What kind of wave is sound, and why does the pitch of a passing siren change?
Describe sound as a longitudinal mechanical wave needing a medium, relate pitch and loudness to frequency and amplitude, and explain the Doppler effect as an apparent change in frequency due to relative motion of source and observer.
A Regents Physics answer on sound and the Doppler effect: sound as a longitudinal wave requiring a medium, the link of pitch to frequency and loudness to amplitude, and the Doppler effect explained by relative motion of source and observer, with worked reasoning examples.
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What this topic is asking
This dot point covers sound as a specific kind of wave and the Doppler effect that changes its apparent pitch. The Physical Setting/Physics course asks you to describe sound as a longitudinal mechanical wave that needs a medium, to relate pitch to frequency and loudness to amplitude, and to explain the Doppler effect through the relative motion of source and observer. The Regents tests the Doppler effect conceptually (a passing siren) and sound through the wave equation.
Sound as a longitudinal wave
Because sound is mechanical, it needs something to vibrate: air, water or a solid. This is why an alarm clock inside a jar makes no sound when the air is pumped out, and why there is no sound in space. The speed of sound depends on the medium (faster in solids and liquids than in air) and on temperature, but it is not on the Reference Tables, so a value is given when needed.
Pitch and loudness
This maps the general wave properties onto how we perceive sound. Tuning a guitar string raises its frequency and so its pitch; plucking it harder increases the amplitude and so the loudness. The Regents may ask which property to change to alter pitch (frequency) versus loudness (amplitude).
The Doppler effect
The classic example is a passing siren: as the vehicle approaches, the pitch is higher than normal; as it passes and moves away, the pitch drops noticeably. The same effect applies to light (a receding star's light is shifted toward the red, lower-frequency end, the "redshift" that shows the universe is expanding). The key reasoning point is that it is the relative motion that matters, and the apparent change reverses as the source goes from approaching to receding.
Reference Tables note
The wave equation and on the Reference Tables apply to sound, but there is no Doppler-effect equation on the Physical Setting/Physics tables, so the Doppler effect is assessed qualitatively. The speed of sound is not a listed constant (unlike the speed of light), so it is provided in any problem that needs it. You recall that sound is longitudinal, needs a medium, and that pitch and loudness map to frequency and amplitude.
Try this
Q1. State why sound cannot travel through a vacuum. [1 point]
- Cue. Sound is a mechanical wave that needs particles of a medium to vibrate; a vacuum has none.
Q2. State what happens to the pitch heard by an observer as a sound source moves away from them. [1 point]
- Cue. The pitch heard is lower than the actual pitch (the waves are stretched, lowering the apparent frequency).
Exam-style practice questions
Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Regents (style)1 marksPart A (multiple choice). As a fire truck with its siren sounding approaches a stationary observer, the observer hears a pitch that is (1) lower than the siren's actual pitch (2) higher than the siren's actual pitch (3) the same as the siren's actual pitch (4) steadily decreasing. Justify your choice.Show worked answer →
A 1-point Part A item on the Doppler effect. The answer is (2).
As the source approaches, each successive wavefront is emitted from a closer position, so the waves reach the observer more frequently, raising the apparent frequency and therefore the pitch. After the truck passes and recedes, the pitch drops below the actual pitch. The trap is (3): the pitch only matches the actual pitch when there is no relative motion.
Regents (style)2 marksPart B-2 (constructed response). A sound wave travels through air at m/s. State why this sound cannot travel through the vacuum of space, and calculate the wavelength of a Hz sound in air.Show worked answer →
A 2-point constructed-response item combining a concept and a calculation.
Vacuum (1 point): sound is a longitudinal mechanical wave that needs a material medium for its particles to vibrate; a vacuum has no particles, so sound cannot travel through it.
Wavelength (1 point): m.
Markers reward stating that sound needs a medium and the correct wavelength from the wave equation.
Related dot points
- Define amplitude, wavelength, frequency and period, distinguish transverse and longitudinal waves, and apply the wave equation and the period-frequency relationship .
A Regents Physics answer on wave properties and the wave equation: amplitude, wavelength, frequency and period, transverse versus longitudinal waves, and the Reference-Table equations linking wave speed, frequency and wavelength, with worked examples.
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A Regents Physics answer on reflection and refraction: the law of reflection, the absolute index of refraction, and Snell's law for the bending of light between media, using the Reference-Table equations, with worked examples.
- Describe diffraction as the spreading of waves around obstacles and through openings, and explain interference as the superposition of waves, distinguishing constructive and destructive interference and standing waves.
A Regents Physics answer on diffraction and interference: the spreading of waves around obstacles and through gaps, the principle of superposition, constructive and destructive interference, standing waves with nodes and antinodes, and how interference shows light is a wave, with worked reasoning examples.
- Describe the electromagnetic spectrum as a family of transverse waves travelling at the speed of light in a vacuum, ordered by frequency and wavelength, and apply to electromagnetic waves.
A Regents Physics answer on the electromagnetic spectrum: the family of transverse waves from radio to gamma rays, all travelling at the speed of light in a vacuum, ordered by frequency and wavelength, and how to apply the wave equation, with worked examples.
- Describe uniform circular motion, calculate centripetal acceleration with and centripetal force with , and identify the real force that supplies the centripetal force in a given situation.
A Regents Physics answer on uniform circular motion: why circular motion is accelerated even at constant speed, how to calculate centripetal acceleration and force with the Reference-Table equations, and what real forces supply the centripetal force, with worked examples.
Sources & how we know this
- Reference Tables for Physical Setting/Physics — NYSED (2006)
- Physical Setting/Physics Core Curriculum — NYSED (2010)