Posts Tagged ‘Wave’

Sound Wave Reflection Within a Room

Regardless of the source, all sound waves exhibit certain characteristics when they change mediums. Sound waves are actually vibrations which travel from molecule to molecule through most substances. For instance, sound waves can easily travel through means such as air, water, drywall, metal and concrete, an inherent capability that presents the primary challenge in many sound reduction and soundproofing endeavors.

If you are in an enclosed room and yell your name, a portion of the sound waves you create will attempt to pass through molecules of the walls, ceiling and floor of the room, while the balance of the wave reflects from these surfaces and remains within the room. Sound transmission refers to the passing of a portion of the energy through the surfaces enclosing the room, while sound reflection refers to the behavior of the energy which reflects and remains within the room.

Sound reflection, or the sound energy that reflected back into the room as you yelled your name, can be further classified by a measurement of the time lapse between the end of the sound’s introduction and its reentry back into the room. Signals with such a time lapse lasting less than 0.1 second are reverberations, while signals exhibiting a time lapse greater than 0.1 second constitute echoes. To understand the distinction between reverberations and echoes, imagine once again yelling your name within an enclosed room. Since you are in close proximity to the walls, ceiling and floor of the room, the sound energy produced reflects quickly back to your ears. Due to the human ear’s inability to distinguish sound signals as recurrent as 0.1 seconds apart, reverberations are interpreted as one lingering sound.

Now suppose you are standing in a giant canyon, enclosed by walls one hundred feet away on either side, and you yell your name with the same intensity as you did in the small room. The longer distance to the walls of the canyon causes sound energy to take longer to reflect off of a wall and return to your ear. With a lapse greater than 0.1 second passing before the reflected sound energy once again reaches your ear, you are able to interpret two separate sound signals, a scenario which exemplifies the defining characteristics of an echo.

The time it takes for a reverberation to weaken by 60 or more decibels and become inaudible is a metric known as reverberation time (RT). Each room has a fixed RT value, which is influenced by such variables as the size and shape of the room, characteristics of the surface textures and the intensity of the original sound energy. With reverberations lasting more than 1.5 – 2 seconds, the human ear can no longer accurately interpret individual sounds, and background noise becomes an issue. For this reason, targeting the capture of sound reflections, eliminating background noise and maintaining RT values below two seconds are common goals across many sound reduction and soundproofing projects.

About the Author: Mark Rustad is President of NetWell Noise Control, based in Minneapolis, MN. Founded in 1991, NetWell is a leading supplier of soundproofing products including acoustic foam as well as online acoustical consulting services. NetWell?s sound management skills are packaged into the industry?s premier website. Discover first hand why so much of NetWell?s business stems from the referrals and repeat orders they receive from satisfied clients around the world.

Be the first to comment - What do you think?  Posted by admin - March 11, 2010 at 4:04 pm

Categories: Audio   Tags: , , , ,

Sound Wave Transmission Through Surfaces

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.

About the Author: Mark Rustad is President of NetWell Noise Control, based in Minneapolis, MN. Founded in 1991, NetWell is a leading supplier of soundproofing products including acoustic foam as well as online acoustical consulting services. NetWell?s sound management skills are packaged into the industry?s premier website. Discover first hand why so much of NetWell?s business stems from the referrals and repeat orders they receive from satisfied clients around the world.

Be the first to comment - What do you think?  Posted by admin - February 28, 2010 at 3:03 pm

Categories: Audio   Tags: , , , ,

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