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Reflection Vs Absorption

Dennis Foley December 5, 2017 No Comments
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Today we’re going to talk about reflection versus absorption. Absorption is the term we use a lot of times and we need to understand the tools that are available in this process so let’s first define our reflections. Reflections are energy that’s greater than 100 cycles let’s use that as our starting point in our definition for this discussion and of course all of us are working in rooms so reflections are that energy that strikes the room surface the room boundary surface and comes back somewhere. Now when it does that it’s striking a surface so it’s going to be altered doing that then it’s going to come back and strike something else.

A good example is the primary, secondary and tertiary reflections we all have in our two channel systems because our primary is the boundary surface closest to the speaker and then that reflection travels across from that let’s just say right side wall to the left side wall, there’s our secondary and then the secondary travels back to its source. So you have this ping-pong effect of reflections going back and forth when we really want to hear just that straight direct energy from our loudspeaker so real acoustic design and analysis is all about balancing the reflected energy room sound with the direct energy from the speaker’s which is more pure.

So how do we do that? Well the most common way today is absorption, it’s very cost effective, it’s cheap, see a lot of building insulation, rocks old, DuPont and you have to be very careful with that stuff because it has its own rate and levels of absorption and you must match the rate and level of absorption with the position in the room, the usage, all kinds of variables. There is no one-size-fits-all and I get this a lot every day with phone calls and emails. Well I bought some of this, I bought some of that and I put it here and when I say why did you do that they thought that that was the thing to do because they’d read it, saw it in forums or places like that so you have to be careful with these half-truths, you have to be careful with hyperbole, exaggeration, innuendo all of these crazy things that we go through in the English language, the marketing people… You’ve got to be really careful here, you’ve got to look at this science, look at what you’re trying to do and look at the process. Don’t blindly accept what somebody says, don’t blindly accept what I say, challenge what we say also here and we’ll get into a discussion about it. I’ve been building rooms for 40 years so I think I have a pretty good handle on what works and what doesn’t but I’m always open for suggestions and new direction so please don’t hesitate to engage us in any form or fashion.

What is absorption? It’s molecular velocity, remember in some of our videos we talked about a cubic yard of air in your room and it’s full of little molecules and when those molecules aren’t moving and touching each other we have silence but when they start to move and they start to bump into each other then we have noise or sound. So it’s molecular movement and then what we do is we run that movement across the surface building insulation, foam, draperies, any fabric, things like that that are absorptive nature and what does that do? Well those changes create friction which changes the energy to heat and that’s how we get absorption; it’s an energy process change from one form to another so friction and heat. Here’s an interesting factoid, there is no temperature, and people actually measure this you know.

If you touch a drape or you touch a piece of foam and it’s absorbing energy you’re not going to feel any heat but there are some minute heat differences depending on amplitude and quantity of energy so anything above 500 cycles there’s no increase in the temperature of this process so a little bit interesting. Here’s another factoid that we’ve discovered, I think the literature will bear us out on this too. Air movement, the velocity from the surfaces its best and greatest at 2 – 4 inches from the surface. That’s why when companies that really know what they’re talking about design a sound absorption product they make sure that the product is spaced out from the wall because they know this rule, they know that 2 – 4 inches from the wall surface the air movement is going to be the greatest and there’s all kinds of reasons for that, we won’t go into it in this video right now but let’s just accept that right now as a given.

Our foam technology actually has its own mounting system which keeps it two inches from the wall, that way you’re absorbing on both sides. What does that do? It cuts down the amount that you need, saves you money so keep that in mind when you’re working with sound absorbing technology. What’s another way to absorb sound? Resonating chambers, air movement across an opening or a cavity into a chamber and remember all these absorption processes are about air movement or velocity, here we’re running air across bulb or some limp mass material, here we’re using a resonating cavity and of course the friction against the opening and the walls inside the cavity produce the absorption. So the cavity is designed for resident frequency, everything above that resident frequency is absorbed, everything below is not so you can design and tune these absorbers in this case the Helmholtz resonator to this particular problem in the room. Hard to do, you have to be real good at it, it’s not something that the hobbyist can really do because the calculations are extreme. So here are the two types we’ve discussed today, limp mass bulb and the Helmholtz resonator, so two types of absorption processes to deal with reflection.

This is an unedited transcript from our video series from Acoustic Fields. There will be some errors in grammar and sentence structure that occur during this translation process.

For complete understanding and comprehension, please view the video which is included in this text. For any additional information regarding this topic or others relating to room acoustics, please contact us directly at:

P: 520 – 392 – 9486

info@acousticfields.com

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Dennis Foley

I am an acoustic engineer with over 30 years’ experience in the business. My technology has been used in Electric Lady Land Studios, Sony Music of New York, Cello Music and Films founded by Mark Levinson, and Saltmines Studios in Mesa, Arizona, along with hundreds of others. Connect with me on Google+

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