ggovind@uceng.UC.EDU (Girish Govind) (06/12/90)
I think there seems to be some confusion between the terms Noise cancellation and Active Noise Control (or Acoustic Noise Control). These are different, Noise cancellation (in its use in this area) is the term used for electrical cancellation, while Active Noise Control is when one tries to cancel the noise by acoustic means (generate noise which is 180 out of phase etc.). I think what Gary was referring to was Acoustic Noise Control and I will share what I know about it and give a few references. I have lots more if someone is interested.... Active noise control concerns itself with the generation of "anti-noise" to acoustically cancel an existing or otherwise uncontrollable unwanted noise. Examples of these are like in ships, aircrafts, industries where one has no control over engine noise (or some cooling fan noise) etc. The scenario is similar to that of Adaptive Noise cancellation except that one has to consider the transfer function of the propagation of anti-noise in addition to that of the noise. Two microphones are used, one that picks up only noise (placed as close to the unwanted noise source as possible) and one in the place where cancellation is desired. The Microphone 1 provides teh input to the adaptive filter and the signal from Microphone 2 is used as the error signal to adapt the adaptive filter. The output of the adaptive filter is fed into a speaker which generates "anti-noise". Now people must be wondering here why one needs adaptive well the problem here is that the acoustics will change all the time. To use it in a room for instance it will change when people move around, when the door is opened. Well anyway, the adaptive filter is used to identify a composite transfer function of the sound propagation paths between the noise source, the loudspeaker (genarating anti-noise), and the two microphones. Issues particular to active noise control (and absent in noise cancellation): 1) Cancellation is primarily done only at the location of microphone 2. Surrounding region cancellation depends on the geometry of the area and noise frequency. 2) The microphone 1 (which is supposed to only pick up noise) may pick up some of the anti-noise which makes it more difficult as then there is correlation between them. 3) To increase the area where cancellation is achieved one may try using multi-microphone, multi-speaker cases but then anti-noise from one speaker will be fed into the microphone of another such system affecting the anti-noise from that system so basically all these systems are coupled now. 4) Proximity of microphone 1 to the noise source is very important (because of issue 2 above) 5) Cancellation is IMPORTANT -- one cannot afford addition to the noise at any cost so closed loop system MUST be stable. 6) The noise generating source must be small as compared to the wavelength of noise it generates and the cancelling speakers should be located near (Lambda/3 to Lambda/4) the source to achieve good cancellation. 7) Due to Issue 6 one can clearly see that Lambda has to be large and typically thus most places where noise control is tried is where noise is < 400 Hz. 8) The anti-noise source must emit signals prior to the noise generator if destructive interference is to occur near the noise generator (as required by Issue 6) so the adaptive filter has to sort of predict the noise ahead of time and cancel it. The noise control system has also been used in cases where there is too much vibration due to a machine. There people use actuators to cancel out such vibrations which may affect the precision of work in other machines. Other examples that come to mind is the one used by Dick Rutan in the Voyager flight -- these were headphones made by Bose (this came out in an old Time Magazine). And it has been used for active sound attenuation in a duct. Gary mentioned about noise control in a car to cancel out road noise and engine noise (apparently an old 60's model used to increase the radio volume when the car used to go on the highway etc.!!). But I would think that it would be a pretty good idea considering the fact that one already has two/four speakers in the car and this signal can be added to the signal from your usual radio station! Well the Lexus commercial talks about cutting out engine noise and radio noise and "making your own noise", wonder if they use it! Guess you people would want to cut out all this noise that I am putting on here! But let me just give some references "Active Attenuation of Noise -- State of the Art" by G Warnaka (Noise Control Engineering, May/June 1982) "Selection and Application of an IIR Adaptive filter for use in active sound attenuation" by L.J. Eriksson et al (IEEE Trans. ASSP, April 87) Oh well, Now back to my regularly scheduled programming ..... Girish Govind (ggovind@uceng.uc.edu) Mail Location 30, Dept. of ECE University of Cincinnati Cincinnati, OH 45221-0030 ---------------------------------------------------------------------------- **************************************************************************** Why is a graduate student kind of like a mushroom? Well he is kept in the dark and fed shit..... ****************************************************************************