Impedance and impulse response measurements using low‐cost components

A thin 27‐mm‐o.d. disk transducer (from Radio Shack 273‐060 signaler) is RTV sealed to the end of a measurement tube whose 15 mm i.d. not only approximates the transducer disk's nodal circle but also the i.d. of a clarinet, sax, or trumpet mouthpiece. A 10‐mm‐o.d. electret microphone element (Radio Shack 270‐092) mounted through the tube wall near the piezo driver picks up the resulting pressure signal. When the high mechanical impedance driver is excited by a sinusoidal sweep via a 6‐dB/oct integrator, the resulting pressure signal gives an excellent view of the input impedance of the air column over the musically useful range to 3.5 kHz since the driver resonance is at ω₀/2π≃5 kHz and bandwidth g≃ω₀/6. A ramplike drive voltage (running between op‐amp supply limits) of the form $A (1 − e−gt/2)$ for θ⩽t⩽2π/ω₀, and constant thereafter, gives a clean pressure pulse ≃ const.(1 − cos ω₀t) whose FWHM≃0.1 ms is convenient for impulse response studies. Drive signal spectrum, piezoelectric coupling, and driver‐to‐tube coupling each minimize transducer second‐mode ringing effects. Driver‐to‐mike proximity effects are small, visible, calculable, and easily allowed for in both impulse and sinusoidal drive usage. The driver is not quite rigid, perturbing the air column slightly, but corrections are straightforward. Applications for research and in teaching labs will be described. [Assisted by NSF.]