Sound Waves: Characteristics and Applications · Lesson 11 of 14
Echo
“An echo is a reflected sound heard separately after a sufficient delay, making round-trip distance calculations possible.”
• Define echo. • Explain why echoes are not heard in every room. • Use the minimum time separation criterion. • Calculate minimum reflecting distance. • Solve round-trip echo problems.
A shout near a distant cliff can return as a clearly separate repetition. The original sound has travelled to the reflecting surface and then back to the listener.
Echo
A reflected sound that returns to the listener after the original sound and is heard as a separate sound.
The human auditory system requires roughly 0.1 s between two arriving sounds for them to be distinguished separately in the echo discussion. If the reflection returns sooner, it merges perceptually with the original rather than forming a clearly distinct echo.
Minimum Distance for a Distinct Echo
Using 340 m s⁻¹, sound travels 34 m during 0.1 s. But this is the full outward-and-return path. The reflecting surface is therefore half that distance away.
Problem
Using v = 340 m s⁻¹ and minimum separation time 0.1 s, find the minimum reflecting distance.
- 1.Calculate total sound path: vt = 340 × 0.1 = 34 m.
- 2.The 34 m includes outward and return travel.
- 3.Divide by 2: d = 17 m.
- 4.The reflecting surface must be about 17 m away under these conditions.
Problem
A clap produces an echo after 0.5 s. If sound speed is 340 m s⁻¹, find the distance to the wall.
- 1.Use d = vt/2.
- 2.Substitute v = 340 m s⁻¹ and t = 0.5 s.
- 3.d = 340 × 0.5 ÷ 2.
- 4.d = 85 m.
- 5.The wall is 85 m away.
Problem
An experiment requires an echo at least 0.2 s later. If v = 343 m s⁻¹, find the minimum reflector distance.
- 1.Use d = vt/2.
- 2.d = 343 × 0.2 ÷ 2.
- 3.343 × 0.2 = 68.6 m for the round trip.
- 4.Divide by 2 to obtain 34.3 m.
- 5.The reflector must be at least about 34.3 m away.
Echo numericals nearly always involve round-trip travel. If you forget to divide vt by two, you calculate the total path length rather than the one-way distance.
Quiz
Which description best matches Echo?
Which term matches this description: A reflected sound that returns to the listener after the original sound and is heard as a separate sound.
Which statement is a key takeaway from this lesson?
Which additional statement is also a key takeaway from this lesson?
Which further statement is also a key takeaway from this lesson?
Practice Problems
- Why is a distinct echo usually not heard in a small room?
- Calculate reflector distance for an echo time of 1.0 s when v = 340 m s⁻¹.
- Why do hard smooth surfaces give stronger echoes?
- Why do rough surfaces make clear echoes less likely?
Key Takeaways
• An echo is a reflected sound heard separately from the original sound. • A sufficient time gap is needed for the ear to distinguish the reflected sound. • Because sound travels to the reflecting surface and back, echo calculations involve a round-trip distance. • Distance to the reflecting surface can be determined from sound speed and measured echo time.