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 1,000 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 140 dB. This range of 0 dB to 140 dB is not the entire range of sound, but is the range relevant to human hearing (Figure 3).
Octave bands, a type of frequency band, are a convenient way to measure and describe the various frequencies that are part of a sound. A frequency band is said to be an octave in width when its upper band-edge frequency, f2, is twice the lower band-edge frequency, f1: f2 = 2 f1.
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Each octave band is named for its center frequency (geometric mean), calculated as follows: fc = (f1f2)1/2, where fc = center frequency and f1 and f2 are the lower and upper frequency band limits, respectively. The center, lower, and upper frequencies for the commonly used octave bands are listed in Table II-1.
The width of a full octave band (its bandwidth) is equal to the upper band limit minus the lower band limit (bandwidth = f2 - f1). 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.
Presbycusis is a gradual sensorineural hearing loss associated with aging. The onset and the degree of hearing loss can vary considerably and is related to genetics, other impacts such as an accumulation of diseases, medications, and the cumulative effect of noise in the modern environment. Presbycusis and noise induced hearing loss appear to be additive and both can contribute to hearing loss in older people. Both types of hearing loss affect the upper range of an audiogram. A sloping audiogram with tapering to the lowest levels at 8,000 Hz often indicates that the hearing loss is aged-related, but after years of exposure, noise-induced hearing loss can have the same pattern. As humans begin losing their hearing, they often first lose the ability to detect quiet sounds in the high frequency range. This progresses to difficulty understanding conversations in noisy environments, even when amplified by hearing aids.
The upper frequency of audibility of the human ear is approximately 15 to 20 kilohertz (kHz). This is not a set limit: some individuals may have higher or lower (usually lower) limits. The frequency limit normally declines with age.
Example: Many ultrasonic welders have a fundamental operating frequency of 20 kHz, a sound that is at the upper frequency of audibility of the human ear. However, a good deal of noise may be present at 10 kHz, the first subharmonic frequency of the 20-kHz operating frequency, which is audible to most people.
Controlling noise by locating or relocating the source should be considered for the design and equipment layout of new plant areas and for reconfiguring existing production areas. A simple rule to follow is to keep machines, processes, and work areas of approximately equal noise level together, and separate particularly noisy and quiet areas by buffer zones having intermediate noise levels. In addition, a single noisy machine should not be placed in a relatively quiet, populated area. Reasonable attention to equipment layout from an acoustical standpoint will not eliminate all noise problems, but it will help minimize the overall background noise level and provide more favorable working conditions.
Octave bands: Sounds that contain energy over a wide range of frequencies are divided into sections called bands, each one octave. A common standard division is in 10 octave bands identified by their center frequencies, 16; 31.5; 63; 250; 500; 1,000; 2,000; 4,000; 8,000, and 16,000 Hz. For each octave band, the frequency of the lower band limit is one-half the frequency of the upper band limit. This is the most common type of frequency analysis performed for workplace exposure evaluation and control. An alternative frequency band, the one-third octave band, is defined as a frequency band such that the upper band-edge frequency, f2, is the cube root of two times the lower band frequency, f1: f2 = (2)1/3f1. The level of detail provided by one-third octave bands, however, is rarely required for occupational noise evaluation and control.
Ultrasound is any sound whose frequency is too high for the human ear to hear. (The upper frequency that the human ear can detect is approximately 15 to 20 kilohertz, or kHz, although some people can detect higher frequencies, and the highest frequency a person can detect normally declines with age.) Most of the audible noise associated with ultrasonic sources, such as ultrasonic welders or ultrasonic cleaners, consists of subharmonics. Even though the ultrasound itself is inaudible, the subharmonics it generates can affect hearing and produce other health effects.
Chief Barrera joined the Fire Department in November 2004 where she served as the Assistant Chief and Chief of the Financial Management Division. In June 2021, Chief Barrera was appointed as Deputy Fire Chief of the Administrative Bureau and worked closely with internal and external stakeholders to improve standard business practices, ensure administrative and fiscal compliance, and foster a workforce that is representative of the communities we serve. In October 2022, Chief Barrera was assigned as Acting Chief Deputy of Business Operations. 2ff7e9595c
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