grade 7 Waves & Sound

Waves & Sound

A wave transfers energy in a medium. Amplitude, wavelength, and frequency describe it; transverse waves vibrate perpendicular to travel, longitudinal parallel. Sound pitch follows frequency, loudness follows amplitude, and timbre follows waveform. Change amplitude and frequency and wave shape and the three traits of sound appear.

What Is a Wave?

Amplitude8 cm

Frequency3 Hz

💡 Wave Vocabulary

①Wave: energy transferred through a medium

②Amplitude (A): distance from baseline to crest (or trough)

③Wavelength (λ): distance crest-to-crest (or trough-to-trough)

④Frequency (f): number of vibrations per second (Hz)

⑤The wavelength number below (100/frequency) is a screen-fit tick, not the air wavelength at 340 m/s.

Transverse vs Longitudinal Waves

🔍 Wave Types

①Transverse: vibration ⊥ direction of travel (water waves, light)

②Longitudinal: vibration ∥ direction of travel (sound)

③Longitudinal waves alternate between compression and rarefaction

④Sound is transmitted by vibrating air molecules

Three Properties of Sound

ComparisonHigh vs Low Pitch

Wave Speed

v = fλ (speed = frequency × wavelength)

Speed depends on the medium; frequency and wavelength are inversely related

🎵 Three Properties of Sound

①Pitch: determined by frequency — higher Hz = higher pitch

②Loudness: determined by amplitude

③Timbre: determined by waveform — every instrument differs

Exam Key Points

Core Summary

v = fλ | Pitch=frequency | Loudness=amplitude | Timbre=waveform

Link wave-speed formula with the three sound properties

🎯 Exam Key Points

①Transverse: vibration⊥travel (water) / Longitudinal: vibration∥travel (sound)

②v = f × λ

③Higher pitch = larger frequency; loud = larger amplitude

④Sound speed in air ≈ 340 m/s (varies with temperature)

⑤Sound cannot travel through vacuum (needs a medium)

Examples and Unit Test

Example 1

What is the speed of a wave with a frequency of 50 Hz and a wavelength of 4 m? (v = fλ)

The speed of a wave is found with v = fλ (speed = frequency × wavelength).

v = 50 Hz × 4 m = 200 m/s.

▸ 200 m/s

Wave speed is v = fλ, the product of frequency and wavelength.

Example 2

What determines the pitch and the loudness of a sound?

The pitch of a sound is determined by frequency (higher frequency means higher pitch).

The loudness is determined by amplitude (larger amplitude means louder sound), and timbre is determined by the waveform.

▸ Pitch = frequency / loudness = amplitude

The three elements of sound are pitch (frequency), loudness (amplitude), and timbre (waveform).

Unit-test style

Which statement about waves and sound is correct?

①The speed of a wave is found with v = f ÷ λ

②The pitch of a sound is determined by frequency

③The loudness of a sound is determined by frequency

④Sound is a transverse wave

⑤The unit of frequency is m

▸ ② The pitch of a sound is determined by frequency

The pitch of a sound rises as frequency increases, so it is determined by frequency.

Wave speed is v = fλ (①), loudness is determined by amplitude (③), sound is a longitudinal wave (④), and the unit of frequency is Hz (⑤).

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