Acoustic measurements are inherently difficult. Firstly because transducers are by far the least perfect part of the equipment inventory. Speakers and microphones commonly have frequency responses that vary by ten decibels or more from flat, and there are heated arguments about just what flat response means anyway. Flat power output into the room? Flat response at the listener’s ear? Because speakers and mics are directional, there can be wide differences between the acoustic and electrical “flat”. Then there is the room’s acoustic response which often swamps the effect of the transducer. Standing waves produce resonances and cancellations many tens of decibels deep.

As an initial method, I have measured a few mics with a closely spaced driver. The direct sound overwhelms the room sound, the driving level doesn’t need to be very high (the dog doesn’t bark every time I start a sweep), and the speaker can be a small wide range unit. As I gain familiarity with the test software (Faber ElectroAcoustic Toolbox for Mac - $300 from the App Store), I should be able to refine the test method so as to cancel out the response of the speaker and other anomalies.

But the first test results are interesting enough to post anyway. The test setup is MacBook with a TASCAM US-144 mk II interface driving a 25W amp driving an Apple Pro Speaker with damping material added inside the ball enclosure. The speaker is pretty flat from 100 Hz to 12KHz or so, with breakup in the aluminum cone occurring above 14 KHz. There are some small ripples due to reflections off the mic and speaker cone which can be minimized by having the two at an angle to each other. So here’s the calibrated measurement mic, a Dayton EMM-6 and the speaker to give an idea of the basic setup: