Transduction of sound analog - A transduction example using a wave height sensor [video]

Abstract

Capacitor Discharge - A Capacitor TutorialThis demonstration is done in a course on transduction. The transducer is a sensor that measures a surface wave on a liquid by converting the wave height to voltage. The sensor is a good visual example of a transducer, and one that can be readily understood. A channel of liquid has a surface wave maker at one end. The wave maker is a wedge that is vertically attached to a loudspeaker. The sensor consists of two parallel bare wires and a simple electrical box. The resistance between the wires varies as the height of the liquid varies. The application of a voltage across the wires then yields a current that varies with the height of the liquid. We use an ac (sinusoidally-oscillating) voltage because a dc (constant) voltage causes electrolysis which produces unwanted effects. A fixed series resistor is added to the circuit, because Ohm’s law shows that the voltage across this resistor varies linearly with the depth of the wires if the series resistance is chosen to be small compared to the typical resistance between the sensor wires. The applied ac voltage (“carrier”) is chosen to have a frequency that is much greater than the typical wave frequency. A lock-in amplifier is used to demodulate the signal (that is, remove the high-frequency carrier). We are then be left with what we desire: a change in voltage that is proportional to the instantaneous wave height at the location of the sensor. How small of a wave amplitude can be detected? The answer depends upon the relative amount of noise in the system, which is quantified by the signal-to-noise ratio. In our case, we can detect amplitudes at which the wave maker appears to be motionless to the unaided eye. If we further reduce the amplitude, we eventually see that noise starts to become important. The sensor is very sensitive. If the table is just lightly tapped, there is a “huge” signal on an oscilloscope, which shows that the surface of the liquid has been disturbed.