Q1. A ship sends an ultrasonic pulse and receives the echo from the seabed after 4 s. If the speed of sound in water is 1500 m/s, find the depth.
d = v·t/2 (the sound travels down and back)
d = 1500 × 4 / 2
d = 3000 m
Sound — Class 9
Sound is a wave you can do maths with. Use the wave calculator to connect frequency, wavelength and speed, then practise echo and SONAR numericals.
Sound waveA longitudinal mechanical wave — it needs a medium and travels as compressions and rarefactions. Cannot travel through vacuum.
Frequency (f) & Wavelength (λ)Frequency = waves per second (hertz, Hz); wavelength = distance between two compressions (m). Pitch rises with frequency.
Wave speedv = f·λ. In air at room temperature, sound travels at about 340 m/s (much faster in water and solids).
Echo & ReverberationAn echo is a distinct reflected sound (heard if the obstacle is ≥ 17 m away). Persistence of sound by repeated reflection is reverberation.
Sound wave calculator Interactive
Link frequency, wavelength and speed, and find how far an echo-causing surface is.
Wave speed, v = f·λ—m/s
Surface distance, d = 340·t/2—m
The human ear hears 20 Hz to 20,000 Hz. Below is infrasound, above is ultrasound (used in SONAR and medical scans).
Numericals practice Interactive
Wave-speed and echo problems with worked steps.
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Solved numericals
Q2. A tuning fork of frequency 340 Hz produces sound in air (v = 340 m/s). Find the wavelength.
v = f λ ⇒ λ = v/f
λ = 340 / 340
λ = 1 m
Formula sheet
| Quantity | Formula | SI unit |
|---|---|---|
| Wave speed | v = f λ | m/s |
| Frequency–time period | f = 1 / T | hertz (Hz) |
| Echo / SONAR distance | d = v·t / 2 | m |
| Speed of sound in air | ≈ 340 m/s (at ~25 °C) | m/s |
Common mistakes & exam wins
- Sound cannot travel through a vacuum — it always needs a material medium.
- In the echo/SONAR formula divide by 2, because the sound covers the distance twice (to the surface and back).
- Speed of sound: solids > liquids > gases (opposite to light).
- For a distinct echo, the reflecting surface must be at least 17 m away (so the delay is at least 0.1 s).
Frequently asked questions
What type of wave is sound?
Sound is a longitudinal mechanical wave that travels through a medium as compressions and rarefactions.
What is the relationship between speed, frequency and wavelength?
Wave speed = frequency × wavelength, v = f·λ.
What is an echo?
An echo is the repetition of a sound caused by reflection from a distant surface (at least 17 m away so it is heard distinctly after 0.1 s).
What is ultrasound used for?
Ultrasound (above 20,000 Hz) is used in SONAR to measure sea depth, in medical imaging, and to clean and detect flaws in machine parts.