It could be a calculated value based on the assumption that the measured sound field was diffuse.
I used an IVIE IE35 with the RTA function and some japanese sound pressure meter with a "strip chart" function.
None of these measuring approaches make any assumptions about the sound field, they simply measure the rate of decay of SPL.
By the way, in my room above the 160Hz boundary the decays were all very smooth. (Most below 160 were also fairly smooth.) Smooth reverberent decay is generally about the number of room modes in a given bandwidth. As you go up in frequency that number always goes up and in any sized room becomes sufficient at some point.
David
David, Patronis, "Sound System Engineering", 8.3 Small Room Reverberation Times:
"What is often overlooked in the attempted measurement of RT60 in small rooms is that the definition of RT60 has two parts, the first of which is commonly overlooked.
1. RT60 is the measurement of the decay time of a well-mixed reverberant sound field well beyond Dc.
2. RT60 is the time in seconds the reverberant sound field to decay 60 dB after the sound source is shut off.
Since in small rooms, there is no Dc, no well-mixed sound field, hence no reverberation but merely a series of early reflected energy, the measurement of RT60 becomes meaningless in such environments.
What becomes most meaningful is the control of the early reflections because there is no reverberation to mask them."
I know measurements from class rooms (which are acoustically even larger than our listening spaces). Same RT value but very different ratings for speech clarity.
Just look at an ETC. If the sound field would be diffuse then we couldn't see any discrete reflections but we do. However, an ETC still doesn't tell us where a reflection is coming from and what the spectral content is.
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This is a totally silly statement. "No reverberation"?
As I keep pointing out, acoustics of a room are a scalable. If, while designing a concert hall I want to build a scale model 1/10th the size and measure it with wavelengths of 1/10 the dimension, this is considered a valid approach. Such models have even been tested with frequency shifted music, to directly evaluate their sound. You never hear the caveat: "but there is no reverberation because the model is too small".
Reverb time is reverb time. The noise stops and the sound decays and you look at the slope of decay. You can concentrate on early decay or later decay, or whatever, but the concept doesn't stop when the room hits a certain size.
I understand that sparcity of modes, or dominance of some, can make the measurements "non-classical". Above the Schroeder frequency in the rooms that I have measured this has not been a problem.
Do your measurements show otherwise?
David S.
Uh oh, deja vu
I refer people back to my ETC measurements in post #835 earlier in this very thread:
http://www.diyaudio.com/forums/mult...y-pattern-stereo-speakers-17.html#post2709656
Energy decay is almost a dead straight decay from about 10ms onwards, and the last identifiable discrete reflection is at about 36ms, and is only just identifiable. That's in a fairly small, lively room.
Considering the RT is 0.4 seconds, 9/10th of the decay period is beyond the last observable discrete reflection, and the overall decay has already stabilised on its ultimate slope by the time the early reflections have all dispersed.
I don't understand why people can't accept that small rooms have reverberation and decay time. There's a really easy test for it - set some music playing at a moderately loud volume and hit pause, the reverb tail is easy to hear and doesn't localize in any particular direction. (Assuming a fairly uniform reflectivity/diffusivity/absorption in the room...)
It's not that people would not accept something, it's just that there is no diffuse sound field in an acoustically small room. This is a physical fact and it can and has been measured:
B. N. Gover, J. G. Ryan, and M. R. Stinson, “Measurements
of Directional Properties of Reverberant Sound
Fields in Rooms Using a Spherical Microphone Array,” J.
Acoust. Soc. Am., vol. 116, pp. 2138–2148 (2004).
Obsessing about RT values is counterproductive if we want to talk about how sound is perceived in an acoustically small room.
B. N. Gover, J. G. Ryan, and M. R. Stinson, “Measurements
of Directional Properties of Reverberant Sound
Fields in Rooms Using a Spherical Microphone Array,” J.
Acoust. Soc. Am., vol. 116, pp. 2138–2148 (2004).
Obsessing about RT values is counterproductive if we want to talk about how sound is perceived in an acoustically small room.
You really need to explain why sound waves don't respond to factors of scaling. Whether we are talking the wave equation or looking at specular reflections or even image models, you need to explain why one set of rules applies for large rooms yet not for small. What is the transition point? Where do the rules change? Why do the rules change?
Its okay to say that you think RT isn't the most appropriate way to view small room performance, or that diffusion tends to be sub par in the make up of domestic rooms, or that concentrating on strength and direction of early reflections makes more sense than thinking about reverberation, but you can not say that the laws of acoustics change with room size!
The concept of RT applies equally to large rooms, small rooms, speaker cabinets and microwave cavities, it is merely a statistical approximation/simplification that allows us to predict with some accuracy the decay of fields in rooms where some basic parameters are known. Its accuracy is not a function of size.
You are mis-reading your references.
David S.
A necessary, but not sufficient condition of a room being difuse would be the monotonic fall of the impulse response. Simon's data does look very good after about 35 ms. But the early part is clearly not difuse. It is well known that perceptually it is the early ppart of the decay that is important.
Markus point about reverb time being somewhat less than useful in a small room is well recognized. Even Toole mentions this in his book. I never use it in small rooms.
Markus point about reverb time being somewhat less than useful in a small room is well recognized. Even Toole mentions this in his book. I never use it in small rooms.
Again, not the point. Look at impulse responses in a concert hall and you will see that they also start with a few sparse reflections until the reverberent field builds up. The point was never whether RT was the end-all be-all psychoacoustic criterion, but whether it exists in a small room.
David S.
You might be surprised given a good test. You can probably locate blind in a concert hall much better than you think you can. I know I was surprised. Not that sight isn't a factor, but you can do better without it - even thru the acoustics - than you might think.
I've been to a couple of the better concert halls on this planet and there was lots of spaciousness but no "pin-point imaging". Certain seats might allow for better localization.
Acoustically small rooms are not "Sabine spaces". You can't do anything useful with RT as a measure because the assumption of a diffuse sound field is wrong.
Of course this doesn't change the laws of physics and the sound field in a small rooms decays just like it does in a large space but I wouldn't use the term reverberation.
Yes, I really closed my eyes. Next time I go to Tonhalle Zürich I'll take a picture just for you
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