. Elements of acoustical engineering. Electro-acoustics; Sound. 194 MICROPHONES to the particle velocity in a sound wave. The response of a pressure gra- dient microphone as a function of the distance from a point source and the frequency is shown in Fig. ^. B. Velocity Microphone'^^'^^''^'^. — During the past few years free rib- bon microphones have been used for all types of sound collection. Essen- tially these microphones consist of a loosely stretched ribbon suspended in the air gap between two pole pieces (Fig. ). In addition to sup- plying the flux to the air gap the pole pieces
. Elements of acoustical engineering. Electro-acoustics; Sound. 194 MICROPHONES to the particle velocity in a sound wave. The response of a pressure gra- dient microphone as a function of the distance from a point source and the frequency is shown in Fig. ^. B. Velocity Microphone'^^'^^''^'^. — During the past few years free rib- bon microphones have been used for all types of sound collection. Essen- tially these microphones consist of a loosely stretched ribbon suspended in the air gap between two pole pieces (Fig. ). In addition to sup- plying the flux to the air gap the pole pieces serve as a bafile for acous- tically separating the two sides of the ribbon. The configuration and dimensions of the baffle determine the effective sound path between the two sides of the ribbon. Under the influence of a sound wave the ribbon is driven from its equilibrium position by the difference in pressure between the two sides. The motion of the ribbon in the magnetic field induces a voltage between the two ends of the ribbon. The electrical output of this system under certain conditions cor- stDE VIEW responds to the particle velocity in a Fig. The essential elements of a ^ound wave. Accordingly, the term velocity microphone. velocity microphone has been applied to the free ribbon microphone. In past analysis it has been customary to treat the system as an acoustic doublet. This method is essentially accurate when the effective dimen- sions of the baffle are small compared to the wavelength. When the effective dimensions are comparable to the wavelength, there Is consider- able discrepancy between the simple doublet theory and the actual per- formance. It is the purpose of this section to develop the theory of the conventional baffle type velocity microphone. Approximate solutions for the diffraction of sound by a circular and square plate have been obtained.^^ These analyses may be applied to the problem of the baffle type ribbon FRONT VIEW 20 Olso
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