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Pitch and loudness Knowledge-Based Study

Physics · Oscillations & Waves · Physics · Sound

Physics topic explanations on Pitch and loudness.

Study notes

  1. The Nature of Sound

    Sound is a mechanical, longitudinal wave produced by vibrating objects. As an object vibrates, it disturbs the surrounding medium, creating alternating regions of high pressure (compressions) and low pressure (rarefactions).

    Two fundamental auditory perceptions characterize every sound we hear: pitch and loudness. While people often use these terms interchangeably in everyday conversation, they correspond to two distinctly different physical properties of a sound wave:

    • Pitch relates to how high or low a tone sounds, determined by the wave's frequency.
    • Loudness relates to the perceived volume or strength of the sound, determined primarily by the wave's amplitude.

    What Determines Pitch?

    Pitch is the subjective perception of the frequency of a sound wave. Frequency is the number of complete wave cycles or vibrations that occur per second. It is measured in hertz (Hz), where 1 Hz equals 1 vibration per second.

    The Frequency-Pitch Relationship

    • High Frequency = High Pitch: When an object vibrates rapidly, it produces sound waves with short wavelengths and high frequencies. Examples include a whistle, a flute, or a bird chirping.
    • Low Frequency = Low Pitch: When an object vibrates slowly, it produces sound waves with long wavelengths and low frequencies. Examples include a bass drum, thunder, or a foghorn.

    Human Hearing Range

    A healthy human ear can typically detect sound frequencies between 20 Hz and 20,000 Hz (20 kHz):

    • Frequencies below 20 Hz are termed infrasound (e.g., seismic waves, elephant rumbles).
    • Frequencies above 20,000 Hz are termed ultrasound (e.g., bat echolocation, medical imaging).

    What Determines Loudness?

    Loudness is the psychological perception of sound energy, closely related to the physical quantity called intensity. The intensity and energy carried by a sound wave depend directly on its amplitude.

    The Amplitude-Loudness Relationship

    • Amplitude is the maximum displacement of the vibrating particles from their equilibrium position (or the maximum change in pressure within the wave).
    • Greater Amplitude = Greater Energy = Louder Sound: Striking a tuning fork or drumhead firmly creates large vibrations, which produce sound waves with larger amplitudes and louder sound.
    • Smaller Amplitude = Less Energy = Softer Sound: Striking a surface gently creates smaller vibrations and waves of lower amplitude, producing a quiet sound.

    Measuring Sound Level

    Sound level is measured on a logarithmic scale in decibels (dB):

    • 0 dB: The threshold of hearing (the quietest sound an average human ear can detect).
    • 60 dB: Typical conversational speech.
    • 120–130 dB: The threshold of pain, where exposure can cause immediate ear damage.

    Independence of Pitch and Loudness

    An essential principle of acoustics is that pitch and loudness are completely independent properties. Changing one does not necessarily change the other.

    Summary Comparison

    • Pitch: Governed by frequency (and wavelength). It dictates musical notes. Changing how fast a string vibrates alters pitch.
    • Loudness: Governed by amplitude (and energy). It dictates volume. Changing how hard a string is plucked alters loudness.

    Four Distinct Combinations

    • High pitch, Loud: A screeching car siren or a referee whistle blown hard.
    • High pitch, Soft: A mosquito buzzing faintly near your ear.
    • Low pitch, Loud: A booming crack of nearby thunder or a powerful bass cannon.
    • Low pitch, Soft: A gentle, quiet hum of an electric refrigerator motor.

    On an oscilloscope display representing a sound wave over time, increasing the pitch compresses the wave horizontally (more cycles shown per unit time), while increasing the loudness stretches the wave vertically (taller crests and deeper troughs).