Microphone array recommendations

In this article, you learn how to design a microphone array customized for use with the Speech SDK. This is most pertinent if you are selecting, specifying, or building hardware for speech solutions.

The Speech SDK works best with a microphone array that has been designed according to these guidelines, including the microphone geometry, component selection, and architecture.

Microphone geometry

The following array geometries are recommended for use with the Microsoft Audio Stack. Location of sound sources and rejection of ambient noise is improved with greater number of microphones with dependencies on specific applications, user scenarios, and the device form factor.

Array Microphones Geometry
Circular - 7 Microphones 7 mic circular array 6 Outer, 1 Center, Radius = 42.5 mm, Evenly Spaced
Circular - 4 Microphones 4 mic circular array 3 Outer, 1 Center, Radius = 42.5 mm, Evenly Spaced
Linear - 4 Microphones 4 mic linear array Length = 120 mm, Spacing = 40 mm
Linear - 2 Microphones 2 mic linear array Spacing = 40 mm

Microphone channels should be ordered ascending from 0, according to the numbering depicted above for each array. The Microsoft Audio Stack will require an additional reference stream of audio playback to perform echo cancellation.

Component selection

Microphone components should be selected to accurately reproduce a signal free of noise and distortion.

The recommended properties when selecting microphones are:

Parameter Recommended
SNR >= 65 dB (1 kHz signal 94 dBSPL, A-weighted noise)
Amplitude Matching ± 1 dB @ 1 kHz
Phase Matching ± 2° @ 1 kHz
Acoustic Overload Point (AOP) >= 120 dBSPL (THD = 10%)
Bit Rate Minimum 24-bit
Sampling Rate Minimum 16 kHz*
Frequency Response ± 3 dB, 200-8000 Hz Floating Mask*
Reliability Storage Temperature Range -40°C to 70°C
Operating Temperature Range -20°C to 55°C

*Higher sampling rates or "wider" frequency ranges may be necessary for high-quality communications (VoIP) applications

Good component selection must be paired with good electroacoustic integration in order to avoid impairing the performance of the components used. Unique use cases may also necessitate additional requirements (for example: operating temperature ranges).

Microphone array integration

The performance of the microphone array when integrated into a device will differ from the component specification. It is important to ensure that the microphones are well matched after integration. Therefore the device performance measured after any fixed gain or EQ should meet the following recommendations:

Parameter Recommended
SNR > 63 dB (1 kHz signal 94 dBSPL, A-weighted noise)
Output Sensitivity -26 dBFS/Pa @ 1 kHz (recommended)
Amplitude Matching ± 2 dB, 200-8000 Hz
THD%* ≤ 1%, 200-8000 Hz, 94 dBSPL, 5th Order
Frequency Response ± 6 dB, 200-8000 Hz Floating Mask**

**A low distortion speaker is required to measure THD (e.g. Neumann KH120)

**"Wider" frequency ranges may be necessary for high-quality communications (VoIP) applications

Speaker integration recommendations

As echo cancellation is necessary for speech recognition devices that contain speakers, additional recommendations are provided for speaker selection and integration.

Parameter Recommended
Linearity Considerations No non-linear processing after speaker reference, otherwise a hardware-based loopback reference stream is required
Speaker Loopback Provided via WASAPI, private APIs, custom ALSA plug-in (Linux), or provided via firmware channel
THD% 3rd Octave Bands minimum 5th Order, 70 dBA Playback @ 0.8 m ≤ 6.3%, 315-500 Hz ≤ 5%, 630-5000 Hz
Echo Coupling to Microphones > -10 dB TCLw using ITU-T G.122 Annex B.4 method, normalized to mic level
TCLw = TCLwmeasured + (Measured Level - Target Output Sensitivity)
TCLw = TCLwmeasured + (Measured Level - (-26))

Integration design architecture

The following guidelines for architecture are necessary when integrating microphones into a device:

Parameter Recommendation
Mic Port Similarity All microphone ports are same length in array
Mic Port Dimensions Port size Ø0.8-1.0 mm. Port Length / Port Diameter < 2
Mic Sealing Sealing gaskets uniformly implemented in stack-up. Recommend > 70% compression ratio for foam gaskets
Mic Reliability Mesh should be used to prevent dust and ingress (between PCB for bottom ported microphones and sealing gasket/top cover)
Mic Isolation Rubber gaskets and vibration decoupling through structure, particularly for isolating any vibration paths due to integrated speakers
Sampling Clock Device audio must be free of jitter and drop-outs with low drift
Record Capability The device must be able to record individual channel raw streams simultaneously
USB All USB audio input devices must set descriptors according to the USB Audio Devices Rev3 Spec
Microphone Geometry Drivers must implement Microphone Array Geometry Descriptors correctly
Discoverability Devices must not have any undiscoverable or uncontrollable hardware, firmware, or 3rd party software-based non-linear audio processing algorithms to/from the device
Capture Format Capture formats must use a minimum sampling rate of 16 kHz and recommended 24-bit depth

Electrical architecture considerations

Where applicable, arrays may be connected to a USB host (such as a SoC that runs the Microsoft Audio Stack (MAS)) and interfaces to Speech services or other applications.

Hardware components such as PDM-to-TDM conversion should ensure that the dynamic range and SNR of the microphones is preserved within re-samplers.

High-speed USB Audio Class 2.0 should be supported within any audio MCUs in order to provide the necessary bandwidth for up to seven channels at higher sample rates and bit depths.

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