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Natural Science Forum / Physics / Acoustics / January 2004Direct excitation of partition
I believe it is possible to estimate the contribution from a path via one partition, to the sound breakthrough between one room and another, by the following method: (1) Hit the partition in question with a hammer, measure its velocity with an accelerometer, and measure the resulting sound level in the receive room, (2) Generate noise in the source room, measure the level, and measure the resulting velocity on the partition in question. There are obvious differences between the two ways of exciting the partition but apart from that, it seems that it should in principle be possible to find the answer this way. Obviously one would have to do quite a lot of averaging. My question is, has anyone done this, if so does it work, and what practical difficulties I should foresee? I was proposing to measure the various levels with a 1/3 octave sound level meter, I suppose a dual channel FFT might be better but I haven't got one readily available. Tony Woolf > I believe it is possible to estimate the contribution from a path via one > partition, to the sound breakthrough between one room and another, by the [quoted text clipped - 13 lines] > > Tony Woolf I think in theory you are right. Think I might consider using a vibration exciter rather than a hammer. I think it is critical that you measure the velocity on the receiver side of the wall if it is not homogeneous. Then, why do this rather than measure the sound directly. Just a minor thought, Tony you don't say if you are speaking about lighweight partitions or heavy walls. With heavy walls it's mostly easier to play (above coincidence = radiation degree 1 + more stable behavior). With lightweight wall below coincidence it's often more difficult. I played with lightweight steel partitions for the industry, but found it very difficult to use it for subsequent calculations. The differences versus exact spot were very significant. Maybe I don't understand the circumstances correctly, but I assume that you want to check the individual contributions of certain building parts on the emission site to certain individual parts on the immision site? As Noral said a vibration exciter allows continuous noise. If the measurement is relative the absolute exitation value doesn't matter that much. If the immission site building parts are heavy (concrete, masonry etc) then vibration measurements are usefull, otherwise I think that Intensity measurements are the way to go. I once (long ago) compared Intensity measurements with vibration measurements (with a former B&K coleague) on heavy walls in function of defining partial contributions, and that worked perfectly. The same procedure with lightweight partitions don't. But of course if possible, the simplest way as Noral suggested is the way to go. Kind regards Eric > > I believe it is possible to estimate the contribution from a path via one > > partition, to the sound breakthrough between one room and another, by the [quoted text clipped - 21 lines] > velocity on the receiver side of the wall if it is not homogeneous. Then, > why do this rather than measure the sound directly. > Just a minor thought, > > Tony you don't say if you are speaking about lightweight partitions or heavy > walls. > With heavy walls it's mostly easier to play (above coincidence = radiation > degree 1 + more stable behavior). > > With lightweight wall below coincidence it's often more difficult. Thanks, also to Noral. I'm trying to avoid calculating radiation efficiency, that's why I want to hit the partition and measure velocity and resulting SPL. It is actually a floor, and if it turns out to be simple solid concrete I could try calculating the radiation efficiency. I have had moderate success with brick walls below the coincidence frequency using other people's results for radiation efficiency. But I don't like this sort of calculation - too difficult. I prefer to measure if possible. The situation is that there is considerable flanking transmission via air routes (stairways and lift shafts) that could be improved. But to see if the project is feasible, the client wants to find out what is the limiting transmission via the separating floor. There is a lightweight cover to the top of the floor that cannot be removed so all direct access to the floor is from below. I haven't visited the site yet so I don't know the construction - at the moment I am trying to get some ideas of ways to proceed. One of the things I was wondering is how likely I am to be able to get measurable SPL over the frequency range of interest (from about 63 Hz upwards) from hitting the floor, or in practice the ceiling, with a padded hammer. Any experience of this, ladies & gentlemen? I agree that a vibration exciter would be better. However I don't think it's practical in this case as the top of the floor is not directly accessible. Tony Woolf |
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