Boundary and mean frequencies of octave bands
Laboratory work 6
Study of production noise. Noise spectrum.
Measurement methods
Objective: studying the main characteristics of industrial noise and principles of its regulation, acquaintance with the methods of measurement and means of protection against noise, draw general conclusions and proposals on the protection of workers from occupational noise.
Basic concepts and definitions
The main sources of noise in buildings and structures of various purposes, and at the sites of industrial enterprises are machines, equipment, vehicles and etc.
Causes of noise can be mechanical, aerodynamic and electromagnetic phenomena. Mechanical noise is caused by impact processes, friction in the machine parts and others. Aerodynamic noise arises in the flow of liquids or gases. Electromagnetic noises occur during the operation of electrical machines.
People react differently to noise. The same dose of noise exposure in some people causes hearing damage, while in others not, some of these injuries may be harder than others. Noise – a different kind of sounds, hinders the perception of useful signals, breaks the silence or has harmful effects on the human body.
Sound is an oscillation of the medium (solid, liquid or gaseous), in which it is distributed. Sound propagating in the air is called air sound, and spreads throughout the material (designs) is called structural sound.
It includes affordable for measurement characteristics – intensity of sound I, sound pressure P and the speed of sound c.
The intensity of sound is characterized by a flow of energy that carries the sound per unit area (W/m2). The interrelation between sound intensity I and the sound pressure P as follows:
, (6.1)
where P – sound pressure (the difference between the instantaneous value of the total pressure and value of medium pressure, which is observed in medium in the absence of the sound field); ρ – density of the medium kg/m3; c – the speed of sound in the medium, m/sec.
For the measurement of sound intensity and parameters such as pressure and sound power, a relative logarithmic unit is introduced, called the sound pressure level, or the level of intensity of Li,
(6.2)
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where I0 – intensity of the sound corresponding to the threshold level,
I0 = 10-12 W/m2.
The human ear and many acoustic instruments react not to the intensity of the sound, but on the sound pressure P:
(6.3)
where P0 – threshold sound pressure (audibility threshold), P0 = 2∙10-5 Pa.
The interrelation between the intensity level and sound pressure level from the formula
(6.4)
whereρ0, c0– accordingly medium density and speed of sound under normal atmospheric conditions (t = 20 ° C, P0 = 2∙ 0–5 Pa).
The sound level is measured in decibels (dB), 1 dB = 0.1 B.
If you have the room a few noise sources the total sound pressure level:
(6.5)
where n – number of noise sources; Li–noise levels from each sources.
If there are «n» identical noise sources with Li levels, the overall sound pressure level
, (6.6)
The summation of sound pressure levels is done in accordance with Code 51.13330.2011 «Sound protection. Actually operating edition of SNiP 23-03-2003».
The noise spectrum
The important characteristic of a sound is the dependence of its level on frequency (f). The lower limit of perception by a person of a sound is about 20 Hz, and top limit – about 20000 Hz. Dependence of level of a sound on frequency is called a noise spectrum.
Definition of a sound energy flux density for each frequency would demand the infinite number of measurements therefore all possible frequency range is divided into octaves. An octave frequency band – a frequency band in which the top boundary frequency (f1) is twice more larger than lower (f2). For each octave count geometric mean value of frequency:
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. (6.7)
Boundary and geometric mean (within these limits) frequency are shown in Table. 6.1.
Table 6.1
Boundary and mean frequencies of octave bands
| Boundary frequencies of octave bands, Нz | 45-90 | 90-180 | 180-335 | 335-710 | 710-1400 | 1400-2800 | 2800-5600 | 5600-11200 |
| Geometric mean frequencies of octave bands, Hz | 63 | 125 | 250 | 500 | 1000 | 2000 | 4000 | 8000 |
Depending on the frequency, which accounts for the maximum sound pressure level, the spectrum character can be:
a) Low frequency (maximum – below 300 Hz);
b) Middle range (maximum – in the 300 ... 800 Hz);
c) High frequency (up to – 800 Hz above).
On character of a noise spectrum can be subdivided also:
– broadband, with continuous spectrum width of more than one octave; this means that each frequency octave corresponds to a certain level of noise (for example, operation of the fan);
– tonal (narrowband), in which range are available to hear discrete tone (for example, noise during the work of a circular saw).
According to temporary characteristics noise are subdivided:
– constant which sound level for an 8 hour working day changes in time no more than on 5 dBA at measurements on the temporary characteristic «slowly» of an audio-noise meter;
– inconstant, which sound level for an 8 hour working day changes in time more than 5 dBA at measurements on the temporary characteristic «slowly» of an audio-noise meter.
Inconstant noises are subdivided:
– fluctuating on time which sound level continuously changes in time;
– intermittent which sound level sharply falls to the level of background noise; and duration of intervals when sound level remains constant and exceeding the level of background noise for 1 second or more;
– impulse, consisting of one or several audible signal, everyone lasting less than 1 second, at the same time sound levels of dB measured at characteristics «slowly» and «impulse» of audio-noise meter, differ not less than on 10 dB.
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