Engineer, do you know the microphone sensitivity?

Sensitivity, which is the ratio of analog output voltage or digital output value to input pressure, is a key indicator for any microphone. When the input is known, the mapping from the acoustic domain unit to the electrical domain unit determines the amplitude of the microphone output signal.

This article will discuss the differences in sensitivity specifications between analog and digital microphones, how to choose the microphone with the best sensitivity according to the specific application, and also discuss why adding one (or more) digital gain can enhance the microphone signal.

Analog and digital

The microphone sensitivity is generally measured at a sound pressure level (SPL) of 94 dB (or pressure of 1 Pa) with a 1 kHz sine wave. The analog or digital output signal amplitude of the microphone under this input excitation is a measure of microphone sensitivity. This reference point is just one of the characteristics of the microphone, and does not represent the entire performance of the microphone

The sensitivity of an analog microphone is simple and not difficult to understand. This index is generally expressed in logarithmic units of dBV (decibels relative to 1 V), which represents the volts of the output signal at a given SPL. For analog microphones, the sensitivity (expressed in linear units mV / Pa) can be expressed in decibels in logarithm:

Where OutputAREF is 1000 mV / Pa (1 V / Pa) reference output ratio.

With this information and the correct preamp gain, it is easy to match the microphone signal level to the target input level of the circuit or other parts of the system. Figure 1 shows how to set the microphone's peak output voltage (VMAX) to match the ADC's full-scale input voltage (VIN) with a gain of VIN / VMAX. For example, with a gain of 4 (12 dB), a maximum output voltage of The 0.25 V ADMP504 is matched to an ADC with a full-scale peak input voltage of 1.0 V.

Figure 1. Analog microphone input signal chain with preamplifier to match microphone output level to ADC input level

Figure 1. Analog microphone input signal chain with preamplifier to match microphone output level to ADC input level

The sensitivity of a digital microphone (in dBFS, decibels relative to digital full scale) is not so simple. The difference in units indicates that there is a subtle difference in the definition of the sensitivity of digital microphones and analog microphones. For analog microphones that provide voltage output, the only limitation on the size of the output signal is actually the limitation of the system power supply voltage. Although not practical for most designs, physically speaking, analog microphones can have a sensitivity of 20 dBV, where the output signal for the reference level input signal is 10 V. As long as the amplifier, converter and other circuits Can support the required signal level, can fully achieve this level of sensitivity.

The sensitivity of the digital microphone is not so flexible, but only depends on one design parameter, namely, the maximum acoustic input. As long as the full-scale digital word is mapped to the microphone's maximum acoustic input (in fact, this is the only useful mapping), the sensitivity must be the difference between this maximum acoustic signal and the 94 dB SPL reference signal. Therefore, if the maximum SPL of a digital microphone is 120 dB, its sensitivity is –26 dBFS (94 dB – 120 dB). Unless the maximum acoustic input is reduced by the same amount, it is impossible to adjust the design to make the digital output signal of a given acoustic input higher.

For digital microphones, the sensitivity is expressed as the percentage of the full-scale output produced by the 94 dB SPL input. The conversion formula of the digital microphone is

Where OutputDREF is the full-scale digital output level.

Now let's compare the last very difficult point, digital and analog microphones are not consistent in the use of peak level and rms level. The microphone's acoustic input level (in dB SPL) is always a rms measurement, regardless of the type of microphone. The output of the analog microphone is referenced to 1 V rms, because the root-mean-square measurement is more commonly used to compare analog audio signal levels. However, the sensitivity and output level of digital microphones are expressed as peak levels because they are referenced to full-scale digital words (ie peaks). In general, when configuring downstream signal processing that may rely on accurate signal levels, you must remember the convention of specifying the output of a digital microphone with peak levels. For example, dynamic range processors (compressors, limiters, and noise gates) usually set thresholds based on the rms signal level, so the output of the digital microphone must be scaled by reducing the dBFS value from peak to rms . For a sinusoidal input, the rms level is 3 dB lower than the peak level (that is, the logarithm measurement of (FS√2)); for more complex signals, the rms level between the rms level and the peak level The difference may be different. For example, the ADMP421, a MEMS microphone that provides a pulse density modulation (PDM) digital output has a sensitivity of –26. A 94 dB SPL sinusoidal input signal will produce a peak output level of –26 dBFS, or an rms level of –29 dBFS.

Since the output of digital microphones and analog microphones use different units, it may be difficult to understand when comparing the two types of microphones; but the two have a common measurement unit in the sound domain, SPL. A microphone may be Analog voltage output, the other is the modulated PDM output, and the other is the I2S output, but their maximum acoustic input and signal-to-noise ratio (SNR, that is, the difference between the 94 dB SPL reference level and the noise level) can be directly Comparison of. Using the sound range rather than the output format as a reference, these two specifications provide a convenient way to compare different microphones. Figure 2 shows the relationship between the acoustic input signal and output level of analog and digital microphones at a given sensitivity. Figure 2 (a) shows the ADMP504 analog microphone with a sensitivity of –38 dBV and a signal-to-noise ratio of 65 dB. When the sensitivity is changed relative to the 94 dB SPL reference point on the left, the result will result in the following: sliding the dBV output upward The bar will decrease the sensitivity, and sliding the output bar downward will increase the sensitivity.

figure 2

Figure 2. (a) Mapping acoustic input level to voltage output level (analog microphone)

(B) Mapping the acoustic input level to the digital output level (digital microphone)

Figure 2 (b) shows the ADMP521 digital digital microphone with a sensitivity of -26 dBFS and a signal-to-noise ratio of 65 dB. The schematic diagram of the digital microphone input to output level mapping shows that adjusting the sensitivity of the microphone will destroy the maximum acoustic input and The mapping between full-scale digital words. Compared with sensitivity, SNR, dynamic range, power supply rejection ratio, THD and other specifications can better show the performance of the microphone.

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