STC RATINGS EXPLAINED

Sound vibration decreases as it travels from one side of a window/wall to the other. To understand the value of STC ratings, we need to quantify and measure this loss. Sound Transmission Class (STC) is an integer rating of how well a building partition attenuates airborne sound. In the USA, it is widely used to rate interior partitions, ceilings, floors, doors, windows and exterior wall configurations (see ASTM International Classification E413 and E90).

Sound Transmission Basics:

DECIBEL (dB) is simply a measurement of how loud a noise is. Think of dB as the volume knob on your receiver. 50 dB is quiet while 140 dB is so loud it can immediately injure your ears.

TRANSMISSION LOSS is a measurement of the dB (volume) difference on either side of a wall. Let's say we have a 100 dB tone on one side of a wall. Pretty loud. We measure this same tone on the other side of the wall and find we have 75 dB. We would say that at this tone or pitch, we have 25 dB Transmission Loss. 25 dB less sound energy made it through the wall to the other side.

FREQUENCY, written as Hertz or Hz, is the measurement of the tone or musical note of a sound. It might have a high pitch like a flute might make (2000 Hz) or a low pitch as from a tuba (as low as 29 Hz).

Most humans are born with the ability to hear frequencies from about 20 Hz (low) up to 20,000 Hz (high) but that range shrinks as we get older. Interestingly, a test tone with a different pitch sent through that same wall might only see a 4 dB transmission loss. The performance of a wall will vary greatly, depending upon the tone (frequency) of the sound. What are STC ratings? In 1961, STC was introduced as the method for comparing various wall, ceiling, floor, door, and window assemblies. STC is calculated by taking the Transmission Loss (TL) values tested at 16 standard frequencies over the range of 125 Hz to 4000 Hz and plotted on a graph. Your curve is compared to standard STC reference curves. If your window/wall STC graph is closest to a standard STC 35 curve, your window/wall is said to have an STC rating of 35.