Natural Science Forum / Physics / Acoustics / September 2006

I'd welcome some advice on this noise problem.

The man in the flat above me has breathing problems and has to use
something called an 'oxygen concentrator'. Every few seconds, it makes
a sort of thudding sound. To give you an idea of the frequency of the
sound it produces, I'll give the size of the machine: it's about 1ft
6ins tall, 1ft wide and about 1ft deep, and the outer case is tough
plastic.

Because of the noise it makes, he himself sleeps in the living-room,
with the machine in his bedroom - which is above my bedroom. There is
nowhere else in his flat where he can put it.

We both thought about how to stop the sound coming through my ceiling,
and I hit upon putting a wooden board, 2ft x 4ft x 0.5inches, on the
double bed in that room, and putting the machine on top of this, with a
carpet tile between the machine and the board.

I'd hoped that, with the bed between the machine and the floor, the
sound would be stopped completely, but unfortunately, I can still hear
the sound coming through my ceiling. The sound it now *very* slight -
I'd say only about 5-10% of what it was, but it's so regular, I can't
help but listen out for it, like a dripping tap, and it's driving me
crazy!

So I'm writing here to ask if anyone here can suggest how to get rid of
that last bit of sound. The machine itself actually doesn't make that
much noise when you're in the same room as it is - it's only in my
bedroom that the slight clicking/tapping it makes becomes something
more like 'thudding'.

The machine is so quiet that I can't understand why putting it on the
bed hasn't stopped the sound completely - I was wondering if perhaps
the board I've put it on is acting as a sounding-board. If this is the
case, would a smaller piece of board be better? (but it can't be too
small, or else the machine won't balance on the mattress). Or a thicker
piece of board than 0.5 inches?

My other idea was to cover the whole board with thick foam rubber. One
problem here is that the machine has an air inlet underneath it, so
putting it on a very soft surface would result in it sinking and
blocking this intake. I was thinking of the sort of rubber mats they
have in gyms.

Though I don't know much about acoustics, my guess is that it's because
the noise is quite low in frequency that it's so difficult to block (it
sounds a bit like someone tapping their finger  softly on a table top,
to give you some idea of the frequency).

So, I'd greatly appreciate any advice anyone here can give me. The
sound is now so slight that I can only just hear it, so it might be
possible to remove it altogether. I hope so - I'm having to wear foam
earplugs to get to sleep, and I hate living like this.

Thanks in advance

Brian