One problem most people have when starting to
test their speakers using computer-based testing software and hardware is
the mic. The mic is arguably the most important piece of the process, but
the availability of high-quality, commercial mics with relatively flat
response from at least 20hz to 20khz is low and those that are available
are quite expensive. The cheapest mic on the market that has been regarded
as accurate is the
Mitey Mike II, designed by Joe D'Appolito. The MM2 is sold with the
CLIO testing hardware/software for $110 or sold separately for $179
(calibrated, $139 uncalibrated) through
Old Colony Sound Labs.
I purchased the MM2 when I bought the
CLIO system because it is calibrated by Joe, himself, and I wanted to
be sure that my investment in CLIO wasn't wasted with a poor quality
testing mic. I have no complaints or regrets about purchasing the
MM2. But those of you needing a good testing mic for use with free
programs like
Speaker
Workshop or some of the high-quality commercial products like LspCad,
Soundeasy, or LspLAB (no longer available) probably don't want to shell
out an extra $140-$200 if you don't have to, right? Well, you don't.
Plain and simple, you can build a mic using the same electret capsule that
goes into the Mitey Mike II for chump change, leaving some money leftover
for nice drivers to test.
The MM2 uses a variant of the
Panasonic WM60AY electret capsule, available from
Digikey
for $2.64 apiece as
part number P9959-ND. I'm not sure that the MM2 uses this exact
capsule since Panasonic makes a wide variety of almost identical electret
mic elements, but the response is nearly identical which is all that
matters (the sensitivity may be different, but the calibration is what you
give up by building it yourself. This won't cause you much trouble,
since exact sensitivity calibration will only let you measure the exact
sensitivity of a speaker. When designing a speaker, dealing in
relative terms is all that matters.
The whole process for building this mic
yourself should take under an hour, but you'll need a few things. Of
course, you'll want to call Digikey and order a few of these elements (I
suggest getting a couple of extras since they're hard to solder and you
might pork up one or two). Next, you should get some 30awg Kynar
wire from Radio Shack or wherever you normally buy 30awg Kynar. I
bought some red and some blue just to keep them straight. You can
buy whatever colors you want, or just a single color and mark one lead
with an ink marker. Also, some heat shrink tubing is good for
sealing the whole thing up at the end. A brass wand is the a good
mounting device, as well, and is what the MM2 uses. Finally, make
sure your soldering iron has a fine point tip on it. If not, the 15W
soldering iron at Radio Shack costs about $7 and is useful for fine
soldering applications like this. Assuming you have the soldering
iron already, a roll of Kynar wire and some heat shrink tubing is going to
cost you about $5.00. As for the cable that will connect your mic to
your PC, that will depend on the type of connector you need -- minijack or
RCA. Clio has RCA jacks, but most soundcards have minijacks on them.
Either way, the way to get the connects and cable is to buy a good quality
minijack or RCA cable of a sufficient length to go from your mic to your
PC or preamp. Don't go buying Monster cable or anything like that.
I used a 10' RCA cable that was sitting in my big box 'o cables. I
cut one end off and stripped the leads.
Shown in Figure 1 is the panasonic element in
my "helping hands" thingy that I got at "the shack" a few years ago.
It's really a great little tool for under $10. The first thing you
need to realize is that these capsules are tiny. They're about 1/2
the size of the capsules that Radio Shack sells and they work much, much
better. Also, in figure 1, notice that one of the terminals is
grounded to the case via a small drop of solder (more on this later). You
can go ahead and solder a wire to each terminal and use the one that comes
from the terminal grounded to the case as the (-) lead and the other as
the (+) lead to attach to your RCA jack or minijack. You want to
keep the phase straight, if possible, but remember that your amp or preamp
may invert the phase. So, prior to importing your data into a
crossover design program, check your impulse response -- if the initial
impulse goes up, you're all set. Otherwise, reverse the phase (do
this by reversing the leads going to the speaker or driver you're
testing). This is important if your crossover design software uses
the phase imported from your measurement equipment. I use Calsod Pro
and strip the phase data from my frequency response data, to allow Calsod
to calculate the minimum phase (and be sure to read Dave Ralph's article
on modeling the "tails", but that's beyond the scope of this article).
But wait. That's too easy. And the
truth is that it really is that easy... unless you want to get fancy.
If you do (and I think you do), read on. Otherwise, connect and twist your wires,
as shown in figure 1a,
and attach them to your cable that will run to your computer, cover the
thing with heat shrink tubing (I use a thin piece to hold the wires and a
thicker one to hold the case to the wires for stress relief to make sure
you dont accidentally pull the wires off the terminals on the capsule.)
Here's the "fancy" part. We're going to
make one slight modification suggested by Sigfried Linkwitz (of
Linkwitz-Riley fame) that will improve the SPL handling of the mic and
reduce distortion. This modification requires you to cut the lead
that connects the ground terminal to the case and dab a little silver
solder to connect the other terminal to the case (where the (+)
lead connects). I have no idea why this works, so don't ask.
If you think there's something wrong with this, please take it up with
Sigfried. It does work, though.
Figure 2 illustrates what the capsule looks
like after I scratched the lead to the case off with a sharp utility
knife. After connecting the wires, I dabbed some solder to connect
the lower lead (use the dot for reference) to the case and connected up my
wires to the cable. As I said, I don't know why this works
better, but I've built it both ways and it's worth the little extra
effort. When connecting your wires to the case, if you screw one up,
it's likely to be because you had trouble with the solder. If that
happens, get some desoldering braid from radio shack and remove any errant
solder before starting over (or just chuck the screwed-up capsule in the
trash and go at it with a fresh one). Also, with the Kynar wire,
strip about 1/4" of the insulation off the end, then use snippers to cut
the exposed portion of wire down to about 1/16". Lay the exposed
portion of the wire onto the solder dab of the capsule's terminal and
touch the tip of the soldering iron to the wire. The solder on the
capsule should melt quite quickly and the exposed kynar wire will sink in.
Hold the wire for about 10 seconds while the solder hardens and then
continue onto the second lead. Once done with that, go back and drop
a dab of solder to connect the appropriate lead to the case.
IMPORTANT: use a multimeter set to test for conductivity to check and make
sure that 1) you have fully cut the original connection between the
terminal and the case; 2) that your connected wires are connected only to
the appropriate terminals; and 3) that the case is connected to the
appropriate terminal.
Figure 3 shows my nice, new DIY testing mic
(top) piggybacked on the Mitey Mike for some comparative testing.
And how did it do? Pretty darn fine! Figure 4 is a response
curve for a 5 1/4" woofer on an 8" baffle made with the MM2 and overlayed
with the response obtained with our DIY mic. There's no point
telling you which curve is for which mic... there's virtually no
difference (and from a crossover design standpoint, there is absolutely no
difference that would affect the design of the speaker whatsoever).
Next, figure 5 is a comparison of highpass
measurements. This is where you see the only real difference between
the mics. The Mitey Mike produced the curve that is slightly below
the curve for the DIY mic from about 5khz to 20khz (where they meet).
The difference is about 1 db and could slightly affect the selection of
tweeter padding resistors and possibly alter your choice in highpass caps
(very, very slightly). Most likely, this difference is due to the
fact that the tweeter measurement was obtained with the mics piggybacked
on one another, causing a slight reflection off the lower mic that caused
a slight increase in SPL at the upper mic (the DIY mic). The woofer
measurements were taken with each mic individually mounted. For that
reason, there's really no difference between the mics making it worth
spending $179 plus shipping. If I knew then what I know now, I'd
have saved the $110 bucks I paid for the calibrated mic (the price when
you buy it with Clio) and spent it on beer, CD's, or whatever. To be
honest, the difference in response when factoring the calibration file for
my Mitey Mike are so small, accurate crossover design can be accomplished
without calibrating these capsules.
That's it. Good luck!
