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Acoustic book

Curves perception

Curves of equal perceptual level (Fletcher and Munson curves)

Although the connection between stronger and weaker sound intensities is felt of both 10 and 12, we do not feel a so wide difference in perception between a weak sound and a strong one. This happens because the connection between intensity, which is a physical characteristic of the sound, and sound sensation, which is a subjective characteristic of the sound, implies physiological and psychological processes into ear and brain. Several experimental psychophysical tests were made to measure the exiting relations between the physical characteristics of a sound and the subjective characteristics as perceived by a person.
Here it is one example.
To calibrate the intensity of a sound frequency f, a subject does a comparison measure with a sound sample, which has, let’s say, a frequency of 1000 Hz and an intensity level of 60 dB. The intensity of the sound to calibrate is varied until the subject considers that it produces the same sound sensation of the1000 Hz sample. Then the test is repeated on a certain range of frequencies until obtaining a curve with a progression as that marked 60 in Figure 15 of Fletcher and Munson curves. For example, a signal of 100 Hz must have a 72 dB intensity level to be heard as a signal of 1000 Hz at 60 dB. Figure 15 also shows the curves that are obtained when the sound sample of 1000 Hz is given to the different levels of intensity. From these curves it appears that the human ear is more efficient for sounds with frequencies between 3000 and 4000 Hz.

Figure 15 Curves of equal perceptive level (Fletcher and Munson curves)

In another experimental test, the intensity of a frequency signal f is decreased until the subject does not feel it anymore. Repeating the test on a wide range of frequencies the curve of a persona auditory acuity is obtained.
In Figure 16 are shown the auditory acuity curves obtained from thousands of American samples. The number on the right in each curve indicates the percentage of population able to hear signals that are below the curve. The U.S. Public Health Service formulated the chart of these tests. The one percent of those people that cannot hear below the level of 99 % requires hearing auxiliary equipment because can’t hear a normal conversation at 60 dB. Only children and young adults are able to hear very weak signals, because acuity decreases with age. These curves have a subsidence between 3000 and 4000 Hz, and this shows again that in this frequency range hearing is the most sensitive sense.

Figure 16 auditory acuity curves in USA

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