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Where To Place Acoustic Panels

Dennis Foley March 7, 2014 4 Comments
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Working out where to place acoustic panels in your home theater, listening rooms, or professional recording studio is always an issue. Deciding on which acoustical panel to purchase is enough of an issue to begin with, now you have to find the correct place.

So, you need a direct and precise answer. Well here it goes… There is no real right answer. Sure there are guidelines but room tuning, and this is the process we are discussing when we speak about working out where acoustic panels should go, is a balance of science and opinion. Lets start with the listening position and look at each room surface from that vantage point.

Dealing With Reflections

When you’re sat in your listening chair, you are effectively sitting in box a surrounded on all 4 lateral sides by walls. Under your feet is the floor and above your head is the ceiling. When you press play, of these six room surfaces, which reflection do you hear first at your listening position? Hopefully, the reflections are not initially over powering and you get to hear first the direct or straight line energy from your speakers.

Next reflection you hear is from the floor and ceiling. Adding all of this up, you have the wanted direct sound from your loudspeakers and then two reflections, one from the floor and one from the ceiling, both with different time signatures, arriving at your ears. What’s next?

Reflections

Next, you have side wall or lateral as they are called reflections. You have speaker to side wall, right side wall to listener, back to other side wall and repeating this process as it washes all this time delayed, reflected soup at your ears.

After this mess, we now add the front and rear wall energy that is looking for a new “home”. The timing sequence looks something like this: direct (wanted) floor, ceiling, side, front and rear walls, all (not wanted). Now, that you have the sonic picture defined, lets look at what room surface boundary reflections you have to deal with and in what order of importance they all are to each other.

Floor And Ceiling Reflections

Floor and ceiling reflections have an impact at the listening position. The ceiling reflection energy is responsible for sound stage height. The existing physical height of the ceiling can be “acoustically exceeded” through the proper use and application of diffusion. It does vary with room acoustic use.

A listening room works best with a combination of absorption and diffusion technologies. A control room may prefer more of an absorptive approach so that a more critical listening sound stage can be realized. All room surfaces contribute to your sound stage.

Side wall Absorbing panels

Side Wall Absorbing Panels

Sound Stage

A sound stage is literally that, namely a “stage of sound” that a two channel system is capable of achieving in an acoustically treated room. It is that space between your left and right channel speakers, but it is also that space that extends wider and even taller than your loud speakers themselves.

Three dimensions of sound space can be created between your speakers. You can have a height, width, and yes, even depth in your sound stage. The speakers themselves give you energy to start with, but you must take that energy and then the room energy and make them both work together. They must get along because they must live in the same house together.

Side Wall Reflections

Side wall reflections must be slowed down. They are simply moving way too fast. When they cut across the straight line or direct sound from our loudspeakers, these side wall reflections try and push themselves into the direct sound stream. These reflections with their different time signatures cause image shifting in our sound stage.

What is image shifting? That is when the singer who should be center stage is off to left or right stage position. Side wall reflections are like that guy when you are driving down the freeway and moving right along with traffic and then from the on ramp, this guy cuts in front of you and all the other lanes just to get to the car pool lane even though he is only one person. Now you understand side wall reflections.

Rear Wall Quadratic Diffusers

Rear Wall Quadratic Diffusers

Side Wall – Absorption

Side wall reflections are best managed through absorption. It cannot be just any type. The proper absorption treatment must be selected that can absorb side wall reflections at the correct rate and level. Both rate and level of absorption must be considered when dealing with side wall reflections.

It is not necessary to absorb 100% of all side wall reflected energy. You must choose how much and at what frequency you want the reflections to intermix with the direct sound. There is no need to destroy the reflected energy by changing all of it to heat. Just some of the more unfriendly parts.

Front And Rear Walls

Rear and front walls contribute to depth and yes, even length, although this one is never easy. This is only accomplished however, by treating each front wall and rear wall surface with diffusion. Quadratic diffusion will give you consistent and predictable performance. You need to use different frequency diffusion ranges depending on front or rear wall, but it can be done and done very well.

Poor sound diffusion is a room distortion that has a huge impact on sound and 95% of rooms have poor diffusion. When you hear proper sound diffusion, your acoustical life will be changed forever.

In Summary
I hope this explanation helped further your understanding of this important room acoustic issue. Please leave any comments below so I can get back to you. Don’t be afraid to hit those Facebook like, Google+ and Twitter buttons on the left hand side so other people can see this post. And if you want to learn more about this subject please sign up for my free room acoustic treatment videos and ebook which provide step by step instructions. Get instant access by signing up here.

Thanks
Dennis

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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4 thoughts on “Where To Place Acoustic Panels

  1. I’m building a ported sub cabinet and I was wondering how many sides should i place acoustic panels on, 3 I hope…back, side, and top 1.5 in thick egg-crate. 15″ sub 3.0 cu.ft.

  2. Hello Dennis,
    Does it make any difference if I mount my 2’x4’x2” panels off the wall 3” to get the absorption from both sides?

    • P, It depends on the panel type, absorption coefficient, and position in relation to a source. Remember that middle and high-frequency absorption works by air flow across a surface. If the panel is absorptive on both sides, then allowing for more airflow will increase the surface area absorption rates. This may or may not be desirable.

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