Understanding Microphone Sensitivity

8/22/2019 Understanding Microphone Sensitivity

1/3Analog Dialogue 46-05 Back Burner, May (2012)

Understanding

Microphone SensitivityBy Jerad Lewis

Sensitivity, the ratio o the analog output voltage or digitaloutput value to the input pressure, is a key specifcation oany microphone. Mapping units in the acoustic domain to

units in the electrical domain determines the magnitude othe microphone output signal, given a known input.

This article will discuss the distinction in sensitivity specifca-tions between analog and digital microphones, how to choosea microphone with the best sensitivity or the application, andwhy adding a bit (or more) o digital gain can enhance themicrophone signal.

Analog vs. Digital

Microphone sensitivity is typically measured with a 1 kHzsine wave at a 94 dB sound pressure level (SPL), or 1 pascal(Pa) pressure. The magnitude o the analog or digital outputsignal rom the microphone with that input stimulus is a

measure o its sensitivity. This reerence point is but onecharacteristic o the microphone, by no means the wholestory o its perormance.

The sensitivity o an analog microphone is straightorwardand easy to understand. Typically specifed in logarithmicunits o dBV (decibels with respect to 1 V), it tells how manyvolts the output signal will be or a given SPL. For an analogmicrophone, sensitivity, in linear units o mV/Pa, can beexpressed logarithmically in decibels:

=dBV

AREFOutputySensitivit PamV

ySensitivit /10log20

where OutputARE F is the 1000 mV/Pa (1 V/Pa) reerenceoutput ratio.

Given this inormation, with the appropriate preamplifergain, the microphone signal level can be easily matched tothe desired input level o the rest o the circuit or system.Figure 1 shows how the microphones peak output voltage(VMAX) can be set to match an ADCs ull-scale input voltage(V IN) with a gain o VIN/VMAX . For example, an ADMP504with 0.25 V maximum output voltage could be matched toan ADC with 1.0 V ull-scale peak input voltage by using again o 4 (12 dB).

ANALOG

MICROPHONE:

VMAXPEAK

OUTPUT

VOLTAGE

PREAMP:

GAIN =

VIN/VMAX

ADC: FULL-SCALE

INPUT LEVEL VIN

Figure 1. Analog microphone input signal chain

with preamp to match microphone output level to

ADC input level.

The sensitivity o digital microphones, with units dBFS(decibels with respect to digital ull scale), is not sostraightorward. The dierence in units points to a subtle

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contrast in the defnition o sensitivity o digital microphonecompared to that o analog microphones. For an analogmicrophone with a voltage output, the only limit to the size othe output signal is the practical limit o the systems voltagsupplies. Although it may not be practical or most designsthere is no physical reason why an analog microphoncouldnt have 20 dBV sensitivity, with a 10 V output signaor a reerence-level input signal. This sensitivity could beaccomplished as long as the amplifers, converters, and othecircuits could support the required signal levels.

Sensitivity o a digita l microphone is less exible; it dependon a single design parameter, maximum acoustic input. As longas the ul l-scale digital word is mapped to the microphonemaximum acoustic input (the only sensible mapping, reallythe sensitivity must be simply the dierence between thimaximum acoustic signal and the 94 dB SPL reerence. Soi a digital microphones maximum SPL is 120 dB, then itssensitivity will be 26 dBFS (94 dB 120 dB). There isno way to tweak a design to make the digital output signahigher or a given acoustic input, unless the maximumacoustic input is lowered by the same amount.

For digital microphones, sensitivity is measured as a percentago the ull-scale output that is generated by a 94 dB SPL inputFor a digital microphone, the conversion equation is

= FSdBFS

DREFOutput

ySensitivitySensitivit %10log20

where OutputDREF is the ull-scale digital output level.

One last very conusing piece o this comparison is theinconsistent usage o peak and rms levels between digitaand analog microphones. The microphones acousticinput levels in dB SPL are always rms measurementregardless o the type o microphone. The output o analomicrophones is reerenced to 1 V rms, as rms measurement

are more commonly used or comparing analog audiosignal levels. However, the sensitivity and output level odigital microphones are given as peak levels because theyare reerred to the ull-scale digital word, which is a peakvalue. In general, this convention o using peak levels tospeciy the output o digital microphones must be kept inmind when confguring downstream signal processing thamay rely on precise signal levels. For example, dynamicrange processors (compressors, limiters, and noise gatestypically set thresholds based on rms signal levels, so adigital microphones output must be scaled rom peak torms by lowering the dBFS value. For a sinusoidal inputhe rms level is 3 dB (the logarithmic measure o (FS/2below the peak level; this dierence between rms and peak

may be dierent or more complex signals. For examplethe ADMP421, a MEMS microphone with pulse-densitymodulated (PDM) digital output, has a sensitivity o

26 dBFS. A 94 dB SPL sinusoidal input signal wi ll give a26 dBFSpeak output level, or a 29 dBFS rms level.

As the outputs o digital and analog microphones havedierent units, comparing one type to another can bconusing; however, they share a common unit o measure inthe acoustic domain, SPL. One may have an analog voltageoutput, another a modulated PDM output, and a third anI2S output, but their maximum acoustic input and signato-noise ratio (SNR, the di erence between the 94 dB SPLreerence and the noise level) can be directly compared
http://www.analog.com/ADMP504http://www.analog.com/ADMP504http://www.analog.com/ADMP504http://www.analog.com/ADMP421http://www.analog.com/ADMP421http://www.analog.com/ADMP421http://www.analog.com/ADMP421http://www.analog.com/ADMP504


2/32 Analog Dialogue 46-05 Back Burner, May (2012)

By reerring to the acoustic domain, not the output ormat,these two specifcations provide a convenient way to comparedierent microphones. Figure 2 shows the relationship betweenan acoustic input signal and the output levels o analog anddigital microphones or a given sensitivity. Figure 2(a) showsthe ADMP504 analog microphone, which specifes 38 dBVsensitivity and 65 dB SNR. Changing its sensitivity, relativeto the 94 dB SPL reerence point on the let, would result insliding the dBV output bar up to decrease sensitivity or downto increase sensitivity.

(a)

(b)

120

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0

MAXIMUM ACOUSTIC INPUT

REFERENCE SPL (94dB)

NOISE FLOOR OF MICROPHONEWITH 65dB SNR

0

10

20

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120

SENSITIVITY (38dBV)

DYNAMICRANGE

dB SPL INPUT

dBV OUTPUT

SIGNAL-TO

-NOISERATIO

110

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0

MAXIMUM ACOUSTIC INPUT

REFERENCE SPL (94dB)

NOISE FLOOR OF MICROPHONEWITH 65dB SNR

0

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60

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80

90

100

110

120

SENSITIVITY (26dBFS)

DYNAMICRANGE

dB SPL INPUT dBFS OUTPUT

SIGNAL-TO-NOISERATIO

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

level for an analog microphone; (b) digital output level for

a digital microphone.

Figure 2(b) shows the ADMP521 digital microphone, whichspecifes 26 dBFS sensitivity and 65 dB SNR. This illustrationo the input-to-output level mapping or a digital microphoneshows that the sensitivity o this microphone cannot be adjustedwithout breaking the mapping between the maximum acousticinput and the ull-scale digital word. Specifcations such asSNR, dynamic range, power supply rejection, and THD arebetter indicators o microphone quality than sensitivity.

Choosing Sensitivity and Setting Gain

A high sensitivity microphone isnt always betterthan a lowsensitivity microphone. Sensitivity tells something about thecharacteristics o the microphone but not necessarily aboutits quality. A balance between the microphones noise level,clipping point, distortion, and sensitivity determines wheth-er a microphone is a good ft or a particular application.A microphone with high sensitivity may need less preampgain beore the analog-to-digital conversion, but it may haveless headroom beore clipping than a microphone with lowersensitivity.

In near-feld applications, such as cell phones, where themicrophone is close to the sound source, a microphonewith higher sensitivity is more likely to reach the maximumacoustic input, clip, and cause distortion. On the other hand,

a higher sensitivity may be desirable in ar-feld applications,such as conerence phones and security cameras, where thesound is attenuated as the distance rom the source to themicrophone increases. Figure 3 shows how the distance othe microphone rom the sound source can aect the SPL.The level o an acoustic signal decreases by 6 dB (one-hal )each time the distance rom the source is doubled.

1 8 16 32

DISTANCE FROM SOURCE (Inches)

87dB SPL

69dB SPL

63dB SPL

57dB SPL

Figure 3. Sound pressure level at the microphone is

reduced as the distance from the source increases.

For reerence, Figure 4 shows the typical SPL o various

sound sources, rom quiet recording studios (below 10 dBSPL) up to the threshold o pain (above 130 dB SPL), thepoint at which the sound causes pain or the average person.Microphones can rarely cover allor even mosto thisrange, so choosing the right microphone or the requiredSPL range is an important design decision. The sensitivityspecifcation should be used to match the microphonesoutput signal level across the dynamic range o interest tothe common signal level o the audio signal chain.
http://www.analog.com/ADMP521http://www.analog.com/ADMP521


3/3Analog Dialogue 46-05 Back Burner, May (2012) 3

120

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0

STORES AND NOISY OFFICES

dB SPL

130

PROPELLER AIRCRAFT

THRESHOLD OF PAIN

HEAVY MACHINE SHOPS

SUBWAY TRAINS, NIAGARA FALLS

THRESHOLD OF HEARING

AUDIENCE NOISE, MOVIE THEATER

TARGET PERFORMANCE FOR RECORDING

STUDIOS AND CONCERT HALLS

QUIET HOMES

GOOD BROADCAST STUDIOS

AVERAGE FACTORY

NORMAL CONVERSATION AT 1m

Figure 4. Sound pressure level of various sources.1

Analog microphones have a wide range o sensitivities.Some dynamic microphones might have sensitivity as lowas 70 dBV. Some condenser microphone modules haveintegrated preamps so they have extra high sensitivity o

18 dBV. Most analog electret and MEMS microphoneshave sensitivity between 46 dBV and 35 dBV (5.0 mV/Pato 17.8 mV/Pa). This level is a good compromise betweenthe noise oorwhich can be as low as 29 dB SPL or theADMP504 and ADMP521 MEMS microphonesand themaximum acoustic inputwhich is typically about 120 dBSPL. An analog microphones sensitivity can be tuned inthe preamp circuit that is oten integrated in the packagewith the transducer element.

Despite the perceived inexibility o a digital microphonessensitivity, the level o the microphone signal can be easilyadjusted with gain in the digital processor. With digitalgain, there is no danger o degrading the noise level o the

signal as long as the processor has a sufcient number obits to ully represent the dynamic range o the originalmicrophone signal. In an analog design, every gain stage willintroduce some noise into the signal; it is up to the systemdesigner to ensure that each gain stage is quiet enough tokeep its injected noise rom degrading the audio signal.As an example, we can look at the ADMP441, a digital(I2S) output microphone with a maximum SPL o 120 dB(26 dBFS sensitivity) and an equivalent input noise o 33 dBSPL (61 dB SNR). The microphones dynamic range is thedierence between the largest (max SPL) and smallest (noise

oor) signals it can aithully reproduce (120 dB 33 dB =87 dB or the ADMP441). This dynamic range can bereproduced with a 15-bit data word. A 1-bit shit o the data ina digital word results in a 6 dB shit in the signal level, so even 16-bit audio processor with a 98 dB dynamic range coulduse 11 dB o gain or attenuation beore the original dynamicrange is compromised. Note that in many processors, thedigital microphones maximum acoustic input is mappedto the DSPs internal ull-scale level. In this case, addingany amount o gain reduces the dynamic range by an equaamount and lowers the systems clipping point. Using theADMP441 as an example, adding 4 dB o gain in a processowith no headroom above ull scale would cause the system toclip with a 116 dB SPL signal.

Figure 5 shows a digital microphone, with either I 2S oPDM output, connected directly to a DSP. In this signachain, no intermediate gain stage is necessary because themicrophones peak output level already matches the DSPull-scale input word.

DIGITAL

MICROPHONE:

0dBFS PEAK

OUTPUT

DSP:

0dBFS PEAK INPUT

Figure 5. Digital microphone input signal chain

connected directly to a DSP.

Conclusion

This article explained how to understand a microphonesensitivity specifcation, how to apply it to a systemgain staging, and why, although sensitivity is related toSNR, it is not an indication o the microphones qualityas is SNR. Whether designing with an analog or digitaMEMS microphone, this should help a designer choose the

best microphone or an application and to get the ullesperormance rom that device.

References

Designing with MEMS Microphones. http://ez.analog.comcommunity/ask_the_expert/archived/mems-microphones .

Lewis, Jerad. AN-1112 Application Note.Microphone Specifcations Explained. Analog Devices, 2011.

MEMS Microphones. http://www.analog.com/en/audiovideo-products/mems-microphones/products/index.html.

1John Eargle, The Microphone Book, Elsevier/FocalPress, 2004.

Author

Jerad Lewis [[email protected]] is aMEMS microphone applications engineer atAnalog Devices. He joined the company in2001 ater getting his BSEE rom Penn StateUniversity. Since then, Jerad has supportedvarious audio ICs, including SigmaDSP,converters, and MEMS microphones. He iscurrently pursuing an MEng degree in acoustics at PennState University.
http://www.analog.com/ADMP441http://www.analog.com/ADMP441http://ez.analog.com/community/ask_the_expert/archived/mems-microphoneshttp://ez.analog.com/community/ask_the_expert/archived/mems-microphoneshttp://ez.analog.com/community/ask_the_expert/archived/mems-microphoneshttp://ez.analog.com/community/ask_the_expert/archived/mems-microphoneshttp://www.analog.com/static/imported-files/application_notes/AN-1112.PDFhttp://www.analog.com/static/imported-files/application_notes/AN-1112.PDFhttp://www.analog.com/en/audiovideo-products/mems-microphones/products/index.htmlhttp://www.analog.com/en/audiovideo-products/mems-microphones/products/index.htmlhttp://www.analog.com/en/audiovideo-products/mems-microphones/products/index.htmlhttp://www.analog.com/en/audiovideo-products/mems-microphones/products/index.htmlmailto:jerad.lewis%40analog.com?subject=mailto:jerad.lewis%40analog.com?subject=mailto:jerad.lewis%40analog.com?subject=http://www.analog.com/en/audiovideo-products/mems-microphones/products/index.htmlhttp://www.analog.com/en/audiovideo-products/mems-microphones/products/index.htmlhttp://www.analog.com/static/imported-files/application_notes/AN-1112.PDFhttp://ez.analog.com/community/ask_the_expert/archived/mems-microphoneshttp://ez.analog.com/community/ask_the_expert/archived/mems-microphoneshttp://www.analog.com/ADMP441

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Understanding Microphone Sensitivity