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Most of the benefits of using acoustical insulation are realized in the construction of lightweight partition walls. However, the benefits of systems with high STC ratings can be lost because of improper installation or poor construction details. Sound flanking paths, sound leaks and structural short circuits due to fasteners are a few conditions that decrease the effectiveness of sound insulating systems.
The ASTM standard practice, E 497, “Installing Sound-Isolating Lightweight Partitions,” provides recommendations for preventing situations or conditions that will detract from the acoustical performance of various types of partitions, such as wood and steel stud walls, floor-ceiling assemblies and roof-ceiling systems. Combinations of dense sound barrier materials, like gypsum board, sheet metal or wood framing, and acoustical caulk can block many above ceiling, between floor-ceiling, and below floor sound flanking paths.
Sound waves can travel through any media, which includes air, water, wood, masonry or metal. The type of media through which sound travels determines whether the sound is either airborne or structureborne. Airborne sound is directly transmitted from a source into the air. All sound that reaches your ear is airborne. Some examples of airborne sound are passing traffic, music or voices from an adjacent room, or the noise from machinery and aircraft.
Structureborne sound travels through solid materials, either from direct contact with the sound source or from an impact on the material. All structureborne sound must eventually become airborne sound in order for people hear it, otherwise, the disturbance is felt as a vibration. Examples of structureborne noise are footsteps, door slams, plumbing vibrations, mechanical vibrations and rain impact.
Most noise control situations require that both airborne and structureborne sound be considered. Effective sound control addresses both sound paths by controlling, or reducing, noise at the source, reducing paths or blocking noise along its path, or shielding the receiver from the noise.
Sound transmission loss is the decrease in sound energy – expressed in decibels of airborne sound –as it passes through a building construction. The metric used to quantify that reduction is the sound transmission classification, STC. The STC value indicates how well sound is controlled room-to-room, including through walls or through floor/ceiling assemblies.
ASTM E 90 is the standard covering airborne sound transmission class or STC. This is a single number rating that evaluates the efficiency of systems in reducing the transmission of airborne noise. In this class the higher the STC rating the better. The rule of thumb is that a 10-point increase in STC means a decrease in the perceived noise by one-half.
ASTM E 1414 is the standard covering ceiling attenuation class or CAC. The rating is similar to STC but in this case measures the efficiency of a suspended ceiling connected by a common air plenum at reducing airborne noise between two rooms. The higher the CAC number the better.
Environmental acoustics studies the characteristics and performance of materials, products, systems and services related to the science of sound and the effect on the surrounding environment. Sound waves travel through air creating very small changes in atmospheric pressure. The sensation of hearing, produced by vibrating the eardrum, is due to these small pressure changes.
The sound wave’s alternating fluctuation, above and below the static atmospheric pressure creates a sound pressure. Sound can sometimes be perceived through the vibration of a body or surface. However, sound is generally regarded as a disturbance in the air, like waves in the sea, but instead of just spreading out in circles on the surface, sound spreads out in spheres in three dimensions, like expanding soap bubbles, one inside the next.
Impact sound transmission loss is expressed in decibels of airborne sound. This decrease in sound energy is measured after the impact noise that's generated above transfers through the floor-ceiling assembly and is transmitted into the air below. Imagine someone hopping around upstairs, over your head. That’s impact sound transmission. It’s rated using an impact insulation class number, an IIC number.
The standard for measurement is ASTM E 492. The impact insulation class number, the IIC number, is a single number rating that estimates the impact sound insulation performance of floor/ceiling systems. The number is an estimate of how much the sound energy is reduced. The higher the number, the better the system.
Sound absorption, is the ability of a material to absorb sound waves rather than reflect sound waves. When we talk about absorption, building materials are measured for their noise reduction coefficient, or NRC. There’s also a second measurement method to calculate absorption, the sound absorption average, SAA. Fundamentally, sound absorption, or the lack of it, is concerned with controlling sound energy within rooms and enclosed spaces.
Sound absorption of a building material is measured using ASTM C 423. NRC is an arithmetic average (rounded off to the nearest 0.05) of the sound absorption capability of a product at only four frequencies: 250, 500, 1000, and 2000 hertz. These frequencies are representative of the center range of human speech. NRC is a single decimal rating between 0 and 1, used to express the absorption properties of materials. Generally speaking, an NRC of 0.55 is average performance and anything above an NRC of 0.70 is considered good for acoustical ceiling tile systems. The higher the NRC the better the material is at absorbing sound energy.