How do we describe a sound wave, and how do amplitude, frequency, period and wavelength relate to what we hear?
Sound as a longitudinal pressure wave: amplitude and loudness, frequency and pitch, period, wavelength and the wave equation, and the audible frequency range.
A focused answer to the Edexcel 9MT0 principles of sound, covering sound as a longitudinal pressure wave, amplitude and loudness, frequency and pitch, period, wavelength, the wave equation and the audible range.
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What this dot point is asking
Edexcel wants you to describe sound as a longitudinal pressure wave and to use the core wave quantities precisely: amplitude (and its link to loudness), frequency (and its link to pitch), period, wavelength, and the wave equation, plus the audible frequency range. These quantities are the vocabulary you will use to analyse every recording and to make every production decision, so they must be second nature.
The answer
Sound as a longitudinal pressure wave
Because sound is a pressure variation, it needs a medium and cannot travel through a vacuum. When we draw a sound wave as a smooth curve, that graph is pressure (or the equivalent voltage from a microphone) plotted against time. The curve is a convenient transverse picture of a wave that is physically longitudinal.
Amplitude and loudness
In a digital audio workstation the amplitude of a waveform is shown as its height in the editor, and the level meters read how large that amplitude is at any moment. Clipping happens when the amplitude exceeds the maximum the system can represent, flattening the peaks and adding harsh distortion.
Frequency and pitch
The note A above middle C is standardised at Hz (concert pitch). Musical intervals are ratios, not differences: the octave is , the perfect fifth is close to . This is why pitch is perceived logarithmically, and why a synthesiser's pitch control and a graphic EQ both use octave-based spacing.
Period, wavelength and the wave equation
A low bass note at Hz has a wavelength of m, which is why bass is hard to control acoustically in a small room and why subwoofer placement matters. A kHz cymbal harmonic has a wavelength of only cm, so it is highly directional and easily shadowed.
The audible range
The human ear responds to roughly Hz to kHz. Below Hz is infrasound (felt rather than heard); above kHz is ultrasound. The upper limit drops with age and exposure to loud sound, so an adult may hear only to kHz or less. This range is why audio systems are designed around a Hz to kHz bandwidth and why CD audio samples fast enough to capture it.
Examples in context
When you set a high-pass filter at Hz to clean up a vocal, you are removing energy whose wavelength (over m) is too long to be useful and that only muddies the mix. When you choose a sample rate, you are choosing how many times per second to measure the amplitude, fast enough to capture frequencies up to kHz. When you tune an oscillator to Hz, you are setting the cycle rate that the ear reads as the note A. Every one of these decisions rests on the relationship between amplitude, frequency and wavelength.
Try this
Q1. State what amplitude and frequency each control in a sound we hear. [2 marks]
- Cue. Amplitude controls loudness; frequency controls pitch.
Q2. A note has a period of ms. Find its frequency. [2 marks]
- Cue. Hz.
Q3. Calculate the wavelength of a Hz note if the speed of sound is m per second. [2 marks]
- Cue. m.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 9MT0/03 20193 marksA sustained synthesiser note has a fundamental frequency of Hz. Calculate its period, and state how the pitch would change if the frequency were doubled to Hz.Show worked answer →
The period is the reciprocal of the frequency: s, which is about ms.
Doubling the frequency from Hz to Hz raises the pitch by exactly one octave, because an octave is a frequency ratio.
Markers reward the formula , the value about ms, and the statement that the pitch rises by an octave (not simply "it gets higher").
Edexcel 9MT0/03 20224 marksExplain the difference between the amplitude and the frequency of a sound wave, stating what each one controls in the sound we hear, and describe the audible frequency range for a typical young listener.Show worked answer →
Amplitude is the maximum change in air pressure from the rest (atmospheric) value as the wave passes; it controls the loudness of the sound, because a larger pressure variation carries more energy and is perceived as louder. Frequency is the number of complete pressure cycles per second, measured in hertz; it controls the perceived pitch, because a higher frequency is heard as a higher note.
The audible range for a typical young listener is roughly Hz to kHz. Below Hz is infrasound and above kHz is ultrasound, both inaudible, and the upper limit falls with age and noise exposure.
Markers reward amplitude linked to loudness, frequency linked to pitch, the units (hertz), and the Hz to kHz range.
Related dot points
- The decibel as a logarithmic ratio: the power formula and the amplitude (voltage) formula, dBFS and headroom, the relationship between decibel change and perceived loudness, and dynamic range.
A focused answer to the Edexcel 9MT0 decibel content, covering the decibel as a logarithmic ratio, the power and amplitude formulae, dBFS and headroom, how decibel changes map to perceived loudness, and dynamic range.
- The harmonic series and timbre: fundamental and harmonics, how the relative levels of harmonics shape tone, the waveform shapes of basic tones, the frequency spectrum and the phase relationships that create a sound's character.
A focused answer to the Edexcel 9MT0 harmonics content, covering the harmonic series, fundamental and harmonics, how relative harmonic levels shape timbre, the basic waveform shapes, the frequency spectrum and phase.
- Analogue-to-digital conversion: sampling rate and the Nyquist theorem, aliasing and the anti-aliasing filter, bit depth and quantisation, dynamic range and quantisation noise, and common audio resolutions.
A focused answer to the Edexcel 9MT0 digital audio content, covering analogue-to-digital conversion, sampling rate and the Nyquist theorem, aliasing, bit depth and quantisation, dynamic range and common audio resolutions.
- Subtractive synthesis: oscillators and waveforms, the voltage-controlled signal path (VCO, VCF, VCA), the filter and resonance, the ADSR envelope, the LFO and modulation, and how these combine to design a synth sound.
A focused answer to the Edexcel 9MT0 subtractive synthesis content, covering oscillators and waveforms, the VCO, VCF and VCA signal path, the filter and resonance, the ADSR envelope, the LFO and sound design.
- Equalisation: the frequency bands, high-pass and low-pass filters, shelving and parametric EQ, cut and boost, the Q (bandwidth) control, and using subtractive EQ to create space and corrective and creative EQ in a mix.
A focused answer to the Edexcel 9MT0 EQ content, covering the frequency bands, high-pass and low-pass filters, shelving and parametric EQ, cut and boost, the Q control, and subtractive, corrective and creative equalisation in a mix.
Sources & how we know this
- Pearson Edexcel A-Level Music Technology (9MT0) specification — Pearson Edexcel (2017)