The Sounds of Silence – Sweep

Sweep in ultrasonic lingo simply means varying the ultrasonic frequency up and down.  A sound clip in an earlier post demonstrated the sound of sweeping the ultrasonic frequency.

Single Frequency Sweep

In an earlier post we also learned that ultrasonic transducers have a property called “resonance.”  Very simply, this means that they will make a louder sound (higher amplitude vibration) when driven at their resonant frequency.  The following sound clip illustrates what the sound coming from an ultrasonic transducer might sound like as the frequency is changed from the resonant frequency to a lower frequency then back through the resonant frequency to a higher frequency and then back to the resonant frequency.  Notice the change in the loudness of the sound as the frequency is changed.

Frequency Sweep Demonstrating Resonance

Looking at the waveform, the effect of changing frequency looks something like this – –

Illustration showing resonance at the center frequency of a frequency sweep
Ideally, the resonant frequency of the transducer is in alignment with the frequency sweep as shown above. Resonance occurs twice during each frequency sweep cycle.

The trouble with this picture is that the peak amplitude at resonance occurs in a regular pattern.  If an object being cleaned exhibits a resonance at the frequency equal to twice that of the sweep rate, the sweep will excite the object into resonance potentially causing damage.

In order to eliminate the possibility of damage due to resonance at twice the sweep frequency, today’s ultrasonic generators are capable of varying the rate of sweep with the result shown below.

Illustration of the effect of random sweep
In the lower example above, the sweep frequency is changing to prevent regularly spaced resonance intervals.

Sweep can be varied either on a regular basis as can be heard in the sound clip below

Varying Sweep Rate

Or the sweep rate can be varied randomly as demonstrated in the following sound clip

Random Sweep Rate

For now, this will be the end of the Sounds of Silence series.  The series, however will resume when we discuss more complex waveforms where center frequency, amplitude and sweep are all varied in a single ultrasonic cleaning tank.

–  FJF  –

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