# Resonance of an Air Column

• john-b
In summary, when a sound is created in a pipe, it is because of the vibrations of the air molecules. Closing the end of the pipe decreases the pitch of the sound because the standing wave is no longer created.

## Homework Statement

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When a stream of air is directed over the end of a 40 cm long piece of plastic pipe, open at both ends, a sound is produced.
(a) Explain why this occurs
(b) If the bottom end of the pipe is covered, what will happen to the pitch of the sound? Explain

## Homework Equations

Ln = (n x λ)/2
F1 = v/(2 x L1)
Ln = (2n-1) x (λ/4)
F1 = v/(4 x L1)

## The Attempt at a Solution

For a), my best guess is that the stream created vibrations in the air molecules which caused the plastic pipe to resonate at its natural frequency and that created a sound

For b), I recreated this question by blowing over a straw and noticed the pitch decreases when the bottom is covered, and I know that a decrease in pitch means a decrease in frequency, but I do not know why this happens.

A nudge in the right direction would be appreciated. Thank you.

You are creating a standing wave in the straw. Opening or closing the end of the straw changes how the wave reflects of that endpoint.

I see. Is the standing wave created by vibrations in the pipe from the moving air?

john-b said:
I see. Is the standing wave created by vibrations in the pipe from the moving air?
A standing wave arises when a wave is reflected back to the source with little attenuation.
Any change in medium tends to create a reflection, and that includes the end of a pipe, whether open or closed.
At a closed end, the air is unable to vibrate along the pipe, so the phase of the reflection is so as to cancel the movement of air at that point.
At an open end, the air pressure cannot change, so in that case the phase of the reflection is so as to cancel the pressure variation there.

## 1. What is resonance of an air column?

Resonance of an air column refers to the phenomenon in which a column of air vibrates at a specific frequency due to an external force or sound wave. This frequency is known as the resonant frequency.

## 2. How is resonance of an air column produced?

The resonance of an air column is produced when a source of sound, such as a musical instrument or a speaker, creates sound waves that travel through the air and enter the column. These sound waves then reflect back and forth within the column, creating a standing wave that resonates at a specific frequency.

## 3. What factors affect the resonant frequency of an air column?

The resonant frequency of an air column is affected by the length, diameter, and shape of the column, as well as the speed of sound in the medium and the air temperature. For a closed column, the resonant frequency is also affected by the air pressure inside the column.

## 4. How is the resonance of an air column used in musical instruments?

Musical instruments, such as flutes and clarinets, use the resonance of an air column to produce sound. The musician changes the pitch of the sound by altering the length of the column through covering or uncovering holes, thereby changing the resonant frequency of the column.

## 5. What are some practical applications of the resonance of an air column?

The resonance of an air column has various practical applications, such as in musical instruments, acoustics, and medical devices. It is also used in measuring instruments, such as air column manometers and Helmholtz resonators, to measure air pressure and flow rates. Additionally, the phenomenon of resonance is utilized in designing and tuning sound systems and in the production of sound effects in movies and TV shows.