Sound intensity levels

Sound intensity level is defined in a similar manner to sound pressure level. In this case the equation is ... 

It can thus be seen that it is important not only to express the unit but also to state sound pressure level, sound intensity level or sound power level. 

Table 3-7 Sound Intensity Levels for a Variety of Common Activities1...

Figure 13.3 Decibel scale and the relation between the Sound Power Level (Lb ), Sound Intensity Level (L/), and Sound Pressure Level (Lp).

The sound-energy flux density level. In decibels of sound, intensity level equals 10 times the logarithm to the base 10 of the ratio of the intensity of this sound to the reference intensity. See also Decibel (dB). 

Sound pressure refers to a pressure change arriving at some location. The sound pressure level Lp in decibels is the pressure level relative to a value of 2xl0 N/m Sound intensity at some location is the average rate at which sound energy moves through a unit area normal to the direction of propagation. Units for sound intensity are joules per square meter per second (J/m /s). Sound intensity level Li in decibels (dB) is the sound intensity relative to 10 W/m. ... 

The reference sound pressure used here is the sound pressure of a sinus sound with a frequency of 1 kHz the human ear is barely able to perceive it (hearing threshold). The sound energy penetrating through the area unit per unit of time is described as sound intensity, sound density, or sound level J. The value of sound intensity at the hearing threshold is approximately /(,= 10 W/m. This value permits a definition of the sound intensity level with the help of Equation (6-2) ... 

Figure 6.48 shows examples of various sound intensity levels. 

Figure 6.48 Examples of different sound intensity levels. The sound intensity factor reveals the amount of sound by which a specific sound is louder than a specific sound close to the MAF. Most sound measurements are conducted approximately 7m distant from the source of the sound or noise.

An essential environmental factor which may affect safety is sound. Vibrations of the air of frequencies from between 16 to approximately 20,000 Hz are perceived by the human ear in the form of sound. The hearing threshold lies approximately at 20 iPa (0.2 x 10 bar) of sound pressure, corresponding to a sound intensity level of 0 dB. At a sound pressure (intensity) level of approximately 130 dB, man s pain threshold will be reached. Some actual sound levels, measured at various distances, can be taken from the following table ... 

The sound intensity level generated by a source of sound within its effective range depends on the properties of the source of sound (sound output, directional characteristics, sound spectrum), the location of the source and the receiver, the terrain (and on any obstacles in the path of sound), the local propagation conditions determined by local topography, growth and cultivation, and from meteorological conditions. 

The results of application of the sound propagation model for a technical installation situated in a level environment without any structures are shown in Figures 7.37 and 7.38. Therein, the lines of equal sound intensity level are depicted in increments of 5 dB. 

Figure 7.37 Sound propagation emanating from a technical installation defined by lines of equal sound intensity level at a light SE breeze (.2 m/s). 

Sound intensity and sound intensity level Sound intensity 7 is the sound power radiated in a specified direction through a unit area normal to the direction of propagation in units of W/m. The sound intensity level (IL) is the designation in decibels of the ratio of two intensities and is expressed by ... 

The quantity pc is called the specific acoustic impedance of the medium it denotes the ratio of the acoustic pressure to the velocity of the vibrating particle. In acoustics the relative sound intensity level is usually expressed by means of a logarithmic quantity k, measured in decibels (db). If sound of intensity/i is compared with sound of intensity J2,k is given by ... 

Li, Ij2 = sound pressure levels, dB LI = sound intensity level, dB... 

Tlie relationship between sound pressure level and sound intensity level. When the sound intensity level is measured in a free field in air, then the sound presstme level and sotmd intensity level in the direction of propagation are numerically the same. In practice most measurements of the sound intensity are not carried out in a free field, in which case there will be a difference between the sotmd pressure and intensity levels. This difference is an important quantity and is known by several terms, such as reactivity index, pressure-intensity index, P-7 index, phase index, or LK value. This index is used as a field indicator to assess the integrity of a measurement in terms of grades of accuracy or confidence limits. This will be considered in more detail later in this section. 

The equipment under test is divided into a convenient number of subareas that are selected to enable a well-controlled probe sweep over the subarea. Guidance is given on the sweep rate and the line density. Measurement accuracy is graded according to the global pressure-intensity index, LK This is the numerical difference between the sound intensity level and the sound pressure level. If this... 

Referring again to Fig. A-14, the sound intensity level measured by the casing of the ventilation fan was 94 dB(A). It would not normally be possible to measure the output from this fan accurately, using a sound level meter, in this situation because of the relatively high sound pressure level in this area due to other sources. 

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