Broadside Array Microphone Beam-forming

Broadside array microphone beam-forming utilizes the difference in time delay between signals received at different microphones in the array. As such, the microphones are placed further apart so the information received at each microphone is sufficiently different. The width of a broadside array beam is based on the wavelength of the signal divided by the length of the aperture. So, at low frequencies (longer wavelength), the beam will need to be wider than that of higher frequencies (shorter wavelength).

Due to the need to process the difference in time delay, and the need to capture frequencies between 300Hz to 3.3KHz, the broadside array microphone needs to be at least 30mm apart. This brings about many limitations.

To understand why, please look at figure 1. In this example, the 2 microphones are facing 0 degrees, meaning that the beam center is the y-axis. Now, let’s assume the signal source at point A is playing at the same dB level as the signal source at point B. Let’s also assume that point A and point B are the same distance away from the center of the array. In this case, the signal from source A will be suppressed because the array microphone can obviously detect that source A is outside the beam (time delay to Mic 1 is much longer than time delay to Mic 2). However, the signal from source B will not be suppressed, because to the traditional array microphone, source B is effectively in the middle on the beam, since the difference in time delay is exactly the same to Mic 1 as to Mic 2. This limitation applies to every plane throughout the z-axis, as well as directly behind the array (180 degrees). Thus, the traditional array microphone can only effectively suppress noise in a 2-D manner (in our example, only noise on the xy-plane is canceled). Please see figure 3 for the effective beam.

Figure 1. Traditional Array Microphone Setup

Small Array Microphone (SAM) Beam-forming

SAM beam-forming technology is unlike traditional setups. SAM beam-forming technology uses 1 uni-directional microphone and 1 omni-directional microphone. Since these 2 microphones can be placed right next to each other (no distance limitations), the information coming to both microphones is highly correlated (virtually the same). Consequently, the beam-forming capability relies on the intelligence of Fortemedia’s AMBIN algorithm to decipher this information.

Because microphones of SAM can be placed virtually right next to each other, the effective beam is a 3-D cone shaped beam. This has many advantages compared to the traditional array microphone. To understand the advantages, please refer to Figure 2. In this example, the setup is exactly the same as Figure 1, except the receiving device is a small array microphone instead of the traditional array microphone. To SAM, the signals from source A and source B are exactly the same (in this case, both outside the beam). This applies throughout the y axis, forming a 3-D cone-shaped beam. Noise above, below, and behind the beam is effectively suppressed. Please see Figure 3 for the effective beam.

Figure 2. SAM Setup

Figure 3. Beam Comparison

Related pages:

1. SAM Benefits
2. SAM in Automotive Hands-free Applications
3. SAM in PC/VoIP Applications
4. SAM in Handheld Applications