Sound
Sound is the alteration in pressure, stress, displacement of particles and the speed of these, which propagate in an elastic material.
Audible sound is the sensation produced in the ear by very small pressure fluctuations in the air.
Sound pressure
Sound pressure is the intensity of sound. Sound pressure, Lp in decibels is the ratio of the measured pressure P in air to a reference sound pressure Po = 2x10-5 Pascal after the following formula:
Lp (dB) = 10 log10 (∆ P2 / ∆Po2)
The most important point to understand about the pressure level is that,sound pressure level is what is currently measured when a sound information needs to be recorded.
Microphones that measure sound are pressure-sensitive devices and are calibrated to convert sound pressure waves into decibels.
Sound power
Sound power is theenergy of sound. Sound power, LW, in decibels is the calculation ratio between the sound power W, for a reference power, Wo = 1 picowatt, according to the following formula:
Lw (dB) = 10 log10 (W/Wo)
The most important point to remember is that the sound power level is not a measurable value, but is calculated on the basis of the sound pressure measurement.
Sum of multiple sound sources
Since the decibel is a logarithmic function, the numbers are not added together in linear form. Therefore, two sources of 73 dB of sound added together are not equal to 146 dB. The resulting sound would actually be 76 dB. The following table shows how to add decibels from two sound sources.
SOUND SCIENCE AND COOLING TOWERS
Frequency of sound.
Fan noise
• Low/medium frequencies travel long distances, through and around walls and obstructions.
• Very difficult to attenuate. Reduce fan noise by using low-sound fans.
• Understand what is measured and heard in the cooling tower and in a sensitive part of the place.
Water noise
• High frequencies that naturally attenuate with distance. Easily attenuated by walls, trees or other obstructions.
• Fully concealed and drowned out by fan noise a short distance from the cooling tower.
SOUND PRESSURE – THE WEIGHTED SCALE A.
The weighted scale - A, dB (A), is a means of translating what a microphone records to measure sound as the human ear perceives sound.
Sound Highlights:
• +/- 1 dB(A) is inaudible to the human ear
• Decreasing a noise source by 10 dB(A) sounds half as loud to the human ear.
SOUND SPECIFICATION – THE PERFORMANCE OF A TECHNICAL SPECIFICATION
Specify the sound pressure in dB (A) measured at 5 feet above the fan discharge during full-speed operation.
• All manufacturers can achieve the performance of a specification with low-noise alternatives.
• Fan noise is what matters. 5 feet above the cooling tower fan is where it matters.
Place of measurement
By the Institute of Refrigeration Technology (CTI), ATC-128 standard
The location of the sound microphone should be 5 feet above the cooling tower fan and tower edge at a 45° angle.
This position ensures an accurate sound measurement by removing an uncertain source by placing the microphone outside the fan's high air discharge speed.
Easy Verification
At 5 feet from the cooling tower, a sound meter records only that of the cooling tower. Interested parties can easily check the actual noise coming from the cooling tower against the specified sound data with good security.
If sound is specified at 50 feet or greater distance from a sound-sensitive location, there is increased uncertainty in measuring the measured data due to other possible sound sources within the 50-foot radius of the sound microphone.
Sound quality
The sound coming from the top of the cooling tower is composed of fan noise at low and medium frequencies. The low and medium frequencies of the fan that "rumble" are very difficult to attenuate. The rumbling of the fan travels through and around everything, which is audible anywhere sensitive to sound.
The sound coming from the sides of the cooling tower is composed of the noise of water at high frequencies, which is much less objectionable than the noise of the fan and is naturally attenuated by distance.
Acoustic shadow*
"Subjective reactions of the total noise generated by cooling towers indicate that as one moves away from an air intake of a tower, one reaches a point where the noise of the water is concealed by the noise of the fan.
This point coincides with the point at which one comes out of the acoustic shadow of the tower structure, which shields the noise of the water in the suction of the air from the noise of the fan discharge."
* Seelbach and Oran, "What to do about the noise of the Cooling Tower", Industrial Acoustic Company.
The sound measured on the side of a cooling tower is within the acoustic shadow of the noise emitted from the top. Outside the acoustic shadow, the low and medium frequencies of the fan completely hide the noise of the high-frequency water.
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