Velocity of Pressure Waves in Compressed Air Pipe Flow

A1: "Seek and ye shall find."

A2: No.

1. Yes; 2. No.

Can sound travel in vacuum?

Not in an absolute vacuum. Why the question?

Thanks PWSlack for confirmation.

But here Joshi's doubt is very much relevant. It is extremely difficult to create absolute vacuum. At high vacuum condition (below 0.001 mbar absolute pressure) will the velocity of sound be the same as ambient air pressure?

You just moved the goal posts. In the OP, it was only 0.5 bar vacuum.

OP's question "1. Can we presume that velocity of sound at same temperature will be same for 0.5 bar vacuum and 25 bar g pressure?"

What I would like to know is "will the velocity of sound be same for 0.5 bar vacuum and 25 bar g pressure?"

Yes Tarnado, you are right. I moved the goal post from 0.5 bar(a) to 0.001 mbar(a) pressure, just to better understand the theory behind it.

Theory says sound require a medium to travel, it can't travel in absolute vacuum. At the same time theory says velocity of sound in gaseous medium is independent of its pressure. Then at constant temperature velocity of sound should be the same at different pressures like 25 bar (a), 0.5 bar(a), 0.001 mbar(a) or even lower. So now the questions are:

1. Where is the cut-off point of gas pressure when sound will suddenly refuse to travel?

2. Is the sound waves travel through the molecules of gaseous medium which collied each other and transmit it and when collision reduces/stops velocity also reduces/ stops?

I'd say that at a certain point of reducing pressure, the concept of sound ceases to be valid, as the molecules rarely collide. Not sure though about whether that's starting to happen at 0.001 mbar. I'd guess not as air at that pressure still contains about 3*10¹³ molecules per millilitre (or cm³). Avogadro's no. 6*10²³/gmol/22.4 litre molar volume/10³ ml/litre/10⁶ 0.001 mbar/bar.

Incidentally, for gas flow in pipes, at very low pressure where individual molecules are moving along the pipe, behaviour is different and it's called Knudsen flow.

GA Codemaster for understanding what I wanted to ask further. I was busy answering to Tarnado's comment before seeing your's.