OHS 314 Chapter 3 Part 3: Physiology of the Auditory System

In Ontario, what is the maximum exposure limit to noise? (5)

1) 85 dBA maximum exposure standard in Ontario 2) Very expensive to change standards to account for noise induced hearing loss 3) 5% of individuals exposed to 80dBA noise levels develop significant hearing loss 4) 5-15% for 85dBA exposure 5) 15-25% for 90dBA exposure

What are the 3 internationally standardized weighting networks for sound measurement?

1) A scale (dBA) 2) B scale (dBB) 3) C scale (dBC)

How do sound level meters use weighing scales? (20)

1) A sound-level meter is designed to give readings of sound-pressure level. 2) Sound pressure in decibels is referenced to the standard reference level, 20 μPa. 3) The human hearing response is not flat across the audio band. 4) For example, our hearing sensitivity particularly rolls off at low frequencies, and also at high frequencies. 5) Moreover, this roll-off is more pronounced at softer listening levels. 6) For this reason, to emulate human hearing, sound level meters usually offer a selection of weighting networks designated A, B, and C, having frequency responses 7) The networks reduce the measured sound-pressure level at low and high frequencies. 8) The A network is an inversion of the 40-phon hearing response, 9) the B network is an inversion of 70-phon response, and the C network is an inversion of 100-phon response. 10) Network selection is based on the general level of sounds to be measured (background noise, jet engines, and so on). 11) For example: • For sound-pressure levels of 20 to 55 dB, use network A. • For sound-pressure levels of 55 to 85 dB, use network B. • For sound-pressure levels of 85 to 140 dB, use network C. 12) These network response shapes were designed to bring the sound-level meter readings into closer conformance to the relative loudness of sounds. 13) However, the B and C weightings often do not correspond to human perception. 14) The A weighting is more commonly used. 15) When a measurement is made with the A weighting, the value is designated as dBA. 16) Such dBA readings are usually lower than unweighted dB readings. 17) Since the A weighting is essentially flat above 1 kHz, any difference between a dBA and unweighted reading primarily shows differences in the low-frequency content of the signal. 18) For example, a large difference in readings shows that the signal has significant low-frequency content. 19) Simple frequency weightings such as these cannot accurately represent loudness. 20) Measurements made with simple weightings are not accepted as measurement loudness levels, but are only used for comparison of levels. Frequency analysis of sounds is recommended, using octave or 1/3-octave bands.

Why does hearing loss occur at 80dB for some but not others? (4)

1) Auditory system is different for individuals. 2) Some people are more sensitive to higher levels of noise while others can tolerate more. 3) The standards and data implemented are based off of population averages. 4) Therefore, the standard may be protective for most of the population but not all

Explain the hearing threshold of humans (4)

1) By definition, 0 dB is set at the reference sound pressure (20 micropascals at 1,000 Hz, as stated earlier). 2) 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. 3) On the decibel scale, the threshold of pain occurs at 140 dB. 4) This range of 0 dB to 140 dB is not the entire range of sound, but is the range relevant to human hearing.

How does noise filtering work? (2)

1) 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. 2) 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.

What is sound pressure weighing?(4)

1) Electronic circuits with sensitivity that varies with frequency similar to human hearing have been developed 2) These weighting methods (e.g., the A weighted network, or scale) are intended to indicate perceived loudness and provide a rating of industrial noise that indicates the impact that particular noise has on human hearing. 3) There are three different internationally standardized characteristics called weighting networks A, B, and C. 4) The weighting networks are the sound level meter's means of responding to some frequencies more than to others. 5) By definition, a weighted-frequency scale is simply a series of correction factors that are applied to sound pressure levels on an energy basis as a function of frequency.

How do octave band analyzers work? (2)

1) Electronic instruments called octave band analyzers filter sound to measure the sound pressure (as dB) contributed by each octave band. 2) These analyzers either attach to a type 1 sound level meter or are integral to the meter.

How are equal loudness curves interpreted? (4)

1) Equal-loudness contours of the human ear for pure tones. 2) These contours reveal the relative lack of sensitivity of the ear to bass tones, especially at lower sound levels. 3) When given a dB sound pressure value for the y axis, match it to the frequency x value then follow contour curve to its equal loudness phons reading 4) For example an 80dB 20Hz sound will sound equivalent to 10 phons (dB)

Why is using Filter A controversial? (4)

1) Filter A is mandated for all frequencies when detecting noise 2) It was originally meant to detect lower level noise at lower frequencies 3) High level noise= Filter C 4) A scale would thus report a misrepresentation of noise level being reported; misrepresents what the worker is hearing

What is loudness?

1) Loudness is the subjective human response to sound. 2) It depends primarily on sound pressure but is also influenced by frequency

What is noise filtering? (5)

1) Most noise is not a pure tone, but rather consists of many frequencies simultaneously emitted from the source. 2) To properly represent the total noise of a source, it is usually necessary to break it down into its frequency components. 3) One reason for this is that people react differently to low-frequency and high-frequency sounds. 4) 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. 5) As a general guideline, low-frequency noise is more difficult to control.

What are features of sound weighing networks/filters?

1) Most sound level meters have three ''weighting'' networks, called the A-, B-, and C-scales (ANSI S1.4, 1971). 2) Originally, the A-scale was designed to correspond to the response of the human ear for a sound pressure level of 40 dB at all frequencies. 3) The B-scale was designed to correspond to the response of the human ear for a sound pressure level of 70 dB at all frequencies. 4) The C-scale was approximately flat (constant) for frequencies between 63 Hz and 4000 Hz. 5) The B-scale is rarely used at present. The A-scale is widely used as a single measure of possible hearing damage, annoyance caused by noise, and compliance with various noise regulations. 6) The sound levels indicated by the A-scale network are denoted by LA, and the units are designated dBA. 7) The weighting for the A- and C-scale is shown in Table 2-4. These values are also plotted in Fig. 2-6. 8) The large negative weighting factor for low-frequency sounds corresponds to the fact that the human ear is not as sensitive to low-frequency sound as it is for sound at frequencies in the 1 kHz to 4 kHz range. 9) For example, a sound having a sound pressure level of 40 dB at 63 Hz would be perceived by the human ear as having a sound pressure level of approximately (40-26=)14 dB. 10) Alternatively, a sound that was perceived to have a sound pressure level of 40 dB for a frequency of 63 Hz would actually have a sound pressure level of (40 + 26Þ=) 66 dB. 11) Because the human ear does not respond as significantly to low-frequency sounds, noise at low frequencies (63 Hz, for example) is generally not as damaging or annoying as sound at high frequencies (2 kHz, for example). 12) If the sound pressure level spectrum is measured or calculated for each octave band, the A-weighted sound level may be calculated, using the A weighting factors (CFA) 13) LA= 10 log10[sum10^(Lp+CFA)] where the summation is carried out for all octave bands.

What is the borderline of intensity that hearing loss can begin to develop? (5)

1) Noise levels are measured in decibels, known as dBA. 2) You can listen to sounds at levels from 0-70 dBA for as long as you want. 3) Very Loud 80-90 dBA = blow-dryer, kitchen blender, food processor; Sounds louder than 85 dBA may cause damage if you listen for 8 hours or more. 4) For every 5 dB increase in loudness, the amount of time you should be exposed unprotected is decreased by half. For example, at 95 dBA, you can safely listen for two hours. 5) National Institute for Occupational Safety and Health (NIOSH) recommends that workers in noisy environments 85 dBA or louder for an 8-hour workday limit their exposure at this loudness level.

What are octave bands (frequency bands)? (5)

1) Octave bands, a type of frequency band, are a convenient way to measure and describe the various frequencies that are part of a sound. 2) 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. 3) 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. 4) The width of a full octave band (its bandwidth) is equal to the upper band limit minus the lower band limit. 5) 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.

Could a sound of 0dB and frequency of 20Hz be heard? If not then at what sound pressure is 20Hz audible. At what level of frequency can a 0dB sound be heard? (5)

1) Require a certain sound pressure level at a certain audible frequency 2) Higher amplitude and higher frequency are most audible 3) A 0dB sound could not be heard at 20Hz since it is not within the audible range 4) Minimum frequency needed to have 0dB sound be audible is approximately 4000 Hz 5) 20 Hz sound can be heard at 80dB

Overall, why are filters used? (5)

1) Subtracts real world sounds at certain frequencies and amplifies at other frequencies 2) Acts as a filter to replicate auditory system filter of human perception for sound 3) For different frequencies, have different filters 4) Sound level conversions are added or subtracted using each filter (add or subtract amplitude levels at certain frequencies) 5) Used by sound level meters since we want to detect human perception of sound not total environmental sound; used in noise analysis to determine if sound levels perceived by humans would cause damage 6) Measures sound that would be heard not total sound in the environment (this is why filters add or subtract at different frequencies)

What is A weighing?

1) The A-network was designed to approximate the equal-loudness curves at low sound pressure levels 2) The very low frequencies are discriminated against (attenuated) quite severely by the A-network, moderately attenuated by the B-network, and hardly attenuated at all by the C-network 3) Therefore, if the measured sound level of a noise is much higher on C-weighting than on A-weighting, much of the noise energy is probably of low frequency.

Why is the A scale weighting used more frequently? (4)

1) The A-weighted sound level measurement has become popular in the assessment of overall noise hazard because it is thought to provide a rating of industrial broadband noises that indicates the injurious effects such noise has on human hearing. 2) As a result of its simplicity in rating the hazard to hearing, the A-weighted sound level has been adopted as the measurement for assessing noise exposure by the American Conference of Governmental Industrial Hygienists (ACGIH). 3) The A-weighted sound level as the preferred unit of measurement was also adopted by the U.S. Department of Labor as part of its Occupational Safety and Health Standards. 4) The A-weighted sound level has also been shown to provide reasonably good assessments of speech interference and community disturbance conditions and has been adopted by the U.S. Environmental Protection Agency (EPA) for these purposes

What are equal loudness contours?(3)

1) The contours represent the sound pressure level necessary at each frequency to produce the same loudness response in the average listener. 2) 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. 3) The lower, dashed, curve indicates the threshold of hearing, which represents the sound pressure level necessary to trigger the sensation of hearing in the average listener. (The actual threshold varies as much as ±10 dB among healthy individuals)

According to OSHA, what are factors to consider in weighing scales? (8)

1) Three different internationally standardized characteristics are used for sound measurement: weighting networks A, C, and Z (or "zero" weighting). 2) The A and C weighting networks are the sound level meter's means of responding to some frequencies more than others. 3) The very low frequencies are discriminated against (attenuated) quite severely by the A-network and hardly attenuated at all by the C-network. 4) Sound levels (dB) measured using these weighting scales are designated by the appropriate letter (i.e., dBA or dBC). 5) The A-weighted sound level measurement is thought to provide a rating of industrial noise that indicates the injurious effects such noise has on human hearing and has been adopted by OSHA in its noise standards (OTM/Driscoll). 6) In contrast, the Z weighted measurement is an unweighted scale (introduced as an international standard in 2003), which provides a flat response across the entire frequency spectrum from 10 Hz to 20,000 Hz. 7) 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. 8) A previous B-weighted scale is no longer used

What was discovered by Fletcher and Munson in 1933 in terms of the perception of sound? (3)

1) Used experiments designed to determine the response of the human ear to sound 2) Their study presented a 1,000-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. 3) Results of these experiments yielded the frequently cited Fletcher-Munson, or "equal-loudness," contours

Why are filters important? (13)

1) While each sound level meter is slightly different, each may have one or more sound measurement "scales" corresponding to a range of frequencies. 2) Three more common scales are labeled A, B, and C. The A-scale consists of frequencies (20 to 20,000 Hz) closely associated with human hearing and is the mandated scale required for site monitoring activities. 3) The B-scale is not used these days, and the C-scale is used for engineering and maintenance activities. 4) There are four types of approved SLMs commercially available that differ by their certified accuracy. 5) Type "0" has an accuracy of ±0 dB and is the most accurate SLM available. It is predominantly used for laboratory research applications. 6) Type "1" SLMs are instruments that have an accuracy of±1 dB. These are precision units intended for extremely accurate field and laboratory measurements. 7) Type "2" SLMs are general purpose sound level meters with an accuracy of ±2 dB. 8) Type 2 or better SLMs set at A-scale are mandated for use to evaluate the occupational environment for noise. 9) The last type of SLM is the Type "S" or "Special Purpose" sound level meter. 10) It is equivalent to the old Type "3" unit with design tolerances similar to those listed for the Type 1 unit. 11) The Type "S" SLM differs from Type 1 because these instruments are not required to have all of the functions of the units previously listed. 12) While the SLM without octave band electronic filter can be set at a scale (A, B, C, or D) which represents a relatively broad range of frequencies, the SLM/OBA can be adjusted to specific frequencies which represent relatively narrow octave band frequencies. 13) The specific "center band frequencies" are 31.5, 63, 125, 250, 500, 1000, 2000, 4000, 8000, and 16,000 Hz. Each center band frequency represents an individual range or band of frequencies. 14) SLMs are calibrated using a sound generator or calibrator set at a specific level and frequency. 15) The device is simply placed over the microphone of the SLM and the meter is adjusted to the known level. 16) When performing the field survey, it is important to correctly hold the SLM. 17) The proper position to hold the SLM is dependent upon the specific instrument to be used. 18) For example, some instruments require that the unit is held away from the body and at a 90° angle from the sound source while others may be held at a 70 degree angle from the source, and so forth.

What would a 60 dB 40 Hz sound be equivalent to when using the equal loudness contour?

60 dB sound at 40 Hz would sound like 5 phons (5dB)

When is the B weighing scale used?

The B-network was designed to approximate the equal-loudness curves at medium frequencies

When is the C weighing scale used?

The C-network was designed to approximate the equal-loudness curves at high frequencies