PERCEPTION OF SOUND (LOUDNESS)

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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 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.

This table 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			INTENSITY ENERGY UNITS	RELATIVE LOUDNESS
THRESHOLD OF FEELINGS		DECIBELS RE 20 uPA 120 110 100 90 80 70 60 50 40 30 20 10 0	1,000,000,000,000 100,000,000,000 10,000,000,000 1,000,000,000 100,000,000 10,000,000 1,000,000 100,000 10,000 1,000 100 10 0	4096 2048 1024 512 256 128 64 32 16 8 4 2 0
DEAFENING	JET TAKEOFF (200') ARTILLERY ELEVATED TRAIN
VERY LOUD	SUBWAY (20') PRINTING PRESS POLICE WHISTLE
LOUD	VACUUM CLEANER (10') STREET NOISE NOISY OFFICE
MODERATE	LARGE STORE CONVERSATION AVERAGE OFFICE
FAINT	PRIVATE OFFICE QUIET CONVERSATION STUDIO (SPEECH)
VERY FAINT	RUSTLE OF LEAVES WHISPER SOUND PROOF ROOM
THRESHOLD OF AUDIBILITY

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: (the above table) 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).