Turn a tuning knob and a note gets higher; turn the volume and it gets louder. Those are two completely separate controls on a sound wave: frequency sets the pitch, amplitude sets the loudness. Get them confused and half the questions in this topic go wrong. Here you will pin down which property does what, and see how both feed into the energy a sound wave carries.
You sing the same note softly, then loudly. Then you sing a high note and a low note at the same volume.
Before reading: when you make the note louder, which property of the sound wave changes? When you make the note higher, which property changes? Write your predictions.
Higher frequency, higher note, and a shorter wavelength
The pitch you hear is your ear's response to the sound wave's frequency: pack more vibrations into each second and the note climbs higher.
The frequency of a sound wave determines its pitch. A high-frequency wave (many vibrations per second) is heard as a high-pitched note; a low-frequency wave is heard as a low-pitched note. Humans hear roughly $20\ \text{Hz}$ to $20{,}000\ \text{Hz}$. Because $v = f\lambda$ and the speed of sound is fixed by the air, a higher frequency means a shorter wavelength, so high notes have shorter wavelengths than low notes in the same room.
The frequency of a sound wave determines its pitch: higher frequency means higher pitch. Since $v = f\lambda$ with the speed fixed by the medium, higher frequency also means shorter wavelength.
Pause, copy the highlighted statement into your book before moving on.
Did you get this? Two sound notes travel through the same air. The higher-pitched note has the:
Bigger swing, louder sound, independent of pitch
We just saw that frequency controls pitch. That raises a question: what controls how loud the sound is, and is it the same property? This card answers it → no, loudness is set separately by the amplitude.
Loudness is a completely separate dial from pitch: it is set by the wave's amplitude, the size of the particle swing, so a sound can be made louder without changing its note at all.
The amplitude of a sound wave determines its loudness. A larger amplitude, particles pushed further from rest, denser compressions and sparser rarefactions, is heard as a louder sound. Crucially, amplitude and frequency are independent: you can change the loudness without changing the pitch (turn up the volume) and change the pitch without changing the loudness (play a higher note just as softly). On an oscilloscope, a louder sound shows taller peaks; a higher-pitched sound shows peaks packed closer together in time.
On an oscilloscope, sound A shows peaks $2\ \text{cm}$ tall every $4\ \text{ms}$. Sound B (same settings) shows peaks $4\ \text{cm}$ tall every $2\ \text{ms}$. Compare their loudness and pitch.
The amplitude of a sound wave determines its loudness: larger amplitude means louder sound. Amplitude (loudness) and frequency (pitch) are independent, one can change without the other.
Add the highlighted statement to your notes before the check below.
Quick check: True or false: turning up the volume of a speaker (increasing amplitude) raises the pitch of the note.
Pattern A, Plan a fair test
Using a signal generator, a speaker and an oscilloscope (or microphone app), design a practical to test (a) that frequency relates to pitch, and (b) that amplitude relates to loudness. In your book: (1) state the independent and dependent variable for each test; (2) state at least two variables you must keep controlled; (3) state what you would observe on the oscilloscope as you increase the frequency, and separately as you increase the amplitude.
Two truths, one lie: Two truths and a lie about relating frequency, pitch, amplitude and loudness. Which statement is the lie?
Dot point: Conduct a practical investigation, to relate wavelength to frequency, frequency to pitch, and amplitude to loudness
Why louder and higher both mean more energy
We just separated pitch and loudness. That raises a question: how do these two properties feed into the energy a sound wave carries? This card answers it → both a larger amplitude and a higher frequency increase the wave's energy.
A sound wave's energy grows when you crank up either dial: bigger amplitude means particles swing further and carry more energy, and higher frequency means more oscillations per second, also raising the energy delivered.
The energy carried by a sound wave increases with both its amplitude and its frequency. A larger amplitude means each particle moves further from rest, storing and passing on more energy each cycle, which is why a louder sound delivers more energy and can, for example, make a candle flame flicker more strongly. A higher frequency means more compressions arrive each second, so the rate at which energy is delivered is greater too. You can demonstrate the amplitude link by aiming a loudspeaker at a candle flame: the louder the sound (larger amplitude), the more the flame is disturbed, showing more energy is being transferred.
Energy of a sound wave increases with amplitude and with frequency
The energy of a sound wave increases with both larger amplitude and higher frequency. Larger amplitude (louder) transfers more energy per cycle; higher frequency delivers more cycles per second.
Pause, copy the highlighted relationship into your book before the check below.
Last check: Decide if the statement is true or false.
Increasing either the amplitude or the frequency of a sound wave increases the energy it carries.
Fill the gaps: Complete the account of a sound-energy investigation, then press Check.
On an oscilloscope trace, a taller waveform means a larger and therefore a sound. Squeezing more cycles into the same time means a higher and therefore a higher .
Dot point: Conduct a practical investigation to demonstrate how frequency and amplitude relate to the energy of a sound wave
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
ApplyBand 5(4 marks) 1. A musician plays the same note more loudly, then plays a higher note at the original volume. For each change, state which property of the sound wave (frequency or amplitude) changes and what the listener notices. Then explain how each change affects the wave's energy.
1 mark: louder = larger amplitude (frequency unchanged) · 1 mark: higher note = higher frequency (amplitude unchanged) · 1 mark: larger amplitude increases the energy · 1 mark: higher frequency increases the energy
ApplyBand 4(3 marks) 2. A signal generator drives a loudspeaker connected to a microphone and oscilloscope. Describe how you would use this apparatus to show that frequency controls pitch and amplitude controls loudness, naming the independent and dependent variables and one variable you would control.
1 mark: change frequency alone and observe both the pitch heard and the cycle spacing on the trace · 1 mark: change amplitude alone and observe both the loudness heard and the trace height · 1 mark: names an appropriate controlled variable (e.g. distance from speaker to microphone, or the other setting held fixed)
Dot point: Conduct a practical investigation, to relate wavelength to frequency, frequency to pitch, and amplitude to loudness
AnalyseBand 5(3 marks) 3. Two notes are recorded on an oscilloscope. Note X has twice the amplitude of note Y but the same frequency. Explain which note carries more energy, and explain why comparing their pitches would not distinguish them.
1 mark: X carries more energy · 1 mark: energy depends on amplitude (energy increases with amplitude, not with pitch) · 1 mark: pitch is set by frequency, which is identical, so pitch cannot distinguish them
Dot point: Conduct a practical investigation to demonstrate how frequency and amplitude relate to the energy of a sound wave
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Q1 (4 marks): Playing the same note more loudly increases the wave's amplitude while its frequency is unchanged; the listener notices the note is louder but at the same pitch. Playing a higher note increases the wave's frequency while its amplitude is unchanged; the listener notices a higher pitch at the same loudness. Both changes increase the energy carried by the sound wave: a larger amplitude means each particle moves further from rest and transfers more energy per cycle, and a higher frequency means more compressions are delivered each second, so energy is delivered at a greater rate.
Q2 (3 marks): Two runs are needed, each changing one setting at a time. In the first, the independent variable is the frequency set on the signal generator, changed in steps while the output amplitude is held fixed; the dependent variables are the pitch heard and the horizontal spacing of the cycles on the oscilloscope trace, which is seen to shorten as the pitch rises. In the second run the independent variable is the output amplitude, changed while the frequency is held fixed; the dependent variables are the loudness heard and the height of the trace, which grows as the sound gets louder. A controlled variable in both runs is the distance from the loudspeaker to the microphone, since moving the microphone would change the measured amplitude for reasons unrelated to the setting being tested. Holding the other generator setting fixed in each run is what allows the effect to be attributed to one variable.
Q3 (3 marks): Note X carries more energy. The energy carried by a sound wave depends on its amplitude: a larger amplitude means each particle of the medium is displaced further from its rest position, so the compressions are denser and more work is done in propagating the wave. Doubling the amplitude therefore represents a substantial increase in the energy delivered per second, which the ear registers as a louder sound. Comparing the pitches would not distinguish the two notes at all, because pitch is determined by frequency, and the two notes have the same frequency by construction. They would sound like the same note, one simply louder than the other. This is the key point of the investigation: frequency and amplitude are independent properties, and only amplitude governs how much energy the wave carries at a fixed frequency.
You can now separate pitch (frequency) from loudness (amplitude), design a practical to test each, and explain how both feed into a sound wave's energy. Next lesson leaves mechanical waves behind for light, an electromagnetic wave that needs no medium at all.
↩ Back to focus-area hubAt the start you predicted which property changes for louder versus higher. Louder = larger amplitude; higher = larger frequency. The double bass and piccolo can share the same loudness (amplitude) while differing wildly in pitch (frequency), proof that the two are independent controls.
A short spaced-retrieval set, mixing this lesson with earlier ones. It gets harder to guess and easier to remember.
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