# Room measurements using pink noise



## Doctor X (Apr 3, 2007)

I found this bit of information on another forum and I thought I might share it as the poster mentions pink noise not being the most effective signal for measuring the room, but rather, using a gun :

*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 get the feeling that in many cases the problems people are trying to rectify by using some of the systems being used by forum members, are caused by the measurement system they are using.

Are the huge suckouts or peaks visible in plots (but not that audible) a result of the speakers (or other speakers not being used but in the same room) themselves acting like Helmholtz resonators that are summing or cancelling standing waves.

By using the gun method (for want of a better term) you have a baseline to begin with.

Some of my other observations include.
50Hz being a problem frequency.
Our A/C mains is 50 Hz and I have seen a few cases where dodgy power supplies have let this frequency bleed through into audio measurements.
None linear Mics
That due to some external factor (being dropped or misused) have weird frequency response curves.
External noise sources
Participated in some CEDIA training where the room sounded quiet, but three mics picked up a low level hum at quite a high level. This made accurate measurements near impossible.
speaker port positioning and/or port driver phase
Depending on how a measurement is taken, this can cause fluctuations in in room response curves.

Ultimately no single measurement will be able to tell you exactly what is going on in a room.*

I assumed pink noise was the standard for measuring. Is this guy smoking crack or does he have a point?


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## JohnM (Apr 11, 2006)

Pink noise is not the standard for measuring, it is a convenience if looking at frequency response on an RTA. Characterising a room requires measurement of its impulse response, in domestic rooms (and many larger spaces) a log sweep is the best way to do that.

There are strands of truth in the quote, though more relevant for big spaces. To measure the properties of an acoustically large listening space, such as an auditorium or concert hall, a full bandwidth omni-directional source is needed, such as this one or this one. A hand drill would be a very, very poor choice, but starting pistols, balloons or, in very large spaces, what is in effect a small cannon (!) may be used (in each of those cases a direct impulse response measurement is being made, with the source delivering the impulse). 

In domestic-sized spaces we are very rarely interested in the properties of the space independently of the speakers that excite it - what would be the point? Domestic spaces are also not large enough to produce a statistically uniform soundfield, so it would be futile to try to create one. In domestic rooms there isn't any particular need to measure with anything other than the speakers/subs that will be driving the space and at the locations where listeners will be sat. 

If the full modal behaviour of a rectangular room is of particular interest all its modal resonances will be excited by a wide range source in any of the corners, and fully captured by a mic placed in the diagonally opposite corner. As we are only interested in modal resonances at relatively low frequencies a subwoofer does the job effectively, to the lower limits of its response - usually more than sufficient unless the room is very large and has a correspondingly very low first modal resonance, but then we are back in the realm of acoustically large spaces.


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## Doctor X (Apr 3, 2007)

Correct me if I'm wrong, when using REW, does it use a log sweep as opposed to pink noise? Can you specify a log sweep, or is not included? Please correct me if I'm wrong.

The other point which I find interesting are the comments concerning the unused speakers in the room acting as resonators. If you run a frequency response measurement using one speaker or in stereo, wouldn't the additional unused speakers contaminate results, and is there a way to avoid that without dragging them to another room?

BTW, thanks John for the lengthy reply! Much appreciated.


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## JohnM (Apr 11, 2006)

If you use 'Measure' in REW the signal you hear is a log sweep. You would only use Pink Noise (or preferably Pink PN) in conjunction with the RTA.

Speakers are generally very poor resonators (unsurprisingly, otherwise their resonances would dominate their output when using them). Their effect on in-room measurements is inconsequential.


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