Design of a Helmholtz Chamber
When a piezoelectric element is supported in an Edge or Nodal mode, and has no case or tuned enclosure, the resulting sound pressure level (SPL) produced is very low. This is because the acoustical impedance of the piezoelectric element does not match that of any open air loading. However, by constructing a HELMHOLTZ resonating case and by using proper mounting techniques, the acoustical impedance of the piezo element and the encased air can be more closely matched to that of open air.
The mode of mounting influences the resonant frequency, impedance, bandwidth and resulting sound pressure level. Mounting a piezoelectric bender at its nodal circle results in lowest bender impedance, highest resonating frequency, narrow bandwidth and highest sound pressure level.Mounting a piezoelectric bender at its edge results in higher bender impedance, lower resonating frequency, broader bandwidth and lower sound pressure level.
The highest sound pressure levels are obtained when the piezoelectric element excites a resonator with a resonant frequency equal to the resonant frequency of the piezoelectric element regardless of the mounting method chosen. The following equation can be used to design such a resonator, known as a "Helmholtz Resonator".
- fo = Resonant frequency of Helmholtz cavity in Hz
- C = Constant - Velocity of sound @ 344 m/sec @ 24°C
- h = Resonator cavity in height in meters
- D = Resonator cavity (support) diameter in meters
- d = Sound emitting hole diameter in meters
- t = Sound emitting hole length in meters
- K = Constant - @ 1.5
- N = Number of sound emitting holes
- p = Constant - @ 3.14
- 2p = Constant - @ 6.28
- a = Constant - @ 4.0
- fB = Bender vibrating plate diameter in inches
- fA = 0.65 (fB) Bender nodal mount diameter in inches
- fn = Bender nodal mounted resonant frequency
- fe = 0.64 (fn) Bender edge mounted resonant frequency
Resonator cavity volume in cubic meters.
The following assumptions are made:
- That all of the resonator cavity and sound emitting hole dimensions are much smaller than the wavelength of sound at the frequency of interest.
- That the sound emitting hole is tubular in geometric shape.
- That where multiple sound emitting holes are used, all holes are tubular in geometric shape, all holes have equal diameters, and all holes have equal length.
- That where multiple sound emitting holes are used, the term (t) representing their length in the above equation, shall represent a single common non-accumulative length.
- That the bender bonding adhesive is elastic such as a silicone rubber (RTV). A full annular ring.
- That the case material is sufficiently rigid enough to prevent flexure due to bender motion or in use shock vibration.
Bender Cavities (Recommended)
- Cavity dimensions are determined by calculations.
- For nodal mount, "fD" should be set equal to the bender nodal diameter "fA", a calculated result.
- For edge mount, "fD" should be set equal to "fB" less 0.040 inches.
Basic Driver Circuits
The following circuits illustrate how only a few low cost electronic components are required to drive the Piezo Ceramic Benders when mounted on either the edge or nodal points. All of the units are compatible with either CMOS or TTL integrated circuits.
Op AMP or Comparator Drive
Often a spare amplifier or gate can be put to use driving a piezo transducer. For maximum sound output the device should be driven at resonance or the frequency can be swept through the resonant point.
CMOS Logic Oscillator Drive
The effective drive to the transducer can be doubled by driving each side of the bender with an inverted signal of opposite polarity.
Transistor Oscillator Drive
The feedback voltage developed forces oscillation at the resonant frequency of the device and its acoustic enclosure.
Low Cost Pulsing Circuit
Designed for minimum part count, this pulsing circuit will oscillate at resonance. The sound produced is more suitable for an alarm application and the rate of pulsing can be varied by charging the RC time constant.