Sound Physics

Question 1

Sound

Answer 1

• a form a physical energy
• caused by pressure variations from a source
• a vibration that moves air and molecules and generates pulses of increased pressure that radiate from a source in a pressure wave

Question 2

Noise

Answer 2

• any annoying or unwanted sound
• not recognized as harmful until hearing loss occurs

Question 3

Pressure waves

Answer 3

• can travel for long distances
• occurs in moveable or elastic mediums like air, water or solids
• the rate at which a pressure wave repeats itself determines the sound’s frequency
• the faster the cycles, the higher the pitch of the sound
• when simple pressure changes occur the resulting sounds are pure tones b/c the pressure variations occur at only one frequency

Question 4

Sound frequency

Answer 4

• measured in cycles per second (cps)
• also called Hertz (Hz)

Question 5

Pitch

Answer 5

• psychological perception of sound frequency

Question 6

What frequency range can the young, healthy, human ear perceive?

Answer 6

• 20Hz - 20,000Hz

Question 7

What tones do audiometers produce?

Answer 7

• 500, 1000, 2000, 3000, 4000, 6000, and 8000 Hz

Question 8

Frequency

Answer 8

• measured in Hz
• psychologically interpreted as pitch
• perceived as low, medium, and high
• the way that pressure waves repeat themselves

Question 9

Amplitude or intensity

Answer 9

• measured in decibels
• psychologically interpreted as loudness
• perceived as low, soft, moderate, loud, high

Question 10

Time

Answer 10

• measured in Ms, S, Min, Hrs, etc.
• psychologically interpreted by duration
• perceived as short and long

Question 11

Intensity

Answer 11

• pressure/strength of a sound wave
• the greater the pressure = the more intense the sound level = louder we perceive sound
• measured subjectively as loudness and objectivity as sound level or sound pressure level

Question 12

Decibels

Answer 12

• an expression of amplitude
• the units that measure sound pressure are newtons/ square meter (N/m^2) or Pascals (Pa)
• the smallest sound pressure the ear can hear is 0.00002N/m^2
• 20N/m^2 is harmful
• anchored to an arbitrary 0 of 0.00002N/m^2 at a frequency between 3000 and 4000 Hz
• based on ratios which expresses the range of sound pressures we can hear in a few numbers
• scale increases in decibels of 10 = increasing the intensity 10X

Question 13

Decibel ratio scale

Answer 13

• based on ratios which expresses the range of sound pressures we can hear in a few numbers
• Scale increases in decibels of 10 = increasing the intensity 10X
• Ex. 20 Db is 10x more intense as 10 Db
• Ex. 1,000 relative energy intensity = 30 Db
• scale is logarithmic so decibels cannot be added
• Ex. two machines that are side by side each produce 90 Db would NOT produce a sound level of 180 Db, because of the scale they would produce around 93 Db together

Question 14

A-weighting scale

Answer 14

• used to measure sound to filter out different sound frequencies
• excludes some low frequencies and some very high frequencies
• is closest to the frequency response characteristics that the human ear is most sensitive to (250-8000Hz)
• expressed as an A after the decibel level

Question 15

Damage Risk Criteria

Answer 15

• standards established to protect the majority of workers from noise induced hearing loss
• can be met through engineering or administrative controls or through the use of personal protective equipment

Question 16

The trading relationship/ exchange rate

Answer 16

• reflects the relationship between the permitted exposure levels and exposure durations
• If permissible noise exposure begins at 85 dBA for 8 hours, a 5 Db increase to 90 dBA is permissible if exposure time is reduced by half, to four hours.
• the 5dB trading relationship is based on temporary threshold shift and on the theory that the ear rests between exposures
• Less TTS occurs from intermittent noise exposure than from continuous noise
• TTS is NOT a valid indicator of permanent threshold shifts

Question 17

Damage risk criteria in Alberta

Answer 17

• a 3dB trading relationship or exchange rate based on equal energy theory
• equal amount of A-weighted energy results in equal amounts of hearing loss, regardless of the type of exposure.
• an 8 hour limit of 85dBA could accommodate a 3dB increase (to 88dB) if exposure time is reduced by half to four hours
• results in equivalent exposure b/c when intensity is doubled the sound pressure level is increased by 3dB.
• If decibels increase by 3 and exposure time is halved there is a equivalent exposure level or Lex
• If decibels increase by 3 and exposure time is halved there is a equivalent exposure level or Lex
• Lex expresses equivalent exposure level , averages a workers total exposure to noise over the entire work day and adjusts it to an equivalent 8 hour exposure
• Section 219 of the OHS code requires employees to do a noise exposure assessment if they are going to be exposed to noise over 82 dBA Lex

Question 18

Identifying Threshold

Answer 18

• the worker’s threshold of hearing at each frequency can be determined
• want to identify the softest sound they can hear at each frequency
• to determine if a threshold has changes, there has to be at least 2 hearing tests to compare it to

Question 19

Threshold shift

Answer 19

• when hearing is affected and some hearing is lost, the thresholds can shift
• a threshold change at any frequency is a shift
• usually the results of hearing loss and an increase in dB
• a shift is a change of at least 15dB
• changes of 5dB or 10dB is too small to be considered a shift
• can occur still when there is no actual change in hearing

Question 20

Pure Tones

Answer 20

• sounds that only occur at one frequency

Question 21

3dB trading relationship

Answer 21

• Based on equal energy theory
• Equal amounts of a-weighted energy cause equal amounts of hearing loss regardless if the exposure is constant or intermittent or impact vs. Steady state