Soundproofing

Soundproofing is a means of reducing the sound pressure respecting to a specified sound source and receptor. It also includes noise control. There are several approaches to reduce sound: increasing the distance between receiver and source, using noise barriers that are reflectinf or absorbing the energy of the sound, using damping structures like sound baffles, using active antinoise sound generators.

Two different soundproofing problems may need to be considered when you are designing acoustic treatments. So, you need to improve the sound within a room, and reduce sound leakage from-/to adjacent rooms or outdoors. Soundproofing can suppress some unwanted indirect sound waves such as reflections that cause resonances and echoes. These consequently cause reverberation. Soundproofing reduces the transmission of unwanted direct waves from the source to a listener through the use of distance and some objects in the sound path.

Distance

The use of distance is straightforward. Energy density of waves is decreasing as they spread out. Increase in the distance between the source and receiver results in a much lesser intensity of sound at the receiver site. In a normal three dimensional setting, with a point receptor and point source, the intensity of acoustic waves will be attenuated according to the inverse square of distance.

Damping

Damping reduces resonance in the room. It absorbs or redirects (reflection or diffusion) sound. Absorption is able to reduce the overall sound level. Whereas redirection makes possible the unwanted sound to be harmless or even beneficial. It reduces coherence. Damping reduces the acoustic resonance in the air. It also reduces mechanical resonance in the structure of the room itself.

Absorption

Absorbing sound is performed easily. Part of the sound energy spontaneously converts to a very small amount of heat in the surrounding objects, rather than sound being reflected or transmitted.

Absorption within fluids and hard solids is low. Soft materials such as neoprene and lead are better. It is even higher in composite materials. For instance foam made from plastic or rubber containing gas open cells or bubbles, emulsion like bitumen and aerosols such as moisture vapor in humid air. Many layers of material can be used to reduce reflections between the interfaces, and to absorb energy from the waves of panels as well as compress waves within each individual material.

To avoid unwanted reflection care must be taken. Transmission or resonance may arise from using stiffness or mass, rather than damping.

Reflection

In highway engineering and other outdoor environments, panelling and embankments are often used to reflect sound into the sky.

Diffusion

An acoustic diffuser may be applied to the surface if a specular reflection from a hard flat surface is giving a problematic echo. It will scatter sound in multiple directions.

Room within a room

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

Most sound/vibration transfer from a room to the outer world occurs through mechanical means. Vibration passes directly through the woodwork, brick and other solid structural elements. When it meets with an element such as a ceiling, wall, floor or window, which acts as a sounding board, the vibration amplifies and is heard in the second space. A mechanical transmission is much faster and more efficient. It may be more readily amplified than an airborne transmission of the same strength.

The use of acoustic foam and other absorbent material means is less effective against transmitted vibration. It is advised to break the connection between the room that contains the sound source and the outside world. It is named acoustic de-coupling. Ideal de-coupling involves the elimination of vibration transfer in both solid materials and in the air space. So air-flow into the room is often controlled. This has safety implications. For instance proper ventilation should be assured. Gas heaters cannot be used inside de-coupled space.

Cancellation of the noise

Noise cancellation generators are active noise controllers. They are a relatively modern innovation. A microphone is used to pick up the sound. The sound is then analyzed by a computer. After that, sound waves with opposite polarity (180° phase at all frequencies) are spread through a speaker. These waves are causing destructive interference cancelling much of the noise.

Residential soundproofing

Residential soundproofing aims to eliminate or decrease the effects of external noise. The window is the main focus of residential soundproofing in existing structures. Curtains can be used to damp sound waves either through the use of air chambers known as honeycombs or through use of heavy materials. Single-, double- and triple-honeycombs in designs achieve relatively greater degrees of sound waves damping. The main soundproofing limit of curtains is the lack of a seal at the edges of the curtains. Double-pane windows may achieve somewhat greater sound damping comparing to single-pane windows. Significant noise reduction may be achieved by installing another interior window. The exterior window remains in place in this case while a hung or slider window is installed within the same openings of the wall.

Noise barriers for exterior soundproofing

Since the early 1970s it has became common practice in the USA to engineer noise barriers along main highways. These barriers are protecting adjacent residents from intruding highway noise. This practice was later followed later by many other industrialized countries. There is a technology that predicts accurately the optimum geometry for the future noise barrier. Noise barriers may be constructed of masonry, wood, earth or a combination these materials. One of the earliest noise barrier designs was applied in Arlington, Virginia adjacent to Interstate 66. Possibly the first scientifically designed and published noise barrier system was in Los Altos, California in 1970.