The rule of thumb for delay and sum beamformers is that a doubling of the number of microphones leads to a gain. This holds there is no interfering signal which is correlated across all microphones. We derive the expected signal to noise plus interference ratio (SNIR) gains for ULA microphones. Consider a far field source impinging N ULA microphones through an anechoic medium as shown in Figure 1:
Figure 1: N ULA microphones
Suppose the signal at each microphone is given as:
where is the desired speech signal, is the direction of arrival (DOA) of the speech signal with respect to the normal to the axis joining all the microphones, is the directional interfering signal, is the DOA of the interfering signal and is zero mean i.i.d noise with variance .
The input SNIR per frequency bin , denoted is given as:
After the delay and sum beamformer, the output becomes:
The output SNIR per frequency bin $w$, denoted $oSNIR(w)$ is given as
The SNR improvement, SNIRI then becomes:
A sample expected SNIRI at a frequency of is shown in Figure 2 below using microphones for and or 20dB. It should be noted that reduces to conventional N fold improvement as shown on the plot. The desired direction is .
Figure 2: SNRI (dB) for different DOA(degrees) and $latx d$
VOCAL Technologies offers custom designed solutions for beamforming with a robust voice activity detector, acoustic echo cancellation and noise suppression. Our custom implementations of such systems are meant to deliver optimum performance for your specific beamforming task. Contact us today to discuss your solution!