"Jimmy" wrote in message
...
On Sat, 8 Jan 2005 00:07:34 -0000, "andrewpreece"
wrote:
"Jimmy" wrote in message
...
On Fri, 7 Jan 2005 17:03:15 -0000, "andrewpreece"
wrote:
Strictly speaking, whether packing the gap wth rockwool is good or
bad
depends on whether more sound is transmitted by the rockwool, or
whether
more is soaked up by it as the sound first goes through it the first
time,
then reverberates in the gap between the party wall and the
plasterboard.
There are two ways to kill sound, reflection and absorption and I
daresay it
would be better to use both than just one or the other. I suspect
that
you
could hedge your bets somewhat by using rockwool but keeping it out
of
contact with the plasterboard ( or party wall ). The whole subject
of
sound
reduction is a bit difficult, and the best you can do is adopt every
precaution.
The previous poster is right about the studwork needing to be
decoupled
from
the party wall, else it acts as a short circuit for sound to cross
the
airgap. Quite how you go about that is another thing. I've heard
talk of
resilient mounts or affixing the stud work to the floor and ceiling
only, or
a combination of methods. Then there's the question of how you seal
the
plasterboard to the ceiling/walls/floor around its perimeter: leave
a
gap
and you have a leakage path, fix the plasterboard solidly and you
have a
sound 'short-circuit' of sorts! Good luck,
Andy.
Thanks to you and BigWallop for the comments,
Yes, I'm no expert on soundproofing, but I've notice when reading the
Building Regulations about methods of soundproofing ceilings, using a
second suspended ceiling attached at the walls but not touching the
existing ceiling, they rocommend using rockwool or similar in the
space between the old ceiling and the new one. They say it doesn't
matter if the rockwool is touching both surfaces, as long as it's not
packed tight - and that sounds about right to me.
Regarding doing a similar setup to sound-insulate a party wall, I am
thinking of using a timber frame (probably made of 2x2, fixed to the
floor, the ceiling and to the walls at each end of the room that are
90 degrees to the party wall. OK, there is bound to be a little bit of
soundwave transmission through the fixing points, but due to the
weight of the double skin of plasterboard, and the rockwool acting as
damping, I can't envisage much of that vibration acting on the double
layer of plasterboard, turning it into a giant soundboard. If the
resulting soundproofing is insufficient, another layer of plasterboard
can be added. In fact, come to think of it, it would make sense to
start off with a single skin of 12mm plasterboad and see if that's
suficient. If not, add another layer.
Going back to the question of whether or not to let the rockwool touch
both surfaces, I suspect it may be better to let it touch both
surfaces because it will then act as a vibration damper. I really
can't imagine that the rockwool itself will conduct sound waves into
the plasterboard unless it was packed very tightly indeed. As I said,
I'm not an expert. These are just my impressions based on real life
experience of how sound transmission behaves and from reading a few
things on the subject. Comments welcome.
J
I think you are right about the rockwool not transmitting too much sound
if
it is loose: it has very little stiffness when loose, and its mass is
very
little anyway so that sounds OK. You may also be right about it damping
the
plasterboard a little, as the plasterboard will resonate slightly as the
sound strikes it. I have located an audio book, and there is a section
in it
on sound reduction. The subject is complicated, with stiffness of walls
dominating at low frequency, then there's a resonant region, then
there's
the mass-controlled region, then there's the coincincidence controlled
region, none of which is any use to you, but I will reproduce a littlte
of
the more practical advice:-
"Cavity Walls: ...the springiness of the air between the cavities reacts
with the mass of the leaves to produce resonance. The resoant frequency
should be below 100 Hz which is achieved by a wide cavity ( 1m or more )
or
leaves of large mass, or boh.....these resonances can be reduced by
introducing an absorbent lining such as fibreglass into the cavity. It
is
especially useful where practical considerations limit the thickness of
the
wall thus raising the lower resonance above 100 Hz.
The sound reduction of a cavity wall is limited by the necessary
edge,
tie and footing connections,as these form leakage paths to conduct sound
between the leaves. Any practical cnsideration which can miimise or even
eliminate these will greatly improve the sound reduction index"
"Flanking Walls: As these conduct sound into the receiving area, there
is
lttle point in improving the insulation of the intervening wall beyond a
certan level unless the flanking walls can also be isolated
structurally"
There is more info on damping, floating floors, ceilings, doors,
windows,airconditioning vents etc that I could provide on request,
cheers,
Andy.
Thank you for posting that. Strangely enough, I learned a lot about
sound by studying loudspeaker eclosure design technique. Your passage
above touches on the subject of resonance. Where any two parallel
surfaces are involved (speaker cabinet or guitar soundbox or possibly
even a cavity wall) there will be a resonant frequency where a small
frequency range will be amplified. I'm not sure how relevent this is
to cavity walls. probably 'not very' if rockwool is loosely packed
into the cavity.
J
Yes, I missed one sentence out of the excerpt ..."In addition there is a
cavity resonance at about 250Hz which reduces sound insulation": this
followed the last sentence I reproduced in the 'cavity walls' paragraph.
I'm of an electronic background, and very similar effects occur with radio
signals, light or electrical waves going down cables. I would think that for
sound a resonance would occur where the gap between cavities is 1/2 a
wavelegth, as any sound wave of that frequency that reverberates back and
forth in a cavity will build up, rather than cancel out. Probably there will
be resonances at multiples of 1/2 wavelength as well, i.e. 1, 1.5, 2 etc. If
your cavity is 2inches and the speed of sound in air is 335m/s then I reckon
your lowest resonant frequency will be 3350 Hz! LOL, that doesn't tie in
with what the book says! Going with the stuff they talked about earlier viz
a 1m cavity and a resonance below 100Hz, I don't think they are talking
about standard size cavity walls, or 2" studwork.
None of which is very helpful, I suspect most of the noise you want to
eliminate will be below 1 or 2kHz anyway,
Andy.
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