Acoustics 101 - A Succinct Primer on Sound Control


Acoustics 101

Sound control can be daunting.  There are so many acronyms and types of sound.  It is hard to know what sound issues you are even having.  So how do you figure out how to fix them if we don’t even know what the different terms mean?

Let’s break it down.

Sound really is just a bunch of waves moving together.  Those waves are different sizes in height and they can be tight together or far apart and they interact with each other in many different ways.

These sound waves have an amplitude.  That means the height of the wave. The higher the amplitude, the louder the wave. They also have a frequency, which is the number of waves that occur in a given time, so the higher the frequency the more ‘’screechy” whereas a low frequency is more like thumping, or bass.

Acoustics CaraGreen Wave

Sound waves have energy (picture a wave, and all the space under each hump, add them up, and that is how much energy the wave has.)

These waves travel around a space and the following things happen:

  1. They transmit through a wall, or a space, which means some amount of the wave passes through a wall or a structure, and you can hear that amount of sound on the other side.
  2. Some of the sound energy passes through the wall in the form of a new wave.
  3. Some sound gets absorbed by the structure itself.  Based on the surface, the weight of the surface and some other factors, some sound energy is lost in the structure itself (it kind of micro-vibrates the structure and loses its energy).
  4. Some sound gets reflected back into the space.  

All of these things happen to sound in a room at any given time.  If you have flimsy walls, you may get a lot of noise from adjacent spaces.  A lot of sounds will be transmitted through the wall.  If you have heavy soft surfaced walls, a lot of sounds may get absorbed by the wall.  If you have hard, glossy stone walls, a lot of sounds could get reflected back into the space.

Now how do you control each of these?  First, let’s understand the technical terms: STC and NRC.

STC is sound transmission class and is a rating given to a structural element (like a wall) to determine how much sound it can stop from being transmitted through the wall.  Higher STC means more sound can be stopped.  Ratings average in the 50s, but because it is based on decibels (the measure of sound), it is a logarithmic scale so it can get complex.  In general, bigger is better.  The human voice and normal conversation is around a 50-60 STC.  Most wall assemblies are suggested to have an STC of 45-50 to sufficiently control sound (like in multifamily residential structures with adjacent units).

NRC is noise reduction coefficient and applies to materials that can reduce the amount of sound bouncing around (reflected) and absorbed by structures within a space.  This can be furniture, carpet, or wall and ceiling treatments.

It is easiest to control NRC as a designer.  You can use acoustical panels (they have decorative, colorful ones now, like EchoPanel by Kirei), carpet or baffling systems.  Incorporating soft fabrics or wall panels is an easy fix for dampening sound within a room. The fact that this can now be done artistically is a boon for designers, who use to be relegated to the off-white or beige ceiling tiles that were the only option.  Materials have an NRC number between 0 and 1 and the closer to 1 is the closer to absorbing 100% of the sound. 0 would be a complete reflection and 1 would be complete absorption. An example would be an acoustic panel with an NRC of .65.

Kirei Mura Fabric Acoustics

Kirei Mura Fabric in Kome 500.

STC is harder to control.  If transmission through the wall is the issue, this is hard to remediate because the wall structure itself is the issue.  Products like resilient channel are hard to install after the fact. Acoustic drywall like QuietRock is a good option, but it should be specified up front with STC goals and requirements incorporated at the design phase.  STC between floors and ceilings is an even larger issue because hard floors introduce structure-borne sound that gets into joists and framing which cannot be controlled without massive redesign. STC is achieved by decoupling the sound source from the structure of the building, not an easy feat after the fact.

Designers typically can design for comfortable interiors by using high NRC materials that look good. With the shift to more industrial, open spaces, with harder surfaces, exposed decking and polished concrete, designing with high NRC materials is more important than ever.  

STC is not really interior design as much as it is wall structure, so this is likely the architect or engineer’s domain.

So, in a nutshell, sound waves are bouncing around us all the time and now you know a little more about them and what is actually under your control and can help you effectively incorporate acoustics into your design.


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