Re: Acoustics==>reverberation time

[Gary Nelson]

I use Audacity to make RT-60 measurements using a good quality condenser 
microphone. The details of making the recording would be a separate topic.

Microphone placement affects the results. You probably want to know this 
information for humans to sit and listen to something, so put the mike at ear 
level. An omnidirectional microphone is preferable to a cardoid at least for 
first cut results.

Get some large toy balloons and inflate a couple as full as possible. Start the 
recording with Audacity.

Pop a balloon and wait a few seconds and stop the recording. Location of the 
pop also affects results. On stage is a good place as that is source of live 
music for example. See method for using speaker(s) below.

Now select a pulldown menu on the track and choose "waveform in dB."  Tho 
balloon pop approximates an acoustic impulse, so the recording approximates the 
impluse response of the space at the location of the microphone.  In 
Edit/Preferences set the output format range to 60dB. The display will look 
like an arrow. Normalize the recording so that the peak is at zero dB. Now when 
the signal decays to - 60dB, the time from onset to that point is the 
Reverberation Time to -60dB aka RT60.  You may find that the background noise 
in the room is above -60dB so the RT has to estimated by using a straight edge 
to extrapolate the decay down to -60.  

Another way to excite the room is to generate a signal using Audacity's pink 
noise generator at a level near maximum (like .9) and make a pulse a few 
seconds duration (longer that the RT-60).
Play the sound pulse and record the result. You will see the excitation and the 
recording. The recording will show a rise to equalibrium/steady state, flight 
time delay (from the speaker to the mike -- estimate distance in feet from 
speaker to mike -- sound travels 1100 ft/sec -- I use 1ms/ft as a way to get a 
quick estimate), and then the arrow as the signal decays to -60dB.  

For example, if the speaker is 50 feet from the mike, it will take most of 50 
ms for the sound to travel from the speaker to the mike. You will see the 
excitation pulse end and about 50ms later the recorded signal will begin to 
decay.

Pink noise is white noise that has been filtered to decay at 6dB per octave -- 
also known as 1/f noise. Most sound from natural sources has this 1/f quality 
so it is a decent excitation signal.

Audacity also offers a fourier transform tool. It is useful to apply the FFT 
tool to the steady state recordiing and look at results.  If the room has 
resonance(s), they will be visible as peaks in the spectrum.



Gary Nelson, PhD
address@hidden




On Nov 21, 2010, at 4:11 AM, Martin Maxino wrote:

> 
> Can i use Octave for acoustical measurements like Reverberation Time, etc.
> -- 
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