Once that point is located, the intensity level can be determined from the vertical axis. To find the loudness of a given sound, you must know its frequency and intensity level, locate that point on the square grid, and then interpolate between loudness curves to get the loudness in phons. The graph in (Figure) should be referenced to solve this example. (a) What is the loudness in phons of a 100-Hz sound that has an intensity level of 80 dB? (b) What is the intensity level in decibels of a 4000-Hz sound having a loudness of 70 phons? (c) At what intensity level will an 8000-Hz sound have the same loudness as a 200-Hz sound at 60 dB? Strategy At a frequency of 1000 Hz, phons are taken to be numerically equal to decibels. The curves were determined by having large numbers of people compare the loudness of sounds at different frequencies and sound intensity levels. Any sound along a given curve is perceived as equally loud by the average person. Each curve is labeled with its loudness in phons. The curved lines are equal-loudness curves. (Figure) shows the relationship of loudness to intensity (or intensity level) and frequency for persons with normal hearing. Phons differ from decibels because the phon is a unit of loudness perception, whereas the decibel is a unit of physical intensity. Timbre is the shape of the wave that arises from the many reflections, resonances, and superposition in an instrument.Ī unit called a phon is used to express loudness numerically. We call our perception of these combinations of frequencies and intensities tone quality or, more commonly, the timbre of the sound. The reason is that each instrument produces a distinctive set of frequencies and intensities. When a violin plays middle C, there is no mistaking it for a piano playing the same note. Sounds near the high- and low-frequency extremes of the hearing range seem even less loud, because the ear is less sensitive at those frequencies. Frequency has a major effect on how loud a sound seems. But loudness is not related to intensity alone.
At a given frequency, it is possible to discern differences of about 1 dB, and a change of 3 dB is easily noticed. The perception of intensity is called loudness. Musical notes are sounds of a particular frequency that can be produced by most instruments and in Western music have particular names, such as A-sharp, C, or E-flat. For example, 500.0 and 501.5 Hz are noticeably different. Typically, humans have excellent relative pitch and can discriminate between two sounds if their frequencies differ by 0.3% or more. The perception of frequency is called pitch. It can give us a wealth of simple information-such as pitch, loudness, and direction. The human ear has a tremendous range and sensitivity.