The Ultrasonic Motion Detector

In class and in the labs you will use an Ultrasonic Motion Detector which measures distance by emitting ultrasonic pulses and determining the length of time it takes for the reflected pulses to return. We can then calculate a distance from the the time and the known speed of sound.  The word "ultrasonic" refers to sound frequencies higher than the response of the human ear (from about 20 Hz to 20 kHz). Bats can detect sound of frequencies up to 120 kHz. [The unit of frequency is the Hertz, where 1 Hz is 1 cycle/sec and 1 kHz is 1000 cycles/sec.]

The speed of sound is related to the wavelength and frequency by the relation:

                                                    c = f l        speed = frequency x wavelength

The speed of sound in air is about 343 m/sec. The speed of sound in water is about 1500 m/sec. So, for example, the wavelength of sound waves of frequency of 1 Hz in air is 343 m. For f = 10kHz, the wavelength becomes 3.4 cm. The wavelength sets the resolution with which we can measure distances.  Therefore, ultrasonic frequencies (very high frequencies) are good for distance measurements. The ultrasonic range finders operate in the range of about 40 kHz.

Ultrasound is also used in medical applications for imaging and surgery. Techniques now exist to generate sound frequencies of a gigahertz (109 Hz) making resolutions of about 1 micron routine.


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