We all know that the most important thing for making a good SPL Car is testing, than testing and a little more testing! "Real" SPL meters aren't cheep (1 sensor TL is 600+ USD, even more here) so I decided to make something with which I could compare two burps. I did a litle searching on the net and found a program called trueRTA . The free wersion isn't acurate enought to see what's louder so I highly recomend buying the level 4 (1/24 Octave) code ($99.95 if only there was a cheaper way of geting it). TrueRTA alows you to mesure (relative) SPL, it can hold the maximum (which is what you need), get's it's signal from the mic in of the computer and is wery easy to use. It can run on a preaty old machine (I'm using a 233MHz notebook with 96MB of ram running winXP).
First I tried using a normal microphone but it didn't have the linearity to compare diferent frequencies (altho I have heard that a capsule like the Panasonic WM60A should work). So I opted to use a piezo element as the mic/spl sensor. Wireing it up is wery simple. You need a mono 3.5mm audio (mic) jack, some coax cable, a 1uF capacitator (the soundcard's mic imput has some DC voltage on it to "power" normal capacitator mic's and has to be removed), a pot (I had the mic imput cliping the signal that was to big and just put a pot inline so I could lower the voltage untill it was OK (you can check this in TrueRTA with the osciloscope function)).
The piezo has an uper (small) and lower (big) plate. Just solder the wires on, put the cap inline (doesn't matter which Wire goes where) and put the pot inline as well (doesn't matter which wire).
I put the piezo element into a box (about the same size as a TL) with a O-ring beneth it for distance (so it can move) and glued all inplace with a hot glue gun.
Sory, don't have any pics of the process (didn't have time), but here are some pic's of the "finished" product (didn't have any suction cups so I used a bit of foam for distance (about 2cm/ litle below 1") and duct tape for fixing it to the window (12" from the pilar, 4" from the dast up if I'm not mistaken):
The pot for lovering the voltage the computer get's so the signal isn't cliped:
What you get on the screan (a slow sweep with my competition box at low volume to se how (un) linear it was):
Again this won't tell you how loud you are (unless you calibrate it somehow. Curently I'm seeing 141.8dB as 110.xdB :P ). It will just tell you if the change you made was for the better or for the worse.
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