Science Tutorial: How do you characterize sounds?
Sounds are often described as loud or soft; high-pitched or low-pitched. These words are commonly used to describe, or characterize, how sounds are perceived. Scientists, on the other hand, describe sounds with characteristics that can be measured using instruments. Scientists measure intensity and amplitude, which can be related to the common words loud and soft. Scientists measure frequency, which can be related to the common word pitch.
| Perceived Characteristic | Physical Characteristic |
| Loudness | Intensity |
| Pitch | Frequency |
These characteristics relate to how sound travels as a wave, as seen in the animation below. However, it is important to remember that in a sound wave, the particles move back and forth rather than up and down, as this picture might seem to suggest. Intensity relates to how much an individual particle moves and the frequency relates to how often it moves.
Transverse waves on a string. The string is displaced up and down as the wave pulse travels from left to right, but the string itself does not experience any net motion.Image used with permission – copyright: Dan Russell, Grad. Prog. Acoustics, Penn State, https://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html.
The bottom half of the video below shows how particles in a sound wave move. When the pressure is high, the particles are squashed together (compressed under high pressure) and when the pressure is low, the particles are pulled apart (expanding under low pressure). The top half of the video below is commonly referred to as a waveform, which shows the change in pressure with time as a sound wave moves past a fixed location.
Sound is a longitudinal wave. Red dots and arrows illustrate individual particle motion. The black arrow illustrates the motion of a wave as the compression of particles moves through the medium. Video by DOSITS, animation from Dan Russel used with permission.
Some of the characteristics used to describe a sound wave are not audible. One important characteristic that we cannot hear is the wavelength of the sound, which is the distance from the top of one wave to the top of the next wave. Although you can see the wavelength of a water wave, you cannot see the wavelength of a sound wave. Scientists can measure it using special instruments, however.
Another important characteristic of a sound wave that we cannot hear is the phase. Phase specifies the location of a point within a wave cycle of a repetitive waveform. When two sound waves interact, the difference between the phases of the two waves is important in determining the resulting waveform.
Phase Video by DOSITS.
Science of Sound Tutorial Sections: