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ADM - Adaptive Dual Microphone

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About Adaptive Dual Microphone

Adaptive Dual Microphone (ADM) is a patent pending, digital signal processing technology creating directional or noise canceling microphone using only two omni-directional microphones. ADM automatically changes its directional characteristics to provide the best noise attenuation in varying environments while fully preserving the signal quality. The adaptation process is very fast and frequency selective so that multiple interferences may be cancelled simultaneously.

Figure 1. Illustration of polar pattern changes according
to the main direction of noise source

Besides its better directional characteristics, ADM is much less affected by wind than conventional, acoustic directional microphones. It has no proximity effect and it is easy to integrate into mobile voice communication and other devices.

ADM technology allows two types of microphones' configuration: "endfire" and "broadside". In the endfire configuration the sound of interest is supposed to be on the axis (line connecting the two microphones). In the broadside configuration it is supposed to be on the line transverse to the axis.

In the endfire configuration ADM has two modes of operation: "far-talk" and "close-talk". In the far-talk mode ADM works as the best possible directional microphone attenuating signals arriving from back and side directions while preserving the front signal. It is ideal for applications like conference, reporter and surveillance microphones, camcorders, hearing aids, assistive listening devices and mobile phones in "speakerphone mode". In the close-talk mode ADM works as the best possible noise canceling microphone creating a virtual "bubble of silence" around it effectively eliminating distant sounds without affecting the close sound. Relative freedom of acoustic design makes is ideal for mobile handsets with "soft" switch between speakerphone (far) and handset (close) modes of operation.

In the broadside configuration only "far-talk" mode is supported.

Figure 2. ADM principle of operation

The functionality of the processing blocks above depends on the operational mode.

In the far-talk mode the delays are chosen to attenuate all signals but the one coming from the main direction. For the endfire configuration the theoretical polar pattern of ADM microphone is described as half of figure eight. Figure 3 shows ADM polar pattern measured in an anechoic chamber. About 30dB attenuation is achieved for back sounds in the whole frequency range - close to the theoretical pattern.

In the close-talk mode ADM tries eliminate all sounds that create equal (with some specified tolerance) sound pressure level on its two sound pressure sensors. Thus all distant sounds coming from all directions are cancelled. At the same time close sounds that create enough sound pressure difference are preserved without any distortion.

Figure 3. ADM polar pattern

Implementation (platforms)

ADM technology has low latency and it can be easily implemented on any low cost, 16 bit DSP. Currently ADM is available on Analog Devices Blackfin Processor family and Kalimba DSP integrated into CSR Bluetooth multimedia chips.

Interactive audio examples

Use control buttons on the interactive screenshots to play/stop/navigate.

Woman voice while a truck is moving backward nearby (one microphone, original signal):


ADM output in the same conditions (no noise suppression or other processing is used):


Mobile phones, Bluetooth headsets, hands-free kits, video/audio conferencing equipment, voice recorders, intercom systems, voice controlled devices and many others.

Video presentations

White papers

114th AES Convention paper >>
116th AES Convention paper >>
117th AES Convention paper >>
Presentation given at IBM speech seminar (June 2003, Haifa, Israel) >>
ADM technology white paper v3.2 >>

Additional information

ADM leaflet >>

Please contact us for more information, audio examples and real time demo availabilities.


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