3. Psychoacoustic aspects of active control

The first indicator that has been used to measure the effectiveness of active control is the attenuation, in dB, of primary noise. Unfortunately, this reduction in the physical noise level does not translate into an equivalent reduction in the sound level perceived by a listener. From a subjective point of view, attenuations are often less than the physical measurements would suggest. Using dBA to try and assess the reduction in perceived level does little to improve matters:

• the shape of the frequency weighting curve that defines dBA underestimates the contribution of low frequencies to the sound level (figure 25 ). As the effective range of active control is precisely located in these low frequencies, the calculation of attenuation in dBA will therefore...