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WADEAL



Joined: 02/05/08
Posts: 278
why do bass frequencies travel easily thru things?
      #647467 - 16/08/08 08:21 PM
I have been reading Hunter Scott Starks excellent book "Live Sound Reinforcement" and in it he makes the comment that low frequency sounds travel around things easier. He also states that the low frequencies also travel through things easier than high frequencies. Indeed, Environmental Health Officers and anyone who has complained about their neighbours' drum and bass, or reggae parties will no doubt confirm that all you can hear through the wall will be the dull thud of the bass end and that not much top end will be present when listening from the other side of a wall.

Can anyone explain in simple terms why this is the case? Why do bass frequencies pass through barriers easier than high frequencies? I can see why a lower frequency sound (which will have a longer wavelength), can reach further round a corner, but not why two equally loud high and low frequency sounds travel differently through a solid object.

p.s. I only managed "O" level physics, so a simple response would be appreciated.

Thanks

WAL


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Adam Peters



Joined: 14/02/08
Posts: 14
Loc: London
Re: why do bass frequencies travel easily thru things? new [Re: WADEAL]
      #647502 - 17/08/08 08:24 AM
You're on the right lines regarding the wavelength of the waveform. With longer waveforms there's more power, this about the meters on a mixer - it's takes a lot less bass to get them to peak than any other frequency. It also depends on the thickness of the object/wall. A good approximation is that you need something at least a quarter of the wavelength thick to absorb sound at that frequency.

Eg,

An 80Hz wavelength is approximately (off the top of my head) about 3.6 meters long. Therefore to absorb 80Hz you would need an object 0.9meters thick.

It then all depends on how the wall/surface is built - if there are any holes or vents then the sound will escape through that - think about the difference between double glazing being opened 0.5cm and shut - loads of sound comes through when it's only opened a tiny bit.

Then there's the material's absorption co-efficient - different materials absorb different frequencies at different rates.

Hope this helps a little.

There's hundreds of books out there that go into way more depth, but it's a field that people make a career out of so you can see the scope!

--------------------
Professional Audio Company Ltd - Specialists in Live & Location Audio
http://www.professionalaudiocompany.co.uk


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Hugh RobjohnsAdministrator
SOS Technical Editor


Joined: 25/07/03
Posts: 22131
Loc: Worcestershire
Re: why do bass frequencies travel easily thru things? new [Re: WADEAL]
      #647522 - 17/08/08 09:41 AM
Firstly, and painting here with a very broad brush, sound waves will diffract around an object if the object's dimensions are similar to or smaller than the wavelength of the sound. This is basic wave theory and you'll finds lots about this aspect of it on the web or in GCSE/A-level physics books.

As for the bass coming through walls, it's about the ability of a material to absorb and reflect sound energy. And that depends on its mass, deminensions, and construction as well as the actual material in question.

Added to all this is the fact that in most natural sounds (and most music), there is considerably more bass energy than treble energy, so you have more bass to control than treble anyway.

Hugh

--------------------
Technical Editor, Sound On Sound


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