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Discussion Starter · #1 ·
the ideal reverb time?


like we'd take infinite output and zero distortion from a subwoofer, or infinite transmission loss in a wall, or perfect time alignment in a loudspeaker...


but what "ideal" for reverb time?
 

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Accoding to a past thread posted in this forum on RT-60:

Quote:
Quote from Dennis Erskine:

I try to keep it in the range of .35 to .40
the measurement scale is seconds

.35-.40 seconds
 

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Quote:
(what is) the ideal reverb time?
This is a meaningless question.


Tests on preferred RT60 varies heavily on the venue/room size. Larger rooms need longer reverbs, smaller rooms shorter. It also depends heavily on the use of the room. I am assuming you are talking HT stuff, which is it's oen use which may of course vary from a 2-channel setup. What are you trying to do, and what is your room size. And then of course taste comes into play.
 

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The reverberation time of .35 to 0.5 seconds is suggested for multi-channel playback systems. An RT60 of 1 or lower is required for rooms where speech intelligibility is required.


The calculation of RT60 accounts for room volumetrics. While, untreated a larger space (like a gymnasium at a high school) will have longer RT60 times, it is incorrect to suggest longer RT60 times are preferred in larger spaces. Indeed, such spaces need to reduce their RT60 (particularily in indoor swimming pools) for safety reasons.


Here's an interesting site with some RT60 WAV format demonstration files.
http://www.bkla.com/reverb.htm
 

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Quote:
The calculation of RT60 accounts for room volumetrics. While, untreated a larger space (like a gymnasium at a high school) will have longer RT60 times, it is incorrect to suggest longer RT60 times are preferred in larger spaces.
Yes, I realize RT60 fully accounts for the volume of the room, no adjustments or corrections are necessary to the RT60 measurements, but everything I've read suggests that "ideal" (subjectively preferred) RT60 for a certain use *will* vary depending on the room size. So short a decay for a small room such as a HT would be too dead for a very large auditorium theater.


Of course, in making these size comparisons, you have to compare identical uses, because a symphony hall is not at all comparable in design to a movie theater.
 

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I would like to see the recent studies. The several I am familiar with have been disgarded...although the research is several years old. Frankly, I don't buy that in any case; but, will agree that the "ideal" reverberation time for a space should be based upon its primary use. The Stag theater, for example, is .55 as I recall and it's a large room.


In some very large spaces, such as large auditoria, achieving .5 would be a serious challenge (assuming .5 was the 'ideal').
 

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Quote:
I would like to see the recent studies. The several I am familiar with have been disgarded...although the research is several years old. Frankly, I don't buy that in any case
Well it's likely that you are correct, and the book info is out of date, I trust your experience more than my own. However, my own experience seems to match those little graphs that I'm sure you've seen in the past. I don't mind long reverbs in large spaces nearly as much as in small rooms. I know something like a huge stadium or other large venue is a completely different animal, but more than a second or a few is fine for those extremely large spaces. It's an extreme example anyway, but illustrative(though I hesitate because large stadiums definitely have different uses).
 

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RT60 Graphs like the one about 75% of the way down at :
http://www.squ1.com/index.php?http:/...behaviour.html

always seemed a little high to me.


This formula

Control Room Recommended RT60 = 0.25 * (( ProposedRoomVolume / 100 m^3 ) ^ .3333)

which was posted by Terry M and Avare, presumably from somewhere. Possibly an ITU specification.


gives

ft^3 m^3 RT60

63 1.8 0.07 (shower stall - waterproof acoustical treatment required?)

1000 28.3 0.16

1250 35.4 0.18

1500 42.5 0.19

1750 49.6 0.20 (approximately 16' x 16' x 7')

2000 56.6 0.21

2250 63.7 0.22

2500 70.8 0.22

2750 77.9 0.23

3000 85.0 0.24

3250 92.0 0.24

3500 99.1 0.25

3750 106.2 0.26

4000 113.3 0.26

4250 120.3 0.27

4500 127.4 0.27

4750 134.5 0.28

5000 141.6 0.28

5250 148.7 0.29

5500 155.7 0.29

5750 162.8 0.29

6000 169.9 0.30

6250 177.0 0.30

6500 184.1 0.31

6750 191.1 0.31

7000 198.2 0.31

7250 205.3 0.32

7500 212.4 0.32

7750 219.5 0.32

8000 226.5 0.33

8250 233.6 0.33

8500 240.7 0.34

8750 247.8 0.34

9000 254.9 0.34

9250 261.9 0.34

9500 269.0 0.35

9750 276.1 0.35

10000 283.2 0.35

11000 311.5 0.37

12000 339.8 0.38

13000 368.1 0.39

14000 396.4 0.40

15000 424.8 0.40

16000 453.1 0.41

17000 481.4 0.42

18000 509.7 0.43

19000 538.0 0.44

20000 566.3 0.45

22500 637.1 0.46

25000 707.9 0.48

27500 778.7 0.50

30000 849.5 0.51

32500 920.3 0.52

35000 991.1 0.54

37500 1061.9 0.55

40000 1132.7 0.56

50000 1415.8 0.60

60000 1699.0 0.64

70000 1982.2 0.68

80000 2265.3 0.71

90000 2548.5 0.74

100000 2831.7 0.76

125000 3539.6 0.82

150000 4247.5 0.87

175000 4955.4 0.92

200000 5663.4 0.96

225000 6371.3 1.00 (approximately 100' x 100' x 22')


I don't have any rigid fiberglass absorbers in my shower, and I sound pretty good in there !


Those numbers I think are meant to be applied from 200hz to 4khz (±0.05s), with it OK for RT60 to rise on both sides of that. (i.e. longer RT60 at 80hz).
http://www.aes.org/technical/documents/AESTD1001.pdf


I also think they are somewhat of an approximation on a variety of reasons (rooms this small don't have an RT60, ±0.15 is good enough, etc.)
 

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Discussion Starter · #9 ·
with respect to the files at

http://www.bkla.com/reverb.htm


can anyone confirm that those are realistic? even at 0.8 reverb time i can't understand a thing that he's saying, yet the article below suggests that 1 is ideal. conceding my novice stature on room acoustics, that doesn't seem to add up.
 

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brianr820

When I went to reverb.htm I went straight to the 0.8 reverb and didn't understand what was being said. So I played it again, same effect. Then I listened to the 1.5 and sort of understood it, then went to the 0.0 and that made sense, and then I went back to the 0.8 and understood every word.
 

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Quote:
Originally posted by brianr820
with respect to the files at

http://www.bkla.com/reverb.htm


can anyone confirm that those are realistic? even at 0.8 reverb time i can't understand a thing that he's saying, yet the article below suggests that 1 is ideal.
Yup. I can't understand this "0.8 second" reverb speech either!


The thing about speech intelligibility is that it cannot be determined by reverberation time alone. Our current standard metric for speech intelligibility is the Speech Transmission Index (STI). This is calculated from "impulse response" at the listening position -- the sum total of all the sound processing by the room.


If the direct sound is very strong (the impulse response has a very high initial pulse compared to the "tail"), there will be high speech intelligibility regardless of the reverberation time (how rapidly the impulse response "tail" falls off). This impulse response always gives a very high (close to 1) STI.


Consider this thought experiment. You are 2 feet away from another person in an anechoic chamber. The reverberation is virtually nil, and you have no trouble understanding each other's speech.


Now you move to a huge hall with a reverberation time of more than 3 seconds. You are again only 2 feet away from the other person. Despite the high reverberation, you have no trouble understanding each other's speech.


Regards,

Terry
 

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As the BKLA site (in the text) suggests, and as Terry has pointed out, speech intelligibility is, as well, a function of the relative "loudness" of the direct vs reverberent sound. The ITDG plays a role also. By the same token, in a room with an RT >= 1, speech intelligibility is very difficult to achieve short of yelling in someone's ear.
 

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Those numbers, Bob, are very interesting. Your math is correct. Well done. :)


On the other hand, I rather doubt those who had a hand in developing that rule of thumb ever envisioned a 100 x 100 x 22 control room.


Smaller spaces have an RT60; however, as the room becomes smaller with respect to the items in the room (furniture, people), predicting RT60 becomes increasing less acdurate regardless of the method used in the prediction model.
 

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Quote:
Smaller spaces have an RT60; however, as the room becomes smaller with respect to the items in the room (furniture, people), predicting RT60 becomes increasing less acdurate regardless of the method used in the prediction model
My mistake, more like an idiot typo, because smaller spaces do have an RT60.


My current rules of thumb for RT60 are (based on ideas from Scott, Greg and Dan):


a) doing the reverberation calculations for RT60 is useful in determining a rough amount of absorption materials needed to treat the room (Fitzroy style),


b) RT60 in a small room can not be measured. The sound must be well mixed in a way that the angle of incidence on the reflecting surfaces is statistically random. You have to get a good distance away from the source for the sound, called the critical distance, before there is any possibility of the perceived amplitude of the reflected sound equaling the perceived amplitude of the source. In a small room this never happens (the critical distance does not exist) for the simple reason that you can't get far enough away from the sound source for the reflected signal to reach parity in amplitude. Thus you can't measure the time to reach a 60 dB drop from initial amplitude beyond the critical distance because there is nowhere to stand to take the measurement (no critical distance). Inject a steady state sound, like pink noise, into a room. If you are beyond the critical distance you can wander around with a sound level meter and the sound pressure will not change significantly. Within the critical distance any attempt to measure it will depend on where you are in the room and will yield wildly varying results. We are also dealing with modes.


Sometimes I call it Theoretical RT60.

Quote:
On the other hand, I rather doubt those who had a hand in developing that rule of thumb ever envisioned a 100 x 100 x 22 control room.
I just kept going until I hit RT60 = 1 second.
 

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Quote:
Originally posted by BasementBob


b) RT60 in a small room can not be measured.
We measure it all the time, Bob. It's absolutely no different for a small room than a large room. This measure has nothing to do with critical distance, as the direct sound is of no consequence.


The old-fashioned way was to fire an impulse (balloon pop or starter pistol) and measure the decay of the sound. It can and will decay to 60 dB of its initial reverberant energy (NOT the level of the first sound arrival, which is the time-delayed original impulse, and which is irrelevant to the measurement). Paper strip charts recorders used to be the standard way of doing this. Averaged over a number of readings, you get a very good measurement of reverberation time.


We use a more modern technique, of course. We derive the impulse response by deconvolving a stimulus sound of pink noise from its measured recorded sound in the room. This impulse response is then filtered for various frequency bands, and the RT60 for each frequency band measured from the dB slope of the amplitude. Often, we don't have a full 60 dB of dynamic range, but this has nothing to do with the room size. Nevertheless, RT60 can be extrapolated pretty accurately (+/- 5 hundreds of a second) using this method.


Regards,

Terry
 

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Discussion Starter · #16 ·
Quote:
Consider this thought experiment. You are 2 feet away from another person in an anechoic chamber. The reverberation is virtually nil, and you have no trouble understanding each other's speech. Now you move to a huge hall with a reverberation time of more than 3 seconds. You are again only 2 feet away from the other person. Despite the high reverberation, you have no trouble understanding each other's speech.
that seems like a very good analogy. perhaps in line with your initial pulse comment, ... shout across that large room from much farther than 2' and we could still converse with some level of clarity...
 

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Quote:
This impulse response is then filtered for various frequency bands, and the RT60 for each frequency band measured from the dB slope of the amplitude.
Bingo, Terry. It's not really too useful to talk about RT60 without specifying the frequency range. A common mistake people make when they start treating a room is to damp the treble too much and the bass not enough, leading to a skewed RT60 vs. F curve. Generally, to get a good tone balance in the room, they should use fewer and thicker absorbers.
 

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Discussion Starter · #18 ·
ok, so if a room was 2000 cubic feet, i need a very very low RT60 per bobs table. but such a room would have an innately lower RT60 than a gymnasium, ...


however, to drop it to 0.21 would require some ferocious treatment, no?


can i measure RT60 with a mic and an o-scope or an RTA? i'll go download the ASTM standard...
 

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Quote:
Originally posted by brianr820
ok, so if a room was 2000 cubic feet, i need a very very low RT60 per bobs table. but such a room would have an innately lower RT60 than a gymnasium, ...


however, to drop it to 0.21 would require some ferocious treatment, no?

RT60 changes with room size, but so does the treatment required. The psychoacoustics seem to indicate that the ear has an innate sense of absorption coefficient, regardless of the size of the room.


For example, what is generally perceived as a "medium-dead" room has an average absorption coefficient of roughly 0.25, regardless of its size. A small room needs less overall absorption to achieve the same absorption coefficient.


Quote:


can i measure RT60 with a mic and an o-scope or an RTA?
If you had a recording oscilloscope with a log scale, you could do it (using an impulse, or a white noise signal that you chopped off suddenly). An RTA can't measure RT60 because it operates purely in the frequency domain, not the time domain. Rooms with wildly different RT60s may display exactly the same response on a RTA.


Regards,

Terry
 

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Having attended a few intelligibility workshops and presentations in the pro-community, I would caution that while influences of RT60 and inteligibility overlap, RT60 does not predict nor guarantee intelligibility.


Like most any quantity in acoustics, it is but one parameter of the picture, yet requires many others to bring some focus to the expectations. Intelligibility is really a measure of the ability to modulate a frequency band over time. Specifically 10-15dB of modulation is required for "good" intelligibility. Don't quote me on it, but I recall mention of 30-50 ms. to be the window of most interest. In my mind, RT60 and other similar parameters are more a descriptor of the ambience and perceived character that we characterize in the first few moments we walk into a room. It is a good descriptor of how a room may feel, but it does not describe intelligibility. There is a measurement for intelligibility called STI (Speech Transmission Index). Gold-Line has some background information on the measurement on their website: http://www.gold-line.com/dspsti.htm


Where in sound reinforcement applications we find that getting into the "good" range is very acceptable, for the few home theater systems I have casually measured for STI, I have seen good correlation with subjective impressions, but the range from average to good sits at the top of the STI scale in the "good" to "exellent" range of scores. Something I hope to experiment with more in the future.


On the matter of measuring RT60 in a small room, while you can certainly take a measurement, the question is what are you really measuring and what does it tell you. The argument against is that you really have a case of decaying modal response, not a statistical reverberant field. That's not to say that measuring the approximate slope of the decay(which is what most do) is not useful, but we do need to keep in mind the limitations of the measurement.


Cheers,
 
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