Sound: Resonance & Doppler

What this investigation asks

A source approaching a listener can produce a higher measured frequency, but moving closer at a constant approach speed does not continually raise that frequency merely because the distance is shrinking. This investigation separates frequency, loudness, relative motion, and acoustic resonance. Record the source and listener conditions before interpreting a sound change. The resonance tube supplies a second measurement chain for sound speed. Its length pattern comes from boundaries in the air column, while the Doppler pattern comes from motion relative to the wave propagation in the medium.

Design and measure the comparison

For the closed-tube activity, change air-column length and identify successive strong-response positions at a fixed source frequency and temperature. Retain which end is closed and the tube-end correction used in the course. Neighboring resonant lengths differ by half a wavelength under that model. Compare an open-ended tube separately, because its allowed patterns differ. A length recorded at one loud position is insufficient to establish sound speed without the frequency and the corresponding mode or successive-position evidence needed to infer the wavelength from the selected boundary arrangement.

Connect the record to the model

Use frequency multiplied by wavelength to infer speed and compare the result with the temperature-matched reference. Sound is a longitudinal mechanical wave: air particles oscillate locally while compressions and rarefactions propagate. The closed-open boundary supports the specified odd-harmonic pattern. The end correction means actual strong-response lengths need not be exact simple multiples of the shortest measured length. Keep particle displacement and pressure patterns distinct when identifying a node. The same location can have different roles in those two complementary descriptions of the standing acoustic wave.

Interpret the evidence and its limits

In the Doppler comparison, vary source or listener velocity while keeping the source's emitted frequency fixed. Distinguish motion relative to the medium from a changing distance alone. A change in loudness can occur as the source approaches without an additional change in the frequency shift at a fixed velocity. Retain the direction convention and avoid substituting one relative-speed expression into every configuration without checking the course formula. A final explanation should identify whether the evidence comes from changing wave-arrival timing, a resonance boundary, or a temperature effect on the medium's sound speed.

Reference conditions and quantities

At 20 °C, the course reference sound speed is 343.2000000 m/s. For its 384 Hz source, the reference wavelength is 0.8937500000 m. These quantities describe the specified temperature and frequency. Resonant tube lengths additionally depend on the boundary and end correction, so the wavelength should not be equated directly to the shortest observed air-column length.

Use the record to test your explanation

Write a prediction before collecting the comparison, naming the quantity that should change and the condition you will alter. Keep a reference run and vary one factor at a time when the question calls for a controlled test. Record the selected apparatus settings, units, and measurement interval. Where an activity supplies Ideal and Real modes or seeded variation, retain that identity with each record rather than mixing incompatible runs in one calculation.

Separate an instrument reading or counted observation from the model relationship used to explain it. Keep raw values alongside derived results so another student can reconstruct the argument. A graph needs labeled axes and a stated comparison range. If a result differs from a reference, inspect the conditions, rounding, and measurement procedure before changing the explanation.

For a classroom discussion, compare claims supported by the same evidence and identify an additional measurement that would distinguish competing accounts. Finish with a claim, the relevant observations, and reasoning tied to this course's model. State a specific limitation instead of assigning a general error label to every disagreement.

Related investigations

Compare transverse standing waves in Standing Waves on a String, or connect measured spectral shifts to Stellar Spectra & Redshift. Identify the medium, relative-motion model, and observed quantity before transferring an explanation of a frequency shift.