28 Jan
2011
28 Jan
'11
5:04 p.m.
On Fri, 28 Jan 2011 16:44:47 +0100
fons(a)kokkinizita.net wrote:
On Fri, Jan 28, 2011 at 03:37:21PM +0100, Renato
wrote:
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
fons(a)kokkinizita.net wrote:
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.
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? 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,