Echo is the single reflection of sound wave . When you stand in front of a wall, and you make a sound, it travels to the wall, upon hitting the wall, it get reflected, such phenomenon is called Echo. Reverberation is the multiple reflection of sound. In a large telecommunications auditorium, perforated absorbent materials are used to line the ceiling so as to reduce reverberation of sound in the hall. The speed of sound varies from one medium to another. It is fastest in metals, followed by water, and then by air.
The speed of sound depends on the following factors.
Let's take a question on this.
- Temperature: The speed of sound increases as the temperature increases. i.e speed of sound (V) is directly proportional to the square root of Temperature.
-The velocity of sound in air will be doubled if its absolute temperature is ---(a) halved (b) doubled (c) quadrupled (d) constant
Solution
V1=v, V2 = 2v, T1 = T, T2= ?
- Direction of the wind: if sound wave travels in the direction of the wind, the speed will increase but if it travels in opposite direction of the wind, it will decrease.
- It is use to determine the speed of sound in air. It can be calculated by using this formula, V= 2d/t, where 'V' is the velocity of sound in m/s, 'd' is the distance traveled by sound in metres, 't' is the time taken to travel this distance.
- It is also use to determine the depth of sea from a ship.
- A man standing 45m from the foot of a tall cliff claps his hands and hears an echo 0.6sec later. Calculate the velocity of sound in air in m/s
- A ship travelling towards a cliff receives the echo of its whistle after 3.5secs, a short while it receives the echo after 2.5secs. If the speed of sound in the air under the prevailing condition is 250m/s. How much closer to the cliff is the ship?
. The formula to use is this:
V is the speed of sound in air
Vo is the velocity of observer
Vs is the velocity of source
F is the frequency emitted by source.
Several steps will be use to explain this phenomenon.
Step 1: When the observer is moving towards the source at rest. (Vs = 0)
Step 2: When the observer is moving away from the source at rest (Vs = 0)
Step 3: When the source is moving towards the observer at rest (Vo = 0)
Step 4: When the source is moving away from the observer at rest (Vo = 0)
Step 5: When they are moving in the same direction ( Observer chasing the source)
Step 6: When they are moving in the same direction ( Source chasing the Observer)
Step 7: When the observer and the source are moving in opposite direction
Questions
- A car moving at 20m/s with it's horn blowing (F= 1200Hz) is chasing another car going at 15m/s. What is the apparent frequency of the horn as heard by the driver being chased?( V= 340m/s)
This question falls under step 6
Vs =20m/s, F= 1200Hz, Vo=15m/s, V= 340m/s.
- A stationary siren emits a note of frequency 400Hz as a train approaches it with 20m/s. What frequency will be received on the train?( Speed of sound is 331m/s)
This question falls under step 1
V = 331m/s, F= 400Hz, Vo = 20m/s
Exercises
- A boy receives the echo of his clap reflected by a nearby hill 0.8s later. How far is he from the hill? ( Speed of sound in air = 340m/s)