A sound engineering question...

[finch127]

So here is a problem...

My brother and I were having an argument today, and we have a question:

Is hollow concrete block better at blocking low frequency noises (jet airplane noise) or is poured concrete block better? I think that the hollow block is better because there is more dead air space and while it has a lower STC (a 52 vs a 55) it will block the low frequency rumble of an airplane better. What I think, is that the hollow block is like a mini isolation room, with the sound energy travelling into the concrete, into air, and then back into concrete, whilst the poured block is simply air into concrete into air again. The STC is lower but again that is an average over a wide range of frequencies and my only concern is the low end. If it were let's say, the high pitched whine of a police siren, I would concede that poured block is much better because of its density and mass.

Also, is the difference in STC perceivable? I was reading and while it is a logarithmic scale, some researchers report hearing similar things at STCs 3 numbers apart so while one is 1000 times more lossy, the difference is imperceivable? I also think that the poured block has a much smaller range of frequencies that it passes rather than the hollow block, so the human ear comes into play in that the loudest of all frequencies passed will be the only one heard vs. similar bands. From my reading I conclude that poured block has a higher transmission loss but it does not absorb that sound and therefore lets it through. Hollow block may let in more sound but it does absorb more because it is more porous from what I think, and this is akin to a double paned window's sound blocking ability compared to a single paned window.

I am grateful to any of you who reply for your time. I need an expert to help clear this up and either lose or rub it in my brother's face Very Happy

Thanks Again!

[hansdown]

Hi finch127.

I'm no expert, but the folks who wrote the" Noise control manual for the city of Vancouver" are.
http://www.google.ca/search?q=Is+hollow+concrete+block+better+at+blocki ng+low+frequency+noises+(jet+airplane+noise)+or+is +poured+concrete+block+better%3F&ie=utf-8&oe=utf-8&aq=t&rls=com.ubuntu:en-US:unofficial&client=firefox-a
I haven't had any luck uploading PDFs to the forum, so I'll give you a link.

Hope it helps.

[finch127]

Say thanks for that, I have an idea for an arugment then:

Because both poured concrete and regular concrete blocks have similar transmission rates for sound, and poured is worse at absorbing the sound (less pores and air space) it would have a higher perceived loudness of sound. Hollow block would let slightly more sound in but would do a better job of absorbing the available sound because of its porous nature. Therefore the difference is negligible. Any gains in sound blockage afforded by poured block are negated by its inability to absorb frequencies.

[tgalati4]

The answer to your question depends on the dimensions of the walls and what standing frequencies would be excited. Low frequency rumbles (below 20 Hz) can cause everything to shake. Filled block has more mass and is therefore more resistent to this shaking. Mass-loaded vinyl over either type of block will help.

Some sophisticated soundproofing uses many layers of vinyl and sheet rock to reduce sound penetration. Double sheetrock works in a pinch over either filled or unfilled block.

Only testing will really tell. There are several sound estimate programs available, but they are only as good as the detailed measurements of the materials used.

Get a high-quality microphone and Real Time Analyzer (could be software-based) and measure the frequency distribution of several types of A/C that fly over head. Pick out the common frequency bands that are excited in your space and design the sound abatement to those frequencies.

It's a lot of guesswork and trial-and-error, but the better measurements you have, the better you can design a habitable space. Don't forget to triple-glaze your windows.