Sound Transmission
Halcro -- Thu, 04/03/2008 - 21:43
http://www.audioasylum.com/audio/vinyl/messages/74/743600.html
http://www.audioasylum.com/audio/vinyl/messages/74/743602.html
Please click the links above to see my diagrams of basic Structure-Borne and Air-Borne transmissions in a typical USA timber stud wall situation.
ST1 shows Structure-Borne sound in the Floor being transmitted to the wall.
Structure-Borne sound can be induced by:-
• Footsteps, moving furniture, dropping items
• Air-borne sound within the room or adjoining room being 'absorbed' into the structure.
This Structure-borne sound travels along contiguous structure at various frequencies depending on the materials.
Whilst it travels, it is being progressively absorbed by the structure (depending on material) and converted into 'heat'.
At the same time it is travelling, the vibrations release sound back into the air which becomes Air-Borne sound.
The further away from the source of the sound, the more absorption has occurred until it dissipates.
Some materials are GOOD at absorbing sound (fibreglass, wool) whilst others are POOR absorbers but great TRANSMITTERS (granite, concrete, steel).
Sound will travel FAR within the good transmitters, but will be absorbed quickly within the poor transmitters.
Air-Borne sound can be induced by:-
• Talking, music, barking dogs
• Structure-borne sound being converted to air movement
It requires a lot more energy for Air-borne sound to enter the Structure, than vice-versa because the differential densities may be 50:1.
Like ALL sound propagation, the further from the source, the more absorption can occur.
Structure-Borne Frequencies
High frequencies are more easily absorbed (depending on materials) than Low frequencies.
Thus, the sound that travels the furthest within the Structure tends to be long-wavelength LOW frequencies.
These long-wavelength frequencies are unable to be carried effectively by THIN materials.
Also the frequencies that are happily carried by one material may be UNHAPPY in another material.
This is why turntable manufacturers laminate dissimilar materials to form their platters so that various frequencies are prevented from ‘jumping over’.
Decoupling of brackets and shelves
With this knowledge, it is easy to see that the long-wavelength frequencies in the wall are not HAPPY to travel in the thin metal brackets which are attached to it.
Even if SOME frequencies do ‘cross-over’…..they are then faced with a DIFFERENT material for the SHELF……and the shelf being DENSER than the bracket, will scoff at the minute high-frequencies it sees.
Of course if you really want ULTIMATE decoupling, placing some rubber or neoprene between the bracket and wall is even better.
I have tried to make this as simple as I can, but there are many more issues involved and it is infinitely more complex than my basic description.
However the physics and acoustics are undeniably on the side of mounting the turntable off the wall and NOT on a floor mounted stand.
Thanks for the explanation. If I understand correctly, things that can be used in mounting a turntable to dissipate vibrations:
1. distance from sound source
2. changes in materials (not sure what the difference we're looking for is?
wave propogation speed? mass? resonant frequency?)
3. use of materials that do not resonate at the frequencies where vibration travels long distance (i.e. low freq)
4. mass (I am a bit confused about this, but a=f/m, so all other things being equal I assume this is true)
A corollary of point 4 would seem to be that if you can tie the mounting device to a huge object (like a wall) you really have mass working for you.
5. I wonder if changes in angles matter, e.g. floor (x,y) to wall (z) back to shelf (x,y)?
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