An Acoustics 101 Primer

There is a lot of confusion when it comes to acoustics, but it’s not rocket science. Yeah, the basics of rocket science are easy compared to acoustics! With terms like NRC, STC, absorption, diffusion, isolation, decoupling, and on and on. The following is a very quick overview of some of the more important concepts that need to be taken into consideration when designing a room or product to optimize the environment for making professional recordings. Or... in the case of prospective buyers, to choose a product that best suits their needs. This, our first article, outlines some of the acoustic principals important when considering a portable vocal booth. In a follow up article, we will explain how our acoustical design plus quality materials plus quality construction result in what we believe are the best portable vocal booths for professional voice talents in the industry for both at home and on the road.

The Basics

Some quick definitions

• Acoustics - is defined as the science of sound, including its production, transmission and effects. (ANSI/ASA S1.1-2013)
• Sound Pressure - The local deviation from the ambient (average) atmospheric pressure, caused by a sound wave. To our ear, volume or loudness.
• Volume or loudness - This is simply our perception of sound pressure! The less the sound pressure the quieter it is. The greater the sound pressure the louder.
• Frequency - Sound pressure vibrations having a higher or lower number of cycles per second. Low number of cycles corresponds to a low frequency or pitch and higher number of cycles corresponding to higher frequency or pitch.
• Acoustic Transmission - The propagation of sound waves through a medium.
• Reflection - Like light, sound can be reflected back from a surface that it strikes. Also, like light the reflected sound will bounce back at an angle equal to the angle of incidence, i.e., the incoming sound.
• Echo - The reflection of sound or repeated reflection of sound bouncing off multiple surfaces can result in multiple sound waves reaching your ears. If there is sufficient delay between the original and the reflected sound this will result in hearing the Sound a second or more times. This is called an echo
• Reverberation - The persistance of sound from a large number of overlapping reflected waves after the original sound source stops is called reverberation.

Absorption

What it is

In acoustics absorption is the amount of sonic energy that is removed from the space it is in. The better absorption a material has the more sound will be prevented from echoing off that material back into the space it is in. The more absorption a material has the higher the NRC. Again, this is primarily about how sound is absorbed from within a space, not from sound coming in from the outside.

Why it’s important

Absorption is an important property of the acoustical environment that you are recording in. The more absorption, and thus the higher the NRC number, the less roomy boomy echo effect you will get in your recordings. But it’s important to understand that you don’t want all sound to be absorbed otherwise your recording will sound flat. You need a little bit of presence in your recording, or it won’t sound natural.

Diffusion

What it is

Unlike absorption which is about the removal of sound within a space, diffusion is about the redirection of sound within a space. Whatever sound is not absorbed by a material bounces back and it can either echo directly back from the direction it came from or be diffused or redirected in one or more different directions.

Why it’s important

Diffusion is important in evening out sound within the containing environment. This reduces or eliminates echoes and reflections and helps make how something is heard the same regardless of where you are in the room or booth that comprises your acoustical environment. Diffusers are components specifically made to aid in this process but it can also be incorporated into other products like sound absorption materials.

Isolation / Decoupling

What it is

Isolation is an acoustical treatment that separates or “floats” the areas you are trying to treat from the surrounding structures they are contained in. Isolation can be accomplished by either using a high density (aka, heavy) material specifically made for the purpose or by physically isolating the area to be protected from its surrounding environment using a variety of construction techniques. The process of isolating in acoustics is referred to as decoupling.

Why it’s important

When there is a requirement for as little noise as possible on a recording set or any other area regardless of purpose, that area will require some degree of isolation. Absorption techniques can treat the sound from within the area but has minimal effect on noise and vibration coming from outside the confines of the acoustical area being treated. Isolation means separation. In this case separating or decoupling the area being treated and the area outside. The two main ways of creating separation from the outside is through mass and space. This means using construction techniques that limit both sound and vibration transmission.

Measuring Acoustical Properties

The following terms are used in measuring various acoustical properties.

STC

STC stands for Sound Transmission Class and rates how well a material stops the transmission of sound through that material. It is measured by the amount of transmission loss of sound in decibels a material, wall or other partition can provide. The higher the number the better the material is at blocking noise to the other side of itself. This number is derived from the testing of the material at multiple common frequencies and then comparing to standard STC curves of reference. STC will measure how well you have isolated your device or room from its surrounding environment. STC relates to acoustic isolation and so gives a good indication of how well a given material or structure will keep noise from entering the area you wish to protect.

Decibel (dB)

The decibel is a unit of measurement. Although it can be used for a variety of different things it is commonly used to measure an amount of sound pressure or as most of us would say, loudness. Most people know that the more dB’s the louder something is. Less well known is just exactly what a given decibel level means. Decibels increase exponentially so a 10 dB increase means 10 times louder, but a 20 dB increase means 100 times louder!

NRC

NRC stands for Noise Reduction Coefficient. This is a rating of how much sound a material can absorb. The scale for NRC is 0 to 1 where 0 is no sound is absorbed and 1 is all sound is absorbed. This tells us how much sound inside a structure, whether it be a booth or a room, is soaked up by the material on the walls, roof etc. It does not tell us how much sound is prevented from coming in the structure from the outside. NRC gives the amount of absorption a material has.

Hz

Hz stands for Hertz and is used to measure frequency. It is defined as cycles per second. The higher the number of Hz the higher the frequency or pitch. Humans can typically hear between 20 Hz and 20,000 Hz.

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