Echo

An echo, nature’s acoustic fantasy, has long been the protagonist of numerous legends and myths, embodied by the nymph of the same name in Greek mythology. This poetic interpretation served to explain the mystery of sound repetition, an enigma for our ancestors who wove melancholic stories around it, like that of Echo and Narcissus.

Discover FizziQ

How to measure it in class

Steps:

Find a large wall or cliff at a known distance
Use the acoustic stopwatch to time the echo delay
Calculate the speed of sound from the round-trip time
Compare with theoretical values at different temperatures

Learn more

With the Renaissance and its critical spirit, curious minds like Francis Bacon and Marin Mersenne began deciphering the echo phenomenon through a scientific lens, discovering that sound, like light, reflects off surfaces. Their approach moved echo from the realm of myth to acoustic physics.

Pierre Gassendi proposed that sound propagation was due to a flow of reflected particles, while Athanasius Kircher coined the term “phonocamptics” to describe this emerging science. Their work paved the way for scientists like Isaac Newton, who laid the foundations of fluid mechanics and wave theory, essential for modern acoustics.

The scientific understanding of echo has evolved from simple observation and analogy with light toward a more complex science integrating precise physical and mathematical principles. This theoretical enrichment has transformed echo into a valuable teaching tool for secondary students.

In class, echo becomes a concrete and engaging phenomenon. Students can investigate the conditions that create echoes. A perceptible echo depends on several factors: sufficient distance to an obstacle, initial sound power, conducive acoustic environment, and adequate reflecting surface.

Echo is much more than a curiosity or sound effect: it is a window to understanding wave physics, with extensive practical applications from concert hall design to submarine technology.

Formula

Echo delay: t = 2d/v

Where:

t: time between sound emission and echo reception
d: distance to the reflecting surface
v: speed of sound (approximately 343 m/s at 20°C)

Minimum distance for distinct echo: d > v × 0.1/2 ≈ 17 m

Scientific activities on this topic

Several methods allow you to use echo to measure the speed of sound.

1: Speed of sound - Measuring the speed of sound by timing: https://www.fizziq.org/en/activities/speed-of-sound/
2: Seven methods to measure the speed of sound - Complete article: https://www.fizziq.org/post/sept-expériences-pour-mesurer-la-vitesse-du-son
3: Can we see sound? - Visualizing sound waves: https://www.fizziq.org/post/peut-on-voir-le-son

Examples

A hiker shouts facing a cliff and hears the echo 2 seconds later: the cliff is 340 m away
A submarine’s sonar detects an obstacle at 500 m by measuring an echo after 0.67 s
Ultrasound imaging uses ultrasounds (2-10 MHz) to image organs through echoes
Bats use echolocation to navigate and hunt in darkness
Concert halls are designed to avoid disturbing echoes while preserving good reverberation

FAQ

Q: Why don’t we always hear an echo? A: To perceive a distinct echo, an obstacle must be more than 17 m away. Additionally, the obstacle must be sufficiently reflective and the sound loud enough to return audibly.

Q: Does echo change the frequency of sound? A: No, the frequency remains identical. Only the intensity decreases as energy dissipates during travel. If the obstacle is moving, the Doppler effect modifies the frequency.

Q: How do bats use echo? A: They emit ultrasounds and analyze the echoes to locate obstacles and prey. This process is called echolocation.

Q: What is the difference between echo and reverberation? A: Echo is a distinct reflection (delay > 0.1 s). Reverberation is the superposition of multiple closely-spaced reflections that prolong the sound without clear repetition.

Q: Can you measure the speed of sound with a smartphone echo? A: Yes, FizziQ allows you to record sound and precisely measure the delay between emission and echo. This is one of the most accessible methods.

Speed of sound - Wave reflection - Reverberation - Sonar - Ultrasound imaging - Echolocation - Doppler effect