聲波被反射時有180度相位差的條件是什麼?

Q: 聲波從一個相對速度較高的介質入射至一個相對速度較低的介質。有沒有錯呢？
No, it is wrong. There is a phase change of 180 degrees on reflection at the boundary when sound travels from a dense medium to a less dense one.

Q:聲波從空氣入射至水然後被反射回來，有沒有相位差的改變?
Yes, when sound travels from air to water, the reflected sound wave on the air-water boundary has no phase change.

Q: 對著牆大叫，被反射回來的聲波有沒有相位差的變?
No, there is no phase change.

Q:聲波由熱空氣入射至冷空氣，被反射回來的聲波有沒有相位差改變?
In fact, there is little reflection under such situation. Most of the sound waves are transmitted. The reason is because the difference of "acoustic impedance" between cold and hot air is not large.

You may refer to the description under the section "phase change upon reflection" on the following web-page:

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/reflec.html


2011-01-02 01:33:02 補充：
Q:關於聲波的那行敘述是不是說錯了?
The description is correct.

2011-01-02 10:24:58 補充：
Thank you in correcting my mistake. I regret that I did the answer late last night and had not checked it before posting.
The most important condition brought by the web-page is "reflections off a lower impedance medium will be reversed in phase"....(cont'd)

2011-01-02 10:25:21 補充：
(cont'd)...That is, there is no phase change when sound in air is reflected off an air-water boundary, but a phase change when reflected off on a water-air boundary.
The description on the table for sound is wrong. Sorry for giving a wrong answer in my post.