Change the note, not the speed
Speed does not depend on frequency.
Low note, high note
A friend plays a low note, then a high note on a flute. You stand far away.
Do you think the high note reaches you sooner, later, or at the same time?
What this lesson covers
The idea
In most media, including air, the speed of sound depends only on the medium, not on the source or frequency; changing the frequency changes the wavelength while the speed stays constant.
Low note, high note
A friend plays a low note, then a high note on a flute. You stand far away.
Do you think the high note reaches you sooner, later, or at the same time?
- At the same time
- Sooner
- Later
Change the frequency
The speed of sound in air is taken as 344 m s⁻¹ (Example 10.2). Choose a frequency. The frequencies are only examples.
The medium sets the speed
Speed = wavelength × frequency. If the frequency is doubled and the speed stays the same, the wavelength becomes half. The speed is set by the medium, not by the source.
In most media, such as air, the speed of sound depends only on the medium, not on the source or the frequency. If the frequency changes, the wavelength changes while the speed stays constant.
Notes
In most media, speed depends only on the medium, not on the source or the frequency. A new frequency means a new wavelength but the same speed.
Check yourself
In air, a sound of low frequency and one of high frequency travel from the same place. What can you say about their speeds?
The frequency of a source changes and the sound stays in air. What happens to the wavelength?
In air the speed of sound is 344 m s⁻¹. What is the wavelength of a sound of frequency 344 Hz, in m?
Answer: 1 m
λ = v ÷ ν = 344 ÷ 344 = 1 m.
The speed of sound is 344 m s⁻¹. The frequency is raised from 344 Hz to 688 Hz. What is the new wavelength, in m?
Answer: 0.5 m
λ = 344 ÷ 688 = 0.5 m. The speed stays at 344 m s⁻¹.
- The higher frequency travels faster. In most media, such as air, the speed of sound does not depend on the frequency, so a higher frequency is not faster.
- They are the same — correct. Yes!
- The lower frequency travels faster. In most media, such as air, the speed of sound does not depend on the frequency, so a lower frequency is not faster.
- It stays the same. If the frequency of the source changes, the wavelength in the medium changes while the speed remains constant.
- It changes, and so does the speed. Only the wavelength changes. The speed stays constant.
- It changes — correct. Yes!