Decibel

When converted into decibel, the sensitivity of elements should rather stand in the range of +/-3 dB. The time of flight should show very smooth variations corresponding to a mispositioning of the probe or of the active surface, if any. Acceptance criteria depends on the shape of the probe let us just say that for plane probes expected variations should be below... 

For optical transmission, tire parameters of greatest importance are attenuation (i.e. loss) and material dispersion. In effect tliey define tire limits of tire optical communication system. Loss, due to absorjDtion and scattering, limits tire lengtlis between tire transmission nodes. In transmission quality fibre, tire loss is in units of decibels per kilometre. 

Reverberation Control. Reverberation time (T q) is defined as the length of time in seconds for the sound of an instantaneously stopped source in a room to decay by 60 decibels (dB). Reverberation time is one important factor in determining the acoustical character of a space and its suitabiHty for specific activities. For lectures and other speech activities a relatively short reverberation time is desirable so that syllables do not persist and overlap one another, causing difficulty with inteUigibiHty conversely, for music activities, a relatively long reverberation time is desirable to allow blending of the sound and a sense of being surrounded by the music. Without reverberation music usually sounds dull and lifeless. 

I oise Transmission Reduction in HVACSystems. One common use of sound-absorbing treatment is to reduce noise transmission in heating, ventilating, and air-conditioning (HVAC) systems (6). The treatments ate used to reduce the transmission of fan noise and air turbulence noise through ducts into occupied spaces. Noise transmission reduction in duct systems is described in terms of insertion loss, the difference in sound power level or sound pressure level measured at a given location before and after installation of the treatment or sound attenuation, the reduction in sound power between two locations affected by a sound source. The units ate decibels. 

Units and Rating Procedures. The unit of sound pressure level is the decibel (dB), defined as follows where L is the sound pressure level, p is the measured sound pressure, andis the reference sound pressure of 20 p.Pa. TL and AiR also are expressed in decibels. 

AVe in this case can be the peak-to-peak voltage of the oscillator ramp if the method of control is voltage-mode, or the maximum peak voltage representing the primary current within the current-mode method of control. The gain can be converted into decibels, which is shown in Equation B.7. 

This unit is also a decibel. Note that the decibel is a pure number. 

When two sound sources are added, it must be remembered that the decibel is a logarithmic unit. Therefore, they cannot be added arithmetically but must be combined as follows ... 

Sound power is the total energy emitted from a fan that is a function of the fan s speed and point of operation and is independent of the fan s installation and surrounding environment Sound power level is the acoustical power expressed in decibels (dB) radiating from a source. Sound power can be converted into predictable pressure levels (dBA) after the acoustical environment surrounding the fan is defined. Sound pressure for a specific fan varies with... 

When quietness is important, the outlet velocity should be in the range of 1,200 to 2,100 fjtm. The low oudet velocity corresponds to low oudet velocity pressure, and this factor direcdy induences power consumpdon. The velocity should be kept to a minimum, pardcularly when the stadc pressure is low. However, very low oudet velocides (less than 1,000 fpm) are not really desirable, because they produce no advantages, not even quietness. Actual decibel radngs can be attained from the manufacturer, and these are the best indicadon of actual noise level to be expected. 

I.D. of duct, ft decibels sound level polytropic efficiency, = m/m ... 

For convenience, the usual measurement of sound is expressed in decibels (dB), and ratings go from threshold of hearing to threshold of pain (135 dB). Figure 34.11 illustrates the common noise criteria, which can be expressed in sound-pressure levels (SPL). The human ear can detect 1 dB but 6 dB represents a doubling of the SPL, although it would need a 10 dB increase to make it sound twice as loud. 

The power of a source (measured in watts) can be similarly expressed in terms of decibels (in this case, called the sound power level) ... 

Since this would give an inconveniently small scale (it would range from approximately 0 to 14 for a human response) the bell is divided numerically by 10 to give the decibel. The equation therefore becomes ... 

It is possible to use a graph to calculate the addition of decibels, even in the case of multiple additions (Figure 42.2). The graph is used in the following way ... 

Vibration may also be expressed in decibels, and the standard reference levels used are ... 

In practice, measurements are made with the use of an accelerometer. This device is connected to a sound level meter and may make measurements of acceleration in terms of decibels (or by changing scales or use of a device similar to a slide mle, in direct terms). An integrator can be connected between the accelerometer and the meter to express the results in terms of velocity or displacement. 

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