Architectural Acoustics FAQ's   1.1 What is reverberation time? 1.2 What is sound absorption coefficient? 1.3 What is the difference between insulation & absorption? 1.4 How is sound insulation measured? 1.5 How do I improve the noise insulation of my house/dwelling? 1.1 What is reverberation time? The time for sound in a room to decay 60 decibels. Scientific work on room acoustics was pioneered by Wallace Clement Sabine 1868-1919 (see his Collected Papers on Acoustics, 1922). The reverberation time, T, is defined as the time taken for sound energy to decay in a room by a factor of one million (60 dB). It is dependent on the room volume and the total amount of sound insulation or absorption material contained in the room. In metric units 0.161 x room Volume T = ---------------------------------------------- sum of Surface areas x absorption coefficients In US English units, dimensions are in feet and the constant is 0.049. Top of page 1.2 What is sound absorption coefficient? The absorption coefficient of a material is ideally the fraction of the randomly incident sound power which is absorbed, or otherwise not reflected. It is standard practice to measure the coefficient at the preferred octave frequencies over the range of at least 125Hz - 4kHz. It can be determined on small material samples with an "impedance tube" or on large samples in a laboratory "reverberation room". The impedance tube evaluates sound absorption at normal incidence only, and produces absorption values that are sightly lower than those found in the reverberation room where the "Sabine coefficient" is measured over a wide range of incidence angles. For the purposes of architectural design, the Sabine coefficient is preferred, though the normal incidence absorption may be used in the absence of any other information. Interestingly some absorbent materials are found to have a Sabine coefficient in excess of unity at higher frequencies. This is due to diffraction effects. Where this occurs the value can be taken at face value for small material patches and as 1.0 for very large absorbers (entire walls). The Odeon computer program includes a file of absorption coefficients. Top of page   1.3 What is the difference between insulation & absorption? There is often confusion between sound insulation and sound absorption. Sound insulation prevents sound from traveling from one place to another, such as between apartments in a building, or to reduce unwanted external noise inside a concert hall. Material like polyurethane (spray foam insulation) is a very effective material for sound insulation. Sound is absorbed when it encounters a material which will convert some or all of it into heat, or which allows it to pass through not to return. For this reason good sound absorbers do not of themselves make good sound insulators. Sound insulators rarely absorb sound. Sound absorbers contribute little to sound insulation. They are treated separately in sound control design. Top of page   1.4 How is sound insulation measured? The measurement method depends on the particular situation. There are standards for the measurement of the insulation of materials in the laboratory, and for a number of different field circumstances. Usually Test procedures (e.g. ASTM E-90 in the lab and E336 in the field) generate a loud and consistent broadband spectrum of steady noise on one side of a partition or specimen of the material under test, then measure the amount of this sound that passes through that material. The ratio of the incident sound to the transmitted sound is the "noise reduction", usually expressed as 10 time the logarithm of this ratio. If the noise reduction is also corrected for the amount of sound absorption to be found in the receiving room, 10 times the logarithm of the corrected ratio is called the "transmission loss. This is performed for 1/3 octave bands of noise from 100 to 4000 Hz. A single-number rating of that range of noise reductions or transmission losses van be had by fitting them to a segmented curve. In North America, this procedure is ASTM E413. The fitted range is from 125-4000 Hz. The value of that curve at 500 Hz is called the Noise Isolation Class (NIC) or Sound Transmission Class (STC) respectively. Internationally, ISO140-3 produces the noise reduction and transmission loss data in the same way. But the single number rating is according to ISO 717 which uses data in the 100-3150 Hz range. This single number rating is called "R'" and "R" respectively. Similar methods are applied to impact ("footfall") noise (a problem in multifamily residential buildings). A standard tapping machine is used to hammer on the floor, lightly and steadily at the rate of 10 taps per second. The sound pressure level in the room below are measured. ASTM E492 and ISO 140-4 and 717 apply. (See ASTM e-33 Web Site.) Top of page   1.5 How do I improve the noise insulation of my house/dwelling? This is one of the most commonly asked questions of noise consultants. Firstly you should consider whether it is noise insulation or sound absorption (see 4.3) that is really required. Sound insulation is most often asked for in order to keep out unwanted noise, but is occasionally requested for the purpose of minimizing disturbance to others. The method of noise insulation will depend on the exact situation; generalities are extremely difficult to devise. Situations are more often than not unique, depending on the nature of the building infrastructure that the architect or his informal successors have devised. More often than not, successful noise isolation improvement requires the advice of a competent and experiences person and at an early stage of the renovation. However, it's not impossible, just a bit more difficult. The following ideas may serve as initial guidelines. When the noise is from an external source such as a main road it may be possible, if planning authorities permit, to screen with a noise barrier. These can be effective providing that the direct line of sight between traffic and house is concealed by the barrier. The weak point for sound transmission to and from a building is most often via the windows. Double glazing will usually afford noticeably better protection than single glazing, but in areas of high external noise it might be preferable to have double windows with a large air gap (25 to 100 mm) and acoustic absorbent material on the perimeter reveal around that gap. For a few people, the resultant lower room background noise level can make noise transmitted through party walls more apparent. The fitting of new windows may reduce the level of air ventilation, and it will be vital to compensate for this, if necessary with by improving the noise insulation of certain party walls. Noise through party walls can be reduced by the addition of acoustic insulation. This is constructed from a layer of sound insulating material, polyurethane (spray foam insulation), installed in the wall to provide acoustic quilting. Top of page   Contact an All Tech Insulation engineer today (432-897-1533) for a deeper discussion about your specific acoustic requirements.