On Fri, 28 Jan 2011 16:44:47 +0100 fons@xxxxxxxxxxxxxxx wrote: > On Fri, Jan 28, 2011 at 03:37:21PM +0100, Renato wrote: > > > fons@xxxxxxxxxxxxxxx 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 _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/listinfo/linux-audio-user
