Thus, sound intensity levels in decibels fit your experience better than intensities in watts per meter squared. You are unaware of this tremendous range in sound intensity because how your ears respond can be described approximately as the logarithm of intensity. Sound intensity varies by a factor of 10 12 10 12 from threshold to a sound that causes damage in seconds. The ear is sensitive to as little as a trillionth of a watt per meter squared-even more impressive when you realize that the area of the eardrum is only about 1 cm 2 1 cm 2, so that only 10 – 16 10 – 16 W falls on it at the threshold of hearing! Air molecules in a sound wave of this intensity vibrate over a distance of less than one molecular diameter, and the gauge pressures involved are less than 10 – 9 10 – 9 atm.Īnother impressive feature of the sounds in Table 17.2 is their numerical range. One of the more striking things about the intensities in Table 17.2 is that the intensity in watts per meter squared is quite small for most sounds. Table 17.2 gives levels in decibels and intensities in watts per meter squared for some familiar sounds. That is, the threshold of hearing is 0 decibels. The decibel level of a sound having the threshold intensity of 10 – 12 W/m 2 10 – 12 W/m 2 is β = 0 dB β = 0 dB, because log 10 1 = 0 log 10 1 = 0. Table 17.2 Sound Intensity Levels and Intensities
Power is the rate at which energy is transferred by the wave. Intensity is defined to be the power per unit area carried by a wave. The relevant physical quantity is sound intensity, a concept that is valid for all sounds whether or not they are in the audible range. High noise exposure is hazardous to hearing, and it is common for musicians to have hearing losses that are sufficiently severe that they interfere with the musicians’ abilities to perform. In cartoons depicting a screaming person (or an animal making a loud noise), the cartoonist often shows an open mouth with a vibrating uvula, the hanging tissue at the back of the mouth, to suggest a loud sound coming from the throat Figure 17.12. We are all very familiar with the loudness of sounds and aware that they are related to how energetically the source is vibrating. But when a passing motorist has his stereo turned up, you cannot even hear what the person next to you in your car is saying. After settling into bed, you may hear your blood pulsing through your ears. In a quiet forest, you can sometimes hear a single leaf fall to the ground.
Several comments and comprehensive bibliographical references are also provided for each drawn page to enlarge the range of subjects or exercises that can be discussed in courses and foster further readings.Figure 17.11 Noise on crowded roadways like this one in Delhi makes it hard to hear others unless they shout. Seven drawn pages constitute a visual support to explain the origin and history of the decibel, together with examples from acoustics and other domains on the use of logarithmic scales and classical decibel calculations. This paper is an example of how educational comics can be designed and used to introduce one of the most commonly discussed topics when the basics of acoustics are taught: decibel level. While they are valuable resources for researchers, these books might appear a little daunting for a young audience or for people who are new to acoustics. In fact, the fields of acoustics cover an especially broad range of subjects and domains, and comprehensive acoustics textbooks are usually quite thick as a consequence. Acoustics is generally defined as the science that deals with the production, transmission, and reception of sound and the understanding and control of its effects.