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Perception of Sound (Loudness)

The deviation of sound above and below the atmospheric pressure levels is called Sound Pressure. The energy expanded in the the process of sound propagation is labeled intensity (loudness) and is measured in energy units. At this point the science of sound can be a little more complex and intimidating since placing a numerical value on sound is very difficult due to the extraordinary sensitivity of the human ear. Our ears can detect deviations in atmospheric pressure in the order of 1,000,000 to 1 and sound intensities of over a trillion to one.

In order to make the measurement, calculation and perception of sound more manageable, a compact scale has been devised incorporating the decibel (dB). A decibel is a logarithmic unit measure of sound pressure.

(FIGURE 7) Shows sound levels of recognizable sound in decibels with a subjective evaluation from “very faint” to “deafening”. It shows the logarithmic values of intensity of energy units and the relative loudness as perceived by the human ear. Obviously, it is much easier to comprehend the decibel levels.

TYPICAL A-WEIGHTED SOUND LEVELS
THRESHOLD OF FEELINGS THRESHOLD OF AUDIBILITY
DECIBELS
RE 20 uPA
INTENSITY ENERGY UNITS RELATIVE LOUDNESS
DEAFENING Jet Takeoff (200′) 120 1,000,000,000,000 4096
Artillery
Elevated Train 110 100,000,000,000 2048
VERY LOUD Subway (20′)
Printing Press 100 10,000,000,000 1024
Police Whistle 90 1,000,000,000 512
LOUD Vacuum Cleaner (10′) 80 100,000,000 256
Street Noise 70 10,000,000 128
Noisy Office 60 1,000,000 64
MODERATE Large Store
Conversation 50 100,000 32
Average Office 40 10,000 16
FAINT Private office
Quiet Conversation 30 1,000 8
Studio (Speech)
VERY FAINT Rustle of Leaves 20 100 4
Whisper 10 10 2
Soundproof Room 0 0 0

(FIGURE 7)

The Relative Loudness levels are important insofar as they demonstrate that a 10-decibel increase will be perceived as twice as loud as the pervious level or conversely, a decrease of 50% from the previous higher level. It is less important to understand the physics of this relative difference as much as to accept it as an acoustical phenomenon.

Note:(FIGURE 7) expresses the sound pressure levels as single number levels in the A weighted scale. The A weighted scale uses the equal loudness contours to provide a single number value in the same manner as our ears perceived sound. The A weighting discounts the low frequency sound level perception (This will be discussed further under Sound Level Meters).