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Sound Transmission Class – STC

By October 15, 2012No Comments

Sound Transmission Class

We see the term sound transmission class (STC) many times in the acoustic literature. It should probably be called sound transmission class rating for better clarification and understanding. It is a number or rating assigned to different structure materials and types that minimize the transmission of sound from one side of the rated structure to the other side.

Air Borne Energy

We all deal with the transmission of air borne sound energy in our studios. We want to keep the energy generated within our studios in the studio. We also want to keep the sound created from outside sources, outside where it was created. This airborne energy is what sound transmission is all about. STC is all about the sound energy that transmits through the air and then eventually strikes a physical structure. The fist thing one has to do to minimize this sound energy transmission is to place a barrier between you and the source.

Sound Turns Into Noise

We all know that sound energy can pass through openings in our rooms. It can pass through doorways and even air conditioning vents. It can even pass through our walls will virtual ease, especially at lower frequencies. Once it passes through our walls, whether it is entering our room from some outside source or whether it is generated by the drummer within our room, it receives a new name. Now, it is called noise.

Noise Is Bad

Noise, by definition, can cause many issues. Outside noise can reach our microphones and then enter our mixes. For those with in home project studios, how often have you waited for a garbage truck to finish its work and move on down the road before hitting the record button. How about the neighbor’s motor cycle. In an office setting that requires confidentiality, one must keep all energy generated in the room where it belongs and all externally generated noise must stay outside the room.

Sound Transmission Loss

Transmission loss is just what the name implies. It is the loss of sound transmitted through the air. It is the number assigned to the structure that we build to stop the noise from entering or leaving our structure. The number represents the number of decibels or units of measure, that sound is reduced by the wall or partition that we have chosen to use.


Sound pressure levels are measured in decibels or dB. The scale of measurement used to simulate sound across the audible frequency range is denoted as dB or decibels. It is a logarithmic scale where 1 + 1 does not equal 2 but some number within base 100. The human ear perceives changes in loudness caused by even small changes in those levels. Each 3dB increase doubles the sound reception and the annoyance or discomfort that goes with it.

Sixteen Different Frequencies

The actual measurement process takes into consideration, 16 different frequencies which lie between 125-4,000Hz. These 16 frequencies represent the basis for the STC number. One can see that this frequency range is really for speech. It does not go low enough to be of any benefit when it comes to low frequencies. For low frequency control, we must use a different approach.

Barrier Construction

Our partition or barrier construction can use many different approaches. The thickness, materials used, and method of construction are all determined by the noise level generated from inside or outside of our room. Noise from a drum room is different than noise from a vocal room. Each instrument produces a different frequency range that must be dealt with. We must also consider the level of the noise. Noise levels determine the structure composition of our partition.

What Is The Number?

Finding the inside noise level number that one needs for the rooms intended use is critical. If we are recording within it, we need lower levels of noise. I like to see studios below 50 SPL. I really like below 45. Remember, a 5 dB. difference makes a huge difference in the perception of sound in the room. If the room is not used for recording, then that particular pressure level will be used to determine our noise barrier material and type. We do not want to overbuild to achieve our acoustic goals. I am working on a modular building system that will achieve these levels. I want my room so quiet that I can hear the beating of my own heart. Now, I must figure out a way to record it.

Low Frequency Needs Mass

To reduce the sound transmission of lower frequencies, we use mass. The density or weight of a structure is critical when we are dealing with frequencies below 125 cycles. A poured concrete wall will block more sound than a framed construction type. Mass through the addition of certain material types will assist us with better STC numbers. For example, when we add additional mass to our barriers by doubling the density of the total structure, we can increase the isolation ability of our partition by approximately 5 dB. Human hearing can detect a 1 dB change in a sound pressure level.

Air Is Barrier

We can also look to air space to assist us. Air is another material that we can use. It is actually a very powerful material that is basically free for the using. If we use an air space between two freestanding walls, we have added another layer of material or “mass” to our noise barrier. An air space of 1 1/2″ will improve our STC rating by 3 dB. An air space of 3″ will improve our STC by almost 8 dB.

Internal Barrier Fill

If we place sound absorptive materials within our framed structure, we can improve our STC rating by something within the 4-6 dB range. The literature tells us that using specialty insulation in this wall surface area will do no better than standard insulation materials. That maybe true when one is dealing with standard insulation materials. Activated carbon is not standard building insulation materials. There is nothing standard about charcoal. I will be testing the use of activated carbon or charcoal as a barrier material.

STC Rating Application

To use the STC number assigned to different barrier technologies, we need to look at the level of unwanted noise we are building a barrier to stop and the level of acceptable sound pressure that is required by our room use. If we have outside or external noise levels of 90dB and our noise levels inside need to be at 60dB. then we need to attenuate 30 dB. of energy. We will use a barrier technology that will exhibit at least a 30 dB. attenuation. It is usually advisable to seek 25 % more than the numbers tell us.

All Energies Managed

We have two different kinds of energy when we are dealing with our rooms. We have the energy that is generated from outside our room. We also have the energy that is generated within our room. We want to keep the outside energy, outside, where it belongs and the inside created energy within the room. To do this, we measure all these noise levels and then choose the necessary barrier technology that can accomplish this goal. We use the STC rating of our proposed barrier to achieve our air borne noise isolation goals.


I am a structural engineer as well as a master furniture maker. I design cabinets for low frequency, activated carbon absorbers. Connect with me on Google+

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