Introduction To Broadside Microphone Arrays

Have you ever wondered how your TV or smart speaker can hear your voice clearly, even with background noise like a barking dog? This directional audio is possible because of the arrangement of microphones. In this article, we'll learn about the concept of broadside microphone arrays and their role in MEMS microphones, which enables selective sound capture from a specific source. To illustrate how these arrays work, let's consider the scenario of someone speaking to their TV.

A broadside microphone array is a setup that employs multiple microphones positioned side by side to enhance directionality and achieve a more focused sound capture. This technique, known as beamforming, involves spacing out the microphones strategically to amplify the signal from a specific direction. In this configuration, the microphones are arranged in such a way that their pickup patterns are directed perpendicular to the array's axis. To simplify the explanation, we'll use an example of two microphones.

Frequency and Array Design

In this example, we observe someone speaking to the TV using a broadside microphone array consisting of two microphones that are specifically arranged to achieve directional audio from the front of the TV. The objective in this scenario is for the two microphones to capture the sound waves in perfect synchronization, resulting in a doubling of the amplitude of the desired sound. This means that the sound from our intended direction should be amplified louder than the sounds coming from the right or left side. This is because when the sound signals are identical, adding them together results in a doubling of the amplitude. Just like in mathematics, when you have two of the same elements, such as 1+1, the result is 2. In the case of a broadside microphone array, this means that combining two identical waveforms amplifies them into a single waveform with twice the amplitude. Consequently, while the surrounding environment may seem equally noisy, the microphone array effectively captures a louder wave signal from the front of the TV.

Now, let's explore what happens with directional audio when the sound wave reaches the microphones when they are out of sync. In our example with two microphones arranged for directional capture from the front of the TV, let's imagine the person speaking is now to the left of the TV, rather than in front. When the person speaks to the TV from the left side, the sound waves they create will now reach the left microphone first, followed by the right microphone. This time difference in the arrival of the wavelength causes the microphones to be out of sync, leading them to capture different waveforms. This discrepancy arises due to one microphone picking up the signal at a different time, thereby negating the doubling effect. The equation no longer holds as 1+1 because the waveforms are unequal, resulting in the sound wave to not double in amplification. This serves as evidence that the broadside array works as it can capture a stronger signal from the front of the TV when the microphones are in sync, effectively amplifying the desired sound.

While broadside microphone arrays provide directional audio, they do present certain challenges. When creating a microphone array the spacing between the microphones is an important design consideration that can create frequency-dependent effects. Higher frequencies, such as the chirping of a bird, produce shorter wavelengths, while lower frequencies, like bass, generate longer wavelengths. Microphones placed very close together might capture higher-pitched sounds effectively but struggle to block out lower frequencies from unwanted directions. Conversely, a larger spacing between the microphones may provide directional audio at low-frequency sounds effectively, but encounter difficulties with high-frequency sounds. The broadside microphone array can provide directionality, but only for a limited bandwidth of sounds. Where in the audio spectrum the array provides directionality is dependent on the chosen spacing between the two microphones. This challenge highlights the importance of carefully selecting the setup for a broadside microphone array, depending on which frequencies require effective beamforming. Should the focus be on capturing low frequencies, mid frequencies, or high frequencies?

To explore how spacing influences directionality in microphone arrays, you can conduct experiments using the interactive chart below. Simply adjust the slider to modify the distance between two omnidirectional microphones in a broadside configuration. As you make changes, the chart will dynamically display visualized polar plots, illustrating the impact on directionality across various frequencies. Take note as you modify the spacing, noticing the enhancement in low-frequency directionality as it potentially affects high-frequency performance, and vice versa. The goal is to find the ideal spacing that ensures exceptional directionality across the entire frequency spectrum. You may find this difficult!