Appliance Design - August 2008 - (Page 32)

NOISE & VIBRATION CONTROL Fig. 2. In a typical active noise control set up, a reference microphone captures the source noise and transfers it to a system controller, a digital signal processor that generates counter-phase signal and sends it to a speaker that transmits the anti-noise signal to the environment. An error microphone located at the point where noise reduction is required provides information to the controller, allowing it to make corrections and make the reduction signal more accurate. The reference microphone and speaker are typically located in a duct that is used to shape and simplify the unwanted noise source. Duct Noise Reference micro phone Error Micro phone Speaker ANR Controller Calibration UI ducts to allow absorption of the acoustic energy. Since thicker acoustic material inhibits ventilation, this form of noise reduction becomes unacceptable for electronic equipment where heat builds up. Longer ducts and sealing of the noise source creates unwanted resistance for air flow, and restricted air flow adds to the problem of heat dissipation, a critical issue in today’s increasingly powerful electronic equipment. The limitations of passive control with acoustic materials can be overcome, however, by employing an alternative method that uses active noise control. With this alternative, one can generate a virtual barrier for noise without interfering with air flow into and out of the unit. The basic principle behind active noise control has been known for more than 70 years. It involves the production of an opposing sound wave (anti-noise) that will cancel out the unwanted sound. This requires emitting a counter signal that is of the same amplitude, but 180 Deg out of phase from the signal to be cancelled. (See Fig. 1.) While conceptually simple, it can be complex to implement, and the level of attenuation is highly dependent on the accuracy of the system in producing the reductive signal (anti-noise) at the proper amplitude and phase. Refe rence In a typical active noise control set up, a reference microphone captures the source noise and transfers it to a system controller, a digital signal processor that generates counter-phase signal and sends it to a speaker that transmits the anti-noise signal to the environment. (See Fig. 2.) An error microphone located at the point where noise reduction is required provides information to the controller, allowing it to make corrections and make the reduction signal more accurate. The reference microphone and speaker are typically located in a duct that is used to shape and simplify the unwanted noise source. Reducing the complexity of the acoustic field by use of a duct makes it easier for the system to generate the proper noise cancellation Applying the theory of active noise control to the design of actual products poses some challenges. From a practical standpoint, there is a limit on size and weight one can put into a system to reduce noise. In addition, electronic equipment poses design constraints related to air flow due to thermal dissipation requirements. Furthermore, in the real world, one does not typically find the simple, ideal acoustic waves that lend themselves to effective cancellation by mirror waves. There are several challenges to implementing practical active noise control in a pointError Prediction Filter Adaptation block Fig. 3. Silentium’s active noise control solution includes a virtual microphone that eliminates the need for an actual error microphone. to-zone application, where noise originates from a localized source, such as a piece of electronic equipment. One of those is timing, making sure that the unwanted noise signal and its opposite anti-noise signal meet at the same place at the same time. This objective is complicated by a number of factors, including a slight group delay in the dynamic speakers typically used for this purpose. One means for dealing with the timing issue is the use of a prediction filter, a set of mathematical algorithms that predict what the unwanted noise will be at the time the anti-noise signal meets it. Another challenge is echo cancellation. This problem stems from the fact that the speaker generating the anti-noise signal emits it both forward and backward, so the anti-noise signal is picked up by the reference microphone and gets combined with the target unwanted noise signal. This issue can be solved by the creation of an echo cancellation filter within the controller, allowing it to focus only on the unwanted noise signal. Another issue is whether to use feedback control, or feedforward control. With feedback control, the controller attempts to attenuate noise without the use of a reference microphone generating a reference signal. It uses only the error microphone. The problem with this approach is a limited frequency bandwidth of acoustic signals that can be controlled because of speaker dynamics. Feedforward control is generally more effective across a broader bandwidth, but requires the use of both a reference microphone and error microphone. Silentium has overcome these obstacles to practical active noise control by developing a unique scheme, based on real-time, adaptive algorithms running on a digital signal processor. The proprietary algorithms cover a broadband audible spectrum ranging from 32 applianceDESIGN August 2008 www.applianceDESIGN.com http://www.appliancedesign.com

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Appliance Design - August 2008 Contents Editorial Shipments/Forecasts News Watch Plastics Switches Noise & Vibration Control Joining New Products Design Marts Association Report: CEA Advertiser’s Index

Appliance Design - August 2008

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