Pressure microphone
A pressure microphone is a microphone design that corresponds to a sound pressure receiver in terms of its acoustic functionality. In this case, in contrast to that of a pressure gradient microphone, the microphone capsule with the membrane located therein is not provided with an acoustic delay element. As a rule, the back of the capsule is closed.
With the pressure microphone, the electrical signals generated are proportional to the sound pressure . It is therefore a sensor for the alternating sound pressure . The idealized directional characteristic corresponds to a sphere. In teaching, this microphone is also often referred to as an omnidirectional microphone or pressure receiver .
Real omnidirectional microphones can consist of a single capsule or two capsules operated in combination.
Principle and characteristics
In the case of a pressure microphone, the sound-absorbing membrane is attached in front of a cavity closed at the rear. This prevents the sound from migrating around the membrane and affecting the rear side as well. In this way, incoming sound is reproduced largely independently of the direction of incidence, always with the same polarity and intensity. The pressure microphone reacts like a barometer to fluctuations in air pressure, i.e. to the non-directional pressure scalar. Therefore, such a microphone can also be effective at very low frequencies. However, there is always a narrow opening to the outside (capillary) to ensure a balance of the static air pressure, whereby very rapid pressure fluctuations of the sound are not compensated, whereby a directional dependency is created at high frequencies.
Pressure microphones are very suitable for recording low frequencies down to the infrasound range (<16 Hz). Pressure microphones are therefore usually used in measurement technology.
Directional characteristics and special features
The directional characteristic of a sphere is always given for pressure microphones . All microphones with directional characteristics other than those of the sphere, especially those with switchable characteristics, are realized with the design of the pressure gradient microphone.
The directional characteristic of an ideal spherical shape applies to the pressure receiver for various acoustic reasons only at low frequencies .
- For high frequencies at which the microphone dimensions are larger than half the wavelength, the directional characteristic deviates from the ideal spherical shape, partly because of the sound shadowing by the capsule; The directional characteristic is more like that of a cardioid.
- A peculiarity of this construction is the pressure back-up effect , a clearly perceptible increase in height of 6 dB (doubling of sound pressure) with almost vertical exposure of the membrane. The cause of this effect lies in the superposition of incident direct sound with sound reflected by the membrane. At 0 ° sound incidence direction is the maximum of the pressure build-up, the increase of which slowly decreases with inclined sound incidence until it is no longer present at 90 ° incidence to the side on the membrane. The limit frequency of the elevation increase depends on the capsule diameter; The wavelength of the maximum pressure back-up effect of 6 dB is in the order of magnitude of the acoustically effective microphone capsule diameter. The larger the capsule diameter, the lower this frequency increase. See also: boundary microphone, pressure accumulation effect and area dependence
This "normal" microphone with omnidirectional characteristics is called diffuse field equalized . If the pressure accumulation effect is eliminated acoustically or electrically for the sound incidence angle of 0 ° in a special microphone design, then the microphone is called free-field equalized . Pressure microphones are usually designed as a small diaphragm (diaphragm size less than 1 inch = 25.4 mm). In principle, our ear with the eardrum is also a pressure receiver. However, an ear does not have a spherical characteristic, if only because of the greater degree of sound shadowing by the head. The hearing with the brain also processes sound differently - it relates the events of both ears. Microphones don't “hear” like ears.
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- ↑ Thomas Görne: Microphones in Theory and Practice , 2nd edition 1996, page 39.
- ↑ a b Thomas Görne: Microphones in Theory and Practice , 2nd edition 1996, page 45