Beamforming arrays date back over 100 years, and were first used in the military for determining the bearing and location of aircraft, ships, and submarines. Until the mid 1970’s, when microprocessors became widely available, these arrays were 100% acoustic and fairly primitive. Modern software development and digital signal processing techniques have substantially advanced the state of the art of this technology. With good design and engineering, it’s now possible to employ an advanced microphone array system to capture desired sounds and produce a highly intelligible, superior-quality audio signal.
Also known as spatial filtering, beamforming can be applied to both sound waves and radio energy, and thus is sometimes discussed in the context of cellular telephones, wireless networking, biomedicine, sonar, seismology, and even radio astronomy.
This paper focuses exclusively on beamforming as a method of sensing and filtering acoustic waves for enterprise video and telephone conferencing applications, and describes emerging techniques and technologies to improve the capture and processing of acoustic patterns, and the clarity of human speech in corporate conference rooms.
Acoustic Beamforming Theory
Research Scientist Andrew A. Ganse of the Applied Physics Laboratory at the University of Washington explains that “The basic point in beamforming is, when you set multiple transducers next to each other sending out signals [or listening for sound waves], you’re going to get a kind of interference pattern, just like you see in a pond when you throw several stones in at once and create interfering ripples. If you select the spacing between your transducers and the delay in the transducers’ signals just right, you can create an interference pattern that’s to your benefit, in particular one in which the majority of the signal energy all goes out in one angular direction.”
By combining elements [i.e., antennas or microphones] in a phased array, so that signals at particular angles experience constructive interference while others experience destructive interference, beamforming can be used to achieve spatial selectivity and elimination of unwanted signals.
Beamforming microphones are part of a larger family of mics considered to be high-directionality, or perhaps ultra-directional microphones, due to their stunning sound quality, advanced performance, and similarities to phased-array radar.
As all sound professionals know, an omnidirectional microphone is one that picks up sound from all directions. The disadvantage of omni mics is that they often result in a sound feed that is reverberant, or hollow, and full of ambient noise from all directions in a room. A basic directional microphone still in common use today is the cardioid mic, perfected in the early 1940’s. Cardioid mics sound less reverberant and less noisy than omni mics because they attenuate off-axis reflected audio and noise by reducing sound pickup from the sides and rear. But cardioid mics have limited selectivity or filtering capabilities, which can be surpassed using beamforming technology.
Beamforming can be used to select, or focus on, just those sounds or voices that you want conference participants to hear, while rejecting all unwanted sounds. Selecting desired sound sources and rejecting undesirable sounds has been the goal of audio engineers for most of a century. With acoustic beamforming and integrated digital signal processing made possible by modern microprocessors and software, this is now technologically possible.
Traditional directional microphones may be used in conferencing applications to perform limited spatial filtering to improve audio quality. These microphones have an acoustic pattern that selectively picks up desired sound waves in one region of space and rejects others.
Beamforming is a signal processing technique carried out by a processor using input from the beamforming microphone array. The signals from the various microphones are combined such that signals at particular angles experience constructive interference, while others experience destructive interference.
Superior spatial selectivity can be achieved using beamforming techniques so that certain regions (sound directions) can be amplified, and other regions can be significantly attenuated. Beamforming processing components can also be configured to attenuate unwanted sounds originating from the direction of a door or window into the conference room.
To adjust and tune the directionality of the array to focus on desired sounds, a beamforming array system adjusts the phase and amplitude of the sounds reaching each microphone, to create a pattern of constructive and destructive interference in the wave front. Information from different microphones is then combined such that the expected acoustic pattern is preferentially processed and fed into the conferencing audio stream.
Advanced beamforming techniques employ interference patterns to change the directionality of the array, so that information collected from the various microphones can be combined to give preference to the expected acoustic patterns. Adaptive beamforming algorithms may also be included to automatically adapt to different situations and environments.
The most advanced, state-of-the-art enterprise video and teleconferencing systems today employ professionally engineered beamforming microphone arrays and associated components and processors.
We’ll continue our discussion of the Advanced Beamforming Microphone Array Technology for Corporate Conferencing Systems in next week’s blog.