Walker, an exhaust manifold company, has developed an active muffler for reducing exhaust noise (Fig. The Lotus system is claimed to be as effective as about 45 kg of sound deadening material. Some manufacturers are now having a much simpler version, capable of switching between hard and soft settings. The greatest improvements are achieved in small vehicles, where the perceived reduction is as much as 80%.Īnother technology to adaptive noise control, developed by Lotus Engineering, is a hydraulic engine mount controlled electronically in response to the engine vibration. A typical layout of an adaptive noise control system is shown is Fig. A quality loudspeaker system is necessary, which is capable of producing up to 40 W RMS per channel. The maximum active noise control can be achieved in about 70 ms. These are played through the in-car speaker system until, through measuring the error signal from the microphones, a minimum noise is achieved. A digital signal processor measures the average sound pressure energy across the cabin and adjusts the phase and amplitude of the anti-noise signals. The system developed by Lotus Engineering uses eight microphones embedded in the vehicle headlining to sample the noise. Considerable low frequency noise is also generated by low profile tyres and harder suspension. There are several critical speeds at which the vehicle displays unpleasant boom. A typical four cylinder engine running between 6 rpm has a firing frequency of about 20 to 200 Hz. The adaptive noise control system requires a high speed digital signal processor and also a detailed understanding of noise generation dynamics in the vehicle. Conventional techniques solve the problem at certain frequencies, but not cover the complete range. The trend of producing lighter vehicles using thinner grade metal further exacerbates the problem. Today most vehicles are susceptible to some low frequency boom in the passenger compartment, even after the use of a large amount of sound deadening. However there is still a demand to reduce noise further, which is becoming ever more expansive. The techniques used for reducing cabin noise levels can range from the use of simple sound-deadening material to special design of engine mountings, exhaust systems and the use of balance shafts on the engine. Low frequency noise (< 200 Hz), causes boom in a vehicle, and hence it becomes very difficult to reduce by conventional methods. Response time to deliver acceptable result. Layout of an adaptrive noise control system. Three signals, the original noise, the anti-phase cancelling waveform and the residual noise.įig. This is because the wide range of noise frequencies produced need fastįig. While the theory is relatively simple, until recently it has not been suitable particularly for motor vehicle use. Figure 30.61 demonstrates the method used where the original noise is picked up by a microphone, inverted and amplified, and then replayed by a suitably located speaker. Figure 30.60 shows three signals, the original noise, the anti-phase cancelling waveform and the residual noise. The principle of adaptive noise control is to cancel out the original source of noise by using sound, which is identical and 180 degrees out of phase, or in anti-phase.
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