III. Gratuidad, desigualdad y derechos
6. Los términos del debate
Sound pressure measurements are made at both very low and very high levels of sound pressure, as well as at both very low and very high frequencies. Measure-ments are also made in different types of sound field, preferably without disturbing the fields.
However, it is not possible to design one single microphone which can fulfil all needs. Several types of microphone must be designed to cover the many different applications.
Some main design parameters are the stiffness and mass of the diaphragm system.
These two parameters determine the diaphragm resonance frequency which sets the upper limit of the microphones frequency range. The fact that the microphone sensitivity is also a function of the stiffness, makes the stiffness an especially im-portant design parameter.
The stiffness is mainly due to mechanical tension in the diaphragm which is perma-nently stretched like the skin on a drum. The mass is partly composed of the diaphragm mass itself and partly by the mass of the air in the narrow slit behind the diaphragm. Even if the physical air mass is low in comparison with the dia-phragm mass it is important as the air moves with a much higher velocity than the diaphragm. The energy required to accelerate the air mass is therefore of the same order of magnitude as that required by the diaphragm mass. The effective mass of Fig.2.9 The phase of the low frequency microphone response is influenced by the ambient pressure.
The calculated responses shown are valid for a microphone which has 10% air-stiffness at nominal ambient pressure (101.3 kPa): a) 1 bar, b) 2 bar, c) 10 bar, d) 0.5 bar
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the diaphragm system is thus significantly greater than that of the diaphragm itself. Typically, the air-mass makes between 10 and 50 % of the total system mass.
As the air mass varies with ambient pressure, it changes the frequency response at high frequencies. A microphone with a large fraction of diaphragm mass should be selected for applications where large pressure variations occur, for example, in div-ing tanks, as the frequency response of such microphones changes less with ambi-ent pressure than that of other microphones.
Other major design parameters are the diaphragm diameter and the diaphragm damping resistance. For condenser microphones, in contradiction to many other types of transducer, an optimal diaphragm damping may be obtained and main-tained over time. Therefore, such types of microphone may be used in the frequency range around and even above the diaphragm resonance frequency.
The damping is caused by the movement of air in the slit between the diaphragm and the back-plate. Diaphragm movements lead to air movements in the slit which cause viscous loss. The damping resistance may be controlled by holes in the back-plate. By changing the number and size of holes and by varying the back-plate's distance to the diaphragm, various degrees of damping may be obtained.
The influence of damping is illustrated by the curves shown in Fig.2.10. In relation to these, the resonance frequency was kept constant at 10 kHz, which is the typical resonance frequency for 1″ and for high-sensitivity (50 mV/Pa or – 26 dB re 1 V per Pa) 1/2″ microphones. The illustrated degrees of damping may all be obtained in practice and they are utilized in microphones designed for different purposes.
Fig.2.10 Influence of damping on the high frequency microphone response (magnitude). The damping is due to movement of the air in the slit between the diaphragm and back-plate. The damping depends on the microphone design.
Examples of a low (a), critical (b) and a high (c) damp-ing are shown
The critical damping (b, quality factor = 1) is used for pressure (pressure-field) microphones while the high damping (c, quality factor = 0.316) is used for free-field microphones, see 2.5.3 and 2.5.4. Low damping corresponding to the upper curve* on Fig.2.10 (a, quality factor = 10) is used by Brüel & Kjær for a microphone with extremely low inherent noise. Microphones dedicated to the various types of sound field are discussed later in this chapter.
The influence of the diaphragm diameter on the sensitivity and on the frequency response is illustrated by Fig.2.11. For the calculation of these curves, the dia-phragm tension, thickness and quality factor (Q = 1) were assumed to be constant.
The applied parameters correspond to those of typical one 1″, 1/2″ and 1/4″ micro-phones. The results show that the flat frequency range is extended upwards when the diaphragm diameter becomes smaller. The upper operation frequency is inverse-ly proportional to the diameter while the sensitivity is proportional to the square of the diameter. Real microphone specifications confirm this.
Microphone types of equal diameter may have a different sensitivity and frequency range. In practice, this is especially the case for 1/2″ microphones. The main physical difference between the existing high-sensitivity (50 mV/Pa) and low-sensitivity (12.5 mV/Pa) types is the tension in their diaphragms which the designer may select within certain limits.
The two upper curves of Fig.2.11 also illustrate typical sensitivities and frequency responses which are obtainable for 1/2″ microphones by using different diaphragm
Fig.2.11 Magnitude of frequency responses (pressure). The curves are valid for models of microphone with critical damp-ing and different diaphragm diameters (relative scale: 1 (a), 0.5 (b), 0.25 (c)). The numbers chosen for the calcula-tion approximate the parameters of existing 1″, 1/2″ and
tensions. The phase characteristics which correspond to the above magnitude char-acteristics are shown in Fig.2.12.
The above curves were worked out by using a simple mathematical model. The parameters of the model correspond to those valid for a series of 1″ 1/2″ and 1/4″ microphones which make a part of the Brüel & Kjær program. The sensitivities and frequency ranges therefore correspond to those valid for some real microphone types.
The simple models discussed have been chosen to illustrate the influence of the main parameters which determine the microphone response. Other, less important, parameters should be taken into account to relate the models to microphones of the real world. In particular, 1/4″ microphones generally cover a wider frequency range than estimated from the simple models. This is because a resonance between the diaphragm mass and the compliance of the air in the slit behind the diaphragm may increase the response at higher frequencies and extend the frequency range of these microphones.
In principle, any microphone could be improved with respect to frequency range if the diaphragm mass could be reduced. A lower mass would increase the resonance frequency and thus also the highest operation frequency. However, this would mean that a thinner diaphragm foil should be used, and that the tension in the dia-phragm material itself would be increased accordingly. This might lead to sagging and instability in the diaphragm. Therefore, very strong diaphragm materials are needed when good acoustic performance and good long term stability are required, as is the case for measurement microphones.
Fig.2.12 Phase of frequency responses (pressure). The curves are valid for models of microphone with critical damping and different diaphragm diameters (relative scale: 1 (a), 0.5 (b), 0.25 (c)). The numbers chosen for the calculation approximate the parameters of existing 1″, 1/2″ and 1/4″
The above explanation shows that the microphone size, the sensitivity and the fre-quency range are tied together and cannot be selected separately. Very often, the user must make a compromise when selecting a microphone. Generally, small mi-crophones work to higher frequencies and create less disturbance in the sound field, but they also have lower sensitivity and higher inherent noise and thus may not be usable at the lowest sound levels of interest.