29/06/18

THE BASICS ABOUT SENSITIVITY IN MICS

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When we talk about a microphone's sensitivity, it normally refers to microphone's sensitivity to sound; and that's the most important one! This is a specification that is important when we select a microphone for a job as it defines the working conditions for microphone and the preamp. This article focuses on microphone sensitivity to sound but mentions the influence from other phenomena like vibration, magnetic fields, etc.

By Eddy B. Brixen, Audio Specialist


Microphone sensitivity to sound

A microphone is a transducer designed to convert acoustic sound (predominantly sound pressure) into electricity (voltage / current). Sensitivity is the number of volts produced at a given sound pressure level. Depending on the design, the ability to do so – the sensitivity – may vary. This is partly due to the principles of the transducer and partly due to the purpose that the microphone was designed. To be able to compare the sensitivity data of different microphones / brands, almost all manufacturers conform to the same standard; IEC 60268-4.

The definition according to the IEC-standard is as follows: "The sensitivity is the ratio of the output voltage of the microphone to the sound pressure to which it is exposed. […]. The sensitivity M is expressed in volts per pascal."

The practical way of finding the sensitivity of a microphone is to place it in front of – and a certain distance (not too close to) – a loudspeaker. The sound produced by the loudspeaker should be a 1 kHz sine wave. The SPL (Sound Pressure Level) of that sine wave should be 94 dB. (94 dB is chosen because it is the same as air pressure of 1 pascal). Under these conditions, the electrical output of the microphone is measured. The magnitude of this output voltage is then regarded as the sensitivity. If the sound field, generated by the loudspeaker, is undisturbed, we have what is called free-field sensitivity.




Figure 1.Principle for the measurement of microphone sensitivity.


Sensitivity may also be expressed in dB with reference to 1 volt / Pa also mentioned as dBV (see examples).
 

Examples

Example 1. The SPL is 94 dB, and the frequency of the test tone is 1 kHz. If the output of the microphone is 20 mV, it has a sensitivity of 20 mV (@ 1kHz).
Expressed with reference to 1 V/Pal. Sensitivity: 20 * log 0.02 [V]/1 [V] = -40 dBV.

Example 2. The SPL is 88 dB, and the frequency of the test tone is 250 Hz. If the output of the microphone is 10 mV, it has a sensitivity of 20 mV (@250 Hz).

Note, example 2. the SPL is only 88 dB. This is 6 dB lower than the ref SPL (94 dB). The missing 6 dB must be added to the output voltage measured. 6 dB is the same as a factor of 10^(6/20) = 2. So the measured voltage is multiplied by 2; 10 mV * 2 = 20 mV (= 0.02 V).

Expressed with reference to 1 V/Pal. Sensitivity: 20 * log 0.02 [V]/1 [V] = -40 dBV.

 

The sound field

If nothing else is stated, the sensitivity is measured in a free field. This sound field is undisturbed and has only one direction. What is measured is called the microphone's free field sensitivity. However, the sensitivity of the microphone may change with the type of sound field to which it is exposed.

Sometimes the sensitivity is measured in a diffuse sound field (the sound comes randomly from all direction). Hence, this provides the microphone's diffuse field sensitivity.

Further, the microphone's sensitivity can be measured very close to the sound source. This is called near-field sensitivity. This measure is normally only of interest for close-talking microphones (and measured using an artificial mouth).

We may define the sensitivity at all frequencies. 1 kHz is chosen due to the fact it is a frequency in the middle of the active frequency range. However, the sensitivity may be specified at other frequencies for instance at all 1/3 octave bands. (The microphone's frequency response is an expression of the sensitivity at each and all frequencies.)

For measurement microphones (omnis), a specific acoustic calibrator can be applied. With this, the sound field is well defined and there will be no disturbance. The calibrator typically produces 94 dB SPL @ 1 kHz. Sometimes a frequency of 250 Hz is applied. This is a signal produced by a so-called pistonphone (114 dB SPL @ 250 Hz).




Figure 2. Acoustic calibrator (B&K 42XX) generates a 1 kHz sinewave with an SPL of 94 dB. This is used for the calibration of a setup involving a DPA miniature microphone (4060/-61/-62).

 

Sensitivity of condenser microphones

Condenser microphones can achieve higher sensitivity compared to dynamic microphones. However, there are some limitations due to the physics of the transducer.

Increasing the polarization voltage increases the sensitivity. However, the polarization voltage should not be so high that it results in discharging sparks between membrane and back-electrode. High humidity increases the risk of discharge.  

Increasing the capacity of the condenser also increases the sensitivity. This can be done in two ways:

1. By making a capsule with a larger diaphragm area
2. By decreasing the distance between diaphragm and the back electrode

A practical problem is that – due to the physical wavelength – a bigger membrane area may move disturbing diffraction phenomena downwards into the audible frequency range, which we normally try to avoid.

Decreasing the distance between the two electrodes may cause electrical sparks from membrane to backplate (especially externally-polarized capsules). The suction caused by the electrostatic field may cause the membrane to stick to the backplate. This can be avoided by establishing a higher tension of the membrane. However, this again reduces the sensitivity.

In mic design, it is a question of finding the right combination of diaphragm size, polarization voltage, distance between electrodes, backplate geometry, etc., to find the optimal working conditions.




Table 1. Sensitivity of various microphone types. Recording high SPLs takes a low-sensitivity microphone. Recording low SPL takes a high-sensitivity microphone.

 

Sensitivity to other kinds of impact

The microphone should only react to sound – nothing else. However, other physical phenomena may have an impact, which results in a microphone output.  
 

Wind

Wind may produce membrane movement. This results in output noise. Omnidirectional microphones are less sensitive to wind compared to directional types (typically in the range of 10 dB). This is due to the design: Omnis (pressure microphones) have a stiffer membrane in front of a closed cavity. Directional microphones (pressure gradient) often have more sloppy membranes that are open on both sides.

The two figures below show the noise spectrum generated by wind and the directionality of wind sensitivity for a d:screet™ 4060 Miniature Microphone.




Figure 3. Left: d:screet™ 4060 Miniature Microphone and its sensitivity to wind (10 m/s) from different directions. Right: A polar plot of the same microphone's wind sensitivity. It’s a perfect omnidirectional microphone. However, regarding wind, it is less sensitive to wind arriving from the back. (Red, full curve: A-weighted measurement. Blue, dotted curve: C-weighted measurement.)

 

Vibrations

All kinds of vibrations may cause membrane movement. This is why, for instance, handheld microphones should contain damping inside the housing to prevent hand noise from becoming a signal on the output.

Listen to the sound samples here: https://www.dpamicrophones.com/mic-university/10-important-facts-about-acoustics-for-microphone, #9 Mechanical vibrations.

 

Humidity

Humidity can be a problem, especially in externally-polarized condenser microphones (discharging, making clicks in the sound). Electret condenser microphones are generally less sensitive to humidity even though the polarization voltage is higher.

In practice, it is a good idea to keep microphones in dry places. DPA microphones are guaranteed to work in relative humidity up to 90%. However, it is possible to clean the miniature microphones adding purified water. Remember, the microphones must dry out before usage.

 

Temperature

Temperature can affect microphones. Electret condenser microphones can lose their charge if exposed to extremely-high temperatures. Normally, microphones can handle temperatures up to 45°C without problems.     

 

Magnetic stray fields

Dynamic microphones are very sensitive to magnetic fields if not compensated by a counter-wound (noise) coil. (Sometimes you can experience what sounds like acoustic feedback even without any loudspeakers turned on. The feed is running between an induction loop installed for hearing aids and the microphone.)

The same phenomenon exists in single-coil pickups for guitars.

Condenser microphones rarely exhibit a sensitivity to magnetic fields that affects ordinary use. However, the sensitivity can be measured and specified as the output voltage when the microphone is exposed to a magnetic field strength of 1 A/m. Normally, this is measured at 50/60 Hz, 1 kHz, and 16 kHz.

 

Conclusion

A microphone's sensitivity to sound is defined by its output voltage when placed in a sound field providing 94 dB SPL of a 1 kHz sine tone. The sensitivity of the microphone is chosen to fit the microphone preamp. High-SPL sources should be recorded with low-sensitivity mics. Low-SPL sources should be recorded with high-sensitivity mics.

A microphone's sensitivity to other types of impact than acoustic sound should be kept to an absolute minimum.

 

References

[1] IEC 60.268 Sound System Equipment - Part 4: Microphones






 

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