents the reflection caused by the floor. The time delay of 3 ms corresponds exactly to the 1.7 m path length difference between direct and reflected sound. This strong reflection violates the requirements of the standard. It can however be attenuated to a negligible level with little effort by placing a piece of moderately absorbing material (e.g. 50 mm thick porous material, about 1.5 m x 1 m) on the floor at the mirror point, i.e. the point, where the sound is reflected on the floor. The measurement result for this case is presented with the red graph in Figure 11. None of the reflections exceeds 0.3 amplitude relative to the direct sound, which is equivalent to more than 10 dB attenuation."

This is Fig. 11:


So the measurements indicate a reflection from the floor and they aim to remove it per standard requirement. But this is what it sounded like:

3.1.4. Subjective room assessment"

[....]

Regarding the floor reflection, the audible influence by removing this with absorbers around the listener is negative – unnatural sounding. No normal room has an absorbent floor. The human brain seems to be used to this."



Ultimately you have to listen and trust your ears in this instance and put that above measurements with a single microphone and no psychoacoustics applied.
 

Jose Almagro

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Are you trying to build a home listening space or a recording studio?
This year as far I remind I've worked in about 5 studios, 1 home cinema, 2 Hi-End rooms and 2 churches and as this part of the forum belongs to Dr. Olive, I wanted to have an advice from him, especially because he's coauthor of the psychoacoustic papers I'm using.

One has to be very careful of following industry standards for acoustics.
It's more important to realize the difference between a standard and a recommendation. CID Rooms designed by Bob Walker are an example; they have quite higher RT than recommended by the EBU 3276 while he was a member of the EBU P/LIST Group that developed the paper.

Ultimately you have to listen and trust your ears in this instance and put that above measurements with a single microphone and no psychoacoustics applied.
I'm not the kind of person who thinks everything can be solved with microphones and software. Sometimes when I'm giving a lecture I use to say the best instrument an acoustician has are his ears, I even said that when I was working for Brüel & Kjaer :D
 

Don Hills

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Jun 20, 2013
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I can't believe they sell that.

They don't "sell that". You supply your own string.

I use a measuring tape provided that 1 m equals 2,93 ms but nowadays you can use REW with an ECM8000 to get the ETC and use the method to identify the reflections. Also with CAD it's very easy to do that using the mirror image of the source.

String is cheaper. :)
 

KlausR.

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Dec 13, 2010
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I'm designing and measuring some rooms and most of all I use EBU 3276 recommendations. An important part is that about reflections and the criterion is they should be under -10 dB compared to direct sound.


The diagram from Jonathan Sheaffer's thesis you included in your post shows detection thresholds from experiments in anechoic rooms with a single loudspeaker as direct sound source and a single reflection. Experiments performed using a complete sound field with all reflections and reverberation obtain different thresholds, for speech for instance for a single 10 ms lateral reflection it is increased by 4 dB (Olive 1989) or 6 dB (Bech 1995).

Detection thresholds for music depend on the music motive, and are between – 25 and -18 dB re: direct sound, 10 ms single lateral reflection, single speaker for direct sound, anechoic room (Schubert 1966). If detection thresholds for music change in a similar way as they do for speech the figure for a complete sound field would be between about -20 and – 13 dB re: direct sound, which means that for lateral reflections a level of 10 dB below direct sound is probably not enough.

Further one should also take into account the the natural level of reflections and determine whether or not it is below the detection threshold, which would be good. These levels depend on the directivity characteristics of the loudspeaker, the absorption coefficients of the room surfaces, and the attenuation due to distance.

Olive et al., “The detection of reflections in typical rooms”, J. of the Audio Engineering Society 1989, S.539

Bech, “Timbral aspects of reproduced sound in small rooms I”, J. of the Acoustical Society of America 1995, vol.97, no. 3, S.1717

Schubert, “Detectability of single reflections for music” (Untersuchungen über die Wahrnehmbarkeit von Einzelrückwürfen bei Musik), Technische Mitteilungen RFZ 1966, vol. 10, no. 3, p.124

EBU says something like the decays should be even to avoid echoes. But how much? There are echo criteria for big rooms like Dietsch & Kraak's but they don't work for small ones because they require a long delay.

Echo threshold are different for different signals, and are about 1-5 ms (clicks), 10-20 ms (noise), 30-50 ms (speech), 80-150 ms (Blauert 2005, Dietsch 1986, Litovsky 1999).

First reflections in small rooms come at delays of under 20 ms (Devantier 2002), so except for sounds like clicks there is no problem, the precedence effect will do its job.

Blauert et al. (2005), “Acoustical communication: The precedence effect”, Proceedings FORUM ACUSTICUM BUDAPEST, OPAKFI Budapest

Devantier (2002), “Characterizing the amplitude response of loudspeaker systems”, Audio Engineering Society preprint 5638

Dietsch et al. (1986), “An objective criterion for the measurement of echo disturbance during presentation of music and speech” (Ein objektives Kriterium zur Erfassung von Echostörungen bei Musik- und Sprachdarbietungen), Acustica, vol. 60, p.205

Litovsky et al., “The precedence effect”, J. of the Acoustical Society of America 1999, vol. 106, no. 4, pt. 1, p.1633

EBU 3276 actually says: “The reverberation field should be sufficiently diffuse in the listening area to avoid perceptible acoustic effects such as flutter echoes.”

The condition for the generation of flutter echoes is that the duration of the impulse is much shorter than the distance between the reflectors divided by the speed of sound in air:

t0 << l/c

If the duration is longer no flutter echo will be generated but one of the room modes will be excited.

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


Klaus
 

Groucho

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Does any of this work allow for the possibility that what we prefer is novelty rather than a fixed acoustic? If someone fiddles around with some reflectors or absorbers or whatever and I like the result today, it doesn't guarantee that I am going to like the result in three months time of constant listening.
 

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