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Room Reflections

MikeSorensen July 3, 2012 No Comments
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Reflections

Reflections in our personal listening environments, home theater rooms, and professional monitoring rooms are both wanted and unneeded. It is a difficult life for a reflection. Sometimes people want to be around you, sometimes they do not. Most of the time they do not. However, reflections do have benefit and properly regulated can add numerous benefits to our total room sound.

What Are They

A reflection is like a miniature part of the main sound wave that morphed and is striking a room surface. We have the level of the reflection and the delay of the reflection. These are the two variables we must understand and then manage with the proper room acoustic treatment. If we assign numbers to these two variables to help us better understand we would say that a 0dB means that the reflection is the same level as the direct signal. If we take the other end of the scale, we say that a reflection level of -10dB means that the reflection level is 10dB below the direct. It is this region that we must look at for our features and benefits for our room sound.

Spaciousness

The statistical area between the point that the reflection is not audible and then the scales other side which shows the echo level where the reflection is too audible, is the area that we create spaciousness. Another word that we may all be more familiar with would be the “air” in our sound presentation. Air represents the space between the notes in our music.

It is the distance and space between notes that gives us the realism within our closed and small room environments. Air is our way of being outside without room walls bothering us. Back to the numbers for illustration purposes for the first 20 ms., this threshold stays constant. With greater reflection delays, lower reflection levels are required to produce an audible reflection. This is where we get this number you see in the literature all the time. In acoustical design, the 20ms. number is a good benchmark to begin with. Experience tells us that 15 ms. – 20 ms. is a more exacting range to shoot for.

Image Shifting

Back to our numerical scale, so we can understand this concept of image change more easily, we look at 10dB above the audible threshold where this amount begins to change the size and location of the image. With greater delays, the image becomes laced with the reflection and then we get the blurring and smearing in the middle and upper frequency ranges. Within this 10 – 20ms., when we increase the reflections level above the audible threshold, we next get spatial effects occurring.

When we increase the reflection level by 10dB above that thresh hold that allows us to hear, we get image position change. This image change usually leans left center or right center. We can get the same image shifting if we use diffusion and in particular quadratic diffusion to minimize the time signature of side wall reflections. If we place the side wall diffusor too close to the speaker, we can obtain this image shift similar to reflection rate and level of audibility. We can move actually the image left or right center with an acoustic room treatment technology.

Discreet Echos

Echos are another factor in this zone of too much reflection and then the other side of the scale where reflections are so minimal, they are not heard at all. If we look at echos and the characteristics of the reflections that produce them, we see that any reflection that has a level 10dB above the image shift threshold, we have a series of discreet echos which are overlaid upon our images. This is a combination of comb filtering and flutter echo combined and the blurring and smearing that goes on in our middle frequency ranges is not acceptable.

Reflection Delay And Level

Reflections have two major variables that we examine. We look at the delay of the reflection and the level of the reflection. The delay is compared to a base line of between 10 – 20dB. which allows us to say that any reflection delay must be below the 20ms. level to become inaudible. If the level or amount of the reflection exceeds that of the original signal, we have image shifting and distortion. We must use room acoustic treatment that keeps both level and delay of the reflection clearly in mind in its sound absorbing capabilities.

Absorption Treatment

We must first absorb enough of the reflection to slow the speed of the reflection down. We must do this without absorbing the total frequencies which produce the reflection and losing all of this energy to heat. We do not need to destroy the reflect completely in order to manage it. It is never a good idea to destroy energy in order to manage it. We just need to slow it down below that speed of the direct sound.

We do this with room acoustic treatment that respects this reflection/audibility scale with rates and levels of absorption that act smoothly and evenly. Room acoustic treatments that do not over absorb at certain frequencies and those that do not absorb at all with others will not be of assistance in achieving a balance between reflected and direct energy in our small room acoustical environments. Treatment must be constant and even in its performance to match the acoustical design needs of both the reflection and the room.

Predictable And Consistent

Room acoustic treatment for reflection control must be predictable and consistent in its performance. If we are after a certain target number for reflection control in our room absorption treatment, we need to know that the acoustical technology is consistent in performance with every piece of material used. If we need more control, we can easily access how much more we will need from what we have already installed.

Reflections must have their delay times inspected at our acoustic service station. We must also look at their respective levels. We want our reflections to live in a narrow zone which lies between echo on the one side and not being heard at all on the other. Properly managed within this small domain reflections can produce “air” or spaciousness along with image definition and location on our sound stage. Inappropriately managed reflections can produce discreet echos and image shifting. It is a delicate balancing act between just enough and not too many.

MikeSorensen

I am a structural engineer as well as a master furniture maker. I design cabinets for low frequency, activated carbon absorbers.Connect with me on Google+

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