Audiograms often display a 4,Hz "Notch" in patients who are developing the beginning stages of sensorineural hearing loss. The speed at which sound travels, c, is determined primarily by the density and the compressibility of the medium through which it is traveling.
The speed of sound is typically measured in meters per second or feet per second. Speed increases as the density of the medium increases and its elasticity decreases. For example:. The frequency, wavelength, and speed of a sound wave are related by the equation. The vibrations associated with sound are detected as slight variations in pressure. The range of sound pressures perceived as sound is extremely large, beginning with a very weak pressure causing faint sounds and increasing to noise so loud that it causes pain.
The threshold of hearing is the quietest sound that can typically be heard by a young person with undamaged hearing. This varies somewhat among individuals but is typically in the micropascal range. The reference sound pressure is the standardized threshold of hearing and is defined as 20 micropascals 0.
The threshold of pain, or the greatest sound pressure that can be perceived without pain, is approximately 10 million times greater than the threshold of hearing. It is, therefore, more convenient to use a relative e. Noise is measured in units of sound pressure called decibels dB , named after Alexander Graham Bell. The decibel notation is implied any time a "sound level" or "sound pressure level" is mentioned. Decibels are measured on a logarithmic scale: a small change in the number of decibels indicates a huge change in the amount of noise and the potential damage to a person's hearing.
The decibel scale is convenient because it compresses sound pressures important to human hearing into a manageable scale. By definition, 0 dB is set at the reference sound pressure 20 micropascals at 1, Hz, as stated earlier. At the upper end of human hearing, noise causes pain, which occurs at sound pressures of about 10 million times that of the threshold of hearing. On the decibel scale, the threshold of pain occurs at dB. This range of 0 dB to dB is not the entire range of sound, but is the range relevant to human hearing Figure 3.
Decibels are logarithmic values, so it is not proper to add them by normal algebraic addition. See Appendix B for information on the cumulative effects of multiple sound sources on the decibel level. The decibel is a dimensionless unit; however, the concepts of distance and three-dimensional space are important to understanding how noise spreads through an environment and how it can be controlled. Sound fields and sound power are terms used in describing these concepts. Sound fields are categorized as near field or far field, a distinction that is important to the reliability of measurements.
The near field is the space immediately around the noise source, sometimes defined as within the wavelength of the lowest frequency component e. Sound pressure measurements obtained with standard instruments within the near field are not reliable because small changes in position can result in big differences in the readings.
The far field is the space outside the near field, meaning that the far field begins at a point at least one wavelength distance from the noise source.
Standard sound level meters i. A free field is a region in which there are no reflected sound waves. In a free field, sound radiates into space from a source uniformly in all directions.
The sound pressure produced by the source is the same in every direction at equal distances from the point source. As a principle of physics, the sound pressure level decreases 6 dB, on a Z-weighted i.
This is a common way of expressing the inverse-square law in acoustics and is shown in Figure 4. If a point source in a free field produces a sound pressure level of 90 dB at a distance of 1 meter, the sound pressure level is 84 dB at 2 meters, 78 dB at 4 meters, and so forth. This principle holds true regardless of the units used to measure distance. Free field conditions are necessary for certain tests, where outdoor measurements are often impractical.
Some tests need to be performed in special rooms called free field or anechoic echo-free chambers, which have sound-absorbing walls, floors, and ceilings that reflect practically no sound.
In spaces defined by walls, however, sound fields are more complex. When sound-reflecting objects such as walls or machinery are introduced into the sound field, the wave picture changes completely. Sound reverberates, reflecting back into the room rather than continuing to spread away from the source. Most industrial operations and many construction tasks occur under these conditions.
Figure 5 diagrams sound radiating from a sound source and shows how reflected sound dashed lines complicates the situation. The net result is a change in the intensity of the sound. The sound pressure does not decrease as rapidly as it would in a free field. In other words, it decreases by less than 6 dB each time the distance from the sound source doubles. Far from the noise source--unless the boundaries are very absorbing--the reflected sound dominates.
This region is called the reverberant field. If the sound pressure levels in a reverberant field are uniform throughout the room, and the sound waves travel in all directions with equal probability, the sound is said to be diffuse. In actual practice, however, perfectly free fields and reverberant fields rarely exist--most sound fields are something in between. Up to this point, this discussion has focused on sound pressure. Sound power, however, is an equally important concept.
Sound power, usually measured in watts, is the amount of energy per unit of time that radiates from a source in the form of an acoustic wave. Generally, sound power cannot be measured directly, but modern instruments make it possible to measure the output at a point that is a known distance from the source. Understanding the relationship between sound pressure and sound power is essential to predicting what noise problems will be created when particular sound sources are placed in working environments.
An important consideration might be how close workers will be working to the source of sound. As a general rule, doubling the sound power increases the noise level by 3 dB. As sound power radiates from a point source in free space, it is distributed over a spherical surface so that at any given point, there exists a certain sound power per unit area.
This is designated as intensity, I, and is expressed in units of watts per square meter. Sound intensity is heard as loudness, which can be perceived differently depending on the individual and his or her distance from the source and the characteristics of the surrounding space. As the distance from the sound source increases, the sound intensity decreases.
The sound power coming from the source remains constant, but the spherical surface over which the power is spread increases--so the power is less intense. In other words, the sound power level of a source is independent of the environment. Most noise is not a pure tone, but rather consists of many frequencies simultaneously emitted from the source.
To properly represent the total noise of a source, it is usually necessary to break it down into its frequency components. One reason for this is that people react differently to low-frequency and high-frequency sounds.
Additionally, for the same sound pressure level, high-frequency noise is much more disturbing and more capable of producing hearing loss than low-frequency noise. Engineering solutions to reduce or control noise are different for low-frequency and high-frequency noise.
As a general guideline, low-frequency noise is more difficult to control. Certain instruments that measure sound level can determine the frequency distribution of a sound by passing that sound successively through several different electronic filters that separate the sound into nine octaves on a frequency scale. Two of the most common reasons for filtering a sound include 1 determining its most prevalent frequencies or octaves to help engineers better know how to control the sound and 2 adjusting the sound level reading using one of several available weighting methods.
These weighting methods e. The following paragraphs provide more detailed information. Octave bands, a type of frequency band, are a convenient way to measure and describe the various frequencies that are part of a sound. The center, lower, and upper frequencies for the commonly used octave bands are listed in Table II The width of a full octave band its bandwidth is equal to the upper band limit minus the lower band limit.
For more detailed frequency analysis, the octaves can be divided into one-third octave bands; however, this level of detail is not typically required for evaluation and control of workplace noise.
Electronic instruments called octave band analyzers filter sound to measure the sound pressure as dB contributed by each octave band. These analyzers either attach to a type 1 sound level meter or are integral to the meter.
Both the analyzers and sound level meters are discussed further in Section III. Loudness is the subjective human response to sound. It depends primarily on sound pressure but is also influenced by frequency. Three different internationally standardized characteristics are used for sound measurement: weighting networks A, C, and Z or "zero" weighting.
The A and C weighting networks are the sound level meter's means of responding to some frequencies more than others. The very low frequencies are discriminated against attenuated quite severely by the A-network and hardly attenuated at all by the C-network. Sound levels dB measured using these weighting scales are designated by the appropriate letter i.
In contrast, the Z-weighted measurement is an unweighted scale introduced as an international standard in , which provides a flat response across the entire frequency spectrum from 10 Hz to 20, Hz. The C-weighted scale is used as an alternative to the Z-weighted measurement on older sound level meters on which Z-weighting is not an option , particularly for characterizing low-frequency sounds capable of inducing vibrations in buildings or other structures. A previous B-weighted scale is no longer used.
The networks evolved from experiments designed to determine the response of the human ear to sound, reported in by a pair of investigators named Fletcher and Munson. Their study presented a 1,Hz reference tone and a test tone alternately to the test subjects young men , who were asked to adjust the level of the test tone until it sounded as loud as the reference tone.
The results of these experiments yielded the frequently cited Fletcher-Munson, or "equal-loudness," contours, which are displayed in Figure 6. These contours represent the sound pressure level necessary at each frequency to produce the same loudness response in the average listener. The nonlinearity of the ear's response is represented by the changing contour shapes as the sound pressure level is increased a phenomenon that is particularly noticeable at low frequencies.
The lower, dashed curve indicates the threshold of hearing and represents the sound-pressure level necessary to trigger the sensation of hearing in the average listener. Among healthy individuals, the actual threshold may vary by as much as 10 decibels in either direction. Ultrasound is not listed in Figure 6 because it has a frequency that is too high to be audible to the human ear.
See Appendix C for more information about ultrasound and its potential health effects and threshold limit values. The ear is the organ that makes hearing possible. This is the only remote and TV guide you need. Get daily updates on world affairs or play a quick game when you have few minutes to spare.
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