Ch. 4: Sound Physics Flashcards
Define the term sound.
- Vibrations emitted from a source that excites similar movement in air molecules and results in pulses of increased pressure that move out from the original source.
- This vibratory energy travels in waves that move through air or any medium and from one medium to another.
What is meant by the terms frequency and intensity?
-Frequency and intensity are two fundamental characteristics of sound.
-Frequency:
>the rate at which a pressure wave repeats itself.
>the faster the cycles of pressure waves are, the higher the pitch of sound.
>sound frequency is measured in cycles per second (cps) also called hertz (Hz).
>Pitch is the physiological perception of sound frequency.
>The term pitch is related to frequency, but pitch is a subjective term and expresses sensory perception in the mind of the listener. The normal human ear can hear sounds from 20 to 20,000 Hz (or cps).
-Intensity relates to the power or pressure of the sound wave.
>The greater the power or pressure, the louder the sound.
>Loudness is a sensory perception in the mind of the listener.
What are sounds called that occur at only one frequency?
Sounds that occur at one frequency are called pure tones.
What is noise?
- Noise is unwanted sound.
- Defining sound as noise is a subjective assessment and often depends on an individual’s attitude about the source and nature of the sound.
Explain the two primary features of the decibel scale.
- The units used to measure sound pressure are either Newtons per square meter (N/m2) or pascals (Pa).
- The smallest sound pressure the ear can hear is 0.00002M/m2 and a pressure of 20/N m2 can be harmful to the ear. This reflects an increase of about 10 million times the original magnitude.
- Because the normal ear can accommodate such a large amplitude range, it was necessary to develop a convenient scale that could measure amplitude more easily.
- The decibel (dB) is the expression of amplitude that was developed for this purpose and is now used (it expresses the range of sound pressures the human ear can hear in relatively few numbers).
- The reference quantity for expressions of the decibel in sound pressure is 0.00002 N/m2 (0.00002 N/m2 = 0 dB). Thus, a decibel is a ratio between a given sound pressure and a specified reference pressure (the faintest sound a human ear can hear, 0 dB).
- The decibel scale is logarithmic and based on powers of ten. Thus, each 10 dB increase is equal to increasing the relative energy intensity tenfold.
Why can’t decibel levels be added when combining noise level readings to determine whether workers are noise exposed?
- Because the scale is logarithmic rather than linear, decibels cannot be added.
- For example, if two adjacent machines each have a sound level reading of 90 dBA, the combined noise level will increase by three decibels, not by 90 dBA (i.e., 90 dBA and 90 dBA will be 93 dBA).
Which weighting network has the frequency response characteristics closest to those of the human ear?
- The A-weighted network has frequency response characteristics that are most similar to those perceptible to the human ear.
- Usually, a sound level survey includes measurements of sounds in decibels on the A-weighted network (dBA).
- Different weighting networks are used when measuring sound; these are used to filter out certain frequencies.
- The A-weighting scale excludes some low-frequency and some very high frequency sounds and is closest to the frequency response characteristics that the human ear is most sensitive (250-8000 Hertz).
What is the purpose of DRC (damage risk criteria) ?
- It is difficult to say what is safe noise exposure, but damage risk criteria (DRC) have been developed and are used as guides to help us determine what is safe and what is not.
- Damage risk criteria are standards (generally in the form of regulations) established to protect the majority of workers from noise-induced hearing loss (standards may be met through engineering or administrative controls or through the use of personal protective equipment).
- The purpose of DRC is to protect the majority of workers from becoming hearing impaired.
What are the two major sets of DRC?
There are two major sets of DRC:
a. the 5 dB trading relationship
b. the 3 dB trading relationship (used in Alberta)
-The trading relationship, or exchange rate, reflects the relationship between the permitted exposure levels and exposure durations.
-DRCs are always expressed as allowable exposure times relative to dBA levels.
-The 5dB trading relationship “trades” allowable time (which always decreases by half) for each 5 decibel increase in noise level.
>It is based on studies of temporary threshold shift (TSS) and on the theory that the ear rests and recovers between exposures.
-The 3dB trading relationship “trades” allowable time (which always decreases by half) for each 3 decibel increase in noise level.
>Is based on the equal energy theory. In theory, this exchange rate results in equivalent exposure, because when intensity is doubled, the sound pressure level is increased by 3 dB. In other words, when the decibels increase by 3 and exposure time is halved, there is an equivalent exposure level or Lex.
>Lex is the terminology used to express equivalent exposure level. A Lex measurement averages a worker’s total exposure to noise over the entire workday and adjusts it to an equivalent 8-hour exposure.
What is the basis for the 5 dB trading relationship?
- The 5 dB trading relationship is based on laboratory studies of temporary threshold shifts (TTS).
- This relationship considers the fact that hearing loss (or threshold shift) due to noise exposure is temporary; provided that the ear has time to rest (i.e., experiences no noise exposure), it will recover.
- The TTS studies demonstrated that greater TTS is generated by continuous exposure to noise than from intermittent exposure; therefore, intermittent exposure was determined to be less hazardous.
What is the basis for the 3 dB trading relationship?
- The 3 dB trading relationship is based on the equal energy theory that states that equal amounts of A-weighted energy cause equal amounts of hearing loss, regardless of whether the exposure is constant or intermittent and regardless of whether the noise is impact noise or steady-state noise.
- When using this type of DRC, exposure to 85 dBA for 8 hours is equivalent to 88 dBA for 4 hours, or 91 dBA for 2 hours, 94 dBA for 1 hour, and so on.
- In other words, a doubling of intensity – an increase of 3 dBA – combined with a reduction of exposure by one half amounts to equivalent noise exposure.
- This is the DRC currently used in Alberta.
What is a threshold shift (TS)?
- A threshold change at any frequency is known as a threshold shift (you need at least two hearing tests for that worker on which to base a comparison).
- When measuring threshold shifts in hearing tests, a change of at least 15 dB is needed to call the change a threshold shift. Changes of 5 or 10 dB are too small to be called a threshold shift.
