Design Of A Helmholtz Chamber

A piezoelectric element (also known simply as a piezo or piezo bender)—by itself, without a case or tuned enclosure—has a very low sound pressure level (SPL). This is mostly due to cancelation that occurs when the face of a sound-producing device (which creates positive air pressure) is able to interact with the back of the sound producing device (which creates negative air pressure); the same as what happens when evaluating a speaker that is not properly mounted in an enclosure.

By mounting a piezo into an enclosure, that cancelation is mitigated. Furthermore, by tuning the enclosure to a frequency that aligns with the piezo’s resonant frequency, a Helmholtz chamber is created. Within a Helmholtz chamber, a larger volume of air becomes a piston that moves air, more efficiently than the piezo by itself, by more closely matching the acoustic impedance of air.

Mounting the Piezo in a Helmholtz Chamber

The method of mounting the piezo, within the Helmholtz chamber, influences the resonant frequency, impedance, bandwidth and resulting sound pressure level of the whole device.

Nodal Mount

Mounting a piezo at its nodal point—just outside the diameter of the ceramic disc—results in the highest SPL at the expense of reduced bandwidth (overall frequency response). This mounting method takes advantage of the resonant frequency of the ceramic disc and narrow impedance peak. Choose this mounting method when using a piezo to create a single tone.

Edge Mount

Mounting a piezo at the outer edge of its metal plate results in the widest usable bandwidth (overall frequency response). This mounting method distributes the impedance peak over a wider range and increases the moving surface area of the piezo, at the expense of peak SPL. Choose this mounting method when using a piezo to create tones at multiple frequencies.

Helmholtz Resonating Chamber Design

The highest SPL is obtained when the piezo excites a resonator with a resonant frequency equal to, just below, or one octave lower, the resonant frequency of the piezo regardless of the mounting method chosen.

If using a piezo to create multiple frequencies with an edge mount design, set the chamber tuning frequency to (or just below) the lowest frequency to be reproduced. If using a piezo to create a single tone, set the chamber tuning frequency to just below the frequency