Key Specs: Top & Bottom Ports
In addition to analog or digital outputs, there are a few other specifications for MEMS microphones to consider when making the proper selection. This slide and the following slides will discuss those parameters in greater detail.
First, comes the decision of whether to select a top port or bottom port microphone. Top port MEMS microphones have a sound hole fabricated in the top of the metal case that houses the internal circuitry, while the bottom port has a hole drilled on the bottom next to the solder pads. With a bottom port, a hole will also need to be drilled in the PCB to allow sound to enter. In general, the decision to select a top or bottom port MEMS microphone comes down to how the microphone fits in with the overall design.
Key Specs: Sensitivity & Sensitivity Tolerance
A MEMS microphone’s sensitivity rating is a measure in dB of how much output a microphone produces for a given sound level. A larger sensitivity number is better, but the sensitivity number will typically be negative. Therefore, a sensitivity rating of -5 dB is better than a sensitivity rating of -25 dB. Analog outputs usually measure sensitivity in dBV which is decibels relative to 1 volt RMS, while digital is measured in decibels relative to full scale output or dBFS.
Sensitivity tolerance is another important specification especially when using MEMS microphones for noise cancelation or array applications. Typical MEMS microphones carry sensitivity tolerances from ±3 dB down to ±1 dB, which allows for closer matching of sensitivities from microphone to microphone. This is one specification where MEMS microphones hold a distinct advantage over traditional electret condenser microphones.
Ratio of microphone output signal with 1 Pa sine wave at 1 kHz to reference output signal:
- Analog - Sensitivity (dBV) = 20 x log10(SmV/Pa ÷ Ref); Ref = 1000 mv/Pa
- Digital - Sensitivity (dBFS) = 20 x log10(S%FS ÷ Ref); Ref = 1.0
Key Specs: Signal to Noise Ratio
Signal-to-noise ratio (SNR) is an indicator in dB of how much background electrical noise the microphone’s MEMS element and ASIC will introduce into the system. This ratio looks at the reference signal (measured when sound pressure is 1 Pa at 1 kHz) and the residual noise at the microphone output. A larger SNR is desired meaning an SNR of 59 dB is better than 36 dB.
- Ratio of the desired signal to the undesired noise measured in dB
- SNR = 20 log(PS/PN)
- PS – Output signal power level
- Measured at 1 Pa (94 dB SPL) at 1 kHz
- PN – Noise signal power level
- Measured at 20 kHz bandwidth, A-weighted in quiet anechoic chamber
Key Specs: Dynamic Range & Acoustic Overload Point (AOP)
The dynamic range is a measure in dB of the loudness range over which a microphone is useful. The minimum signal represents what the microphone can distinguish from residual noise, while the maximum signal is how high the microphone can perform without distortion. A larger dynamic range number is better, so a dynamic range of 95 dB is better than 63 dB. The maximum signal of the dynamic range can also be referred to as the acoustic overload point (AOP). This is the sound level where distortion rises rapidly and is typically capped at a distortion level of 10%.