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DIY sub-bass SPL tester

Discussion in 'Archived Threads 2001-2004' started by Bill Fagal, Jan 9, 2003.

  1. Bill Fagal

    Bill Fagal Stunt Coordinator

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    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
     
  2. Bill Fagal

    Bill Fagal Stunt Coordinator

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    BTW, just thought of a way around the problem of ELF mic vibration at low freqs.

    Problem: if the mic itself is vibrating (like picking up structural vibrations--and what doesn't vibrate at some frequencies?) motion along the driver/diaphragm axis will induce voltage because of the inertia of the diaphragm.

    Solution: mount a second driver/diaphragm opposite the first wired in parallel in phase (like a push-push bipole). Now any microphone motion along the axis of the voice coils will induce opposite (canceling) voltages.

    To avoid phase cancellations between the mic diaphragms in near-field measurements, position the opposing diaphragms equidistant and 90 deg. from the speaker. IOW, aim the side of the mic at the speaker.

    Bill
     
  3. Jonathan M

    Jonathan M Second Unit

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    Hmmm, sounds like a plausible plan. I'd give it a go if I had a suitable driver to use for the mic. I guess how good it works would depend on how sensitive the voltage calculations would need to be. Have you tried it out? I'd be very interested to see how it turns out.

    I for one have always been a bit uncomfortable with the "Correction tables" for the RS SPL meter. It seems they haven't changed in a number of years, and I'm sure that there has been variation in units over the years.

    Be interested to hear how any experiments go.
     
  4. Bill Fagal

    Bill Fagal Stunt Coordinator

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    Haven't tried it yet myself, just sharing my thoughts as they come, stream-of-consciousness-like.

    PE sells sealed-back mids for
     
  5. Manuel Delaflor

    Manuel Delaflor Supporting Actor

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    I will wait for someone to implement it, sounds interesting... [​IMG]
     
  6. Bill Fagal

    Bill Fagal Stunt Coordinator

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    Can anybody suggest a practical non-magnetic method for accurately measuring/plotting diaphragm excursion?

    Since accurately measuring driver sub-bass output (free- or half-space) is nearly impossible (short of using an empty field or parking lot) I've been thinking that plotting diaphragm excursion may be the way to go.

    The subject will be a prototype of my ultra-excursion driver concept. This R&D mule will have a 10,000+ amp-turn field coil (for adjustable Qes) and a high-current VC, so I'm anticipating interference if I try to sample large-signal performance magnetically, i.e., with another voice coil.

    There are laser measurement systems out there, but I'd like a solution I can do myself on the cheap. After all, this is still just a hobby, at least until I can get the prototype to prove the concept.

    Bill
     

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