How Sound Is Measured
The most common unit used to measure the pressure intensity or volume of sound waves is the decibel (abbreviated dB). Because of the incredible sensitivity of the human ear, decibels use very small (microbar) units of sound pressure. A microbar of pressure is equal to one-millionth (1/1,000,000 ) of normal atmospheric pressure. The threshold of audibility to the human ear equals zero decibels. Changes in dB levels measured against near total silence are exponential. For example, a 10 dB sound is 10 times stronger than a 0 dB sound, a 20 dB sound is 100 times stronger, and a 30 dB sound is 1,000 times more powerful.
Strength of Common Environmental Noises
0 dB – Near total silence (weakest detectable sound)
15 dB – Whisper, ticking of a watch
60 dB – Normal conversation
90 dB – Lawnmower, train whistle (level at which prolonged exposure causes hearing loss)
110 dB – Car horn, power saw, train
120 dB – Rock concert, diesel engine
125 dB – Pneumatic riveter (physical pain and immediate ear damage begin)
140 dB – Gunshot
180 dB – Jet engine afterburner
200 dB – Saturn Rocket
Measuring Sound Control Performance
The effectiveness of windows and other wall or façade components in controlling noise is typically measured in one of 3 ways: STC (Sound Transmission Class), EWR/EWNR (Exterior Wall Noise Reduction) and OITC (Outdoor-Indoor Transmission Class).
STC (Sound Transmission Class)
This rating was first published in 1970 in accordance with ASTM E413, and was designed to provide a performance rating for interior wall partitions against noises from speech, television, radio and office equipment. It is important to note that ASTM E413 specifically states that the STC calculation should not be used to evaluate partitions exposed to transportation noise such as motor vehicles, aircraft and trains.
EWR/EWNR (Exterior Wall Noise Reduction)
This rating was developed in 1977 by Wyle Laboratories for the U. S. Department of Transportation, Federal Highway Administration. It is similar to the STC rating, but, unlike the STC rating, it is designed to provide a performance rating for building facades that are exposed to transportation (traffic) noise.
OITC (Outdoor-Indoor Transmission Class)
This rating was first published in 1990 in accordance with ASTM E1332, and was designed to provide a performance rating for exterior facades and facade elements (windows and doors) that are subjected to transportation noise such as motor vehicles, aircraft and trains. The OITC rating can be assigned to specimens tested in the field and in the laboratory.
For additional information on ASTM and other industry groups that affect how window performance is tested, measured and reported, please visit the Industry Links section of the Learning Center.