An analysis of complex sound by Helmholtz resonator- acoustic physics

What is Helmholtz resonator ?


A musical note is generally a mixture of several tones. The quality of a note is determined by the number of tones present, their frequencies and relative intensities. The analysis of a note means the determination of the frequencies and relative intensities of different tones present in it. A trained ear can detect some of the partials making up a musical note and can also roughly estimate their intensities. However, the presence of tones can be conveniently detected by using Helmholtz resonator quite independently of ear.



Structure construction= 
Simple figure of Helmholtz resonator 

A Helmholtz resonator consists of a narrow metal or glass enclosure which is connected to the open air by a short neck (N) as shown in figure. There is also a narrow aperture (P) at the back, called pip. In one type the resonated is a pear shaped  and has a fixed volume as shown in figure. The resonator may also be a cylindrical enclosure whose volume can be varied continuously. Sound,waves are incident on the neck and ear is placed behind the pip (P) to hear resonance. A thin mica plate whose natural frequency is the same as that of the resonator may be placed at the pip. At resonance the mica plate vibrates vigorously and can be easily detected by reflected spot of light the incident sound contains a tone of frequency equal to the natural frequency of the resonator then the tone will be intensified and can be clearly detected. By using a number of resonators of different sizes and noting the resonances one can identify the tones present in purpose the cylindrical resonator whose volume and hence resonant the incident note. For this frequency can he changed continuously may be used with convenience.

Different between pipe and Helmholtz resonator:-


 In Helmholtz resonator damping is relatively small. As a result the resonance becomes very sharp and selective and the vibration persists for a longer time. On the other hand, in a pipe resonator the damping is relatively high which makes it less selective and the vibration does not persist for a long time.  

Use of Helmholtz resonator:-

(1)  Analysis of a complex, note is possible by using Heimholtz resonator. Highly selective and sharp resonance in the resonator makes it extremely sensitive detector of sound at a particular frequency. To find the harmonic components present in the incident sound a series of graduated resonators of various volumes and areas of the neck can be used. If the incident sound contains a tone having frequency equal to the resonance frequency of the resonator then a resonance can be heard. To cover a wider range of frequencies it is customary to use resonators of continuously variable Highly volume.

(2)   A weak sound of appropriate frequency can get intensified by a Helmholtz resonatod This is caused by more rapid drainage of energy from the source.

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