Bill Fagal
Stunt Coordinator
- Joined
- Jul 8, 2002
- Messages
- 166
I was just thinking about the difficulties of measuring sub-bass, and a little lightbulb clicked on over my head. Yeah, I know the correction tables for RS SPL meters go pretty low, but how 'bout this for a down and dirty DIY solution:
Take any small midrange driver you have lying around and mount it in a very small enclosure (just big enough for the driver to fit). If you can find a sealed-back mid, you can skip the enclosure altogether. The parameters of the driver don't matter, and neither does the absolute size of the enclosure as long as it's tiny and well sealed (though it would probably be a good idea to include a pressure-equalizing pinhole) Attach thin wire leads and connect them to a voltage tester with a peak-hold or averaging function. Congratulations, you've finished your test mic.
The highish Fs driver in the small enclosure will give you a highish Fb of 200-400Hz. That's just dandy because this microphone will only be used in the linear region below its Fb, so the higher the Fb, the better. This idea rips off the ELF concept that makes use of the predictable 12 dB/oct roll-off below Fb, only we're making a microphone instead of a subwoofer.
You'll need to calibrate your mic against some standard, like your trusty old RS meter. With the two mics close together, Play a test tone, say 100-120Hz, where both units are linear. (It should be at least an octave below your DIY mic's Fb, where the 12dB/oct roll-off behavior is in full swing.) Adjust the tone volume until the RS meter shows a nice round dB number. Note the number and the indicated voltage coming from your little ELF mic (peak or average, it doesn't matter). Now you're calibrated.
When you measure and plot data points, just add 12dB per octave below your calibration point. With reference to your calibration voltage, a doubling or halving of the voltage means a 6dB rise or drop.
Unless Thiel and Small were lying, this mic should be nice and linear as low as you want to go.
Bill
Take any small midrange driver you have lying around and mount it in a very small enclosure (just big enough for the driver to fit). If you can find a sealed-back mid, you can skip the enclosure altogether. The parameters of the driver don't matter, and neither does the absolute size of the enclosure as long as it's tiny and well sealed (though it would probably be a good idea to include a pressure-equalizing pinhole) Attach thin wire leads and connect them to a voltage tester with a peak-hold or averaging function. Congratulations, you've finished your test mic.
The highish Fs driver in the small enclosure will give you a highish Fb of 200-400Hz. That's just dandy because this microphone will only be used in the linear region below its Fb, so the higher the Fb, the better. This idea rips off the ELF concept that makes use of the predictable 12 dB/oct roll-off below Fb, only we're making a microphone instead of a subwoofer.
You'll need to calibrate your mic against some standard, like your trusty old RS meter. With the two mics close together, Play a test tone, say 100-120Hz, where both units are linear. (It should be at least an octave below your DIY mic's Fb, where the 12dB/oct roll-off behavior is in full swing.) Adjust the tone volume until the RS meter shows a nice round dB number. Note the number and the indicated voltage coming from your little ELF mic (peak or average, it doesn't matter). Now you're calibrated.
When you measure and plot data points, just add 12dB per octave below your calibration point. With reference to your calibration voltage, a doubling or halving of the voltage means a 6dB rise or drop.
Unless Thiel and Small were lying, this mic should be nice and linear as low as you want to go.
Bill