A Quiet Wind Tunnel

A sonic boom is a relative observation... When you would move with the same speed as the pressure wave there would be no "boom". So When a plane passes you with supersonic speed when you are standing on the ground you will hear the boom as the pressure wave passes you. However in the wind tunnel there is no boom because the air is moving, although the pressure wave caused by an object in the air stream is present. Is this corrctly explained?

I'm still not sure, because if it is relative, then the air is going mach 6 over the stationary plane being tested. The air is also mach 6 compared to the observer so that seems to be like there might be a sonic boom.

Your explaination makes sense, but there is something that you are missing. I believe that it is not necessarily the plane's movement relative to the air that creates the sonic boom. Rather, it is the plane's velocity relative to the velocity of the sound it is creating. You can have the wind moving as fast as you want, and if the plane is stationary, it is not actually exceeding the velocity of the sound it is creating. I think, that your point about the relative observer is correct. A person inside the plane creating the sonic boom will not hear it because they, with the plane, are traveling faster than the sound and therefore cannot hear it. My question then becomes: what if you are trailing the plane by holding on to a tow line? You cannot hear the sonic boom that you are creating. At first I would assume that you would not hear the plane's sonic boom either because of the relative velocity being zero (same speed as plane). However, I think you would hear it because the sonic boom is being created in front of you and must travel past you.

The "boom" we are talking about is not a momentary happening, but it is continuous. Therefore I reffered to the man on the ground hearing the "boom". The "sonic-boom" is only heard when a pressure wave moves in to our ears, beating on our hearing organ. In the wind tunnel the phenomenon can be observed and when the relative moisture is high enough one can see a conus shaped profile coming of from the nose of the object being tested caused by rappid decompression just behind the pressure wave. So even when you would be behind the plane there is no "boom" but only pressure differences between the various locations inline of the direction of movement. Maybe my explanation is confusing you, is there anybody else that can explain it more clearly?

That's a clear explaination. I am somewhat of a layman on the subject, but gave it a shot anyway.