Sound is a sensory experience originating from physical phenomena involving vibration and energy transfer. The relationship between a sound wave’s measurable properties and the subjective experience of hearing is a primary focus of acoustics and psychoacoustics. Understanding this connection requires separating the objective, scientific measurement of the wave from the psychological perception by the human brain. This distinction between the physical and the perceptual is where the concepts of frequency and pitch diverge and connect.
Frequency: The Physical Foundation of Sound
Frequency is an objective, measurable property of a sound wave, defined as the rate at which the sound pressure wave repeats itself. This rate is expressed as the number of cycles per second, with the standard unit of measurement being the Hertz (Hz). For example, a sound wave with a frequency of 100 Hz completes 100 full cycles every second.
Frequency is inversely linked to the physical length of the wave, known as the wavelength. In a constant medium like air, a higher frequency corresponds to a shorter wavelength, and a lower frequency corresponds to a longer wavelength. Low-frequency sounds, such as thunder, are characterized by long, slow pressure oscillations. Conversely, high-frequency sounds, like a whistle, involve rapid pressure changes and short wavelengths.
Pitch: The Subjective Interpretation of Sound
Pitch is the perceptual attribute of sound that allows a listener to order sounds on a musical scale from low to high. Unlike frequency, which is measured by instruments, pitch is a subjective, psychoacoustical quality generated by the brain’s processing of auditory signals. It is the sensation of “highness” or “lowness” assigned to a tone.
The range of frequencies humans can perceive as pitch is typically 20 Hz to 20,000 Hz (or 20 kHz). Within this audible spectrum, the human auditory system is most sensitive to frequencies between 2,000 Hz and 5,000 Hz. The ability to perceive the highest frequencies diminishes with age, a condition known as presbycusis, often dropping to 15,000 Hz or lower for most adults.
The Core Relationship: How Frequency Drives Pitch
The fundamental link between frequency and pitch is direct: a higher measured frequency results in a higher perceived pitch, and a lower measured frequency results in a lower perceived pitch. For simple, pure tones, the two properties are nearly interchangeable, as pitch is determined by the fundamental frequency of the vibrating source. This relationship allows us to identify the difference between a bass drum’s low tone and a flute’s high tone.
However, the way humans perceive pitch is not linear, but logarithmic. This means the perceived distance between pitches is based on frequency ratios, not absolute frequency differences. A classic example is the musical octave, where a tone is perceived as the “same” note but higher.
A musical octave is produced when the frequency of a tone is exactly doubled. For instance, the note A below middle C is often tuned to 220 Hz, and the A one octave higher is 440 Hz (a 2:1 ratio). Doubling the frequency again results in 880 Hz, which is the A two octaves higher. This logarithmic processing explains why a change from 100 Hz to 200 Hz is perceived as the same musical interval as a change from 1,000 Hz to 2,000 Hz, even though the absolute frequency difference is ten times greater in the second case.
Psychoacoustic Factors That Modify Pitch Perception
Pitch is a construction of the brain, meaning that in complex situations, perception can sometimes deviate from the physical frequency measurement. These psychoacoustic effects demonstrate that pitch is not identical to frequency, but rather a closely related interpretation. One factor is the influence of amplitude, or loudness, on perceived pitch, particularly at the extremes of the frequency spectrum.
For very loud low-frequency tones (below 1,000 Hz), the pitch can be perceived as slightly lower than if the tone were played quietly. Conversely, very loud high-frequency tones (above about 2,000 Hz) may be perceived as having a slightly higher pitch. This phenomenon illustrates how the overall volume level can subtly “bend” the subjective perception of pitch, even when the underlying frequency remains constant.
Another significant psychoacoustic effect is the phenomenon of the “missing fundamental.” This occurs with complex tones, such as those produced by musical instruments, which are composed of a fundamental frequency and a series of overtones, or harmonics. If the fundamental frequency is physically removed, the brain can still perceive the pitch corresponding to that missing fundamental based on the remaining harmonic pattern. The auditory system uses the mathematical relationship between the harmonics to calculate the lowest frequency in the series, effectively maintaining the correct perceived pitch.