![]() ![]() The body part normally referred to as the ear is technically called the pinna.įigure 14.12 The illustration shows the anatomy of the human ear. The ear converts sound waves into electrical nerve impulses, similar to a microphone.įigure 14.12 shows the anatomy of the ear with its division into three parts: the outer ear or ear canal the middle ear, which runs from the eardrum to the cochlea and the inner ear, which is the cochlea itself. The sound wave that hits our ear is a pressure wave. The way we hear involves some interesting physics. The perception of frequency is called pitch, and the perception of intensity is called loudness. Sounds below 20 Hz are called infrasound, whereas those above 20,000 Hz are ultrasound. You may have noticed that dogs respond to the sound of a dog whistle which produces sound out of the range of human hearing. Dogs can hear sounds as high as 45,000 Hz, whereas bats and dolphins can hear up to 110,000 Hz sounds. Other animals have hearing ranges different from that of humans. Humans can normally hear frequencies ranging from approximately 20 to 20,000 Hz. It can give us plenty of information-such as pitch, loudness, and direction. A voice becomes louder when air flow from the lungs increases, making the amplitude of the sound pressure wave greater. A voice changes in pitch when the muscles of the larynx relax or tighten, changing the tension on the vocal chords. This vibration escapes the mouth along with puffs of air as sound. As air travels up and past the vocal cords, it causes them to vibrate. These folds open and close rhythmically, creating a pressure buildup. People create sounds by pushing air up through their lungs and through elastic folds in the throat called vocal cords. The sound intensity level is not the same as sound intensity-it tells you the level of the sound relative to a reference intensity rather than the actual intensity. The unit called decibel (dB) is used to indicate that this ratio is multiplied by 10. Since β is defined in terms of a ratio, it is unit-less. ![]() Another example is that if one sound is 10 7 as intense as another, it is 70 dB higher. For example, a 90 dB sound compared with a 60 dB sound is 30 dB greater, or three factors of 10 (that is, 10 3 times) as intense. That is, the threshold of human hearing is 0 decibels.Įach factor of 10 in intensity corresponds to 10 dB. The decibel level of a sound having an intensity of 10 –12 W/m 2 is β = 0 dB, because log 10 1 = 0. I 0 is chosen as the reference point because it is the lowest intensity of sound a person with normal hearing can perceive. Where I is sound intensity in watts per meter squared, and I 0 = 10 –12 W/m 2 is a reference intensity. Where it is assumed that r is given in meters (because the reference distance is in meters).14.7 β (dB) = 10 log 10 ( I I 0 ), β (dB) = 10 log 10 ( I I 0 ) , ![]() In terms of decibel transmission loss (TL), this becomes Transmission loss is a positive number although it represents a loss term for acoustic energy.įor spherical spreading, we found that. The decibel scale is particularly convenient because transmission loss terms along different segments of a total ray path can be added to determine the total loss of signal strength. When using decibels, the term transmission loss, is often used to describe the number of decibels of sound level that are lost over a given distance. Here we will look at formulas for spherical and cylindrical spreading in the decibel scale. Note: Be careful – marine reference values are not the same as in air! ![]() Where and correspond to standard marine reference values, assumed to be measured at 1 meter from the sound source. The sound level in decibels (dB) is calculated from a measured intensity ( I) or sound pressure ( P) amplitude as ![]()
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