Several important microphone specifications include: Signal-to-Noise Ratio (SNR), Acoustic Overload Point (AOP), Total Harmonic Distortion (THD), cutoff frequency, sensitivity, phase response, group delay, and current consumption.
When it comes to designing noise cancellation in headphones, although all of these specifications are relevant, the key parameters that influence microphone selection are: SNR, AOP, cutoff frequency, phase response, group delay, and current consumption. The variation in cutoff frequency and phase response between microphones also plays a critical role.
Signal-to-Noise Ratio (SNR):
The microphone's inherent noise floor must be lower than the surrounding ambient noise to reliably capture external noise signals. In quiet environments, a high-SNR microphone is essential. A higher SNR allows noise reduction algorithms to function more effectively.
For example, a microphone with a 50dB SNR has a self-noise level of 44dBSPL. If the ambient noise is below this, the microphone's own noise overshadows the environment, making effective noise cancellation nearly impossible. A 70dB SNR mic has a self-noise level of 24dBSPL, which is quieter than most real-world environments and close to an anechoic chamber, making algorithmic noise cancellation much easier.
Acoustic Overload Point (AOP):
When positioned close to a speaker, a microphone may receive very high sound pressure levels-sometimes enough to exceed its AOP, leading to distortion over 10% or more. This distorts the signal and makes accurate noise cancellation difficult.
It's important to analyze the mic's THD versus SPL curve and select microphones with distortion below 1%. A high AOP ensures the microphone can handle loud environments without introducing significant distortion.
Cutoff Frequency:
A microphone should have a low cutoff frequency (30Hz or lower) to reliably capture low-frequency noise. If the cutoff frequency is too high, the ANC (Active Noise Cancellation) system may fail to suppress deep bass noise effectively.
Inconsistent cutoff frequencies between multiple microphones can create significant challenges for ANC algorithm implementation and lead to poor or inconsistent low-frequency cancellation. Tight tolerances are required.
Phase Response:
Phase response represents how the phase of different frequencies shifts across the microphone's audio band. The phase response curve shows how the microphone handles the timing relationships between different frequency components in the signal.
Group Delay:
Group delay is the frequency-dependent delay of the microphone, derived from the phase response. It indicates how long it takes for different frequency components to travel from the acoustic input to the electronic output.
Minimizing and stabilizing group delay across all frequencies is essential to avoid phase distortion in the output signal. If the captured noise signal is already distorted, effective cancellation becomes impossible. Therefore, tight control of cutoff frequency and phase response is critical for optimal ANC performance.
Current Consumption:
Current consumption is a key factor, especially for always-on and battery-powered headphone applications. A sleep or standby mode is needed to reduce power draw and extend battery life.
Microphone power consumption often varies with the operating clock frequency. Some datasheets clearly specify how lower clock frequencies reduce power consumption-this must be balanced with performance needs.
