ANC (Ambient Noise Compensation)

The primary use of Ambient Noise Compensation (ANC) is to automatically adjust the output level of a program or page signal based on the ambient noise level in a room.

Consider a room where a program signal is playing and there is little ambient noise. The people in the room hear the program at a particular loudness with good intelligibility. When the ambient noise in the room rises, for example when several people in the room begin talking, there can be significant loss of intelligibility and the program does not seem to be as loud to the people listening. In the extreme case, when the ambient noise is very loud, there is virtually no intelligibility and people in the room may not even perceive that the program is playing.

In a fixed installation, the ANC serves as the hand on a knob maintaining the correct program signal level. The goal of the ANC is to maintain an appropriate program volume, as perceived by the listeners in the room, regardless of the ambient noise present.

tip: The ANC block should be placed near the end of the signal processing chain, usually immediately before an analog output. Blocks capable of dynamically adjusting program gain – dynamics or Level blocks, for example – must be placed before the ANC block program input in the signal chain. Blocks capable of dynamically affecting the ambient mic(s) audio level should not be used in the ambient noise processing chain. External gain changes can interfere with the noise sensing algorithm.

tip: The ambient sensing microphone should be placed as close as possible to the noise source, and as far away as possible from the program speaker(s) to reduce acoustic coupling. Multiple noise sensing microphones can be mixed together using a Mixer block that feeds the ambient input.

tip: Setup suggestion: when you first set up each ANC block, start with the Rate Control set to a fast or low value such as 2 seconds and a Gain value of 3 to 6 dB. This makes the Program audio level change very quickly and noticeably to even short increases in room noise such as clapping your hands or yelling. This helps clearly demonstrate what the ANC block is doing and provides a great starting point for further parameter adjustments.

note: The ANC block uses a modeling algorithm that allows it to significantly reduce its own program material from the ambient noise microphone (a 20 dB reduction of the program material at the noise microphone is typical). The modeling is done continuously during normal block operation and is stored in the internal DSP’s memory.

tip: A 300 Hz 2nd-order high-pass filter is automatically applied to the ambient noise input and a 3 kHz high-cut is also applied. Additional filtering (PEQ blocks) can be placed upstream of the ambient input to further optimize the ANC response for a particular frequency range, if desired. These filters are on the noise processing chain and are not heard in the program audio.

note: The Detected Noise meter displays the rms level, in dBFS, of the ambient noise. The response time determines the averaging window for the rms level.

How do I relate the Detected Noise meter to a dB SPL reading in the room?Place your SPL measuring device near the ambient sensing microphone but where ears will be listening and take a reading, say 55 dB SPL. Now take the current reading of the Detected Noise meter; for example let’s say it reads -40 dBFS. Thus -40 dBFS is approximately 55 dB SPL. It is only an approximation because most SPL measurements are made with an A- or C-weighted filter curve applied, whereas the ANC Ambient Noise input uses a band-pass filter.