Airborne sound converter

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As air transducers are speakers referred to the electric power in ultrasound in air and gases convert and vice versa (→ reciprocity ).

Since gases have a very low density , sound energy can only be generated efficiently if it is well coupled to the medium. Therefore, compared to other loudspeakers, ultrasonic transducers have some special features in order to efficiently generate ultrasound or air-ultrasound. Most membranes are used.

There are several methods:

1. Principle of electrostatic loudspeakers :

A thin, metal-coated plastic membrane that works as one of two capacitor electrodes against a grid as a second electrode. By applying a high DC voltage, which is superimposed with an AC voltage , an alternating electrostatic attraction is created between the two electrodes, which causes the membrane to vibrate.

Disadvantage: High electrical voltages are required and the converter is sensitive to environmental influences (humidity, moisture, ...).

2. Principle of ferroelectric loudspeakers :

• PVDF films have piezoelectric properties and can be made to vibrate by an alternating voltage on applied electrodes.

Disadvantage: low sensitivity as a receiver.

• Piezophone: A flexural oscillator made of a thin metal membrane and a piezoceramic disc glued on . This structure is z. B. used for parking aids in cars.

Disadvantage: The frequency bandwidth of this converter is very small. It is only a few percent (−6 dB bandwidth approx. 3 ... 5%)

• Combination of different oscillation modes (flexural oscillator and thickness oscillator), whereby the membrane is designed as a matching layer at the same time (quarter-wavelength matching layer: the thickness of the membrane corresponds to a quarter of the sound wavelength in the membrane material). This creates constructive interference . The membrane material must have a very low acoustic impedance in order to be able to effectively transfer sound energy into the gas, i.e. H. the density and the longitudinal speed of sound must be very low. This is z. B. achieved with composite materials made of an epoxy matrix with embedded hollow glass spheres.