How Array Microphones Work
Ever wanted to know how microphone arrays work? Read our latest document to find out all the details of how they work!
About 10 years ago, Shure published a document titled “Microphones on the Ceiling? Shure Says NO” whose sole purpose was to discourage people from mounting microphones on the ceiling in meeting rooms. The results would be so disappointing and inconsistent that we did not want Shure products to be associated with such poor performance. But the introduction of the Microflex® Advance MXA910 ceiling array microphone in 2016 changed that.
Using array microphones instead of conventional microphones in a meeting room, classroom, or training room offers several benefits. Speech can be captured from microphones mounted on (or suspended from) the ceiling, providing a more discreet solution that blends harmoniously with the room’s interior design. The excellent rejection of room noise and reverberation provides superior clarity and intelligibility. And the ability to aim coverage where people are sitting without physically moving the microphone allows more efficient installation.
The concept of microphone arrays has been around for decades; a description, equations, and conceptual drawings appear in the 1938 lab notebook of Ben Bauer, the legendary Shure engineer who designed the Uniphase principle that allows a microphone to be unidirectional. Bauer understood microphone directionality like no one else, but the technology available at that time made a multi-element array of microphones impractical to actually build.
As the technology of microphones, electronics, and digital signal processing evolved, however, it began to look feasible, and Shure began experimenting with array microphone designs. Early prototypes required an entire rack full of computers to handle the intense digital signal processing necessary, but the technology eventually reached the point where one processor chip was powerful enough and compact enough to fit inside the microphone itself.
An array microphone uses a number of microphone transducers arranged in a specific pattern and summed together. When their outputs are mixed together, they form a virtual microphone with a more directional pickup pattern than the individual microphones have. The resulting pickup pattern forms a narrow “beam”, so array microphones are sometimes called “beamforming” microphones.
When several microphone transducers are arranged in a row, sounds approaching the array directly from the front (perpendicular to the array) will arrive at each microphone transducer at roughly the same time and therefore will add together constructively. Sounds from the sides arrive at each of the transducers at slightly different times because of the different distance to each transducer. This causes some frequencies to add together and some to subtract from each other, producing a lower audio level overall. So, the microphone array is more sensitive to sounds from the front and less sensitive to sounds from the sides.
The size of the array, the number of transducers, and the arrangement of the transducers all contribute to how directional an array microphone is. As with all directional microphones, the directionality varies with frequency; the microphone array is more directional at higher frequencies and less directional at lower frequencies. Digital signal processing (level adjustment, delay, equalization, etc.) on the signal from each individual microphone transducer can improve the array directionality to be more consistent across the entire frequency range, which delivers better rejection of off-axis sounds, improved intelligibility, and more natural sound quality.
If the transducers in an array are placed in a horizontal line, the beamforming effect only works in the horizontal dimension; the pickup pattern gets narrower from side to side but not from top to bottom. The MXA710 Linear Array Microphone uses this arrangement, so its pickup lobes are shaped like semi-circular arcs or wedges. When the mic is mounted on the ceiling, the pickup lobes can be angled to the left or the right, but not forward or backward. That’s useful in applications where you want to pick up groups of people rather than isolating the voice of one person.
To make a pickup lobe that is directional in both the horizontal and vertical axes -- like the beam of a flashlight -- the transducers need to be spaced apart both horizontally and vertically. That’s why many array microphones (like the MXA920 Ceiling Array Microphone) are either square or round, so the transducers can be spaced apart in two dimensions. This allows the pickup lobe to be tightly focused on one person or just a few people.
By slightly delaying each transducer’s output before they are mixed together, the direction of the pickup lobe can be shifted to point to one side or the other instead of straight down from the microphone. By varying these delays, the pickup lobe can be aimed at a specific part of the room without physically moving the microphone. This allows the microphone array to be mounted where it is practical (without interfering with light fixtures or air vents for example) but still have audio coverage where people are sitting.
The addition of automatic mixing can also allow the microphone array to detect the position of a talker and automatically deploy coverage at the appropriate angle, elevation, and width. Sophisticated processing can determine the difference between a person speaking and a keyboard clicking or a door closing to prevent false activations.
To create multiple pickup lobes, the output from each transducer is split to allow the creation of multiple mixes that can be used to form multiple coverage areas. This allows simultaneous talkers in different parts of the room to be captured without moving one pickup lobe back and forth between them, which would inevitably miss some parts of speech. The MXA901 Conferencing Ceiling Array Microphone and MXA902 Integrated Conferencing Ceiling Array, for example, include Single-Zone Automatic Coverage™ technology where each transducer’s output is split eight ways, enabling the creation of eight simultaneous pickup lobes that can be automatically deployed to completely cover everyone in a 20 by 20 foot (6 by 6 meter) area.
The MXA920 ceiling array mic features Multi-Zone Automatic Coverage™ technology, which allows up to eight distinct coverage areas to be placed over seating areas, a podium, a white board, etc. for more comprehensive coverage. A coverage area can also be muted, which prevents sounds from that zone from being captured (such as a doorway).
Years of development and testing of the acoustic, electronic, mechanical, processing, and software aspects were required to make Microflex® Advance™ array microphones sound natural -- not just in a laboratory but in real rooms with typical acoustics. The Shure Electro-Acoustic Design department is constantly developing new and improved array microphones that work well in a broad set of user applications.
