# Acoustical diffusion, scattering, attenuation

1. Nov 7, 2013

### Misophonia

This is killing me; not sure if I'm over-complicating or under-complicating it. I’m trying to understand if there is any difference between acoustic scattering and diffusion, and how that differs from attenuation. When particles collide, a small amount of heat is created, since the collision is not perfectly elastic. This is attenuation, and since the collision is necessary for both attenuation and scattering/diffusion, doesn’t this all equate to the same thing? If you put soft fibrous material on a wall, an incoming sound wave will congest the particles against each other as well as the fibers of the material, attenuating the sound. Congestion seems kinda the opposite of scattering, though if scattering describes when particles deflect off of a straight course, wouldn’t congestion increase scattering? Just asking that question makes me feel stupid… Installing diffusers instead will break up the wave and bounce multiple waves in multiple directions, having two effects: attenuation (and/or scattering?) through collision, and the division of the magnitude of the initial wave into multiple less powerful waves (converting a single cohesive force into ambient noise). I’ve also read that scattering and diffusion are analogous, and that one is simply a larger form of the other. But wouldn’t “diffusion” of light in fog, for example, be analogous to sound diffused into a fibrous material (attenuation/scattering)?

2. Nov 8, 2013

### Bobbywhy

Misophonia, Welcome to Physics Forums!

Acoustic engineers use a vocabulary of technical terms when communicating. Each term has a specific meaning, and every student of acoustics must learn these meanings. This enables the engineer to communicate accurately, both verbally and in writing, while avoiding ambiguities and misunderstandings. Below are several websites that should help with this process.

“Diffusion in simple terms is the scattering of sound energy. When sound bounces off hard flat surfaces, the energy remains very much intact yielding discrete echoes. These echoes will produce destructive effects like comb filtering, standing waves and flutter echoes which degrade speech intelligibility and music clarity. Installing sound diffusers can deal with this problem. Diffusers interrupt discrete echoes by scattering or diffusing sound energy over a wide area without removing it from the room. This maintains sound clarity and improves speech intelligibility.”
http://www.acousticsfirst.com/educational-videos-acoustic-sound-diffusion.htm

“Diffusion, in acoustics and architectural engineering, is the efficacy by which sound energy is spread evenly in a given environment. A perfectly diffusive sound space is one that has certain key acoustic properties which are the same anywhere in the space. A non-diffuse sound space would have considerably different reverberation time as the listener moved around the room. Virtually all spaces are non-diffuse. Spaces which are highly non-diffuse are ones where the acoustic absorption is unevenly distributed around the space, or where two different acoustic volumes are coupled. The diffusiveness of a sound field can be measured by taking reverberation time measurements at a large number of points in the room, then taking the standard deviation on these decay times. Alternately, the spatial distribution of the sound can be examined. Small sound spaces generally have very poor diffusion characteristics at low frequencies due to room modes.”
http://en.wikipedia.org/wiki/Diffusion_(acoustics [Broken])

“Acoustic attenuation is a measure of the energy loss of sound propagation in media. Most media have viscosity, and are therefore not ideal media. When sound propagates in such media, there is always thermal consumption of energy caused by viscosity. For inhomogeneous media, besides media viscosity, acoustic scattering is another main reason for removal of acoustic energy. Acoustic attenuation in a lossy medium plays important role in many scientific researches and engineering fields, such as medical ultrasonography, vibration and noise reduction.”
http://en.wikipedia.org/wiki/Acoustic_attenuation

You wrote this: “If you put soft fibrous material on a wall, an incoming sound wave will congest the particles against each other as well as the fibers of the material, attenuating the sound.” What does “congest” mean? I’ve never heard that term used regarding sound waves before. Can you please supply us your source of this or your reference?

If the above websites do not answer all your questions and erase all your doubts, then return here and post your questions. Members here at Physics Forums are always ready and willing to assist a true searcher.

Bobbywhy

Last edited by a moderator: May 6, 2017
3. Nov 8, 2013

### Misophonia

Well, I suppose sound will ultimately be completely attenuated over time as hardly anything is an ideal medium; diffusion would simply help to expedite the process while making the sound wave less audible by breaking it up. I guess scattering is what I'm most confused by. Is diffusion simply a larger form of scattering? I've heard definitions of scattering, but I can't quite seem to get a mental picture. Does it mean to say that the molecules are not going in a perfectly linear path, bumping into each other (attenuation), but still going in the same general direction? Is this simply a way of describing the manner in which sound is attenuated, such that adding acoustical absorbers increases incidence of scattering, while attenuation is the "measure" of how much is lost through this scattering? Scattering is also the reason for the fact that the wave spreads out as it moves forward right?

“Congestion” is my way of saying that the particles are put into a tighter space; if two people are walking down a narrow hallway, there will be a greater chance of them bumping into each other vs. a wide hallway. I didn’t really know what term to use to describe it, so I used ‘congestion’ because my thinking is that this is sort of like how increasing the pressure on a contained gas causes it to get hotter. In the case of acoustical absorption, I figure that the velocity of the wave is forcing the particles into more confined spaces in between the fibers of an absorber, increasing the incidence of the particles impacting themselves as well as the fibers, which increases pressure and temperature. Or am I way off? The articles never go into enough detail about this.

4. Nov 11, 2013

### Misophonia

I've tryed researching this topic, but all I ever come up with is info about electromagnetic radiation. Any info I do find about it as it relates to acoustics only ever has one or two sentences about it. I just need someone to clarify this for me, and it seems like that's just bound and determined not to happen.

5. Nov 11, 2013

### jartsa

When a lightning strikes, nearby a sharp bang is heard. Far away a long rumble is heard, because sound waves have scattered from various obstacles.

Two bangs may merge into one rumble. Like two smells merge into one, because of diffusion of gas molecules.

Very far away the rumble is really long and the volume below hearing treshold.

Scatter = Bounce
Diffusion = Mixing
Attenuation = 1: sound volume is decreasing or 2: sound wave is losing energy

Absorption = sound becomes sucked into an object, instead of bouncing off from the object

Last edited: Nov 11, 2013
6. Nov 11, 2013

### Bobbywhy

For an explanation of sound waves scattering, see this:

“(b) Wind and Temperature Gradients: The speed that sound propagates in a gas depends on the temperature of the gas. Higher temperatures produce higher speeds of sound. Since the temperature of the atmosphere is not uniform there are local variations in the sound speed. For example, under normal conditions the atmosphere is cooler at higher altitudes. This results in sound waves being 'bent' upwards (see REFRACTION). This will result in the formation of a shadow zone, which is a region in which sound does not penetrate. In reality some sound will enter this zone due to scattering. Scattering occurs when sound waves are propagating through the atmosphere and meet a region of inhomogeneity (a local variation in sound speed or air density) and some of their energy is re-directed into many other directions. In environmental noise situations, scattering is caused by air turbulence, rough surfaces, and obstacles such as trees. The scattering of sound by rain, snow or fog at ordinary frequencies is insignificant.”
http://www.sfu.ca/sonic-studio/handbook/Sound_Propagation.html

For your other two areas of interest, diffusion and attenuation, see these:

http://www.sfu.ca/sonic-studio/handbook/Diffusion.html
http://www.sfu.ca/sonic-studio/handbook/Attenuation.html