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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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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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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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