Sound pressure level

Fig. 1. Approximate relationship between sound pressure level and distance as a function of room absorption. Numbers on the curves are metric sabins.

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. 

Noise reduction (AIR) is the difference in the average sound pressure level between the source room and the receiving room. When the receiving room is relatively reverberant and the measurements are made in the reverberant fields of the two rooms the relationship between TL and AIR is as follows, where S is the surface area of the sound barrier between the two rooms and is the amount of sound absorption in the receiving room (7). 

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. 

Rotating vanes are provided in the rooms to ensure diffuse sound fields. Sound is introduced into the source room, the average sound pressure level is measured in one-third octave bands in both rooms, and the sound-transmission loss is calculated as follows, where and die average sound... 

The electric motors are often the noisiest component of the centrifuge assembly. Most standard motors in the 75—250 kW range develop noise levels of 85 dbA (weighted sound pressure level using filter A, per the ANSI standard). A quiet motor can reduce this level by 5 dbA and should be used whenever noise is of concern. 

Disturbance or load variable Sound pressure level Manipulated variable Number of constraints Mass flow Mass of reactants Molecular weight... 

Sound strength is therefore location-depeudent and is often quantified as a sound pressure level (Lp) in dB based on the root-mean-square (rms) sound-pressure (ps) value, where... 

For airborne sound, the reference pressure is 2 X 10" Pa (29 X psi), which is nominally the human threshold of hearing at 1000 Hz. The corresponding sound pressure level is 0 dB. Conversation is about 50 dB, ana a Jackhammer operator is subject to 100 dB. Extreme levels such as a jet engine at takeoff might produce 140 dB at a distance of 3 m, which is a pressure amplitude oi 200 Pa (29 X 10" psi). These examples demonstrate both the sensitivity and wide dynamic range of the human ear. 

There are two approaches to fluid-generated noise control—source or path treatment. Path treatment means absorbing or blocking the transmission of noise after it has been created. The pipe itself is a barrier. The sound pressure level inside a standard schedule pipe is roughly 40-60 dB higher than on the outside. Thicker walled pipe reduces levels some at more, and adding acoustical insulation on the outside of the pipe reduces ambient levels up to 10 dB per inch of thickness. Since noise propagates relatively unimpeded inside the... 

FIG. 8-79 Valve -generated sound pressure level spectnims. 

Noise level Motors must conform to the requirements of lEC 60034-9. A safe sound pressure level for a human body to perform better, during an 8-hour... 

The links between levels of exposure and inconvenience caused by ventilation noise are described in an investigation carried out on office workers.- Technical measurements and analyses of the ventilation noise at 155 typical office workplaces were in this study combined with assessments by the office workers of the level of disturbance that they experienced, the effect on working performance, fatigue, stress-related pain, and headaches. The average noise level was about 40 dB(A) at two of the workplaces, while it was about 35 dB(A) at two others. It emerged from rhe narrow-band analyses that the sound pressure levels of rhe infrasound were not in any event of an order that this type of sound frequencies (below 20 Hz) could contribute to any disturbance effects. Any steps taken to counter the sound frequencies of the ventilation noise under 50 Hz, i.e., the point of btersection between the threshold curve of auditory perception and the spectral level distribution curve of... 

Earlier experiments indicate clearly that a lowered sound pressure level can be an effective measure to reduce the inconvenience reactions due to a ventilation noise, provided that it is targeted at the most critical frequency range from the point of view of influence or that the measure results in a general lowering over the entire spectral range of the ventilation noise. 

Velocity, m s , in Allowable sound pressure level, dB(A), in the room ... 

It is important to quantify the sound pressure levels in dB generated by each source and for each frequency (31.5-8000 Hz) in order to establish which noise will be masked or prevalent. It must be noted that when the pressure levels of two noises differ by more than 10 dB, the resulting level is equal to that of the higher-level source in other words, the noise at the higher level masks the noise at the lower level, which will not be perceptible to the listener (or the phonometer). In this case it is useless to reduce the latter noise, as the composed noise will remain the same, being influenced by the higher-level noise only. 

Concerning the sound pressure level, when a noise generated by an HVAC system or an air-handling unit increases the ambient background noise by a dB, the noise increase is just perceptible. On the contrary, an increase oi 5 dB or more is clearly perceptible. 

As regards the noise spectrum, the different situations can be analyzed ap proximately with NC (noise criterion) and NR (noise rating) curves (Fig. 9.6.3). NC and NR curves define the octave band limits of an acceptable back ground noise each of them is characterized by a number representing the sound pressure level at 1000 Hz. 

A more recent method of analysis uses the RC (room criterion) curves. In this case it must be calculated the arithmetical mean of sound pressure level at 500, 1000, and 2000 Hz. The obtained value identifies the specific RC curve. The noise is classified as rumbly (with excess of energy at low frequencies) if it is under 500 Hz and its sound pressure level exceeds the RC value by 5 or more dB. The noise is classified as hissy (with excess of energy at high frequencies) if it is over 500 Hz and its sound pressure level exceeds the RC value by 3 or more dB. 

Unsteady noise can be evaluated by a phonometer, which measures the sound pressure level for a time period of noise fluctuation and gives the time-weighted average value. 

Actual position in band depends on FD fan size and acoustics of boiler house Figure 23.14 Sound-pressure levels for Saacke rotary cup burners (with permission of Saacke Ltd, Portsmouth, UK)... 

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. 

Noise from a compressor plant arises at different sources, and each source has its own pattern of sound pressure levels. Noise levels can be divided into two groups, the low-frequency pulsating air intake sound and the higher-frequency machine noise from compressor, prime mover and fans. Local statutory regulations on noise levels should be determined, and action taken by the supplier to ensure that the noise levels do not exceed those stipulated. 

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. 

The total acoustic power of a source can be related to the sound pressure level at distance r by the following equation (assuming spherical propagation) ... 

If the sound pressure level at a distance fo is known it is possible to calculate the sound pressure level at positions r quite easily ... 

All manufacturers now publish sound pressure levels for their products and such figures should be scrutinized and compared as part of a fan-selection decision. Fans are statically, and sometimes dynamically, balanced by the manufacturer. If it is necessary to dismantle a fan for transport, it should be rebalanced on commissioning, imposing a load close to that ultimately required. 

A plof of fhe real part of the relative heat release response for three Lewis numbers is shown in Figure 5.1.10. This plot was calculated for a reduced activation energy y3 = 10 and a burnf gas femperafure of 1800 K, represen-fative of a lean hydrocarbon-air flame. Note fhaf fhe order of magnitude of fhe relative response of fhe flame is only a little more than unity. This is a relatively weak response. For example, a sound pressure level of 120 dB corresponds to a relative pressure oscillation p /p = 2 X10 so fhe fluctuation in the heat release rate will be of fhe same order of magnifude. 

Note <3>, equivalence ratio modulation frequency SPL, sound pressure level A, mean flame surface area fluctuation of flame area v, mean velocity at the burner outlet and v, imposed velocity... 

The overall sound pressure level reaches 91 dB. The pressure spectrum in Figure 5.2.8b is quite similar to that associated with the flame-plate interaction in Figure 5.2.8a. The presence of harmonics of the fundamental frequency indicates that the pressure signal is also periodic with an oscillation frequency corresponding to the flame oscillation frequency, but that the flame response is nonlinear with a rich harmonic content. These energetic harmonics indicate that the... 

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