[Browntrout]

Hello, I am currently designing my listening room (very small). When assembling my stereo I took into consideration the vibrations within the components and the stand and speakers and how they interact. I couple everything indirectly to the floor exept my source which is isolated from everthing else.
  Now I see panels hung on walls, some sat on the floor and others supported on stands. If these are absorbing the energy from my speakers should I couple them to the floor/wall to complete the circle of energy in my system?
  Thanks for any input or suggestions, Ben.

[richidoo]

The absorbtive treatments damp the air vibrations by converting them into heat through the friction generated when air velocity is turbulated by the FG fibers. Much the same way that sorbothane damps mechanical vibrations. The absorbers don't redirect vibrations into the walls, so there's no need to couple them to the floor.  Since you're into vibration mitigation, you might like to read this:

http://www.grandprixaudio.com/idx_design_wp.php

[Browntrout]

 Thanks for the reply Richidoo, I must confess to not really following the idea that sound is turned into heat. I read the link you posted and have come across this approach in audio before but after some thought and a few little experiments I came to the conclusion that the fast conveyance of vibrations into the ground (from speakers, and components) results in the most natural sound due to it altering the nature of the vibrations (speed and frequency) the least (the opposite of the attenuation approach) through allowing free energy movement in the system.
  Now I understand that acoustic panels are a seperate case to be looked at with an open mind and this is why I'm asking. I would be greatfull if anyone out there has noticed a difference between say a wall hung panel and a stand mounted one in the same location. One is almost completely decoupled from the wall (being suspended) and stereo whilst the other is totally decoupled from the wall but could be coupled (through the use of spikes) to the floor and therefore the stereo. Is the 'ideal' for them to be floating in free space?

[Ethan Winer]

I would be greatfull if anyone out there has noticed a difference between say a wall hung panel and a stand mounted one in the same location.

I have many panels on the walls and many more on stands, and I've never noticed a difference. Rich has the right answer - sound is absorbed at the panel by converting the wave energy to heat. At least for conventional "porous" absorbers. Another type of bass trap is based on a resonant membrane. This type works on an opposite principle (wave pressure rather than wave velocity), but the waves are still converted to heat. Even in the scenario you describe, where waves are damped by coupling to something solid, the energy is converted to heat there too.

--Ethan

[Browntrout]

Hello Ethan, thanks for writing, I've been doing some reading as my knowledge of how these things work specifically is limited at the moment. I've enjoyed watching your videos on youtube so thankyou for taking the time to make them.
  As for sound being converted to heat I think that while some may ultimately end up as heat most still remains as sound. If my thinking is correct the material breaks up the wave by dispersion within very small cavities. So where originally in the air there was a unified wave in the material the movement is broken down into many many small movements in many different directions as determined by the physical shape of the material.
  I understand that in doing so the fibres will move, and in their resistance to being moved they change the direction of the air and that very very small amounts of friction between the fibres will result but to say that the sound energy is converted to heat is still, I think, incorrect. I think that the efficiency of converting sound to heat within an acoustic absorber will probably be under 1% efficient as a guess.
  I reckon that the vast majority of the energy stays as kinetic energy, either in moving air in tiny pockets in many different directions within the material or in moving the tiny fibres of the material very small distances.
  Now I know heat is movement, movement of atoms or molecules but for sound to 'break the barrier' between the macro and the micro would require a heat component to be included in the sound. A bit like air being blown at something and saying the friction between the air molecules and the item cause friction and generates heat against hot air being blown on something and it heating up.
 For ambient temperature air to heat something through movemnet requires speeds that are quite incredible, it usually has the effect of cooling things down as opposed to heating things up. But if the air has heat in it (heat energy) it can only require the speed of a hairdryer to heat something up rather quickly. Though this is still not as efficient a method of heat transfer as solid contact, as in a soldering iron.
  I hope you don't mind me disagreeing with you on this, I don't claim to have much understanding of acoustics and still very much appreciate your answer to my question.
  I think the best thing for me to do is have a play and see for myself, that should be quite good fun, something to do over the winter months. I shall try and carry out some experiment to see what interaction an acoustic panel has with a speaker, ultimately does not a well treated room with plenty of absorbtion change the 'Q' of the driver, if these reflections are physical movements then surely dynamic pressure between driver and facing wall must  drop with the addition of absorbtion panels?
  Anyway thanks to both of you for putting up with me and my ramblings, cheers, Ben Anderson.

[Carl V]

have fun with your experimentations.

But keep a couple of principles in mind
Air pressure & Air velocity.....musical waveforms.
At the wall/boundry interfaces you have Pressure
away from boundries you are in the velocity region.
Two differring sets of circumstances requiring 2 different
solutions....related, even interalated but still unique.


What you want & need for 150 HZ is not the same for 1,500Hz
or 15,000 Hz.

And your example of air being blown acoss a surface is fine
it's just not all that applicable for the the small amounts of air
being moved when reproducing a cello or Trumpet via our
Speakers.

[richidoo]

Hi Ben, it's good to see you thinking it through and trying to understand. Way to go! 

You are correct in your assumptions about how sound is converted to heat. There are two ways.   The first way is by turbulating a fluid, you generate heat. You take laminar (frictionless) flow of a fluid and push it through a labyrinth and cause the molecules to change direction. This changing of direction is resisted by at least 2 physics principles, Momentum and Adhesion. Momentum says the air molecules want to keep going in one direction undisturbed. To violate this causes internal friction in the air when air molecules are forced to rub together. Adhesion is the principle that molecules of fluids want to stick together and stick to the solids they contact. Molecular adhesion is described by viscosity, a variable in sound attenuation equations. Adhesion of air to the FG fibers and to other nearby moving air molecules intensifies the turbulence of the air moving past the obstacle.  An example of this effect is a Vitamix blender. You can boil water, or make hot soup by simply running the mixer on high for a few minutes. The friction of the water molecules in turbulence cause it to heat just by the rapid motion and internal friction of the water. This is how sound is attenuated over distance in free air.

The other way that heat is created in the FG when sound is attenuated is by air molecules impacting the fibers (with added effect of adhesion to other airmolecules also trying to get past same fiber) the fibers will move in equal and opposite direction to the deflected air molecules.  The fibers are forced to flex, this causes internal friction inside the glass fibers, which is another way that attenuation is converted into heat.

Both methods are mentioned in this cryptic definition of sound absorption. http://www.britannica.com/EBchecked/topic/555255/sound/63978/Sound-absorption

The levels of sound in a domestic listening room are low. At 90dBA which is a likely a loud PEAK SPL in normal music listening, the amount of power in the sound is 1/1000 Watts immediately adjacent to the sound source. The power is dissipated as it moves into the volume of air surrounding the source, and is attenuated by moving through the air due to friction. Now move back 8 feet from the source and put up a FG panel covering a small percentage of the total interior area of the room. The original .001W is spread out over the entire interior surface. A panel covers 1/50th of the surface and absorbs 60% of 1/50th of .001W on loudest peaks. Thats 12 millionths of a watt being absorbed converted to heat across an 8 square foot area panel.  You are not going to feel any heat being given off from that panel. You might not even find instruments able to measure the heating effect.  Sound in air is extremely low power. But this does not take away from the fact that sound attenuation is ALWAYS the conversion of kinetic energy to heat, whether the sound is moving inside a diamond or air. It doesn't ring forever, because the movement sound is incites internal friction which converts the sound to heat. 
http://en.wikipedia.org/wiki/Sound_power_level

The point is that sound has very low amount of power, so when converted to heat there is nothing to show for it. Kinda like converting energy back into matter, the ratio is extremely small, but it is real.

[Browntrout]

Once again thanks for the great responses. I can see this aspect of home listening being even more involved than the stereo side of things.
 Can I ask a question?
 If the sound at the first reflection point has different characteristics depending upon height from the floor (assuming high frequencies travel from the speaker differently than low frequencies) why are absorbers that are placed there uniform construction from top to bottom?
 Would I be correct in saying that to create equal absorbtion (actually equal reflection) throughout the Y axis this difference should be factored in, as in lessening treble absorbtion as you move down the panel toward the floor?
 Or is this going to result in an incorrect representation of the original (as emitted by the speakers) stereo image or sound through removing  audible dimension from the listening room?
 Thanks again, Ben.

[Ethan Winer]

to say that the sound energy is converted to heat is still, I think, incorrect ... I hope you don't mind me disagreeing with you on this

I never mind anyone disagreeing with me on anything.

But the law of conservation of energy means that when sound waves are "dissipated" they have to go somewhere. So where else would the energy go? Clearly it doesn't just hang around in a state of suspended animation, so to speak.

--Ethan

[richidoo]

Once again thanks for the great responses. I can see this aspect of home listening being even more involved than the stereo side of things.

I agree. Acoustics is the final frontier for the audiophile, and also the largest and potentially the most expensive part of the sound system if the room is appropriately sized and treated to match the performance of the chosen speakers.

If the sound at the first reflection point has different characteristics depending upon height from the floor (assuming high frequencies travel from the speaker differently than low frequencies) why are absorbers that are placed there uniform construction from top to bottom?

A vertical wall only has one point that can reflect sound into the ear. That's the point that needs treatment. But enlarging the area of the absorbers makes a larger region free of first reflections at the listening position.

High frequency sound and low frequency sound travel through the air in the same way. But high frequency sound is attenuated faster (over shorter distance) because it spends more time at high velocity than does low frequency sound, so there is more opportunity to convert it to heat through friction that increases with velocity. At the distances found in a home stereo system, high frequency attenuation is not an issue. It does happen though, but the music mix and speaker design already account for it. It is the perception of smearing caused by phase distortion that is of concern in treating 1st reflection points, not the SPL level.

Many people prefer 1st reflections remain untreated. Treating them does improve clarity, but it can reduce the pleasurable sensation of spaciousness if overdone. It is somewhat dependent on the speaker design, cabinets shape, driver dispersion and crossover alignment which determine how ugly the side reflections will sound.

Moving closer to the speakers and further from the walls will reduce the 1st reflection smearing without removing the side reflection benefits by changing the arrival times of direct vs reflected sound, but you need a large enough room because every speaker has its minimum listening distance. I personally prefer diffusion on sidewalls and ceiling, but it is more expensive than absorbtion. But where the room size and budget dictate absorption, let your taste be the judge. Experiment with absorber size and thickness.

Would I be correct in saying that to create equal absorbtion (actually equal reflection) throughout the Y axis this difference should be factored in, as in lessening treble absorbtion as you move down the panel toward the floor?
 Or is this going to result in an incorrect representation of the original (as emitted by the speakers) stereo image or sound?
 Thanks again, Ben.

Ears are a single point, as is the tweeter, so 1st reflection only happens at one point on the reflecting surface, not all along a vertical line on the sidewall. Even if a line of reflections were a concern, as with line array speakers or stats, the varying distance between tweeter A and tweeter B to the ear by reflection is not large enough to change the amount of free air attenuation.  It might be 12' to one and 12' - 2" to the other. Those 2" will not make any difference in SPL.

Good questions Ben!
Rich

[Ethan Winer]

Ears are a single point, as is the tweeter

Yes, but sound from a source radiates and spreads outward. So it does more or less "splash" an area larger than a single point on the side walls and ceiling (and floor).

--Ethan

[srlaudio]

The only problem with all these scenarios described here is the speaker itself.  Most speakers have abysmal polar plots, that is to say they are not linear with frequency at all axis.  Perhaps this is where the theory of absorption at the early reflection points comes in.  That is, to attenuate a problematic portion of the speaker's polar plot. Transducers (devices that convert one form of energy to another i.e. speakers and microphones) are the most critical components of an audio system.  Proper diligence in the choice of loudspeaker will make a huge difference in the room treatment equation.  All of a sudden the path of the accepted practice will be upended, and true fidelity will dictate what works well where, and old ways of thinking will become moot.  The sound of real instruments are an indication of where speaker design should go, that is energy that goes equally in all directions, and challenges the acoustics designs of any room in a proper form.