Enough acoustic theory (or is it really like brain surgery?)
November 17, 2018 12:20 PM   Subscribe

I want to understand some acoustic theory to design solutions for outdoor noise (or at least communicate better with acoustic engineers). I know of software for this but want to know just enough of the nuts and bolts. There's lots of info available but I don't want all of it and don't know where to start.

I want to introduce some new methods into exterior acoustic design and acoustic engineers here are reluctant to go beyond what they know. Knowing more would help me come across better.

I'd like to know how I would go about calculating:
A Sound attenuation from source to proposed receiver?
B Sound level/pressure? at receiving boundary?
C What volume or face area of a given material* I would need to (not sure what) to reduce received decibel level from e.g. 75 to 65 Decibels?

*I have info on sound absorbing capacities of various materials and plants (all plants have their own specific level of sound absorption).

BUT - is it even possible to know 'enough' to do the above, or is this like asking about knowing 'enough' about brain surgery?
posted by unearthed to Science & Nature (7 answers total) 1 user marked this as a favorite
It's hard to know quite what you are asking to be honest.

Assuming an anechoic environment I believe the answer to a and b are the same (sound from a point source attenuates according to the inverse square law, this is high school level physics).

The answer to c in a real world situation is complicated and generally I have always relied upon acoustic engineers to answer such a question. I don't think it's plausible for you to do yourself, except if you simplify to the point that the answer is indicative rather than really accurate. (Which to be fair is often enough.)

I've never found that I needed anything beyond what a high school teacher understands to talk to engineers, who have always been happy to engineer whatever was requested as long as their fees were paid. You should be able to get this from text books and probably YouTube these days.
posted by deadwax at 2:29 PM on November 17, 2018 [1 favorite]

I want to know how do do some of the basic math involved in acoustic calculations - I never did high-school physics, so where should I start for the basics? for me this is unknown unknown territory - at the same time I don't need all of math, I just want enough to be more fluent with this issue.
posted by unearthed at 5:07 PM on November 17, 2018

What are you trying to do? Are you trying to change the quality of sound -- through acoustic treatments? Are you trying to stop the transmission of sound from one source to another -- acoustic isolation? Are you trying to redirect/reflect sound coming from, say, a busy highway into the air and wondering what kind of material to use?

1) Knowing enough so that you can communicate with acoustic engineers and have them understand what your intentions are
2) Knowing enough so that you can brainstorm somewhat realistic ideas and work with acoustic engineers to actually make it happen
3) Knowing enough to decide/make it happen/calculate a solution yourself.

Which are you in? 3) is "become an acoustic engineer". 1) is "get familiar with the basic intuitive concepts". And there's a lot of space within 2).
posted by suedehead at 8:19 PM on November 17, 2018 [1 favorite]

I'm interested in using what is known as sonic crystal theory to mitigate environmental sound (especially industrial noise, but also traffic).

This is explored deeply in "Innovative barriers exploiting natural materials" found on the HOSANNA website, a noise control think tank and conference.

1 and 2 is what I'm looking at.

Start with basic concepts (so when I approach an engineer my ideas get treated seriously). No I don't want to become an acoustic engineer.

I'm looking for pointers to good text books, anything useful off youtube (but don't know where to start searching).
posted by unearthed at 10:47 PM on November 17, 2018

In 2004, I taught a course on collaborative design within engineering, where structural and acoustic engineering students had to do a project together. Unfortunately, I've lost the very lovely ppt the acoustics professor had made, though it was my faithful companion for ages. It was simple and very clear. Maybe you could find this type of acoustics 101 lecture somewhere on the internets?
Anyway: it's really helpful to think of acoustics as something similar to light, it follows similar geometrical patterns. It's not exactly the same, but enough of the same for you to be able to work with it in models. The keyword here is geometry, even on the materials scale: the sound bounces off surfaces both on the macro on the micro level, and you want that bouncing off to serve your purpose.
People are often really focused on using sound-absorbing materials, but you can also make a huge difference using spatial geometry. Combining the two will be best. Think of the weird walls of anechoic chambers. It works outdoors as well as indoors but the computer programs built to calculate it are not as efficient for outdoor planning.
posted by mumimor at 4:16 AM on November 18, 2018

This is a bit outside my area, but:

GoogleBooks result for Handbook of Environmental Acoustics

pdf of a paper on outside sound propagation from Penn State University

pdf of a paper from the World Health Organization on the fundamentals of acoustics

Acoustics 101 webpage from, admittedly, a company that seems to sell acoustical treatment materials; there's no math, but it does cover a lot of the basic principles.

pdf of a paper from the US Department on Housing and Urban Development on environmental noise attenuation

The US Federal Highway Administration seems to have put out quite a bit of information on the theory, principles, and construction of barriers and other ways of affecting environmental noise, but their website is being uncooperative at the moment, so no direct links. (I can find things on Google but I can't actually get to the relevant webpages or publications.)

Being that you're not in the US, I would think that similar government departments in NZ should have similar papers and publications.

A Sound attenuation from source to proposed receiver?
B Sound level/pressure? at receiving boundary?

I want to know how do do some of the basic math involved in acoustic calculations

So, here's the thing; the math/physics to figure A and B out are relatively simple - IF you're considering things in a "neutral" environment, an anechoic chamber or in free field. But once you're approaching things from an engineering standpoint, where you have to actually create something in the real world, you need to account for things like reflection, absorption, wind, temperature, humidity and how all of these factors interact with each other, and there the math gets real complicated real fast. Which is why acoustic engineers tend to use a lot of software and tables. So learning the basic principles and concepts and then some of the software seems like it would be of more use to you, because the "basic math" is too simple to be of much practical use, and the engineers are likely not actually doing most of the more complicated math themselves, they're using software.

I'm interested in using what is known as sonic crystal theory to mitigate environmental sound (especially industrial noise, but also traffic).

This is explored deeply in "Innovative barriers exploiting natural materials" found on the HOSANNA website, a noise control think tank and conference.

Having dug into that a little bit (although the website seems not to play very nice with Safari, trying to get to the "downloads" or the "white papers" pages just sends me to the main page), it looks like the readily available pdf Summary Brochure kind of doesn't go into sonic crystal all that deeply, it just includes the use of same in some of their conclusions. However, the info presented in that brochure certainly suggests that some actual studies and measurements using sonic crystals have been done, which you may be able to get from the org or the website, although maybe not for free.

IOW, the HOSANNA group may have basically done a lot of the work for you in terms of providing hard evidence about how well the sonic crystal method works and how to apply it, and presenting engineers with published papers and data certainly seems easier to me than doing the math yourself.

There's also Chong, Yung (2012). Sonic Crystal Noise Barriers. PhD thesis The Open University. (pdf link), which may help in providing evidence for the application of sonic crystals.
posted by soundguy99 at 8:08 AM on November 18, 2018

Thanks mumimor for some sound guidance on thinking and concepting and soundguy for the links and explanation - I can now see that worked examples 'by hand' are not a thing I'd ever want to do.

Reading onward from your terms and links I found a uk company who mak sound meters who have lots of useful info e.g. Octave Band Filters on a Sound Level Meter

The Penn State paper is super useful.
Handbook of enviro acoustics - I'll find a copy.
I've noticed I have two references to Chong's works that I have yet to follow up on from ages ago!

I've also found Engineering Acoustics: An Introduction to Noise Control which sounds clearer than most tech books.

I downloaded most of HOSANNA before they restructured their webpage - seem to be lots of circular refs now.

I feel I have 'enough' to go on with.
posted by unearthed at 3:02 PM on November 18, 2018

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