Reflection of Sound – The basic idea:
When sound travels and strikes a surface, it can bounce back towards the same medium. This bouncing back of sound is called reflection of sound.
For example, when you shout towards a large wall, the sound travels towards the wall. The wall reflects the sound, and the reflected sound travels back towards you.
This simple idea of reflection is important for understanding both echo and SONAR.
What is an Echo?
An echo is a reflected sound that is heard as a distinct repetition of the original sound.
Imagine standing some distance away from a large cliff and shouting: “Hello!”
You first hear your original “Hello!”. After a short delay, you hear another “Hello!” because the sound has travelled to the cliff, reflected from it and returned to you.
That second, separate sound is the echo.
How is distance found using an Echo?
Suppose you stand in front of a cliff and make a sound. The sound does not simply travel from you to the cliff.
It travels:
You \(\to\) Cliff \(\to\) You
Therefore, the sound travels twice the distance between you and the cliff. This means that when we calculate distance using:
\(Distance=Speed\times Time\)
the answer represents the total distance travelled by the sound, not just the distance between you and the reflecting surface.
Therefore:
\(Round\ trip\ distance=v\times t\)
Since the sound travels to the reflecting surface and then comes back:
\(Distance\ of\ reflecting\ surface=\frac{v\times t}{2}\)
Why do we divide by \(2\)?
Students often make a mistake here, so remember the journey of the sound.
Suppose the distance between the source and the wall is \(50\ m\).
The sound travels:
\(50\ m\)
towards the wall and another:
\(50\ m\)
back to the source.
Therefore:
\(Total\ distance=50+50=100\ m\)
So, if a calculation gives us \(100\ m\), that is the round-trip distance.
The actual distance to the wall is:
\(\frac{100}{2}=50\ m\)
What is Reverberation?
Now consider a different situation.
Imagine speaking inside a large, empty auditorium. You say a sentence, but instead of hearing one clear repetition, the sound seems to continue for a short time.
Why? The sound is repeatedly reflected from the walls, floor and ceiling.
These reflected sounds reach your ears very close together and overlap with the original sound and with one another. This persistence of sound due to repeated reflections is called reverberation.
Echo vs Reverberation:
Both phenomena involve reflection of sound, but they are heard differently.
| Echo | Reverberation |
|---|---|
| Reflected sound is heard as a distinct repetition. | Sound persists for a short time due to repeated reflections. |
| The reflected sound is heard separately from the original sound. | Reflected sounds overlap with the original sound and one another. |
| Example: shouting near a distant cliff and hearing the shout again. | Example: speech continuing in a large, reflective auditorium. |
| A clear repetition can be heard. | Speech may become unclear if reverberation is excessive. |
Why does an auditorium need Sound-absorbing materials?
In an auditorium, too many reflections can produce excessive reverberation.
Hard surfaces such as walls, floors and ceilings can reflect sound strongly.
Therefore, suitable sound-absorbing materials such as curtains and carpets can be used.
These materials absorb some of the sound energy and reduce repeated reflections.
As a result:
\(\downarrow\)
Less excessive reverberation
\(\downarrow\)
SONAR – Using reflected sound to find underwater objects:
The same idea of reflection is used in SONAR.
A SONAR device sends ultrasonic sound waves into water.
The waves travel through the water and may strike an underwater object, such as the sea floor or another object.
The waves are reflected and return to the SONAR device.
The device measures the time taken for the reflected signal to return.
Since the speed of sound in the medium is known, the distance can be calculated.