The decibel level of a sound having the threshold intensity of 10 −12 W/m 2 is β = 0 dB, because log 101 = 0. Jet airplane at 30 m severe pain, damage in seconds Loud rock concert, pneumatic chipper at 2 m threshold of pain Noisy factory, siren at 30 m damage from 8 h per day exposure Inside a heavy truck damage from prolonged exposure The bel, upon which the decibel is based, is named for Alexander Graham Bell, the inventor of the telephone. The units of decibels (dB) are used to indicate this ratio is multiplied by 10 in its definition. Because β is defined in terms of a ratio, it is a unitless quantity telling you the level of the sound relative to a fixed standard (10 −12 W/m 2, in this case). Sound intensity level is not the same as intensity. In particular, I 0 is the lowest or threshold intensity of sound a person with normal hearing can perceive at a frequency of 1000 Hz. The sound intensity level β in decibels of a sound having an intensity I in watts per meter squared is defined to be , where I 0 = 10 −12 W/m 2 is a reference intensity. How our ears perceive sound can be more accurately described by the logarithm of the intensity rather than directly to the intensity. The reasons for this choice of units are related to how we perceive sounds. Decibels are the unit of choice in the scientific literature as well as in the popular media. Sound intensity levels are quoted in decibels (dB) much more often than sound intensities in watts per meter squared. Because pressures are higher in the greater-intensity sound, it can exert larger forces on the objects it encounters. The more intense sound is produced by a source that has larger-amplitude oscillations and has greater pressure maxima and minima. Graphs of the gauge pressures in two sound waves of different intensities. This relationship is consistent with the fact that the sound wave is produced by some vibration the greater its pressure amplitude, the more the air is compressed in the sound it creates. The pressure variation is proportional to the amplitude of the oscillation, and so I varies as (Δ p) 2 (Figure 2). In this equation, ρ is the density of the material in which the sound wave travels, in units of kg/m 3, and v w is the speed of sound in the medium, in units of m/s. (We are using a lower case p for pressure to distinguish it from power, denoted by P above.) The energy (as kinetic energy ) of an oscillating element of air due to a traveling sound wave is proportional to its amplitude squared. Here Δ p is the pressure variation or pressure amplitude (half the difference between the maximum and minimum pressure in the sound wave) in units of pascals (Pa) or N/m 2. The intensity of a sound wave is related to its amplitude squared by the following relationship: In equation form, intensity I is , where P is the power through an area A. 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 2. 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. Noise on crowded roadways like this one in Delhi makes it hard to hear others unless they shout. Calculate sound intensity levels in decibels (dB).įigure 1. Define intensity, sound intensity, and sound pressure level.By the end of this section, you will be able to:
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