Passive, tuned acoustic absorbers, such as Helmholtz resonators (HR) and quarter-wave tubes, are commonly used solutions for abating the low-frequency tonal noise in air induction systems. Since absorption at multiple frequencies is required, multiple absorbers tuned to different frequencies are commonly used. Typically, the large size and multiple numbers of these devices under the hood is a packaging challenge. Also, the lack of acoustic damping narrows their effective bandwidth and creates undesirable side lobes.
Active noise control could address all of the above-mentioned issues. Most active noise control systems use feedforward adaptive algorithms as their controllers. These complex algorithms need fast, powerful digital signal processors to run. To ensure the convergence of the adaptation algorithm, the rate of adaptation should be made slow. This might lower the effectiveness of the controller during the transients, e.g., a fast run up of the engine in an induction or exhaust noise control application.
An alternative to the feedforward active noise cancellation is feedback-based active noise control. Feedback noise control strategies are more straightforward and computationally demanding than adaptive feedforward schemes and thus can be programmed in less expensive micro-controllers rather than digital signal processors. Moreover, contrary to feedforward scheme where the microphone and speaker are located upstream of the air filter and thus subject to the environmental elements, in proposed feedback scheme they are placed downstream of the engine air filter and are well protected.
DEICON has developed an active feedback noise control scheme for air induction systems and demonstrated its effectiveness in a laboratory set up. A number of 2nd order filters programmed in a microcontroller were used to control the engine noise at multiples tones. The effectiveness of the actively controlled system matched or exceeded that of the traditional induction system with multiple passive acoustic absorbers.