Acoustical materials are a variety of foams, fabrics, metals, etc. used to quiet workplaces, homes, automobiles, and so forth to increase the comfort and safety of their inhabitants by reducing noise generated both inside and outside of those spaces. Acoustical materials are used in two major ways: as soundproofing, by which noise generated from outside a given space is blocked from entering the space; and, as sound absorbing, where noise generated within a space is reduced inside the space itself. As an example of soundproofing, a school might construct a special wall to isolate the music room from a general classroom next door. As for sound absorbing, a machine shop might install barriers to block and absorb the acoustic energy of a noisy air compressor.

In this article, the focus will be on the blocking and absorbing methods, as those are most traditionally associated with acoustical materials.

There are several specifications of the acoustic properties of acoustical materials that are measured and delineated by manufacturers and which are used to quantify the effectiveness of the material for handling sound or noise. The most common of these include:

The absorption coefficient, or sound absorption coefficient,  is defined as the portion of sound energy incident on a material’s surface that is not reflected.

The specific acoustic impedance is defined as the product of a material’s density and its acoustic velocity. The effectiveness of acoustic material to absorb sound energy depends upon the frequency of the sound, with the mid-to-high ranges being more effectively muted by most materials than the lower frequencies.

A so-called noise reduction coefficient establishes a material’s average absorption coefficient at frequencies of 250, 500, 1000, and 2000 Hz., and is useful for comparing a material’s effectiveness at absorbing noise in general. In special applications, such as recording studios, the noise reduction coefficient is less useful because it does not cover the lower base range frequencies which tend to present the biggest problem. In these situations, using the absorption coefficient at the frequency in question can make a better determination of a material’s effectiveness; unfortunately, noise reduction coefficients for various commercial materials are often published whereas the absorption coefficients generally are not.

Materials used for soundproofing are given an STC, or Sound Transmission Class,  a rating which quantifies how well a material blocks transmission at frequencies associated with speech. Like the Noise Reduction Coefficient associated with absorptive materials, the STC rating does not give a good indication of a material’s effectiveness at blocking low- or high-frequency sounds, such as mechanical noise or music.

Sound for human-occupied environments is measured by an A-weighted sound level scale, which reduces the impact of high and low frequencies to better match the human ear’s response to the middle ranges. This scale, with units of dBA, is sometimes referred to as noise level and is a selectable feature of most sound meters. The dB refers to decibels, which is logarithmic versus linear scale. Table 1 below illustrates the relationship between the decibel scale and the corresponding energy. Note that every 10 dB change adds an order of magnitude (factor of 10) to the energy level.

Acoustic foam does not block sound. It is used in sound absorption applications such as gymnasiums to reduce reverberations as noise travels and bounces off reflective surfaces. To block sound, acoustics specialists rely on mass primarily, while eliminating any gaps in walls where sound can leak through and eliminating, as well, any mechanical couplings between inner and outer walls. Soundproof sheetrock, mass loaded vinyl barriers, and sound isolation clips are some of the products used for soundproofing. Sometimes laminated materials are used to reduce the transmission of sound through walls by disrupting the resonant frequencies of like materials. There are also specialized products available for use in construction such as soundproof windows, sound reduction windows, and sound resistant doors.

The standard unit of sound absorption, or Sabin, is based on the amount of absorption a 1 ft-square section of material will achieve over a frequency range of 125-4000 Hz.

It is often impractical or undesirable to completely cover a room with acoustic material. In these instances, discrete placement of material in small patches proves more effective than placing it all in one spot. Likewise, placing material asymmetrically—on non-parallel walls, for instance—usually achieves a greater quieting effect, as does locating the material near corners and edges rather than in the centers of walls. Situations like these can often be tricky to quiet, and often the advice of an acoustical contractor or an architect familiar with acoustical construction can be helpful.

Commercially available soundproof drywall, or pre-damped drywall, achieves its reduction in sound transmission by sandwiching a layer of viscous damping glue between two sheets of usually dissimilar materials of unequal thickness. Manufacturers claim a several-times reduction in sound transmission over ordinary drywall. Installation of these materials has to abide by the manufacturers’ instructions to achieve their maximum effectiveness. Do-it-yourselfers can sometimes obtain good results by mimicking the idea behind pre-damped drywall. Acoustic caulk or other forms of sound dampening sealants, marketed under a number of tradenames, are available for this purpose. Other forms of materials can be applied to help deaden or dampen sound including Liquid Applied Sound Deadeners (LASD) or sound dampening liquid coatings.

Mass loaded vinyl is a generic term for several commercial products which use heavy vinyl sheeting installed behind sheetrock as a means of blocking sound transmission. By being loosely draped between stud attachments, the material flexes as sound waves impinge upon it, prohibiting the transfer of their energy to the more-rigid interior walls. Again, the materials must be installed according to manufacturers’ instructions to be effective; keeping the product behind sheetrock maintains a structure’s fire resistance.

Another method is lead-sheet soundproofing, also referred to as acoustic or acoustical lead, which, given the material’s high density, proves an effective method of reducing sound transmission through walls. In many applications, the lead is bonded to foam on both sides and then adhered to sheetrock.

Sound isolation clips provide a mechanical means of decoupling inside walls from exterior structures. Usually consisting of rubber-fitted mounts that hold nailing strips, these clips provide a convenient method of isolating a room from external noises. As with any of these soundproofing methods, clips are designed only to work on airborne sound. Noise coming through structures, plumbing, etc. will remain unmuffled by their use.

Soundproofing is used in many other settings besides buildings. Aircraft, for example, are soundproofed to protect crew and passengers from engine noise and high-frequency flight noise. Aircraft insulation soundproofing often has thermal resistance as well to insulate cabins and cockpits from extreme outside temperatures. Vendors also supply materials for general aviation aircraft.

Automobiles are fitted with a variety of soundproofing materials to protect passengers from engine, airflow, and road noise.

Sound absorption materials come in a variety of styles allowing them to integrate aesthetically to indoor offices and the like. They are also available in special laminated versions for industrial applications such as machinery enclosures, automobile headliners, etc.  Acoustic foam is usually of the open cell variety as closed-cell structures tend to reflect sound rather than absorb it. Foam surfaces can be flat or topological—the familiar “egg-crate” pattern being representative of the form.

What is the material used in acoustic foam? One popular product in these applications is melamine foam, which can often be purchased as composite sheets and as 2 x 2 ft. squares and 2 x 4 ft. rectangles in thicknesses of 1 and 2 inches. The material is usually applied to the walls of enclosures, etc. with adhesive. A variety of spray-applied acoustical insulations are available as well which can be applied over large interior walls as a way of reducing reverberation while producing a finished interior surface. Acoustical foams are available in many bulk forms, including sheets and rolls, and also as tape. Melamine foam is manufactured from thermosetting melamine resin which produces a very fine, open cell, fiber-less structure which is fire resistant and easily die-cut, routed, or sliced to produce an abundance of shapes. There are instances where syntactic foam has been used for noise attenuation, and even aluminum foams have been applied for noise control.

For interior spaces, the primary objective is reducing the echoes produced as these make hearing speech difficult. Absorption is the principal method for doing this. Studies have shown that a general inhabited space, one not devoted to sound recording, for instance, can be made too quiet which is uncomfortable for many people. That is where combining absorptive shapes with shapes that diffuse sound has proven effective.

Acoustical absorbers are available both as hanging acoustical baffles and as sound dampening room dividers. Sometimes referred to as sound abatement panels, these come in an array of designs and constructions to complement the décor of the spaces in which they are installed. They can be flat, permanently installed dividers or moveable, soundproof accordion room dividers.

When it comes to sound absorption in machinery enclosures, server rooms, etc., a variety of materials and methods are available. Acoustical blankets are used in industrial settings to set up noise absorption around loud machinery and are typically rated to withstand the high temperatures so often associated with industrial operations. These can serve as temporary or permanent noise absorbers. The same blankets, which are usually made of some combination of acoustical foam for absorption and mass loaded vinyl for blocking, can be built as full, custom enclosures to completely surround offending machinery. Pipelines can also be covered with similar materials to minimize the transference of noise through them. OSHA has specific guidelines for manufacturing plants, among them that the continuous noise exposure in an 8-hour shift should not exceed 90 dBA (a time-weighted average, or TWA), and the highest impact or percussive noise exposure should not exceed 140 dBA.

Where high-frequency noise in plants generated by turbine-driven pumps and compressors can be controlled with barriers, many reciprocating machines produce low-frequency vibrations which are harder to attenuate with absorbent barriers: very often these vibrations are transmitted into the floor and the structure of the building. This noise is best attacked by isolating the machines from the structure with resilient pads or isolation mounts.

There are a few subjective measures associated with sound, among them loudness and quality. Loudness is expressed in units of phons to provide a qualitative measure, a phon being defined as the pressure level in dB of a 1000 Hz. tone against which a person compares another pure or complex tone. Another measure, the sone, is defined as the loudness of a 1000 Hz., 40 dB tone. A relationship can be plotted between phons and sones.

Quality is the subjective measure of the pleasantness or unpleasantness of sound which can make sounds of the same loudness appear more or less disturbing. Music typically combines fundamentals and harmonics to produce a pleasing sound, whereas in the case of noise, the frequencies are irregular and random. Among the attributes used to describe sound quality are pitch and timbre. Pitch is determined mainly by frequency, then intensity and wave shape while timbre is determined mainly by wave shape, then by intensity and frequency. It is these differences that enable us to distinguish the same note played on a brass horn and a woodwind as coming from different instruments.

As mentioned, sound can find its way through the smallest gaps in an otherwise impenetrable barrier, a phenomenon known as “flanking.” Where air passes, so too will sound. Cracks which provide clearance for ordinary doors to function provide enough clearance for noise to pass. Penetrations through walls for piping and ductwork will also permit sound to pass, as will the pipes themselves, and these gaps should be properly sealed. Where barrier walls adjoin other walls or floors, a sealant can be used as well to close up any gaps.

A few rules of thumb apply to soundproofing and sound absorbing. Usually, doubling the mass of a wall will produce a noticeable reduction in the level of sound coming through it. When adding sound absorption to a room, usually it takes a 3 dB reduction in sound levels to affect a perceptible difference.

Most of the material described herein relates to common industrial sound and noise problems found in offices, factories, etc. There are special cases where the acoustics of a room are important from the standpoint of being able to hear the sounds without losing their liveliness: concert halls, recording studios, etc. Here is where diffusors come into play. They create a sense of spaciousness in a room by scattering sound waves to reduce echo and standing waves, giving the sound better clarity.

In addition to organizations that sponsor standards for industrial noise control such as OSHA and ANSI, the following organizations can provide useful information on various aspects of acoustic materials.

This guide provides a basic understanding of acoustic materials and their use in soundproofing and sound absorbing for industrial, residential, and commercial environments. For more information on related products consult our other guides or visit the Thomas Supplier Discovery Platform to locate potential sources or view details on specific products.

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