Soundproofing - AbsoluteAstronomy.com

Soundproofing is any means of reducing the sound pressure

Sound pressure

Sound pressure or acoustic pressure is the local pressure deviation from the ambient atmospheric pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water...

with respect to a specified sound

Sound

Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

source and receptor. There are several basic approaches to reducing sound: increasing the distance between source and receiver, using noise barrier

Noise barrier

A noise barrier is an exterior structure designed to protect sensitive land uses from noise pollution...

s to reflect or absorb the energy of the sound waves, using damping structures such as sound baffle

Sound baffle

A sound baffle is a construction or device which reduces the strength of airborne sound. Sound baffles are a fundamental tool of noise mitigation, the practice of minimizing noise pollution or reverberation. An important type of sound baffle is the noise barrier constructed along highways to...

s, or using active antinoise sound generators.

Two distinct soundproofing problems may need to be considered when designing acoustic treatments - to improve the sound within a room (See anechoic chamber

Anechoic chamber

An anechoic chamber is a room designed to stop reflections of either sound or electromagnetic waves.They are also insulated from exterior sources of noise...

), and reduce sound leakage to/from adjacent rooms or outdoors. Acoustic quieting

Acoustic quieting

One of the major reasons for the development of acoustic quieting techniques was for making submarines difficult to detect by sonar. This military goal of the mid- and late-twentieth century allowed the technology to be adapted to many industries and products, such as computers , automobiles , and...

, noise mitigation

Noise mitigation

Noise mitigation is a set of strategies to reduce noise pollution. The main areas of noise mitigation or abatement are: transportation noise control, architectural design, and occupational noise control...

, and noise control

Noise control

Noise control is an active or passive means of reducing sound emissions, often incentivised by personal comfort, environmental considerations or legal compliance. Practical and efficient noise control is wholly reliant on an accurate diagnosis of what is causing the noise, which first involves...

can be used to limit unwanted noise. Soundproofing can suppress unwanted indirect sound waves such as reflections

Reflection (physics)

Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...

that cause echoes

Echo (phenomenon)

In audio signal processing and acoustics, an echo is a reflection of sound, arriving at the listener some time after the direct sound. Typical examples are the echo produced by the bottom of a well, by a building, or by the walls of an enclosed room and an empty room. A true echo is a single...

and resonance

Resonance

In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...

s that cause reverberation

Reverberation

Reverberation is the persistence of sound in a particular space after the original sound is removed. A reverberation, or reverb, is created when a sound is produced in an enclosed space causing a large number of echoes to build up and then slowly decay as the sound is absorbed by the walls and air...

. Soundproofing can reduce the transmission of unwanted direct sound waves from the source to an involuntary listener through the use of distance and intervening objects in the sound path.

Distance

The energy density of sound waves decreases as they spread out, so that increasing the distance between the receiver and source results in a progressively lesser intensity of sound at the receiver. In a normal three dimensional setting, with a point source and point receptor, the intensity of sound waves will be attenuated according to the inverse square of the distance from the source.

Damping

Damping means to reduce resonance

Resonance

In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...

in the room, by absorption or redirection (reflection or diffusion). Absorption will reduce the overall sound level, whereas redirection makes unwanted sound harmless or even beneficial by reducing coherence. Damping can reduce the acoustic resonance

Acoustic resonance

Acoustic resonance is the tendency of an acoustic system to absorb more energy when it is forced or driven at a frequency that matches one of its own natural frequencies of vibration than it does at other frequencies....

in the air, or mechanical resonance

Mechanical resonance

Mechanical resonance is the tendency of a mechanical system to absorb more energy when the frequency of its oscillations matches the system's natural frequency of vibration than it does at other frequencies...

in the structure of the room itself or things in the room.

Absorption

Absorbing sound spontaneously converts part of the sound energy to a very small amount of heat in the intervening object (the absorbing material), rather than sound being transmitted or reflected.

There are several ways in which a material can absorb sound. The choice of sound absorbing material will be determined by the frequency distribution of noise to be absorbed and the acoustic absorption profile required.

Porous absorbers

Porous absorbers, typically open cell rubber

Rubber

Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...

foams or melamine

Melamine

Melamine is an organic base and a trimer of cyanamide, with a 1,3,5-triazine skeleton. Like cyanamide, it contains 66% nitrogen by mass and, if mixed with resins, has fire retardant properties due to its release of nitrogen gas when burned or charred, and has several other industrial uses....

sponges, absorb noise by friction within the cell structure.

Porous open cell foams are highly effective noise absorbers across a broad range of medium-high frequencies. Performance is less impressive at low frequencies.

The exact absorption profile of a porous open cell foam will be determined by a number of factors including the following:

Cell size
Torosity
Porosity
Material thickness
Material density

Resonant absorbers

Resonant panels, Helmholtz resonators

Helmholtz resonance

Helmholtz resonance is the phenomenon of air resonance in a cavity, such as when one blows across the top of an empty bottle. The name comes from a device created in the 1850s by Hermann von Helmholtz, the "Helmholtz resonator", which he, the author of the classic study of acoustic science, used to...

and other resonant absorbers work by damping a sound wave as they reflect it.

Unlike porous absorbers, resonant absorbers are most effective at low-medium frequencies and the absorption of resonant absorbers is always matched to a narrow frequency range.

Reflection

In an outdoor environment such as highway engineering, embankments or panelling are often used to reflect sound upwards into the sky.

Diffusion

If a specular reflection

Specular reflection

Specular reflection is the mirror-like reflection of light from a surface, in which light from a single incoming direction is reflected into a single outgoing direction...

from a hard flat surface is giving a problematic echo then an acoustic diffuser

Diffusion (acoustics)

Diffusion, in acoustics and architectural engineering, is the efficacy by which sound energy is spread evenly in a given environment. A perfectly diffusive sound space is one that has certain key acoustic properties which are the same anywhere in the space. A non-diffuse sound space would have...

may be applied to the surface. It will scatter sound in all directions.

Room within a room

A room within a room (RWAR) is one method of isolating sound and stopping it from transmitting to the outside world where it may be undesirable.

Most vibration

Vibration

Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...

/ sound transfer from a room to the outside occurs through mechanical means. The vibration passes directly through the brick, woodwork and other solid structural elements. When it meets with an element such as a wall, ceiling, floor or window, which acts as a sounding board

Sounding board

A sound board, or soundboard, is the surface of a string instrument that the strings vibrate against, usually via some sort of bridge. The resonant properties of the sound board and the interior of the instrument greatly increase loudness over the string alone.The sound board operates by the...

, the vibration is amplified and heard in the second space. A mechanical transmission is much faster, more efficient and may be more readily amplified than an airborne transmission of the same initial strength.

The use of acoustic foam

Acoustic foam

Acoustic foam is an open celled foam used for acoustic treatment. It attenuates airborne sound waves by increasing air resistance, this reducing the amplitude of the waves. The energy is dissipated as heat. Acoustic Foam can be made in several different colors, sizes and thickness. It is Best...

and other absorbent means is less effective against this transmitted vibration. The user is advised to break the connection between the room that contains the noise source and the outside world. This is called acoustic de-coupling. Ideal de-coupling involves eliminating vibration transfer in both solid materials and in the air, so air-flow into the room is often controlled. This has safety implications, for example proper ventilation must be assured and gas heaters cannot be used inside de-coupled space.

Noise cancellation

Noise cancellation generators for active noise control

Active noise control

Active noise control is a method for reducing unwanted sound.- Explanation :...

are a relatively modern innovation. A microphone is used to pick up the sound that is then analyzed by a computer; then, sound waves with opposite polarity (180° phase at all frequencies) are output through a speaker, causing destructive interference and cancelling much of the noise.

Residential soundproofing

Residential soundproofing aims to decrease or eliminate the effects of exterior noise. The main focus of residential soundproofing in existing structures is the window

Window

A window is a transparent or translucent opening in a wall or door that allows the passage of light and, if not closed or sealed, air and sound. Windows are usually glazed or covered in some other transparent or translucent material like float glass. Windows are held in place by frames, which...

s. Curtains can be used to damp sound either through use of heavy materials or through the use of air chambers known as honeycomb

Honeycomb

A honeycomb is a mass of hexagonal waxcells built by honey bees in their nests to contain their larvae and stores of honey and pollen.Beekeepers may remove the entire honeycomb to harvest honey...

s. Single-, double- and triple-honeycomb designs achieve relatively greater degrees of sound damping. The primary soundproofing limit of curtains is the lack of a seal at the edge of the curtain. Double-pane windows achieve somewhat greater sound damping than single-pane windows. Significant noise reduction can be achieved by installing a second interior window. In this case the exterior window remains in place while a slider or hung window is installed within the same wall openings.

Noise barriers as exterior soundproofing

Since the early 1970s, it has become common practice in the United States and other industrialized countries to engineer noise barrier

Noise barrier

A noise barrier is an exterior structure designed to protect sensitive land uses from noise pollution...

s along major highways to protect adjacent residents from intruding roadway noise

Roadway noise

Roadway noise is the collective sound energy emanating from motor vehicles. In the USA it contributes more to environmental noise exposure than any other noise source, and is constituted chiefly of engine, tire, aerodynamic and braking elements...

. The technology exists to predict accurately the optimum geometry for the noise barrier design. Noise barriers may be constructed of wood, masonry

Masonry

Masonry is the building of structures from individual units laid in and bound together by mortar; the term masonry can also refer to the units themselves. The common materials of masonry construction are brick, stone, marble, granite, travertine, limestone; concrete block, glass block, stucco, and...

, earth or a combination thereof. One of the earliest noise barrier designs was in Arlington, Virginia adjacent to Interstate 66

Interstate 66

Interstate 66 is an Interstate Highway in the eastern United States. As indicated by its even route number, it runs in an east–west direction. Its western terminus is at Middletown, Virginia, at an intersection with Interstate 81; its eastern terminus is in Washington, D.C., at an...

, stemming from interests expressed by the Arlington Coalition on Transportation

Arlington Coalition on Transportation

The Arlington Coalition on Transportation is a citizens group formed to oppose the construction of Interstate 66 through Arlington, Virginia. The focal activity of ACT was the conduct of a lawsuit filed against the Virginia Department of Transportation in 1971 in district court...

. Possibly the earliest scientifically designed and published noise barrier construction was in Los Altos, California

Los Altos, California

Los Altos is a city at the southern end of the San Francisco Peninsula, in the San Francisco Bay Area. The city is in Santa Clara County, California, United States. The population was 28,976 according to the 2010 census....

in 1970.