The myth of generic optimum room dimension ratios

theophile

Well-Known Member
I have not read this thread. What I will suggest, is that since many of the problems that are associated with objectionable rooms comes from dimensions that are multiples of one another: Why not make the listening room's dimensions all become Prime numbers? For example; A room 23 feet long 17 feet wide with an 11 foot ceiling.

Another factor overlooked when designing listening rooms for Hi-Fi, is that parallel walls create echoes of their own. Even varying the divergence from parallel by only a few inches supposedly disables the formation of those echoes.
 

KlausR.

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Dec 13, 2010
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theophile said:
I have not read this thread.

I suggest that you do, in particular post #41.

Another factor overlooked when designing listening rooms for Hi-Fi, is that parallel walls create echoes of their own. Even varying the divergence from parallel by only a few inches supposedly disables the formation of those echoes.

I attach the pdf of an overview of literature on this particular issue, you will see that making the walls non-parallel does not solve the problem.

Klaus


View attachment Non rectangular rooms.pdf
 

KlausR.

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Ethan Winer said:
Originally Posted by KlausR.
... what’s the use of distributing 50 mode frequencies in an optimized manner when only one mode is driven at any one time?

When two or more modes are close together, they combine to be even stronger than a single mode. This is the reason modes should be evenly distributed, and has nothing to do with how many different bass notes might be played in a given piece of music.

From what I've read so far about room modes I understand that when a second mode lies within the bandwidth of a first mode the second one is driven at the frequency of the first one once the first one is excited. Bandwidth is inversely proportional to reverberation time and is approximately between 2 and 10 Hz. When two such modes are driven and the sound source stops then these two modes decay at their own respective frequencies which results in the two modes beating with reach other (Everest, Master Handbook of Acoustics, 5th edition, p.239-240).

Two modes that are close together are modes "propagating" in different directions of the room. The extent to which each of those two modes is excited depends on the location of the source within the respective interference pattern. A mere look at the mode frequencies will tell you nothing about this. Two modes may be spaced such that both are driven when driving one of them, but assume that for one mode the source is located in a pressure maximum, for the other in a pressure minimum, what happens then? Is the second mode still driven at the frequency of the first mode? Do their amplitudes combine to be stronger? The two mode frequencies as such won't tell you. They further don't tell you anything about the bandwidth of the respective mode, they don't tell you anything about their decay.

Then, all of the above is theory, the question is: what of this is actually happening in a real room?


Klaus
 

KlausR.

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Ethan Winer said:
Originally Posted by KlausR.
what of this is actually happening in a real room?

That's the real question. Maybe you can experiment with REW or similar software and post some graphs.


I don't have any measurement gear or software, and never saw the need to get some, because my ears tell me that in my room at listening position things are ok: no modes beating each other, only the 2nd order width mode excited when playing those 3 particular tracks that contain that frequency, I then move the head and it's gone.

Klaus
 

j_j

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I don't have any measurement gear or software, and never saw the need to get some, because my ears tell me that in my room at listening position things are ok: no modes beating each other, only the 2nd order width mode excited when playing those 3 particular tracks that contain that frequency, I then move the head and it's gone.

Klaus

If you have a decent Omni microphone, and a good stereo ADC and DAC, look at the "FFT Workshop" at www.aes.org/sections/pnw/ppt.htm and there you can get freeware that will allow yout o measure things (within your mic and adc/dac parameters) to a fair-thee-well. For free. Just load "Octave" and have at.
 

KlausR.

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If you have a decent Omni microphone, and a good stereo ADC and DAC, look at the "FFT Workshop" at www.aes.org/sections/pnw/ppt.htm and there you can get freeware that will allow you to measure things (within your mic and adc/dac parameters) to a fair-thee-well. For free. Just load "Octave" and have at.

Thanks for the pointer. Don't have a mike, and for the time being no perceivable problem in the listening/living room. The room is in fact heavily treated (acoustic ceiling: http://asadl.org/jasa/resource/1/jasman/v106/i1/p233_s1), and the speakers are on the longer room wall and have good off-axis response and high(ish) directivity, that too should help, just in case that help is needed.

Klaus
 

KlausR.

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I’m reading the PhD thesis of Bruno Fazenda, “Perception of room modes in critical listening spaces”, and found the following in the conclusion part:

The definition of a suitable aspect ratio to control the modal distribution of a room is a useful technique to avoid the problem of degeneracy, but is not in itself sufficient and does not certify that the room response will be correct and free from perceptible problems. The reliance on frequency spacing statistics makes this technique weaker when compared to other methods that take into account the temporal characteristics of resonances. If, as is often required, adequate isolation is ensured, problems of resonances should always be expected given the rigid boundaries that are usually specified. These resonances are a physical property of cavities that cannot be avoided, and even ‘optimal’ modal distributions will cause perceptible problems when musical notes match specific artefacts of the room response.

Furthermore, the results from this investigation indicate that the temporal response of resonances is an important subjective factor in the detection of the effects of room modes. Given this, and taking into account the large differences between decay rates of a similar bandwidth resonance at two different frequencies, the mere reliance on the modal spacing gives a poor indication of the subjective quality of a room. This is associated with the fact that rooms with more resonances at the lowest audible frequencies will exhibit longer decays than rooms with the same average modal spacing but with less modes at the lower frequency range.

Interesting reading, check it out at

http://eprints.hud.ac.uk/3601/1/Per...l_listening_spaces-BFazenda-PhDThesis2004.pdf

Klaus
 

NorthStar

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Ethan Winer

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Don't have a mike, and for the time being no perceivable problem in the listening/living room.

I don't know how much you have invested in your listening system, but a perfectly competent measuring microphone can be had for $50:

Comparison of Ten Measuring Microphones

The only other things you need are a computer which I assume you already have, and a decent USB sound card with phantom power ($100):

Room Measuring Primer

If you measured your room's response and ringing, I'm pretty sure you'd be surprised by what you see.

Edit: This $90 USB microphone needs no separate sound card:

http://www.parts-express.com/pe/showdetl.cfm?partnumber=390-808

--Ethan
 

j_j

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Or just go for a USB measurement mic! Got mine for ~$50

Actually, Bruce, that complicates things quite a bit. It is better to have the ability to loop back the DAC signal into the ADC signal in one channel. This allows software to detect and analyze things like system latency, and from that it can also provide hints (via looking at the analytic envelope of the impulse response) as to where various reflections, etc, arise, by showing you the total distance speaker->reflection->mike. If you try it using two speakers in different locations, you can pretty much figure out from whence the unwanted comes.
 

KlausR.

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Ethan Winer said:
Originally Posted by KlausR. View Post
Don't have a mike, and for the time being no perceivable problem in the listening/living room.

I don't know how much you have invested in your listening system, but a perfectly competent measuring microphone can be had for $50:

The only other things you need are a computer which I assume you already have, and a decent USB sound card with phantom power ($100):

If you measured your room's response and ringing, I'm pretty sure you'd be surprised by what you see.


Price is not the problem. However, I consider that measuring is appropriate once a problem is spotted, not the other way round, i.e. the measurements tell me that there is a problem that I will inevitably perceive. For all sorts of things there are perceptual thresholds and measurements, as I know them, are not correlated to those. In Fastl/Zwicker is a graph showing what room impulse response looks like once converted to Sone:

SPL_Sone.JPG

The Sone scale is linear, i.e. 6 Sone are perceived twice as loud as 3 Sone. On its own graph in Sone is much less intimidating than the graph in SPL. One now would need the JND (just noticeable difference) to see whether there might be a perceivable problem.

As far ringing is concerned, Cumulative Spectral Decay plots will show both magnitude and temporal behaviour of e.g. modes, but they do not take into account magnitude and temporal masking.

In forums like this one there are many who are literally hypnotized by "bad looking" measurements, without any regard whatsoever as to how much of what you measure you can actually hear, or even hear as being disturbing. Because of all this I do not trust uncorrelated measurements, so as long as I don't spot any audible problem in my setup, I don't see the need to measure.

Klaus
 

j_j

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Price is not the problem. However, I consider that measuring is appropriate once a problem is spotted, not the other way round, i.e. the measurements tell me that there is a problem that I will inevitably perceive. For all sorts of things there are perceptual thresholds and measurements, as I know them, are not correlated to those. In Fastl/Zwicker is a graph showing what room impulse response looks like once converted to Sone:

View attachment 10652

The Sone scale is linear, i.e. 6 Sone are perceived twice as loud as 3 Sone. On its own graph in Sone is much less intimidating than the graph in SPL. One now would need the JND (just noticeable difference) to see whether there might be a perceivable problem.

As far ringing is concerned, Cumulative Spectral Decay plots will show both magnitude and temporal behaviour of e.g. modes, but they do not take into account magnitude and temporal masking.

In forums like this one there are many who are literally hypnotized by "bad looking" measurements, without any regard whatsoever as to how much of what you measure you can actually hear, or even hear as being disturbing. Because of all this I do not trust uncorrelated measurements, so as long as I don't spot any audible problem in my setup, I don't see the need to measure.

Klaus

How are sones being calculated there? The results do not look appropriate, and I would like to see (and in fact you can send it to me if you like) the actual impulse response, which I can (given some time) analyze in terms of actual loudness and see what comes out of it.
 

KlausR.

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How are sones being calculated there? The results do not look appropriate, and I would like to see (and in fact you can send it to me if you like) the actual impulse response, which I can (given some time) analyze in terms of actual loudness and see what comes out of it.

That graph is copied directly from the book, I attach the pdf of the corresponding page.

Klaus

View attachment Scan001.PDF
 

j_j

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Whew - need a way to know how he calculated sones. I have a substantial feeling that bandwidth was not properly considered.

If you see the blog (small, yes) at audioskeptic.blogspot.com and read the loudness discussion, you'll see what I mean.
 

KlausR.

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Dec 13, 2010
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Whew - need a way to know how he calculated sones. I have a substantial feeling that bandwidth was not properly considered.

If you see the blog (small, yes) at audioskeptic.blogspot.com and read the loudness discussion, you'll see what I mean.

Tomorrow we're off for 6 weeks vacation, enough time to read the printout of your blog.

Klaus
 

theophile

Well-Known Member
I suggest that you do, in particular post #41.



I attach the pdf of an overview of literature on this particular issue, you will see that making the walls non-parallel does not solve the problem.

Klaus


View attachment 10601

I read enough to know that I'd rather read jj's replies than yours and the pdf wasn't a particularly illuminating document.
 

KlausR.

Well-Known Member
Dec 13, 2010
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... and the pdf wasn't a particularly illuminating document.

In this case I propose that I mail you the papers and you prepare a pdf which IS illuminating.

Klaus
 

theophile

Well-Known Member
In this case I propose that I mail you the papers and you prepare a pdf which IS illuminating.

Klaus

It's impossible to add to a full vessel. I doubt that I'd bother relaying any knowledge your way.
 

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