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Sound - Consequences of Increased Amplitude


Two of the major variables in the production of mechanical vibration are frequency and amplitude.  The consequences of frequency variation were discussed in a preceding blog.  Today's blog concentrates on amplitude and the power implications of varying either or both frequency and amplitude. As discussed in the blog titled Ultrasonics-Sound-Amplitude, the amplitude of a vibration describes the distance the vibrating object moves during each cycle of vibration.  This distance may also be called the "excursion," "displacement," or "travel."  In the case of a child's swing, the amplitude may be several feet.  The amplitude of ripples on a pond are from a fraction of an inch to several inches.  The amplitude of vibration of a sound wave may be as much as several inches (typical travel of a very powerful sub-woofer in a theater sound system) but are more typically a fraction of an inch.  The amplitude of vibrations in the ultrasonic range is seldom more than a few thousandths of an inch and is often much less.  Higher amplitude equates with louder sound or more intense vibration. Increasing amplitude without changing frequency requires that a vibrating object travel with more speed to travel from one extreme of its excursion to the other.  Acceleration from moving in one direction to moving in the other must also increase.  The following illustration shows this relationship.
The above illustration shows how speed and acceleration are influenced by an increase of amplitude within a sound wave or other vibration. The illustration is for concept only.

In the blog Ultrasonics-Consequences of Increased Frequency it was revealed that speed of motion and acceleration within the sound wave are critial to producing the effects required for ultrasonic cleaning to take place.  Achieving adequate speed and acceleration requires energy or "power."  Increasing frequency while maintaining the same amplitude of vibration requires more energy.  Similarly, increasing the amplitude of vibration while maintaining the same frequency requires additional power.  If frequency is increased without increasing power, the amplitude of vibration will diminish accordingly. The next blog will describe in detail the phenomenon of "cavitation" which is the basis of ultrasonic cleaning.  As we discuss cavitation, one should keep in mind that cavitation won't occur unless certain frequency and/or amplitude requirements are met to achieve the acceleration required to exceed the tensile limits of the liquid to be cavitated.

-  FJF  -