Re: [LAU] OT - impulse response recording

On Fri, 28 Jan 2011 16:44:47 +0100
fons@email-addr-hidden wrote:

> On Fri, Jan 28, 2011 at 03:37:21PM +0100, Renato wrote:
>
> > fons@email-addr-hidden wrote:
> >
> > > Starter guns, electric sparks, exploding balloons or
> > > preservatives, etc. are not the best way to do this. You get a
> > > much more accurate result and a much better S/N ratio using a
> > > sine sweep and deconvolution instead. Aliki (available on my
> > > website) can do this. See
> > > <http://www.kokkinizita.net/papers/aliki.pdf> for an
> > > introduction. The actual application does not have all the
> > > features explained in the paper but it will do the basic IR
> > > measurement.
> >
> > question: why the amp/speakers don't have to be high quality
> > flat-response monitors (as I think Julien is saying), but the mic
> > does?
>
> They have to be of reasonable quality and FR, but any remaining
> small errors can easily be calibrated out. You just take the
> direct sound (for a good speaker + mic this will be the first few
> ms of the IR) as a reference.
>
> A few percent distortion (most speakers will have this) does not
> matter if the measurement is done in the correct way: after the
> deconvolution the distortion products will appear *before* the
> real IR and they can be removed easily.
>
> > Also, do mic vendors offer files with the frequency response of the
> > mic so one can "multiply the inverse of it" with the spectrum of the
> > signal recorded with said mic, taking mic charateristics out of the
> > equation (at least theoretically)?
>
> The high quality ones will have tight specs and usually come with
> a measured FR or other calibration data.
>
> There are a few other points to consider.
>
> Most speakers will be directional at all but the lowest frequencies.
> It depends on your application if this matters or not. If it does
> (e.g. to measure room acoustics according to international standards)
> you need an omnidirectional speaker. These are usually made as a
> dodecahedron of small speakers. See <http://www.gizmag.com/go/3894>
> for an example.
>
> Also omni mics will have some directionality at high frequencies, and
> that is why they come in two forms: calibrated for flat FR on-axis
> (plane wave), or for flat FR in a diffuse field (averaged over all
> directions). The difference matters only at very high frequencies.
> Some measurement come with a small conical diffuser which can be
> attached in front to improve diffuse-field response.
>
> Ciao,
>

Thanks for the detailed response; I realize this is still mostly
rocket-science for me and I'll have to study before asking further
explanations (I'll get there eventually).

renato
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Received on Fri Jan 28 20:15:07 2011