Acoustic room response - measurement and interpretation

I'd like to get your suggestions on a good way to look at acoustic room 
response for purposed of equalization.

I'm thinking that I can play some wideband tones, which get recorded into a PC 
via a microphone.

Does anyone have any suggestions on the tones that would be good to use?

Once the data is recorded in my PC, what type of processing needs to take 
place to 'characterize' the room? (taking the inverse FT, etc.?) If someone 
has a idea on a procedure for this I'd like to hear what you have to say.

Thx.

What is a "wideband tone"?  The standard ways are an impulse, swept
sinusoids, and pink/white noise.  I believe that pink noise is the most
commonly used method.

Once the data is in the PC, an FFT will give you the room's frequency
response.

A common method is to use a real-time spectrum analyzer that basically
displays a live FFT.  While running noise through the speakers, an equalizer
is adjusted in order to make the live FFT response flat.  It's important to
use a microphone with a flat frequency response (generally one designed
specifically for this purpose) or you will be compensating for that as well.

"Larry McFarren" <invalid@address.com> wrote in message
news:wEPWb.21467$tD.2040@newssvr24.news.prodigy.com...
> I'd like to get your suggestions on a good way to look at acoustic room
> response for purposed of equalization.
>
> I'm thinking that I can play some wideband tones, which get recorded into
a PC
> via a microphone.
>
> Does anyone have any suggestions on the tones that would be good to use?
>
> Once the data is recorded in my PC, what type of processing needs to take
> place to 'characterize' the room? (taking the inverse FT, etc.?) If
someone
> has a idea on a procedure for this I'd like to hear what you have to say.
>
> Thx.
>

Jon Harris wrote:

> What is a "wideband tone"?  The standard ways are an impulse, swept
> sinusoids, and pink/white noise.  I believe that pink noise is the most
> commonly used method.
> 

The swept sin method is displacing the noise stimulus in 
situations where there may be signifigant harmonic 
distortion present because that method separates the 
harmonic components from the fundamental components in time.

No approach to this is very good for rooms, however, because 
at best you get the response of the room at one point and 
you don't have to move your mic very far at all to get a 
very different response because of interference effects from 
reflecting objects and surfaces in the room.

Bob
-- 

"Things should be described as simply as possible, but no 
simpler."

                                              A. Einstein

In article <c0gii7$17cg55$1@ID-210375.news.uni-berlin.de>, 
goldentully@hotmail.com says...
>
>What is a "wideband tone"?  The standard ways are an impulse, swept
>sinusoids, and pink/white noise.  I believe that pink noise is the most
>commonly used method.
>
>Once the data is in the PC, an FFT will give you the room's frequency
>response.
>
>A common method is to use a real-time spectrum analyzer that basically
>displays a live FFT.  While running noise through the speakers, an equalizer
>is adjusted in order to make the live FFT response flat.  It's important to
>use a microphone with a flat frequency response (generally one designed
>specifically for this purpose) or you will be compensating for that as well.
>
>

Thanks for the feedback.

Yes, by wideband tone, I was referring to an impulse (wide bandwidth).

OK, so let's say that I play pink noise out my speakers (left and right at 
separate times) and collect the data with my PC. I'm not looking to use 
external eq or spectrum analyzers. This is the functionality that I'd like to 
implement myself. Also, the microphone calibration is something that I can 
take care of.

Assuming that I'm able to record the data into my PC with a fairly high degree 
of accuracy (mic. calibrated correctly - freq. response) and high SNR, I'm 
searching for a procedure to analyze the data.

So, you've mentioned taking a FFT. OK, I believe that I should be able to do 
this with MATLAB. Any suggestions on the size of the FFT? Any other issues to 
contend with regarding the FFT? Sorry, I am new to DSP and really need things 
spelled out at this point.

Now, once I look at the FFT and determine where my peaks are in the freq. 
response, I will want to reduce their amplitude in order to get a flatter 
freq. response. I won't be handling the dips in the freq. response just yet. 
For now, I just want to smooth out the peaks. So how do I go from analyzing 
the peaks in the FFT to creating a filter, perhaps a FIR filter?

Thanks again. Please continue your feedback.

Larry McFarren wrote:

   ...

> Yes, by wideband tone, I was referring to an impulse (wide bandwidth).

   ...

An impulse has the appeal of theoretical simplicity, but is poor in 
practice. The peak-to-average ratio doesn't lend itself to good data 
reduction. A pseudo-random bit generator putting out only 1 ans -1 has 
unity peak-to-average power ratio, the best possible. Autocorrelation 
probably gives a better result than an FFT.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

In article 402bf861$0$3138$61fed72c@news.rcn.com, Jerry Avins at
jya@ieee.org wrote on 02/12/2004 17:04:

> Larry McFarren wrote:
> 
> ...
> 
>> Yes, by wideband tone, I was referring to an impulse (wide bandwidth).
> 
> ...
> 
> An impulse has the appeal of theoretical simplicity, but is poor in
> practice. The peak-to-average ratio doesn't lend itself to good data
> reduction. A pseudo-random bit generator putting out only 1 ans -1 has
> unity peak-to-average power ratio, the best possible. Autocorrelation
> probably gives a better result than an FFT.

to punctuate what Jerry said, folks might want to read about Maximum-Length
Sequences (MLS) to get the whole story.

r b-j

This link is to a (long) but very readable article on acoustic equalization:
    http://www.ethanwiner.com/acoustics.html

Standing waves are quite sensitive to the exact position of the microphone,
and the article shows up to 20 db notches in room response being very
common.

Swept sine measurements need to be done slowly. Because cancellation is due
to reflection and acoustic delay, the measurement needs to be done slowly
enough to allow the standing wave to develop at each measurement frequency.

Impulse measurements can suffer from poor signal-to-noise ratios at some
frequencies.

Pink noise is a common technique, but be sure the sample is long enough in
time to capture the lowest frequencies of interest and the relevant acoustic
delay. If the sample rate is high enough to capture high audio frequencies
(say 48 Ks/s), the FFT size can get pretty large especially for a large
room.

    -- Tom




"Larry McFarren" <invalid@address.com> wrote in message
news:wEPWb.21467$tD.2040@newssvr24.news.prodigy.com...
> I'd like to get your suggestions on a good way to look at acoustic room
> response for purposed of equalization.
>
> I'm thinking that I can play some wideband tones, which get recorded into
a PC
> via a microphone.
>
> Does anyone have any suggestions on the tones that would be good to use?
>
> Once the data is recorded in my PC, what type of processing needs to take
> place to 'characterize' the room? (taking the inverse FT, etc.?) If
someone
> has a idea on a procedure for this I'd like to hear what you have to say.
>
> Thx.
>

TOM wrote:

> Swept sine measurements need to be done slowly. Because cancellation is due
> to reflection and acoustic delay, the measurement needs to be done slowly
> enough to allow the standing wave to develop at each measurement frequency.
>

Not true, I'm afraid.  They don't need to develop in order 
to predict from a measurement that they will if sustained.

If that were true the impulse response method wouldn't work 
for other reasons than poor SNR.


Bob

-- 

"Things should be described as simply as possible, but no 
simpler."

                                              A. Einstein

In article c0higa08il@enews2.newsguy.com, Bob Cain at
arcane@arcanemethods.com wrote on 02/12/2004 23:06:

> TOM wrote:
> 
>> Swept sine measurements need to be done slowly. Because cancellation is due
>> to reflection and acoustic delay, the measurement needs to be done slowly
>> enough to allow the standing wave to develop at each measurement frequency.
>> 
> 
> Not true, I'm afraid.  They don't need to develop in order
> to predict from a measurement that they will if sustained.
> 
> If that were true the impulse response method wouldn't work
> for other reasons than poor SNR.

if i may pile on, theoretically you can drive a linear system with *any*
wideband input, measure the output, and compute the transfer function (or
frequency response or any other property derivable from it) by complex
division of the output spectrum by the input spectrum.

particularly, regarding swept sinusoidal measurements, we talked about this
long ago:

http://groups.google.com/groups?threadm=865%40audioheads.com

and
> 
http://groups.google.com/groups?threadm=01be0820%24c1f3ab20%24720101c0%40joh
n-pc


of course, it is measuring the response from wires driving the loudspeaker
amplifier to the wires coming out of the mic preamp for a particular room
and transducer placement.

r b-j

Bob Cain wrote:

> 
> If that were true the impulse response method wouldn't work for other 
> reasons than poor SNR.
> 

I meant to say the "impulse stimulus method" wouldn't work.


Bob
-- 

"Things should be described as simply as possible, but no 
simpler."

                                              A. Einstein