Acoustic Sensors - Technical Specifications

An acoustic wavelength of 5 mm, corresponding to 70 kHz, will have roughly the same diffraction and resolution properties as a 5-mm electromagnetic (EM) wave [Rayleigh length ka² ~(13 cm^-1) a²]. Also, owing to the large acoustic impedance mismatch between the contents of a container and air, the contents will be a hard scatterer of acoustic waves, just as a good conductor is a hard scatterer of EM waves.

The absorption of sound in air is ~3 dB/m at 70 kHz. The echo obeys the radar equation for maximum range of detection (in meters). Thermal noise, which limits radar sensitivity, is low for acoustic receivers because the bandwidth is generally smaller. The available thermal noise power generated by a receiver of bandwidth Bn (in Hz) at temperature kT (in J) is kTBn, or 4 x 10^-21 W/Hz of bandwidth at room temperature. A three-percent bandwidth generates just 10^-17 W of available thermal noise power.

For a 10-cm resolution at a range of 3 m (an angular resolution ~30 mrad), the Rayleigh criterion dictates an aperture of about 20-cm diameter for a 5-mm wavelength.

The device must operate in a pulsed mode so that the sound generator does not unduly interfere with the detectors. An echo from a 1-m distant object returns to the detectors 6 ms after transmission. To permit imaging of objects as close as 1 m, therefore, and still allow a few ms (hundreds of wave periods) for vibrations to damp at the detector plane, means that the pulse duration should not exceed ~3 ms (~200 wave periods).

To permit an echo to return from objects up to 5 m away means an interpulse spacing of at least 30 ms. A reasonable frame rate, therefore, is about 30 Hz, which is about the frame rate of camcorders. This frame rate with a 3-ms pulse gives a duty cycle of 0.1. Of course, the LED preprocessor can integrate the signal over the entire 3-ms pulse. It probably shouldn't integrate from pulse to pulse, since the eye and brain are likely to do a better job at integrating signals from moving objects.

Acoustic Sensors | Applications

EME Services and Products: