Understanding Acoustics

In buildings, acoustic treatment is added to reduce the impact of sound entering into specific areas.
Sound is controlled by three means:

  1. Firstly treatment of the source (e.g. building an enclosure around the source such as around compressors, or adding acoustic treatment to the area where the sound is coming from)
  2. Secondly treatment of the path of the noise (e.g. erection of barriers such as building walls on freeways)
  3. And finally treatment at the receiver end. (E.g. wearing earplugs)

There are two types of acoustic functions that are of concern in this manual: sound insulation and noise absorption.

The acoustic treatments discussed relate not only to the treatment of the path of the sound, such as walls, but also Hueintek has devised acoustic treatments that not only reduce sound transmission of the path but also treat the source room and receiving rooms, so that noise levels are reduced.

Common Sound Insulation Terms

Rw

Rw is the weighted sound reduction index in dB (decibels) and it describes the airborne sound insulating power of a building element. It is a laboratory-measured value as defined in ISO717 Part 1. It can apply to walls, ceiling/ floors, ceiling/roofs, doors, or windows. The higher the number, the greater the sound insulating power of the building element. It is measured over the frequency range 100 to 3150Hz and replaces the old measure STC (Sound Transmission Class) that was measured over 125 to 4000Hz. In many cases the Rw has a value of one point less than the STC value for the same construction.

An increase in the Rw of a wall by 6 to 10 points will reduce the perceived loudness of sound passing through the wall by about half. Detailed below is how the sound insulating effectiveness of walls depends on their Rw (or Rw + Ctr values).

Rw +Ctr

Rw + Ctr are Rw with the addition of a low frequency sound correction factor Ctr. (a negative number). The use of Rw + Ctr has become more relevant due to the increase in low frequency sound sources such as surround sound systems, traffic and aircraft noise, drums and bass guitars. Two walls can have the same Rw rating, but have different resistance to low frequency sound, thus a different Rw + Ctr.

DnTw

DnTw is the equivalent of Rw, but measured onsite. Rw is the value measured in an acoustic laboratory, while DnTw is measured on-site. An on-site measured value of DnTw + Ctr is permitted to be 5 points lower than the Rw + Ctr value. Where the BCA may call for an Rw + Ctr > 50, the same requirement may be satisfied by measuring DnTw + Ctr > 45 on-site.

Ln,w +Ci

Ln,w + Ci describes how easily impact sound travels through a wall or floor. Impact sound is generated by sources such as dryers, washing machines and heeled shoes on a wooden floor. Unlike Rw values, better performing walls or floors have lower values. Therefore when specified, Ln,w + Ci values are maximums while Rw values are minimums. For example, the BCA requires some floors to have Ln,w + Ci < 62.

Impact Sound Isolation and walls

Walls that have an Impact Sound Insulation requirement are defined in the BCA as walls that do not have any rigid mechanical connection between two separate leaves except at the perimeter. Discontinuous Construction is defined in the BCA as walls that have a gap of at least 20mm between two separate leaves. Double stud plasterboard walls connected only at the perimeter are classed as ‘discontinuous’.

NRC

A materials ability to absorb sound is measured by its sound absorption coefficient and is often expressed in terms of a Noise Reduction Coefficient (NRC) The NRC is the arithmetic average of the product’s sound absorption coefficients at 250, 500, 1000 and 2000Hz. The sound absorption coefficients of products are measured according to ISO140-3. An NRC of 0.4 means 40 % of sound is absorbed by the material. Typical building materials such as plasterboard have an NRC of approximately 0.04 which means only 4% sound will be absorbed.

Glossary of Terms

Absorption : The amount of sound energy incident upon a surface, which is absorbed, changed to heat energy, and not reflected from that surface as sound.

Acoustic impedance : The ratio of sound pressure at the surface to the volumetric velocity through it.

Acoustics : The science of sound including its transmission, production and effects.

Attenuation : The reduction of sound intensity, measured in decibels that is caused by any factor such as distance or the presence of absorbing materials.

British thermal unit (Btu) : The quantity of heat required to raise the temperature of one pound of water by one degree Fahrenheit.

Convection : The transfer of heat by mass movement within a fluid.

Decibel (dB) : A unit of sound intensity. It is the logarithmic ratio, to the base 10, of the intensity of sound to be reported compared to a standard reference sound intensity. Practically, a change in intensity of 2 dB is not noticeable to the ear, 5 dB is noticeable and 10 dB is twice as loud (or soft). The decibel is a commonly reported as dB(A), where the A reference conforms to the response of the ear.

Dew point : The temperature at which a sample of air, with no change in pressure or water vapor content, becomes completely saturated, i.e., attains a relative humidity of 100%.

Emissivity : A rating of the ability of a material to give off heat as radiant energy. It is expressed as the percentage of energy given off by a perfect black body radiator. Flanking transmission: The transmission of sound between two points by an indirect path.

Frequency : The number of vibrations per second. The unit is Hertz (Hz) which represents the number of vibrations per second. Heat capacity: The heat required to raise the temperature of a given mass of a substance by one degree.

Heat transmission coefficient (U) : A unit expressing heat passage through a complete building section, including air films. Technically, heat transmission is measured in Watts per square metre, per degree C of temperature difference from air to air for a composite building section. It is used as a basis for determining transmitted heat loss or gain.

Insertion loss : The sound attenuation achieved by insertion of a sound absorbent material.

Intensity : Sound intensity is proportional to the sound power or energy.

Latent heat : The heat required at constant temperature to cause a change of state in matter.

Mass law : The attenuation or sound transmission loss due solely to the surface density or weight per square metre of a material. In general, a doubling of mass will improve attenuation by 4 to 5 dB for a homogeneous material. A non-homogeneous structure such as a partition wall can give much greater attenuation due to stiffness, vibration and reverberation control. For example, use of infilling in a light-weight partition can improve attenuation by 3 to 6 dB, with little increase in mass of the structure.

Noise criteria (NC) curves : Specify the maximum noise levels permitted in each octave band for a specified single rated NC number. Normally used in architectural specifications.

Noise rating (NR) curves : Similar to noise criteria curves but based on a different standard.

Noise reduction coefficient (NRC) : A single-number sound absorption coefficient used as a guide to the average acoustic performance of a material. It is the arithmetic average of sound absorption coefficient at 250, 500, 1000 and 2000 Hz. (to the nearest 0.05)

Octave : Two sound frequencies are an octave apart when one frequency is twice the other.

Reflectivity : A measure of the ability of a material to reflect heat moving by radiation through air. It is expressed as a ratio of radiant heat reflected by a material to the radiant heat reflected by a perfect mirror under similar conditions.

Relative humidity : The ratio of the partial pressure of water vapour in a given sample of air to the saturation pressure of water vapour at the same temperature.

Resonance : A buildup in amplitude of vibrations of a structure, which occurs when the frequency of the sound impinging on the structure corresponds to the natural frequency of the structure.

Reverberation : The persistence of sound in a room after the sound source has stopped. The greater the absorption in the room, the lower the reverberation. Sensible heat: The heat required to change the temperature of a substance without a change of state. Sound

absorption coefficient (SAC) : The proportion of incident sound energy which is not reflected. Sound breakout: The residual sound energy transferring through the walls of a duct system from a generated sound source within the system.

Sound transmission class (STC) : A single number rating displayed on standardized curves which represents the dB transmission loss performance over all frequencies. The higher the STC, the more efficient the structure.

Specific heat : The ratio of the heat capacity of a given mass of a substance to that of the same mass of water, at a given temperature.

Surface coefficient : Refers to the thermal conductance of an air film immediately adjacent to an exposed surface of a material. Commonly called the `f’ factor.

Thermal conductance (c) : A unit specifying the amount of heat, in Watts, that passes through a square metre of material which has a given thickness and one degree Celsius of temperature difference between its surfaces. Used to compare insulating efficiencies of material with varying but stated thicknesses or materials composed of two or more basic materials.

Thermal conductivity (k) : A unit expressing the amount of heat, in Watts, that passes through a square metre of material which is exactly one metre thick and has one degree Celsius of temperature difference between its surfaces. Used for comparing the insulating efficiencies of homogeneous materials.

Thermal radiation: The transmission of heat through space by wave motion.

Thermal resistance (R m2K/W): The reciprocal of thermal conductance; it is used in calculating overall heat transmission coefficients. It can be specified as material/product R or as a Total system R value.

Thermal resistively (r or 1/k): The reciprocal of thermal conductivity.

Transmission loss: The reduction in sound intensity across a wall. It is the ratio of incident sound to sound transmitted through the wall.

Vapor barrier: A material that does not readily permit the passage of water vapor. Normally a material is rated atone perm or less. (Perm = a vapor transmission rate of 1 grain of water vapor per square foot, per hour, per inch of mercury pressure difference).

Vapor permeability: A rating of material giving the amount of water vapor that passes through a precise thickness of the material.

Watt: A unit of rate of heat loss or gain from a material, measured as joules per second.