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Sound Waves, Frequency, and Pitch
The Physics Behind Music
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Sound travels as a wave, and one of the most important properties of that wave is frequency. Frequency tells us how many wave cycles pass a point each second, and it is measured in hertz, or Hz. In music and everyday listening, frequency is closely connected to pitch, which is how high or low a sound seems to our ears. Understanding this link helps explain why a bass note sounds deep while a flute note sounds bright and high.
A low frequency wave has fewer cycles each second and usually a longer wavelength, while a high frequency wave has more cycles each second and usually a shorter wavelength. In the same medium, sound speed stays nearly constant, so frequency and wavelength change in opposite ways according to v = fλ. Musical notes are often described by their frequencies, such as A4 = 440 Hz. This relationship is useful in music, acoustics, instrument design, and audio technology.
Key Facts
- Frequency is the number of wave cycles per second: f = 1/T
- The unit of frequency is hertz: 1 Hz = 1 cycle/s
- Wave speed, frequency, and wavelength are related by v = fλ
- Higher frequency usually means higher perceived pitch
- Lower frequency usually means longer wavelength in the same medium
- A common musical reference note is A4 = 440 Hz
Vocabulary
- Frequency
- Frequency is the number of complete wave cycles that pass a point each second.
- Pitch
- Pitch is the perception of how high or low a sound seems to a listener.
- Wavelength
- Wavelength is the distance between matching points on consecutive waves, such as peak to peak.
- Hertz
- Hertz is the unit used to measure frequency, equal to one cycle per second.
- Wave speed
- Wave speed is the rate at which a sound wave travels through a medium.
Common Mistakes to Avoid
- Confusing frequency with loudness, because frequency affects pitch while loudness depends mainly on amplitude. A high sound is not necessarily a loud sound.
- Assuming higher frequency means faster sound speed, because in the same medium sound speed stays about the same. Instead, wavelength gets shorter as frequency increases.
- Mixing up wavelength and frequency, because they change in opposite directions when wave speed is constant. If frequency doubles, wavelength is cut in half.
- Using the wrong unit for frequency, because frequency must be given in hertz or cycles per second. Meters measure wavelength, not frequency.
Practice Questions
- 1 A sound wave has a frequency of 220 Hz. How many wave cycles pass a point in 5 s?
- 2 Sound travels through air at 340 m/s and has a frequency of 680 Hz. What is its wavelength?
- 3 Two notes travel through the same air. One has frequency 200 Hz and the other has frequency 800 Hz. Which note has the higher pitch, and which has the shorter wavelength?