 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
Natural Science Forum / Physics / Acoustics / September 2004Active Noise Cancellation for Boat Cabin
Does anyone know an any successful applications of active noise cancellation in a boat cabin to reduce engine noise. The boat in question has a rectangular 24' x 40' cabin with two diesels in an engine room below. The floor is steel. I realize that this may be too complex a situation for ANC, but thought that I would ask. Assuming that most of the vibration is coming directly through the floor, I thought is might be possible to affix drivers below the floor to cancel vibrations in the floor. Is any work being done on such a large scale? Mark, Lord Corporation has been successful using active vibration transducers to reduce noise and vibration in passenger aircraft. Main problem in your case is that someone will have to do system development. > Does anyone know an any successful applications of active noise > cancellation in a boat cabin to reduce engine noise. The boat in [quoted text clipped - 5 lines] > to cancel vibrations in the floor. Is any work being done on such a > large scale? "Noral Stewart" wrote in > Mark, > [quoted text clipped - 10 lines] > > to cancel vibrations in the floor. Is any work being done on such a > > large scale? Noral It has been my understanding that active noise cancellation really only worked in spaces where the noise being reduced was small, such as noise cancelling headphones and A/C duct noise reduction. I thought that it worked in small spaces because there a realtively few modes and thus the small number of modes can be cancelled. I have doubts that it works in a space as big as a ship cabin. Anyway I am not any sort of expert on ANC but would like to hear from someone who is. In terms of reducing cabin noise levels, I think it would be easier to pursue a 1/3 octave analysis of the cabin noise and look at reducing the structure borne and air borne noise transmission by modifying the structure. Regards, Tom Harper > "Noral Stewart" wrote in > [quoted text clipped - 25 lines] > Anyway I am not any sort of expert on ANC but would like to hear from > someone who is. Not an expert, but I have developed a couple of automotive installations, including a non modal (free field) cancellation system. There is no intrinsic problem with a boat cabin, of that size, so long as you can get a well dispersed array of microphones in there, and enough, sufficiently powerful, loudspeakers. There are at least 3 weasel words or phrases in the preceeding paragraph. > In terms of reducing cabin noise levels, I think it would be easier to > pursue a 1/3 octave analysis of the cabin noise and look at reducing > the structure borne and air borne noise transmission by modifying the > structure. Agreed. Conventional noise reduction techniques are invariably cheaper and usually have a less restricted frequency range. In your case, at a guess, consider soft engine mounts to a heavy subframe, in itself isolated from the floor, and then a heavy airtight box around each engine. All hard paths from the engine to the structure of the ship may need to be broken. Engine noise reduction is a field with a long history, quite why ship builders (and submarine builders) feel that the rules don't apply to them is beyond my understanding. Cheers Greg > Not an expert, but I have developed a couple of automotive > installations, including a non modal (free field) cancellation system. > There is no intrinsic problem with a boat cabin, of that size, so long > as you can get a well dispersed array of microphones in there, and > enough, sufficiently powerful, loudspeakers. Can you share with us the frequency range and attenuation amount? For instance, as far as aircraft cabin noise is concerned, noise in the low frequency range from 200 Hz down to perhaps 60 Hz is the most disturbing. In this range, ANC earmuffs work pretty well. Above 500 Hz, ANC is compromised by phase shift between the mic, speaker and ear, but the hard shell of any noise reducing headsets is adequate. In aircraft, and I suspect boat cabins just as well, the weight of walls and windows is important to minimize, so it is important to minimize the transfer of vibrations in the audio frequency range, as well as to minimize the emission of sound from the propulsion system in that same audio range. In boats, acceptable mufflers are reasonably effective, but engine and gearbox vibrations will dominate unless special and appropriate vibration isolation (audio frequency vibration isolation) measures are applied. You have to perform an informed Survey, investigating for the airborne sound modes separately from sound transfer via structure borne vibrations. For example: I did such an investigation several years ago in small aircraft. From the specific study, I concluded that the primary source of cabin noise therein was exhaust sound out of the tailpipe opening under the cabin floor. By means of an accelerometer survey in flight, I determined that audible sound transfer to the cabin via structure borne vibrations was negligible. To isolate the exhaust sound from the cabin, to evaluate the propeller noise transfer, I had them duct the exhaust to the aircraft tail. In the mode, the cabin quieted to be just below 85 dBA. This was reported at a Noise Con meeting in the early 1980's. The propeller noise, after all, is emitted primarily into the plane of the propeller disk, and relatively little noise along the the rotation axial direction, where the cabin of a single engine aircraft is usually located. This fact is not widely circulated, nor is the fact that the exhaust noise is dominant in single engine aircraft cabins. The industry has yet to appreciate this (20 years after I demonstrated it). My solution is to wear ANC muffs when flying single engine aircraft. So for a boat cabin, you need to perform the same sort of informed survey. Angelo Campanella >> Not an expert, but I have developed a couple of automotive >> installations, including a non modal (free field) cancellation system. [quoted text clipped - 3 lines] > > Can you share with us the frequency range and attenuation amount? At the time we were limited by the speed of the DSP hardware that was affordable, in context. This limited us to 200 Hz maximum, with a maximum of three simultaneous harmonics of the trigger signal. The lower frequency limit is set by the audio hardware - but in practice 30 Hz is achievable with nothing very elaborate in the way of speakers and amps. The first 'cavity' mode of a car's cabin is at around 50 Hz, typically, but there are very annoying resonances in the exhaust system at around 30 Hz. The best attenuation I remember seeing was around 15 dB, at any one frequency, but in a car this was not a practical solution, since it is more important to get a good first guess, rather than a well optimised final reduction. In practice we'd get 5 dB or so off all the peaks in a second gear wide open throttle run. In overall dBA the results were less impressive, obviously. Typical slew rates would be around 30 Hz/second. I did start investigating the distortion (or added noise, more generally) we got as a result of these high slew rates, but left that company before I got to any meaningful results. > For instance, as far as aircraft cabin noise is concerned, noise in the > low frequency range from 200 Hz down to perhaps 60 Hz is the most > disturbing. In this range, ANC earmuffs work pretty well. Above 500 Hz, > ANC is compromised by phase shift between the mic, speaker and ear, but > the hard shell of any noise reducing headsets is adequate. Yes, close coupled solutions are a better bet where they are acceptable to the customer. > You have to perform an informed Survey, investigating for the airborne > sound modes separately from sound transfer via structure borne vibrations. We used to do a survey of the acoustic modes of the cabin, and used the results of that to help place the mics and speakers. We did not do a noise path analysis, as such. Nowadays I would. Perhaps one of the more interesting results was that generally when tuning a system we needed several hundred watts of amplifiers, and subwoofers to match, but a correctly optimised system would cancel just fine with just 20 W of amps, and typical 6 inch speaker cones. Cheers Greg Locock |
Reply to this Message |
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2009 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.