Sound Wave Transmission Through Surfaces

By: Mark Rustad

We are so accustomed to the many sounds audible to us at any given time that we hardly pause to distinguish what we are actually hearing or why. Hearing a combination of sounds around your home, such as the dishwasher running, cars passing by and a neighbor mowing the lawn is so commonplace that it does not generally warrant a second thought. However, there is an intricate science behind sound behavior that explains why we hear certain sounds and with what intensity.

A sound wave is an organized vibration which passes energy from particle to particle through any molecular structure. You hear the voice of someone in the same room speaking to you because the energy travels through the molecules of the air and reaches your ear. Likewise, you perceive sounds from outside such as traffic or a lawn mower because the vibrations produced by these sound sources travel through the molecules of the air and through the structure of your home to deliver a portion of the sound energy to your ears.

In this illustration, when the sound vibrations produced by the neighbor's lawn mower change mediums, a portion of the sound is reflected off of the outer surface of your home while the remainder transfers through the surface and becomes audible within your home. The energy bouncing off of your home and remaining outside is sound reflection, while the energy traveling through the structure and into your home demonstrates sound transmission. Since wood, drywall, concrete and water are made up of molecules, sound energy travels as easily through such structures as it does through air.

Sound transmission can be measured using a metric known as the Sound Transmission Coefficient (STC). The STC rating is a numeric value which describes a wall, floor or ceiling's propensity to prevent sound transmission. The typical wall with studs and drywall averages an STC rating of 38. A room with an STC rating of 38, for instance, would affect a 38 decibel drop in the audibility of incoming sound in a controlled environment.

While decibels describe the intensity of a sound wave, Hertz is a measure of its frequency, or the number of cycles occurring over a specified time. One Hertz is one vibration, or cycle, per second. Sounds with higher frequencies have a higher pitch. Low frequency sounds, such as bass, have flatter, longer sound waves which travel more easily through surfaces than do higher pitch sounds. Higher frequency sound waves, having a high Hertz value and thus more up and down movement in the cycle, are less successful at traveling through surfaces because they must overcome more distance and time within the structure. Comprehending frequency allows you to better understand why you are able to hear the low pitch lawn mower next door yet you cannot hear the neighbors having a conversation from the same distance.

The behavior of sound waves of differing decibel levels and frequencies accounts for the highly individualized nature of soundproofing and sound reduction projects. It is advised to consult with a reputable soundproofing supplier prior to any soundproofing endeavor so as to ensure that an effective solution for your application is reached.

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