Convolutions and Reverb

[dmcclain]

Hi,

I just got an Earthworks M-30 measurement Mic and I tried some experiments to obtain the room response. One of these involved popping a balloon and recording the "impulse response" of the room.

I notice that the initial pulse is bipolar -- meaning that it is a rising pulse followed immediately by a negative going pulse at about the same amplitude, and then followed by some ringing before the first few reflections start arriving. I suspect that this has to do with (a) the finite duration of the impulse -- a balloon popping is not a perfect impulse, and (b) conservation of mass -- which would imply that the sum amplitude through a pulse of air would have to equal zero.

Anyway, when I try to convolve music using this measured impulse I end up with a very annoying high-pass filtered sound, not at all like music played through loudspeakers in this room. The bipolar pulse at the front of the recording would indeed have a high-pass character to it.

If I convolve white noise with this pulse then the resulting noise is very colored and displays peaks at frequencies where I would expect the room modes.

It appears to me that I don't really have the "impulse response" but rather the convolution of the balloon popping waveform and the true impulse response of the room. I can guestimate the location of the first few reflections and create an artificial impulse response file containing those first few impulses. When I do that the results sound a bit more like the actual room -- very comb filtered.

So my questions to all you audio experts out there are these...

When you want to get a sample of live reverb, do you use a popping balloon, or some other technique?

And if you use a popping sound, do you do any kind of preprocessing to the recorded reverb (like I did with eyeball estimation of reflection impulses) or do you actually use the raw recording itself?

And if you use the actual recorded waveform for your reverb convolution kernel, how do you deal with the high-pass character of the sound?

Cheers,

- DM

[resistor man]

Wish I could help... haven't even got into the convolution aspects of CEP and don't know when I will, especially since I bought Reaktor, which takes up all that part of my brain.

[Kihoalu]

.

It sounds to me like you may have had the ballon too close to the mike. If I was trying to get the impulse response of my little studio I would pop the balloon near the speakers with the mike near the listening position. Since you are AC coupled somewhere in the mike/amp chain then the response MUST be bipolar but it sounds like your leading edge of the impulse is too sharp ergo the extra HF when you convolve.



[This message has been edited by Kihoalu (edited 10-24-2001).]

[dmcclain]

Hmmm, interesting thoughts...

I tried another experiment where I fed an impulse 1-sample wide to the speakers across the room and recorded those impulse responses. They are much sharper even than the balloon popping, and they actuall had a huge amount of bass.

The balloon pop gave about -24 dB bass relative to the high-end. Overall, those speaker impulses sound more like what my room sounds like when convolved with music.

But I realize I measured the combination of room + speaker when I made those recordings.

I also tried playing with some chirp sounds and I got a good cross correlation between the source chirp and the first arrival from the speakers -- 11 ms delay. And I can clearly see the reverb tail extending about 100 ms past the first arrival on the sonogram displays. But for the moment the fine details about reflections allude me on this kind of processing.

I'll keep digging here. Thanks for the suggestions!

- DM

[dmcclain]

> Since you are AC coupled somewhere is the
> mike/amp chain the response MUST be
> bipolar

...that's actually a very interesting comment, and it took me a while to understand what you were really saying here.

Thanks!

- DM

[JohnBussoletti]

You can also try a sharp hand clap, a la Flamenco. Cup one palm slightly and strike it with three fingers of the other hand, trapping air in the pocket. Practice till you get a clear, short pop.

Pistol shots come to mind as well, but they might overload the SPL limit of the mike (as well as alarm anybody in your vicinity). Has anyone tried caps, as in the cap guns of our youth?

Also, a steel ball dropped on a hardwood floor?

I like the single sample through the speakers. Can you estimate the frequency response of the speakers (via analytical definitions of sine wave sweeps played through the speakers and recorded into your calibrated mike) and then de-convolve the speaker coloration from the impulse?

[Kihoalu]

.

I guess I did not say that quite right. For good HF response the leading edge will be the same as the impulse - right? i.e. a vertical line. For totally flat response without echoes the returned response would be the same impulse as presented (that's obvious). Now if you reduce the HF the leading edge will either lay-over or reduce in amplitude or both. Conversely, if you increase the LF the area of the pulse will increase and the leading and trailing slopes will tend to be slower as well. Now add in reflections which may have all of these characteristics and you get the total shape. This is all I was trying to communicate.

You could be overdriving the mike as well (even with just a balloon pop) if you are too close and this will screw-up you impulse as well.

Can you go somewhere dead and small and pop the ballon to see if it makes a good impulse?? I always heard that popping balloons was a good way to do it - but I have never tried to get an impulse that way yet.

...K