SoundPressureLab - Sound Pressure Measurement Objectives:...

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Sound Pressure Measurement Objectives: 1. Become familiar with hardware and techniques to measure sound pressure 2. Measure the sound level of various sizes of fan modules 3. Calculate the signal-to-noise ratio of acoustic measurements 4. Investigate the noise of the fan modules as a function of size and operating speed Background Sound can be described as a tiny variation in air pressure, detected by some receiver. Typical sound pressure amplitudes range from a few micropascals to a few hundred Pascals that fluctuate around atmospheric pressure (~101,000 Pascals). This variation in air pressure travels as a longitudinal wave, i.e. a compression wave, which propagates throughout the medium (air) at a velocity called the sound speed. In air at 20 ° C, the speed of sound is about 344 m/s. In general, the tiny variation of pressure oscillates very rapidly in time around the atmospheric pressure. In the audio-range, these oscillations occur between 20 and 20,000 times per second. In other words, audible sound is made up of frequencies between 20 and 20,000 Hz. The frequency components in this wave are perceived as different tones and pitches by the human ear. 20 Hz corresponds to a very low pitch while 20,000 Hz corresponds to a very high pitch. Most noises are composed of many different frequencies. The Brüel and Kjær Type 2236 Precision Integrating Sound Level Meter, as shown in Figure 1, can be used to measure the magnitude of sound pressure levels (SPL’s). Common reasons that sound pressure level is measured include: 1. The evaluation of noise emanating from a product to determine how the product could be made less annoying to customers. For example, automotive companies perform these measurements to determine the sound that a car engine produces when it is running. The Department of Transportation takes SPL readings to measure noise coming from interstate highways. 2. The measurement of noise to evaluate if an environment is damaging to human ears. For example, a factory must evaluate the noise level that the workers are exposed to while working. This involves comparison of the levels measured to published standards by regulating agencies such as OSHA (Federal Occupational Safety and Health Administration). One of the most important components of a sound level meter is the microphone Most of the high quality sound level meters have a condenser microphone. This type of microphone is constructed with a thin metal foil stretched like a drumhead over a frame. It is always protected by a guard because it is fragile. When the slight changes in pressure, caused by a sound wave, reach this diaphragm, the diaphragm moves relative to a rigid metal backplate. The capacitance (the ability of two closely spaced metal plates to store energy in an electric field) changes with the movement of the diaphragm. This change in capacitance is converted to a milli-volt signal, which goes on to a preamplifier. The preamplifier in a microphone-sound measurement system is
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This note was uploaded on 09/08/2010 for the course ME 120 at San Jose State University .

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SoundPressureLab - Sound Pressure Measurement Objectives:...

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