Earl Kiosterud wrote:
"Adam S" not.valid@nosuchaddress wrote in message
u...
Hi,
I'm fooling around with a simple audio analyzer program I wrote for my PC. It basically
does a sine sweep and measures phase & mag difference between input channels of the sound
card and then dumps the measured response to a text file. I'm using the tools Gnuplot and
Scilab for the rest of my analysis. My goal is to measure the response of individual
drivers to aid the design and optimization of a crossover.
I know that most people use 'gated' sampling of impulse/MLS excitation signal. I'm
hopping to stay with sine waves and slow sweeps for its brilliant S/N ratio (over 60dB
range). Below is link to a plot I made of the drivers I'm testing.
http://members.optusnet.com.au/~eseychell/bode.png
As I expected at 1 meter microphone distance ,the response is very much effected by room
reflections. (room size 10 x 3 x 5 meters) . At 30mm, the response curve is significantly
smoother. I'd like to know if this "near field" data can be used to extrapolate the
response for 1 meter ? Can I assume that only overall gain will be changed between a
measurement at microphone distance of 1 meter vs 30mm ?
Adam
Adam,
I read your post with considerable interest, and have these comments.
"Close mic'ing" is often done with LF drivers, and I think generally valid, At the lower
end of the plot, the cone is pretty much a piston, the wavelengths long, so the pressure
generated is pretty much the same anywhere. There is virtually no wavefront, as the
wavelength is very large compared to the size of the cone. I wouldn't trust it at the
higher end of plot. Wavefront is important, and you're only sampling one part of it. You'd
expect a big diffraction peak, unless the inner part of the cone is starting to operate
independently, and if that's the case, all bets are off as to the wavefront that it's
creating. That appears to be the case with the quite-smooth on-axis response of your driver
in the 1-4 KHz range. To increase your confidence there, you might
want to run several close-mic'ed plots
with the mic moved to various places near the center of the woofer and see what kind of
correlation there is. Probably a better test would be to do this part outdoors, not
close-mic'ed, with the woofer aimed upwards, and away from anything reflective. You'll get
all kinds of comb effects in the 100-500 Hz range from ground reflection, and you might
want to position the cabinet the same distance as it is from the wall when it's in use
indoors to get an idea of the back-wall reflection effects there -- the're usually not
insignificant, though often overlooked. There won't be any significant diffraction in the
1-4 KHz range to go around to the back and bounce back, so the results in that range should
be quite valid. You can still move the mic around and look for variances. You'll also want
to know generally how much the off-axis response sags as the frequency goes up, and look at
the crossover frequency to see if that is of concern.
In any event, you'll also want in-room, not close-mic'ed, both-woofer measurements. There
is back wall reflection comb-filter effects that I mentioned earlier that can be
substantial, at least the first null. There is "coupling" where the pressure from one
woofer actually changes the radiation resistance (favorably) seen by the other woofer. And
and there is room effect at very low frequencies where the long wavelengths make the room
essentially a pressure container, also causing a rise in output.
I'm not at all sure how much I'd trust close mic'ed tweeter measurements, but the outdoor
test can be done there quite readily. Also, do some off-axis plots too.
I'd love to try your program. My email address is at my web site, www.smokeylake.com/excel
at the bottom of the page, if you'd be willing.
Thanks Earl for showing an interest in my project and your informative
reply. Its nice to hear that I'm not completely going off track with
this idea of near field measurement across full audio range for all
drivers in the system. I will definitely do as you suggested and try
various positions on a parallel plane a short distance from the woofer
and tweeter. You have given me plenty of ideas to experiment with. I'm
not sure what you mean about the "diffraction peak". If your referring
to diffraction around the speaker box, the isn't this energy normally
not contributing in standard anechoic measurement techniques ?
I am still setting up my homemade measurement jig and plan to buy some
long lengths of cable to run the measurements outdoors. The beauty of
sine sweep is its inherent selectivity to the signal source. The
bandwidth can be low as few hertz or less if your prepared to wait
longer for the sweep, about 1 minute or so. This allows outdoor
measurements with minimal affect from ambient noise. As you also
mentioned, outdoor measurement will confirm any differences between near
field and far field plots.
To avoid ground reflections, will it help laying the speaker horizontal
facing up, and raised 1 or 2 meters above the ground covered with tall
grass ?
I basically wrote this little program because I couldn't find anything
that would do arbitrary sine sweep vector plots using the PC soundcard.
It actually started it few years back, but never got anywhere. Since
then I've had more practice at C++ and had some drivers gathering dust
ready to be put into speaker boxes I built some time ago. I should warn
you its quite in its early stages and only has command line interface.
This suits me as it integrates well with most of the other scripting
and/or command line based software tools I use. A GUI might be useful
but I don't have time to devote to aesthetics. It first accurately
calibrates soundcard latency, then monitors input levels to allow
pre-adjustment of the soundcard volume controls before commencing the
sweep(s). I'll be happy to clean it up and make it GNU free software,
thats assuming of course if anyone else out there would find such a tool
useful. Currently only raw results are produced. Math tools like Scilab,
Octave or Mathcad can easily read the resulting text file for further
processing.
Adam