Acoustic Basics

For the Control Room and Mix Environment

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Although the acoustic treatment required to optimize the sound is different for every room, every setup, and each unique application, there are still some basic acoustic concepts that are applied universally in a properly treated room. In this section of our web site we will present 5 of the more fundamental acoustic topics addressed in a properly treated control room or other mixing environment.

  1. Room Resonance Control: Bass Traps
  2. Comb Filtering for speakers and microphones
  3. Flutter Echo
  4. Reflection Control
  5. Reverberation Time

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1. Room Resonance Control: Bass Traps

Sound is conveyed through waves in the air. Waves that exist between a pair of surfaces can create standing wave resonances whenever the distance between the surfaces is any even multiple of one-half of the wavelength. At resonant frequencies (tones), the sound is louder and decays much more slowly than at non-resonant frequencies, causing uneven tonal quality and interference with clarity. Resonant frequencies occur mainly in the bass range, due to the relationship between the wavelengths of low-frequency sounds and the typical sizes of people's rooms.


This wave is in a standing wave resonance 
since it's wavelength equals the distance 
between the pair of surfaces.

Every room has its associated resonant frequencies. Rooms built using preferred dimensions ratios have potentially more even distributions of these resonant frequencies. Room built with angles walls or ceilings have more complicated resonant modes than typical rectangular rooms and the resonances can be potentially less severe. But, no matter what the size or shape of the room, resonant frequencies can be controlled through the use of bass traps.

"Bass" frequencies occupy all the notes on the left half of the keyboard (Everything below middle C). Since this is such a large portion of the musical spectrum, and because every room has potential resonant frequency problems in this bass range, it is imperative that the low frequencies be the first issue to address in improving any room's acoustics. Of course, each specific room's geometry, setup, and application dictate how to best optimize the bass performance. However, there are some general enhancements that can be made using ASC Tube Traps that are sure to offer improvement in any room.

2. Comb Filtering/ Phase Interference

Sound and music propagate through waves and, therefore, must abide by the laws of wave physics. This means that when 2 waves "collide", they do not bounce off one another as is the case with physical objects. Instead, at that location in space and moment in time, they either add their combined amplitudes to some degree or cancel their combined amplitudes to some degree.

Waves exactly in phase add to make a wave with twice the amplitude.          
Waves exactly out of phase add to make a wave of zero amplitude.
  Waves out of phase to a small degree add to make a wave with slightly higher amplitude than either wave individually.

The wavelength of the 2 sound waves and the difference in the distances they have traveled determine whether they add to or subtract from the combined resulting amplitude. This means that there are a series of adds and cancels at various frequencies of sound for any given room setup.


There are many potential reflection points that can cause a sound launched from a source to return to that source and interfere with itself. There are also many potential ways for sounds to travel from one source to another and cause interference. Likewise, there are many ways for sounds launched from single or multiple sources to arrive at the mix position or mic position at different times and interfere with one another there. All of these interfering waves cause the resulting amplitude of the sound to either increase or decrease to some degree depending upon the frequency (tone) of the wave. The resulting adjustment to the amplitudes at each frequency is called a comb filter.

Comb filtering effects are reduced by placing acoustically absorptive materials at the reflection points responsible for the interfering waves. The materials must be of a size and type to properly address the frequencies of each specific problem. Rearranging the speaker or mic setup will simply shift the locations of reflections and alter the problem frequencies, but does not remove the problem.ASC Sound Panels, Sound Planks, and Fractional Tube Traps are often used to control comb filter reflections, with the appropriate device chosen based upon the frequency of the problem. Although locating the precise positions of problem reflections can be a complex task, there are a few locations where controlling the reflected wave is sure to make an improvement to the sound.


3. Flutter Echo

There are certain paths for sound that produce a repeating loop. Every time the wavefront passes the engineer or artist, it is heard as the sound is intended, but with a twist. Just as when you "click" the individual prongs on a comb in quick succession, the quickly repeating sound of the wavefront continuously passing the listener produces a distinctive "zinging" tone. This is known as flutter echo and is due to our brain's desire to interpret air pressure fluctuations at some frequency as a particular tone. For this is exactly what is occurring as the wavefront continuously passes your ear at some rapid rate.

The flutter paths are most commonly located along lines between parallel surfaces. Speakers or recorded sound sources located between parallel surfaces are constantly sending sonic wavefronts into the repeating loops of these flutter paths.


Speaker Flutter in the Mix Environment
Flutter in the Tracking Room (top view)


Placing ASC Tube Traps, Sound Planks, or Sound Panels at the reflection points for these flutter paths breaks up the flutter. This removes the tonal discoloration caused by the "zinging" sounds our brain interprets from the repeating wavefronts it encounters.

4. Reflection Control

As seen in sections 2 and 3, controlling room reflections is fundamental to creating accurate sound reproduction in any room. In addition to utilizing precisely selected panels addressing comb filter and flutter problems, it is also generally desired to include the proper combination of absorption and diffusion to control sounds reflected throughout the room. The desired balance of absorption and diffusion is obtained through selection of appropriate absorptive material and proper placement to create diffractive diffusion and/or multiple time-delayed specular diffusion.

  Edge-effect diffractive diffusion
  Multiple time-delayed specular diffusion

The proper placement and selection of panels to attain the desired reflection control is determined on a case-by-case basis due to the large number of variables involved.

5. Reverberation Time

Sound produced within any enclosed space will continue to exist in that space for some amount of time after it is created, decaying away until it is inaudible. If this decay time, known as the room's reverberation time, is too long, sounds will linger within the space and begin to overlap with new sounds being made, creating an unintelligible cacophony.

  Long reverberation time = Poor Intelligibility
  Short reverberation time = Good Intelligibility

A sufficient amount of acoustic absorption is required at all audible frequencies of sound in order to keep the reverberation time in a room short enough to have good intelligibility. The measurement of the reverberation time in a room is often referred to as RT60. The desired RT60 at an frequency varies from room to room. All ASC acoustic treatments alter the RT60 of a room to some degree. Acoustic treatment is developed with desired RT60 levels in mind.