Wave reflection in Closed End Wind Instrument

[mayer]

Hi, I am a little confused with the phase change that occurs in closed end wind instruments. According to http://newt.phys.unsw.edu.au/jw/flutes.v.clarinets.html, the phase does not change when the sound wave reflects off the closed end of the instrument. I thought that the phase changes by 180 degrees after reflecting off a denser medium?

Thank You

 

[Greg Bernhardt]

I'm sorry you are not finding help at the moment. Is there any additional information you can share with us?

 

[AlephZero]

Remember sound waves in a fluid are longitudunal waves, not transverse waves like vibrations of a string.

For the pressure reflection from a closed end, think of it this way: if there is a positive pressure pulse approaching the end, there is more air in the tube than there would be if there was no pulse. So if the phase inverted to give a negative pressure pulse, where is that "extra" air going to end up? There is nowhere for it to go, and the pressure pulse doesn't invert.

At the open end, the pressure pulse leaves the tube and dissipates in the outside air, leaving a partial vacuum behind it - which looks like a negative pressure pulse, or a phase inversion.

Note, the velocity pulse at the closed end does invert. But the velocity doesn't invert at the open end, because the air "in fromt of" the reflected negative pressure pulse is traveling towards the open end, to "fill up" the region of negative pressure as the pressure pulse moves further into the pipe.

 

[mayer]

Ahhh, I see. Well that cleared it up! Thanks to the both of you!

 

[sardar]

for your question! I can understand your confusion about the phase change in closed end wind instruments. Let me explain the concept of phase change and its relationship to wave reflection in these instruments.

First, let's define what we mean by phase change. In simple terms, phase change refers to the shift in the position of a wave's crest or trough. In the case of sound waves, it can also refer to the shift in the compressions and rarefactions of the air molecules that make up the wave.

Now, when a sound wave travels through a medium, it causes the particles in the medium to vibrate in a particular direction. When this wave reaches the closed end of a wind instrument, it reflects back in the opposite direction. However, unlike light waves that experience a phase change when reflecting off a denser medium, sound waves do not experience a phase change when reflecting off a closed end.

This is because the closed end of a wind instrument is not a denser medium compared to the air inside the instrument. The air molecules inside the instrument are still the same density as the air outside. Therefore, there is no change in the phase of the sound wave upon reflection.

In fact, the phase of the sound wave remains the same throughout the instrument, except for when it reaches the open end. At the open end, the sound wave is able to escape and continue traveling in a straight line. This change in direction is what causes the phase of the wave to change by 180 degrees.

I hope this explanation has cleared up your confusion about the phase change in closed end wind instruments. It is important to remember that the phase change only occurs when a wave reflects off a denser medium, and in the case of sound waves in wind instruments, this only happens at the open end. Thank you for your question and keep exploring the fascinating world of sound and waves!