# Sound insulation, absorption and diffusion. Can they be part of the same process when wanting to both soundproof and acoustically treat a room?

I understand that insulation, absorption and diffusion in acoustics are different concepts. Notwithstanding, would it be correct to assume they may be part of the same process when all are used to achieve 'sound proofing' and 'acoustic treatment' in a certain scenario?

To put it differently, is it correct to say that by treating the sound within a room (with proper absorption and diffusion techniques) we can limit (up to a certain extent) the pressure of certain sound waves that will exit the room?

More specifically, I would think that by treating a room with high absorption coefficients, a certain amount of sound energy will be transformed into heat energy (for instance using mineral wool). Shouldn't this mean that less energy will 'get out' of the room? Also, shouldn't this mean that we could potentially plan a room's construction with both sound proofing and acoustic treatment at the same time, without them being completely separate activities at the design stage?

Therefore, in the aforementioned example could we not say sound insulation, diffusion and absorption may be part of the same process instead of being treated separately. When a room's structure (both inner and outer) is defined can't we limit sound transmission to a certain degree by adding non parallel walls for instance, thus decreasing vibrations?

• The phrase "part of the same process" is very vaguely defined in your question. – Paul Feb 4 '16 at 16:44
• In general, almost any factor you wish can be part of a process or considered in a design. This question is overall very vague and unclear. – DLS3141 Feb 4 '16 at 17:16
• I define process as the activities entailed when planning and implementing a solution for a (or several) given problem. I was wondering whether It is feasible (and efficient) to 'plan' both sound proofing and acoustic treatment in a space at same time, instead of first having to sound proof and then acoustically treat the space. Are there models that can allow us to come up with solutions that synergistically work together to achieve the aforementioned goals. Does this help clarify the question? Thank you. – Drumagog Feb 4 '16 at 18:01

## 2 Answers

This is a very good question and is in fact subject to in-depth research.

The factors one has to take into account involve differential acoustability (a fairly newly constructed term in order to address the various changes that occur when synergistic processes in acoustics take place).

One has to consider the net rate of absorption, diffusion and reflection per square metre (this is a definition similar to intensity). One then would have to treat the average density of a room as roughly uniform, and then partition it by assigning values from a continuum ranging from 0 to

(Net rate) * (Average power of sound output) * (differential acoustability)


This then will allow one to get a spatial-temporal function of density-like quantities to be Fourier-transformed in order to get the average rate.

There is a fairly comprehensive analysis on the matter published here:

http://www.multi-science.co.uk/ba-room_acoustics.htm

Acoustic impedance is very similar (mathematically the same as) to electrical impedance. When an electrical wave hits something with infinite impedance (like unconnected wires), electricity doesn't "leak" out of the wire. Instead, the wave is reflected back at the source.

Intermediate impedances allow some of the wave to pass while reflecting the rest back at the source. Only when the impedance of the load matches that of the source will there be no reflected waves. This is called "impedance matching" and is studied a lot in power systems.

Impedance matching is the reason why doctors use ultrasound gel - without a significant amount of acoustic energy is lost at the probe/air interface and at the air/skin interface.

Acoustic insulation doesn't really use the same transfer mechanism that thermal insulation uses. Acoustic signals are pressure waves and act like such. Material thickness doesn't make much difference in attenuating sound, but the material impedance does; this is how things like acoustic wallboard works - not by making the drywall thicker but by layering different materials. Each layer represents a new acoustic interface, which means a new opportunity to reflect sound waves.

All this is to say that you can't modify an interior chamber's acoustic properties without affecting how that chamber passes (or doesn't pass) acoustic waves to its surroundings. Churches and concert halls will typically have a very long reverb time because their (stone, typically) walls are very high impedance, which means that acoustic sources will produce higher sound pressure levels. Read more.

• Thank you for clarifying. Therefore, would it be more effective to design a solution that takes into account both elements (transmission and treatment) rather than thinking of them as two separate activities? It seems that many companies separate the two as if they were de facto different projects all together, thus my initial confusion. – Drumagog Feb 4 '16 at 23:16
• @Drumagog - I'm not sure what you're referring to when you say "treatment", but regarding transmission, I would reiterate the electronics analogy: acoustic impedance is frequency dependent. This means you can tailor the frequency characteristics in a particular space based on the material choices you make at the boundaries. – Chuck Feb 5 '16 at 0:06
• by treatment i refer to conditioning the room so that the frequency response stays somehow even across the domain, particularly in a defined listening position. Your answer is clear. However, doesn't the property of transmission change when we may have amplifications because of reflections in certain spots? To give a more concrete example, wouldn't a frequency that is reflected and boosted as a result be more likely to penetrate further the adjacent material? All i want to know is whether I can plan a room's acoustics/sound proofing by taking into account both properties. – Drumagog Feb 5 '16 at 8:46