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Ultrasonics - Sound - The Power of Resonance


Resonance, as discussed in a previous blog, can be either or both beneficial and/or detremental.  This blog will explore the beneficial qualities of resonance. Consider the model of a child's swing discussed in the preceding blog.  We all know that by "pushing" or "pumping" at the right time can easily make the swing go higher and higher.  However, if the pushing or pumping is NOT timed correctly, the result may range from non-productive to disasterous.  Clearly, one does not want to try pushing when the swing is returning from its furthermost position unless you enjoy losing teeth (or worse). Resonant objects want to vibate at their natural frequency and do so with the slightest provocation.  They resist vibration at frequencies other than their natural frequency.  A very small amount energy at the proper frequency can result in major motion of resonant objects.  In the case of the swing described above, the person pushing the swing might not be able to lift the person in the swing off the ground but through the effect of resonance (and pushing at the right time) the person in the swing is, in effect, lifted as high as the swing allows through the benefit of resonance. Resonance, in fact, can amplify sound by changing pressure into displacement.  The resonant item is not really providing increased energy but is converting pressure energy into displacement energy.  The following example may help clarify.  Consider a bell.  As  the clapper in the bell swings and hits the bell, the clapper imparts a particular amount of energy through its high pressure collision with the bell.  If it were possible to isolate the actual sound of the clapper hitting the bell, it would be a single, and not very intense shock wave.  Once the bell is struck, however, the energy imparted by the clapper goes into exciting the bell into resonant vibration which may last for a considerable period of time.  In effect, the "ringing" of the bell is releasing the energy delivered by the clapper in a single blow. In musical instruments, resonance is used to increase the sound intensity of the source.  A violin or guitar string vibrating in air makes very little sound by itself.  But, when the vibration of the string sets the body of the violin or guitar into resonant vibration, the result is sound that can be heard throughout a concert hall.  One may ask how a violin or guitar, then, manages to play more than one note if its sound is the result of resonance.  A bell as described above, has only one (major) resonant frequency and can play only one note.  In the case of the violin or guitar, however, the string is the resonant source (similar to the bell).  The body of the violin or guitar is designed to have a wide range of resonant modes of vibration which can pick up and transmit a wide range of frequencies.  The design of the instrument's body or sounding board is critical to the sound of the instrument as it provides the majority of the sound delivered by the instrument.  Prized instruments are able to produce a wide range of resonant tones.  In wind instruments like flutes, trumpets and clarinets, the vibration of a column of air within the instrument is the resonant source.  These instruments are designed so that their resonance can be changed by varying the length of the air column to play a variety of notes.  In general, wind instruments rely on only the resonant column of air and not a resonating sounding board to produce musical notes.  Since there is no secondary resonance involved, they are usually capable of more intense sounds than string instruments.  An orchestra may have only two or three trumpets while the same orchestra utilizes 12 or more violins to produce the desired sound intensity. Resonance can be very useful in designing sound sources.  But, it can also be troublesome.  Resonance at an unwanted frequency can lead to disaster.  An upcoming blog will discuss the "down" side of resonance.

-  FJF  -