Whether you are recording an acoustic guitar with a quality microphone or you are listening to music in your favorite seat, reflections are at play. These reflections can be pleasing (reverb) but can also be problematic (short delays). As sound waves interact physically, they are also changing physically. Fundamentals and harmonics are doing all kinds of higher-ED math as they collide. These physical anomalies cause “cloudy” phasing issues, namely comb-filtering and the spots within the room that have frequency excess and deficiencies I spoke of in pervious posts. Most of these interactions whether direct (straight from the sound source) or indirect (reflected), alter our perceived clarity, smearing their clean, original tones. In order to clear up the studio or listening space, these reflections should be either reduced (absorbed) or scattered into different directions (diffused).
Absorption can be likened to “friction for sound” in that absorptive materials impede the sound from being reflected. All materials have some absorptive quality. The calculations for Absorption coefficients and Noise Reduction Coefficients are very sciencey. These technical specs are determined by the makeup, mass and thickness of a surface and are frequency dependent. The simplest way of understanding this is knowing that the thickness of the material is generally related to the wavelength of the frequency. Meaning, the thicker the material, the lower the frequency that can be absorbed. Higher frequencies, since their wavelength’s are shorter, are absorbed easier than the lower. More on this later. Typically thick, porous, soft materials are great for absorption. Closets full of clothes, carpets, plush-cloth-couches and thick curtains are examples of common absorptive surfaces. Here is a picture of a portion of our old coffee shop/studio. Notice the decorations.. ahem, the absorption materials?
When it comes to sound absorbing materials, there are many commercial products available to help reduce reflections. Google it! Many of these work very well but they also can be very expensive. As you may know, I am kind of a DIY’er and for a DIYer, some of the best options for absorption can be items designed for use in construction. Rigid compressed fiberglass and other insulation materials can work wonders. I’ve had success using 2″ thick, 4 foot by 10 foot sheets of compressed fiber to line walls and doors. I have a good buddy who works in the HVAC industry and has been able to supply me with some of these materials at cost. One of my favorite products is a “green” solution to insulation made of recycled denim and cotton. This stuff is amazing and itch free. I’ve used it in 2 foot by 4 foot wooden frames as absorption panels. When designing Amusement Park Studios in Lubbock, we wanted to eliminate as much floor to ceiling reflection as possible in this rented, low-drop-ceiling building. The whole ceiling received this treatment as shown in this photo. The panels shown also house this material.
For the most part, 2″ to 4″ thick sound absorbing materials are very effective at controlling mid and high frequencies. Frequencies below 500Hz are the hardest to control due to their wavelengths. Remember: wavelength is determined by the speed of sound/frequency. (ex. 1130/100Hz= 11.3 foot wavelength). Bass frequencies tend to build up at walls, ceilings and floors. “Cornering” absorption materials or spacing them a few inches off from the wall to create an “air gap” helps effectively increase their thickness. [An example of this is displayed in the pic above located in the right corner.] Thicker materials or those purposfully engineered to battle these low frequencies can me employed to get the job done as well. These tools are commonly referred to as “bass traps”.
Bass traps are absorbers with thicker design to help battle low frequency reflections. Used to help control inherent and active low frequency issues within a space, these can come in various forms. Following the same concepts of absorption these are usually placed in corners where the build up is greatest. Sometimes this can tamed with a large amount of foam or insulation materials in the corner. One of the cooler options (IMO) are resonant absorbers called “tube traps”. The ones I built for my space are a version of some that we built for the studios during my time teaching audio courses at South Plains College in Levelland Texas.
These “tube traps” are designed to resonate in response to lower frequencies (bass) creating a “vacuum” for these large waveforms. They are about 15 inches in diameter and 6 foot tall. The front side being absorptive is positioned in and the back semi-reflective (diffusive) side is turned outside. These are effective down below 100Hz and I have 11 of them. Here’s how I built these if you’re interested. Here is pic of what they look like. The gray ones.