Lowest frequency of a water with drinking straw

[desmond iking]

Homework Statement

the ans given here is 'using the opened pipe method'. but i solve it using closed pipe method. which i have sketched using the pencil on the right of the diagram. why can't i solve it using 'closed pipe method' , i just couldn't understand.

Homework Equations

The Attempt at a Solution

 

[CWatters]

A straw is open both ends. It would only be closed one end when you put it into the drink. That is not stated in the problem. How do you know how far the straw is inserted into the drink? If that information was provided you could work it out by treating one end open and the other closed.

 

[CWatters]

PS Do you understand why you get a node at a closed end and an antinode at an open end?

 

[desmond iking]

The air will vibrate at open end so it is antinode. The air wouldn't vibrate at closed end so it is node.

 

[BruceW]

Homework Statement

the ans given here is 'using the opened pipe method'. but i solve it using closed pipe method. which i have sketched using the pencil on the right of the diagram. why can't i solve it using 'closed pipe method' , i just couldn't understand.

This is a good point. It is hard to tell if the straw is closed at one end, or open at both ends. This website http://www.phys.unsw.edu.au/jw/flutes.v.clarinets.html might help a bit. But anyway, to find out I guess you could physically try to get a sound wave to go through a straw. If your mouth is fully closed around the straw, then it is closed at one end. But if you can only make the vibrations when you don't have a tight seal around the straw, then it is open at both ends.

 

[CWatters]

Problem says "blows at the end". I guess not 100% clear but I would take that to mean blow across the end from a distance like you would a bottle.

 

[CWatters]

Perhaps worth looking at the foot note in that link Bruce posted..

It's worth adding that the flute is not entirely open at the embouchure: the hole across which the player blows is smaller than the cross section of the pipe. This narrowing does have an acoustic effect. Nevertheless, it is sufficiently open that large oscillating flows of air can enter and leave the pipe with very little pressure difference from atmospheric. Low pressure, high flow: this boundary condition is a low value of acoustic impedance. The clarinet is not completely closed by the reed: a small, varying aperture is left, even when the player pushes the reed towards the mouthpiece. However, this average area is much less than the cross section of the clarinet so the reflection of the acoustic wave is almost complete, and the acoustic flow is very small, in spite of the large acoustic pressure produced by the vibrating reed. High pressure, low flow: it is a high value of acoustic impedance. See Flute acoustics and Clarinet acoustics for details.

If you seal your lips around the outside a straw the cross section at that point isn't going to be reduced compared to the diameter of the straw. On the contrary the mouth is much larger. eg it's more like a Flute than a Clarinet. I reckon it would be more open than closed.