Sounds of Silence – Pulse Period and Duration

Today we are, again, going to bring ultrasonic sounds into the realm of human audibility by lowering the frequency to ones that most people with normal hearing can hear.  Several preceding blogs have discussed the effects of varying the pulse width and pulse period of an ultrasonic generator in an ultrasonic cleaning system.  If you’ve missed these blogs or want to know more before preceding here, just type “pulse” into the search window.

Pulse simply involves turning the ultrasonic energy on and off repeatedly in a defined way.  The effect produced depends on two variables, the pulse period and the pulse width.

Pulse Period –

The pulse period (sometimes called the pulse rate) is the time between the initiation of pulses.  In the sound clip below, all of the pulses are of the same length but occur at an increasing rate until there is no separation between them at all (a solid tone).  The pulse rate is then decreased making the pulses, although still of the same duration, further apart.  The following illustration is representative of this effect although  (because of space limitations) it does not exactly represent the sounds heard in the clip below it.

Illustration showing varying pulse period

The pulse length remains the same but the time between the initiation of pulses varies.

Pulse Varying Period

Pulse Duration –

The pulse duration is the length of each pulse.  In the sound clip below, the times between pulse initiation are all the same.  The duration of each pulse, however, changes.  At first the pulse duration is short.  Each successive pulse lasts a longer and longer time until the pulse duration completely fills the time between pulse initiations making a solid tone.  The pulse duration is then shortened to increase the “quiet time” between the still equally spaced pulses.  The following illustration is representative of this type of variation.

Illustration showing varying pulse duration

In this case, pulses are initiated at regular intervals but last a varying period of time.

Varying Pulse Width

Complications –

Adjusting the pulse period and duration can be complicated.  This is especially true in the case that both are adjusted as time which is the case in some ultrasonic generators.  For example, let’s say that the pulse period is 1 second meaning that a pulse is initiated once every second.  If the pulse duration is then, for example, set at .5 seconds, the ultrasound will be on for .5 seconds then off for .5 seconds.  If, however, the pulse period is set for .25 seconds and the pulse duration is set for .5 seconds the result will be that the ultrasound is on ALL the time since the duration of each pulse does not expire before the initiation of the next pulse.  In effect, there is NO pulse at all!  This complication can be eliminated by using controls to set the pulse period (time between pulse initiations) and the “duty cycle.”  The duty cycle defines the portion of time between each pulse initiation that the ultrasonic energy remains on.  Unfortunately, as you may remember, duty cycle is also one of the ways that ultrasonic power can be controlled.  It is important to realize that although similar, control of pulse parameters and power may or may not be the same thing.  This interaction of parameters led many users of the initial ultrasonic generators offering these features to utter frustration.  Hopefully, the above will help eliminate that frustration if it still exists.  As always, questions on the above or any other blog topic are welcomed in the “comments” section below or by email to

–  FJF  –

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