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MCAT Sound Practice Questions: Doppler Effect, Decibels and Standing Waves

By StudyStore · Published 2026-10-27 · 9 original questions · about 14 min

Nine free MCAT-style sound practice questions on wave speed, decibels, standing waves in strings and pipes, the Doppler effect and ultrasound, with full explanations.

Quick concept primer

Sound is a longitudinal pressure wave, and v = fλ ties its speed, frequency and wavelength together. The medium sets the speed: sound moves fastest in stiff solids, slower in liquids, and slowest in gases, so changing the source frequency in one medium changes only the wavelength. Loudness is measured on a log scale, β = 10 log(I/I0), so every factor of 10 in intensity adds 10 dB. Intensity from a point source falls as 1/r2.

Standing waves form when reflected waves overlap. A string fixed at both ends or a pipe open at both ends supports every harmonic, fn = nv/2L. A pipe closed at one end supports only odd harmonics, fn = nv/4L with n = 1, 3, 5. The Doppler effect shifts the heard frequency, f' = f(v ± vobserver)/(v ∓ vsource), using the top signs when source and observer approach. The 4D light, sound and optics pack adds flashcards and more questions on waves and optics, and the free sampler lets you try sample cards first.

Traps: decibels do not add linearly, a source moving toward you raises pitch but does not change the speed of sound, and ultrasound depth uses half the echo time.

9 practice questions

Try each question before opening the explanation. Click "Show answer and explanation" to check yourself. All questions are original and written for study practice.

Question 1

A pump produces a sound level of 60 dB at a technician's bench. A second change in the room raises the sound intensity at the bench by a factor of 1000. What is the new sound level?

  1. 63 dB
  2. 90 dB
  3. 120 dB
  4. 180 dB
Show answer and explanation

Answer: B. 90 dB

Each factor of 10 in intensity adds 10 dB, so a factor of 1000 (three factors of 10) adds 30 dB, giving 90 dB. The 63 dB choice adds log(1000) without the factor of 10, 120 dB doubles the decibel value, and 180 dB multiplies 60 by 3, treating the scale as linear.

Question 2

A small speaker acts as a point source in an open field. A meter 2.0 m away reads 70 dB. Ignoring absorption, about what does the meter read at 4.0 m?

  1. 35 dB
  2. 67 dB
  3. 64 dB
  4. 58 dB
Show answer and explanation

Answer: C. 64 dB

Doubling the distance spreads the energy over four times the area, so intensity drops to one quarter. The change is 10 log(1/4), about −6 dB, giving about 64 dB. The 67 dB choice assumes intensity halves, 58 dB uses a factor of 16, and 35 dB halves the decibel value as if the scale were linear.

Question 3

A tube 0.85 m long is closed at one end and open at the other. The speed of sound in the air inside is 340 m/s. What is the lowest resonant frequency of the tube?

  1. 100 Hz
  2. 200 Hz
  3. 400 Hz
  4. 50 Hz
Show answer and explanation

Answer: A. 100 Hz

A tube closed at one end has a node at the closed end and an antinode at the open end, so its fundamental wavelength is 4L = 3.4 m and f = 340/3.4 = 100 Hz. The 200 Hz choice uses the open-open formula v/2L, 400 Hz uses v/L, and 50 Hz uses v/8L.

Question 4

A pipe closed at one end has a fundamental frequency of 120 Hz. All of the following are resonant frequencies of this pipe EXCEPT:

  1. 360 Hz
  2. 600 Hz
  3. 240 Hz
  4. 840 Hz
Show answer and explanation

Answer: C. 240 Hz

A pipe closed at one end supports only odd multiples of its fundamental: 120, 360, 600, 840 Hz and so on. These are the 1st, 3rd, 5th and 7th harmonics. 240 Hz is an even multiple (the second harmonic), which would need an antinode at the closed end, so it cannot form.

Question 5

A string 0.60 m long is fixed at both ends, and waves travel along it at 240 m/s. What is the frequency of its third harmonic?

  1. 200 Hz
  2. 400 Hz
  3. 1200 Hz
  4. 600 Hz
Show answer and explanation

Answer: D. 600 Hz

For a string fixed at both ends, fn = nv/2L. The third harmonic is 3(240)/(2 × 0.60) = 600 Hz, with a wavelength of 0.40 m. 200 Hz is the fundamental, 400 Hz is the second harmonic, and 1200 Hz uses v/L instead of v/2L for each step.

Question 6

An ambulance siren emits 700 Hz while the ambulance drives straight toward a stationary pedestrian at 20 m/s. Take the speed of sound as 340 m/s. About what frequency does the pedestrian hear?

  1. 661 Hz
  2. 700 Hz
  3. 741 Hz
  4. 744 Hz
Show answer and explanation

Answer: D. 744 Hz

For a moving source approaching a still observer, f' = f × v/(v − vs) = 700 × 340/320, about 744 Hz. The 741 Hz choice uses the moving-observer formula 700 × 360/340, which applies when the listener moves, not the source. 661 Hz is the receding value, and 700 Hz ignores the shift.

Question 7

A tuning fork in a room is replaced by one whose frequency is twice as high. Air conditions do not change. How do the wavelength and speed of the sound in the room change?

  1. Wavelength halves and speed stays the same.
  2. Speed doubles and wavelength stays the same.
  3. Both wavelength and speed double.
  4. Wavelength doubles and speed stays the same.
Show answer and explanation

Answer: A. Wavelength halves and speed stays the same.

The speed of sound is set by the medium, here the same air, so it does not change. From v = fλ, doubling f at constant v cuts λ in half. B and C wrongly let frequency change the speed, and D has the inverse relationship backward.

Question 8

An ultrasound pulse travels through soft tissue at about 1540 m/s and reflects off a boundary. The echo returns to the probe 26 µs after the pulse was sent. About how deep is the boundary?

  1. 1.0 cm
  2. 2.0 cm
  3. 4.0 cm
  4. 0.20 cm
Show answer and explanation

Answer: B. 2.0 cm

The pulse travels to the boundary and back, so depth = vt/2 = (1540)(26 × 10−6)/2, about 0.020 m, or 2.0 cm. The 4.0 cm choice forgets to halve the round trip, 1.0 cm halves twice, and 0.20 cm slips a power of ten.

Question 9

At room temperature, in which of these media does sound travel fastest?

  1. A steel rod
  2. Liquid water
  3. Air
  4. Helium gas
Show answer and explanation

Answer: A. A steel rod

Sound speed rises with the stiffness of a medium relative to its density, and solids like steel are far stiffer than fluids, giving speeds of several thousand meters per second. Water is next at roughly 1500 m/s. Helium carries sound faster than air because its atoms are light, but it is still a gas and well below water and steel.

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Written by StudyStore from standard scientific facts as a study aid. These are not official exam questions and are not affiliated with or endorsed by any testing organization. Always check values against the reference provided with your exam or course.

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