Sound levels, addition

If a number of measured values of equivalent accuracy are available, the determination of the average or mean can be done arithmetically, so long as the sound levels do not differ by more than lOdB. If the differences are larger, quadratic (i.e., energy-related) averaging will have to be applied. The rules of sound level addition can be adopted for the purpose the average is given by... 

Physics-based synthesis can provide extremely high quality and expressivity in a very compact algorithm. Such computational models can provide extremely low bit rates at very high quality levels for certain sounds. In addition to data compression applications, such models can also provide a foundation for the future evolution of musical instruments, moving it from the real world of wood and metal into the virtual world where formerly impossible modifications are easily tried out. 

In this section we very briefly review the principles of sound reflection. Additional introductory material is presented in the preceeding papers in this publication, and a more complete analysis appears in standard acoustics textbooks such as that of Kinsler and Frey (1), or Pierce (2), or on a broader introductory level that of Crawford (3). 

The sound level experienced at a distance from an explosion can be heavily depended on transient atmospheric conditions (e.g. temperature inversions, wind shear, and even gusts of wind). The low frequency air pressure from a blast can cause vibration in buildings, in addition to its startle effect. Such vibration can give rise to concern about possible building damage at relatively low over-pressure levels. The subject has been investigated in depth by the United States Bureau of Mines, whose conclusions are summarised in [33.11]. 

Unless specified otherwise in applicable standards, the sound level should not exceed 85 dB at 1 m from casing/surface and 1.5 m above the equipment base. The control of sound to meet this requirement shall be the sole responsibility of the vendor. Suitably designed silencers or acoustic enclosures should be provided by the vendor at no additional cost to purchaser. 

If the number of sources is small, the addition procedure for arriving at a total sound level can be simplified by using a nomogram (Fig. 17). The higher sound level is increased by an amount depending on the difference in level. If there are more than two levels, the procedure is done step by step. It is advisable to perform the addition with values of one decimal place and round the final result to a whole number of dB. 

Addition of dedbels Since decibels are logarithmic units, they are not added arithmetically. Decibels are added by converting them to power, intensity, or pressnre adding these quantities arithmetically and then converting them back to decibels. The mathematics involved may be somewhat complex for people not accustomed to working with logarithms, so charts have been developed for convenience. See Fig. 4.97 for one example of such a chart. To add the sound levels of 80 dB and 74 dB that are produced individually by... 

Audible alarm signals - it is essential that audible alarm systems are sufficient in nature and volume that all those persons for whom it is intended are able to recognise it for what it is. In general a sound level of 60 to 65 dB(A) is considered appropriate. Additional requirements will be required when workplaces have higher background noise levels or where people will need to be roused from sleep. 

A corresponding composite probe with the same frequency and crystal size, however, detects the test flaw much better the echo has a 12 dB higher amplitude (see Fig. 4) and in addition, the noise level is much lower, resulting in an improved signal to noise ratio. This effect is especially observed at high sound attenuation. However, in materials with low attenuation or in case of shorter sound paths the standard probe yields a comparable good signal to noise ratio. 

Since rms pressure variations have to be measured in the range 20 x 10 N/M to 200 N/M (a range of 10 ) it can be seen that an inconveniently large scale would have to be used if linear measurements were adopted. Additionally, it has been found that the ear responds to the intensity of a sound (a P ) in a logarithmic way. The unit that has been adopted takes these factors into account and relates the measured sound to a reference level. For convenience, this is taken as the minimum audible sound (i.e. 20 x 10 N/M) at 1 K. 

Alloying additions are made to improve the performance of an anode material. Of equal importance is the control of the levels of impurity in the final anode, since impurities (notably iron and copper) can adversely affect anode performance. Thus careful quality control of the raw materials used and the manufacturing process adopted is essential to sound anode production. This too is discussed below. 

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