Sound Energy: Two Types
We have two major types of sound energy we must deal with when it comes to recording studios. First, we must deal with the sound energy created by outside our studio such as trains, planes, and cars. The energy within our studio must be managed and be contained as much as possible within the studio walls. To accomplish both of these objectives, we must turn to vibrational acoustics for answers.
Gas vs. Solid
When sound energy travels through the air from human activity, it strikes our room’s surfaces. At the moment of impact it is transformed from a wave suspended in air to a vibration traveling through a solid material. Air or gas is different in many ways from a solid when it comes to vibrational energy.
Vibrations Or Oscillations
Vibrations are a repetitive and if you will, oscillatory response of any mechanical system. The rate of the vibration cycles produced by our sound wave oscillation is termed ” frequency”. If we look at repetitive motions that are long and occur at lower frequencies, we call these oscillations. This same behavior at higher frequencies is called vibration. Vibration and oscillation are synonymous.
Wall – Mechanical System
Our wall in our recording studio is a mechanical system. It moves because of vibrations. It has a resonant frequency at which frequencies above the frequency of resonance are absorbed and below are not. We want to lower the resonant frequency of our recording studio walls, not only for outside noise coming in but also from studio noise leaking out into adjacent rooms.
The way we slow vibrations down is by putting individual barriers in front of them. These materials or barriers, must have a different density or weight to them. We want to make the vibration work really hard to get through some materials and maybe not as hard through others. Look at Kit Kat candy bar. Bite off the end. Notice all the different layers of materials or sugar. These candy bar layers are made of just two or three different materials. To slow vibrations down, we need many layers of different types and weights of “sugar”.
A layered Approach
Arrangement and assembly processes of one’s chosen layers, must also be considered. Depending on the amplitude of the outside energy trying to get in, we may use different materials in different layering arrangements. If we are trying to minimize lower frequency vibrations, we may choose a completely different approach in materials and the assembly or layering of those materials than if we are dealing with a middle or higher frequency range.
What kind of materials can we use? What are good “layers” to use? Concrete is one, along with steel. Lead is a good layer. Vinyl can even be a layer of material in our studio wall. The air we breath and even the insulation we put in our walls to keep heat and cool in and cold or hot temperatures out can be considered as a potential layer. Plant materials are popular today. Cork would be an example for this category.
Step On The Scale
Now that we have our materials, we must look at the densities or weight per square foot of each one of our materials. We want to arrange our materials in a manner that reduces vibrations as the energy travels through the wall. If we put barriers in front of the vibration that are different densities, we can cause the vibration to slow down at different rates and levels. This process produces a number called a sound transmission loss rating or STC.
Sound Transmission Loss – STC
STC is the number assigned to our layering process about how well we did selecting materials and arranging them in ways that reduced the vibrational energy we were fighting against. Whether a car or a dog bark, we choose materials to isolate those frequencies. The STC number we created with our wall layers is our sonic report card. With studio wall construction, a STC of 75 would be great and a STC or sound transmission loss of 35 would be bad. Lets build a wall to see how all this works.
Low Frequency Vehicles
Our goal is isolation from cars and trucks in our studio. These are low frequency producing devices with long wavelengths. We need a big “gun”. Lets choose a STC of 75 as our isolation goal. First, lets use concrete. Not poured concrete but “cinder block” as it is called in the trades. You know the one with the two holes in them. We now have two layers. We have concrete and air. Remember, air is a layer that we can use.
High Mass First
Inside the two holes that are filled with air, we will place a sound absorbing material such as building insulation. Now, we have concrete, air, and building insulation materials to bring our total to three layers. On the outside surface of our concrete blocks, we place six inches of Thermafiber. Thermafiber is a material that is spun from the slag in steel mills. It is the slag left in the big bucket that pours the liquid steel into ingots and then is spun into a “fabric”.
Drywall is a high density finish type material. Lets elevate its performance on our STC chase, by placing a vinyl high mass material between two layers of drywall each with different thicknesses or densities. Lets vary the drywall thickness on the inside of our studio wall by selecting 1/4″ and 3/8″ thickness. For the outside wall, lets use 3/8″ and 1/2″ with a vinyl layer. Lets also vary the thickness of our vinyl between each of our drywall sandwich.
We now have a wall that is over 20″ thick, with different materials of different densities. A vibration that moves through this damping sandwich would act like a car that is stopped on the freeway. You move forward a little then you stop. You start again, but this time slower. Eventually, after 4 hours on the LA freeway, you run out of gas or patience. As far as a vibration is concerned in studio acoustics, we will take either one.