Building Acoustics Measurements
Building acoustics is the name given to many different types of noise measurements that are carried out in buildings to test the construction of houses, offices, cinemas etc. It can also be used to study the performance of various materials that might be used to control the amount of noise within an enclosure such as a large factory space as part of a noise control program. Noise nuisance problems that at first appear to be caused by excessively noisy neighbors may in fact turn out to be partly due to under-performing building elements and a good understanding of the characteristics of walls and floors will very often assist the noise control engineer in his investigation of an initial domestic noise complaint. Suitable instruments to measure the characteristics of noise within enclosed spaces are the new CEL-400 and CEL-500 series of real time analyzers.
CEL-500 series used for the investigation of room acoustics
One of the primary measurements required in this type of work is to find the reverberation time in an enclosed space. This is important because it helps to describe the liveliness of the enclosed space. The reverberation time varies with frequency and so these measurements are almost always taken in third octave bands or octave bands at the very least. Results of these types of measurements are used to measure the effectiveness of a partition such as a wall or a floor at keeping unwanted noise out of another room. Results of building acoustic measurements are also used to measure the absorption coefficient of various types of materials used in the construction of the room. This information can also be used in industrial situations to reduce the noise level in a factory where there is a large amount of reverberant noise at typical worker stations.
reverberation time decay curve on CEL-500 series analyzer
A controllable electronic noise source is used as a stimulus signal for these measurements which are then measured under the control of the measurement instrument. This is usually done with a real time analyzer because a large number of measurements are required at a number of different positions in the enclosed room in order to get an average result which is representative of the room as a whole. At least 6 measurement positions are usually required from 100 Hz to 3150 Hz third octave band center frequencies. This is at least 96 results for reverberation time analysis and nearly 400 when transmission loss measurements have to be acquired across a partition.
third octave band real time analysis from 12.5 to 20 kHz
Using serial or sequential frequency filters in this situation would be extremely time consuming so real time or parallel frequency analyzers are almost always used. The benefits of real time analysis are the speed at which the multiple data sets can be acquired. This helps to keep the time on site down to a minimum and the interruptions to any other persons in the area is also minimized. Warnings should always be given to other people when high levels of steady or impulsive noise are going to be generated. The measurements are obtained over all the frequency bands in the instrument giving a complete picture from 20 Hz to 20 kHz. The CEL-513 Pink noise generator is an ideal accessory for any of the CEL-500 real time analyzers fitted with the building acoustics option.
pink noise generator to act as controllable noise source
Measurements consist of finding the transmission loss across a part of the building elements such as party walls or from a floor through a ceiling. Noise sources typically used are pink noise generators which produce noise energy over the whole audio bandwidth of interest. This broadband signal is fed into a high power amplifier and loudspeaker so that accurately controlled noise levels may be produced. The real time analyzer turns the noise source on and off at the right time and uses special switching signals to gather the correct measurements for the analysis of the raw data. As an alternative, a high noise level impulse is often used as the stimulus signal. This type of noise source has the advantage of being easier to carry around and can be generated from a starting pistol or a helium balloon but the noise level produced may not contain enough energy at all the frequencies of interest. Impulses generated like this are also somewhat variable in output and so may not trigger the data capture reliably and correctly every time.
measuring the reverberation time in a room
Spectral noise levels are required on both sides of a common party wall or from outside to inside when assessing the impact of traffic noise on a building facade. Frequencies cover the range from 100 Hz up to 4 or 5 kHz typically when carrying out the measurements to international standards. Methodologies are laid down in many ISO and ASTM standard documents. Reference should be made to these standards to ensure that the correct measurement results are collected before leaving site so that the calculations can be carried out when results have been transferred to a laptop computer. Special software is available to perform these calculations according to the appropriate methods for each task.
noise reduction through a wall as a function of frequency