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Room measurements using pink noise
Yahzi
post Jan 15 2013, 11:54
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I was browsing around and found this bit of information which I don't quite understand, so anyone who is well versed in acoustics can perhaps set a few things straight :

Accurate room measurement is complex in that you need.
1: A way to record what is going on in a room.
2: Away to create the sound in a room that 1 will measure.

Currently 1 is reasonably easy and cost effective to achieve as there are a number of accurate (or accurate enough) systems available that do much of the number crunching in software.

The most common way to achieve 2 is to use the loudspeakers already in the room and this while convenient, leads to inaccuracies in measurements.

The reasons for this are as follows.

To accurately excite all the modes in a room you need a source that is broadband and truly omnidirectional.
While most decent speakers can be broadband -but often are a little deficient at low frequencies-, few if any are truly omnidirectional.
This means that when using a speaker as an audio source in room measurement, you are in reality only measuring the combined response of your speaker and the room. The in room measurement of Bi or Di polar speakers that had the same frequency response would be totally different.

Ideally you want a sound source that as already mentioned is truly omnidirectional as this will excite all room modes equally. Any one who hasever looked at polar plots of speakers will know that even the best claimed omni speakers rarely are truly omnidirectional.

A gun shot on the other offers typically a wide bandwidth, is omnidirectional and has the added benefit of being loud. Loud enough so that true reverberation time and modal measurements can be made.

It has even been suggested, for those that don't want to fire their magnums in their listening rooms, that something as simple as a hand drill could be used as a sound source. These are cheap, the results are repeatable and will certainly put a lot less stress on a system than trying to belt out Pink noise at high SPL.


I thought pink noise was the standard method of measuring speakers in a room. He seems to be saying that a gunshot rather than pink noise is preferable as the levels are higher, or at least that's what I think he's saying. Is this guy completely on crack or does he have a point?
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DVDdoug
post Jan 15 2013, 19:33
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Typically, you want to measure the speakers and room together since the speakers are usually used to reproduce music in the room. Of course the location of speakers (or other sound source) and listening position are very important. If you move something, the results will change.

Typically, pink noise or a frequency-sweep is used to measure the frequency response of the speakers in the room.

In theory, you can use an impulse to measure the frequency characteristics and in real life it's how you measure the reverb in a room. In fact, a convolution reverb does just that... You supply an impluse from your favorite concert hall and you can digitally reproduce the sound of the hall on your recording.

Other than for measuring reverb time or simulating room reverb, I'm not sure how useful impulse measurements are for measuring frequency response. (Sometimes, I clap my hands to get a sense how "live" or "dead" a room is.) But, I'm sure lots of different kinds of measurements are made at various listening positons when building concert halls and theaters. And, I'm sure the final tweaks are done by ear with "real" sound sound or real music!

In the real-physical world, ideal impulses (infinite amplitude and zero time-duration) do not exist. I don't know how well a gunshot represents an impulse and I actually don't know how it's most-commonly done, but here's Wikipedia. I would guess that the sound characteristics of the particular gunshot (or whatever) have to be subtracted-out of the results.

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Ethan Winer
post Jan 15 2013, 21:48
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QUOTE (DVDdoug @ Jan 15 2013, 13:33) *
Typically, you want to measure the speakers and room together since the speakers are usually used to reproduce music in the room.


Exactly. I didn't realize an audio forum in Zambia could be so popular, but this is the third time today I've seen someone quote that post and ask if it was valid. No, it's not valid, even if it's well-intentioned.

QUOTE
In theory, you can use an impulse to measure the frequency characteristics and in real life it's how you measure the reverb in a room.


Modern room measuring software uses a sine-wave sweep as a source signal, then uses convolution to convert that to an impulse to derive the various graph displays such as frequency response, ringing, early reflection timing and spectrum, RT60, and so forth. Using a sweep is much better than trying to create an impulse that's suitably linear, and also loud enough to get a useable s/n ratio. A sweep also allows using a "tracking filter" to further improve the quality of the measurement.

--Ethan

This post has been edited by Ethan Winer: Jan 15 2013, 21:49


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Yahzi
post Jan 16 2013, 10:16
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Hi Ethan. So measuring the room on it's own is really just pointless? Because we don't hear just the room? I know with a gating measurement you can eliminate the room and "see" the raw speaker response. But how would you see the room response without the speaker affecting it? It sounds really silly, come to think of it.

This post has been edited by Yahzi: Jan 16 2013, 10:17
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Arnold B. Kruege...
post Jan 16 2013, 14:08
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QUOTE (Yahzi @ Jan 16 2013, 04:16) *
Hi Ethan. So measuring the room on it's own is really just pointless? Because we don't hear just the room? I know with a gating measurement you can eliminate the room and "see" the raw speaker response. But how would you see the room response without the speaker affecting it? It sounds really silly, come to think of it.


Measuring the room on its own is pointless for audio because that isn't how we use the room. Speakers are different enough from each other that they generally each cause the room to respond in a different way.

Reciprocally, the room has a very strong influence on the sound of the speaker, such that it is of questionable benefit to audition speakers in any room other than the one in which they are going to be used.
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DVDdoug
post Jan 16 2013, 19:19
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QUOTE
Hi Ethan. So measuring the room on it's own is really just pointless? Because we don't hear just the room?
Not always... If you are working on room acoustics it would be helpful to measure the room acoustics alone. If you are building a theater, concert hall, or "music room" you might be using it for live acoustic instruments.
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Ethan Winer
post Jan 16 2013, 19:29
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QUOTE (DVDdoug @ Jan 16 2013, 13:19) *
If you are building a theater, concert hall, or "music room" you might be using it for live acoustic instruments.

That's what I would have said if you didn't beat me to it. I deal with recording studio "live" rooms too, where the sounds are created by acoustic instruments and guitar amplifiers etc rather than playback speakers.

--Ethan


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Yahzi
post Jan 17 2013, 20:04
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On the subject of guns, do people still use them to measure rooms? Or exploding balloons? Or is it just one of those things that were used a few decades ago but people moved on from? What do speaker designers use when measuring speakers, software and condenser mics, or something else entirely?

BTW, thanks Ethan, Arnold and Doug!

This post has been edited by Yahzi: Jan 17 2013, 20:08
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DVDdoug
post Jan 17 2013, 20:23
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QUOTE (Yahzi @ Jan 17 2013, 11:04) *
On the subject of guns, do people still use them to measure rooms? Or exploding balloons? Or is it just one of those things that were used a few decades ago but people moved on from?


Ethan says:
QUOTE (Ethan Winer @ Jan 15 2013, 12:48) *
...Modern room measuring software uses a sine-wave sweep as a source signal, then uses convolution to convert that to an impulse to derive the various graph displays such as frequency response, ringing, early reflection timing and spectrum, RT60, and so forth. Using a sweep is much better than trying to create an impulse that's suitably linear, and also loud enough to get a useable s/n ratio. A sweep also allows using a "tracking filter" to further improve the quality of the measurement.

--Ethan


Differnet caliber guns have a different sound (different spectrums), and a balloon sounds differnet from a gunshot too. So, I would have assumed that a gunshot or balloon is not ideal for measuring anything other than reverb time.
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Arnold B. Kruege...
post Jan 17 2013, 23:09
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QUOTE (Yahzi @ Jan 17 2013, 14:04) *
On the subject of guns, do people still use them to measure rooms? Or exploding balloons? Or is it just one of those things that were used a few decades ago but people moved on from? What do speaker designers use when measuring speakers, software and condenser mics, or something else entirely?

BTW, thanks Ethan, Arnold and Doug!



I did some room measurements about 2 years ago, where I wanted a quick confirmation of the source of the most noticeable reflection in the room and had only simple equipment and software at hand. The sound source was a cap pistol. Qucik and dirty. Did the job!

BTW, the bang from a cap pistol is far from being any kind of a pure impulse. But its better than a hand clap, which I also tried.

Spark gaps were also used back in the day.

In general the use of impulsive sound sources has SNR issues because the energy that you can pack into a single very short sound is relatively small.

With FFTs, we can obtain the "Impulse Response" of the room from measurements involving just about any reasonable kind of known, broadband stimulus signal including pink noise, white noise, music, impulses, swept tones, etc.
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