The ratio between two amplitudes is commonly discussed in terms of decibels (abbreviated dB). Rather than discuss amplitude using such a wide range of numbers from 0 to 1,000,000, it is more common to compare amplitudes on a logarithmic scale. As mentioned earlier, the softest sound we can hear has about one millionth the amplitude of the loudest sound we can bear. Nevertheless, there is a correlation-even if not perfectly linear-between amplitude and loudness, so it's certainly informative to know the relative amplitude of two sounds. (This might possibly be due evolutionarily to the importance of hearing speech and many other important sounds which lie mostly in that frequency range.) We're much more sensitive to frequencies in the range from about 300 Hz to 7,000 Hz than we are to frequencies outside that range. Furthermore, our sense of loudness varies considerably depending on the frequency of the sounds being considered. Experiments find that for most listeners, the (extremely subjective) sensation of a sound being "twice as loud" requires a much greater than twofold increase in amplitude. First of all, our subjective sense of "loudness" is not directly proportional to amplitude. The subtractive difference between amplitudes is 0.5 in the first case and 0.125 in the second case, but what concerns us perceptually is the ratio, which is 2:1 in both cases.ĭoes a sound with twice as great an amplitude sound twice as loud to us? In general, the answer is "no". For example, on an arbitrary scale of measurement, the relationship between a sound of amplitude 1 and a sound of amplitude 0.5 is the same to us as the relationship between a sound of amplitude 0.25 and a sound of amplitude 0.125. Without trying to explain all of those factors, we can at least point out that our sense of the relative loudness of two sounds is related to the ratio of their intensities, rather than the mathematical difference in their intensities. The relationship between the objectively measured amplitude of a sound and our subjective impression of its loudness is very complicated and depends on many factors. Amplitude is a wave's maximum variation in air pressure (from normal atmospheric pressure). Sound travels as a longitudinal wave, in which particles move back and forth along the line of the traveling wave. It is a wave characterized by the transmission of energy in the form of compression (increased pressure) and rarefaction (decreased pressure) through a medium. For example, ocean noise pollution produced by ships may be as great as 200 dB expressed in the sound pressure level, where the more familiar sound intensity level we use here would be something under 140 dB for the same sound.Sound is variation in air pressure detectable by the human ear. It is beyond the scope of most introductory texts to treat this scale because it is not commonly used for sounds in air, but it is important to note that very different decibel levels may be encountered when sound pressure levels are quoted. This scale is used particularly in applications where sound travels in water. It should be noted at this point that there is another decibel scale in use, called the sound pressure level, based on the ratio of the pressure amplitude to a reference pressure. For example, a 56 dB sound is twice as intense as a 53 dB sound, a 97 dB sound is half as intense as a 100 dB sound, and so on. Note that because only the ratio I 2 / I 1 I 2 / I 1 is given (and not the actual intensities), this result is true for any intensities that differ by a factor of two. This means that the two sound intensity levels differ by 3.01 dB, or about 3 dB, as advertised. 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 17.14.
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. Figure 17.13 Noise on crowded roadways like this one in Delhi makes it hard to hear others unless they shout.