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Noise Frequency Analysis

  1. Jul 15, 2010 #1
    I am responsible for investigating excessive noise problems in the cabin of an aluminum fireboat. I have no idea if the problems are with structure borne noise or airborne noise, so here is what I was thinking about doing, please let me know if there are any problems with this method.

    I was going to bring my laptop onto the boat and record the noise in the cabin over the range of RPM’s of the engine using a program like Audacity. I would then use the built in FFT to determine what frequencies these noises are occurring at.

    I would then repeat the same procedure by recording the noise at the engine, and determine the frequencies there. If the two spectra were not comparable I would know that the noise was a structure borne problem.

    Alternatively I guess I could go around looking for vibrations to determine if it is structure borne, however knowing the exact frequency of the problem would be good in order to design a solution.

    One question I have is when recording the noise, in order to get telling results, what quality of microphone would I need? Would the built in mic on my laptop work okay, or would I need a more advanced mic?
  2. jcsd
  3. Jul 15, 2010 #2
    Are you trying to measure the noise or find the source(s) of excessive noise.....

    Open and close the cabin door(s) while listening...you'll probably find the noise level doesn't change much..meaning air is not carrying most of the noise.

    In a boat, it's usually hull and bulkhead vibration that causes most noise and noise power is carried mostly by lower frequencies...typically engine or machinery vibrations are transmitted throughout the boat. And any metal boat carries such energy more effectively than wood or fiberglass....Are engines and machinery such as generators mounted on shock absorbers like rubber engine mounts...or firmly bolted to the engine bed stringers....I'll bet it's the latter....

    The standard way to measure noise is a db meter, the kind many new boat noise reports are based on.

    Are you with the NYFD evaluating their new boats?? they have some 125 footers or so on the way....
  4. Jul 15, 2010 #3
    The theoretical problem with your method is the assumption about the transfer function. If the engine is producing sharp frequency peaks, and you don't find those, then the source is almost certainly not from the engine.

    But is the spectrum is broad, all kinds of things can happen on the way to your cabin, some structures might dampen high frequencies, some might transfer mostly high frequencies, or come into resonance. All of this will transform the spectrum on the way to you. I am not in the noise measuring business though, so I don't know if it will work anyhow.
  5. Jul 15, 2010 #4
    Are there any good ways of pinpointing what part of the structure is transmitting the vibrations resulting is the noise? I have been told the engine and other machinery are mounted of shock absorbers, however I haven't seen this first hand.
  6. Jul 15, 2010 #5
    Just some thoughts... Any sorta OK external electret (one with a battery) mic should do well for first examination. But you may be more interested in very low (under 100Hz) vibrations, and those mics are going to get expensive. You could also try to find contact mics, or even old phono-cartridges, which you can attach to structural members. Straing gauges might go up high enough in fre
  7. Jul 15, 2010 #6
    dang it....continued:

    (strain gauges) might go up high enough in frequency too.

    If it's really acoustic rather than structural you can probably start sticking different
    absorbers on the walls. If it's structural you may need some more strategic bracing to damp out the vibrations.
  8. Jul 18, 2010 #7
    The issues in the original post likely requires some practical experience....so why not call a few boat manufacturers armed with a few questions....

    "Are there any good ways of pinpointing what part of the structure is transmitting the vibrations" I mounted some 12 volt fans in my former Hatteras fiberglass boat with wood bulkheads. Mounting one on a forward stateroom bulkhead created enough vibration noise to be really annoying...more noise than the fan itself...so I ended up taking out the screws and using Velcro....but that bulkhead did not make any noise that could be heard when underway from hull vibration...or maybe it could not be heard over engine noise....lots of lessons in that example....
    I have no practical idea how you'd find a particular resonant frequency in a hull or interior component...the only crude thing I can think off...press you hand or foot against different surfaces and see if the noise level changes...maybe you can dampen the noise inducing vibrations???
    Last edited: Jul 18, 2010
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