# Is the speed of sound relative?

## Main Question or Discussion Point

drunken musings..... if i was sat on the wing of a jet travelling at mach2 would i be able to hear the the noise of the engine behind me?

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mathman
No. Sound speed is Mach 1.

Pengwuino
Gold Member
So yes, speeds are relative if there must exist a medium of propagation (aka everything besides light). Actually that's kind of a cool thought experiment, almost makes me want to strap myself to a supersonic jet.

AlephZero
Homework Helper
No, the sound can only travel at Mach 1 rellative to the speed of the air.

You don't have to sit on the wing. The same effect occurred in Concorde when it accelerated through Mach 1 and the engine noise disappeared in the passenger cabin, apart from a small amount of noise transmitted through the structure of the plane itself and not through the air.

At a more "down to earth" level, this is the cause of the Doppler effect.

Gold Member
It depends on what the speed of the flow is where your head is. Moving at supersonic speeds, shockwaves develop and slow the flow down. Behind the shocks, air is subsonic normal to the shock but may be supersonic overall. If it is subsonic everywhere, you could hear the engines. If it is supersonic, you couldn't. In all likelihood you wouldn't hear the engines.

Pengwuino
Gold Member
You don't have to sit on the wing. The same effect occurred in Concorde when it accelerated through Mach 1 and the engine noise disappeared in the passenger cabin, apart from a small amount of noise transmitted through the structure of the plane itself and not through the air.
Certainly not as cool of an experience, however.

The speed of sound is relative to the density of the medium it is travelling in. Thus it is faster in water than in air and faster at ground level than at altitude. Although Mach 2 airflight is commonplace nowadays, no vehicle has achieved Mach 1 at ground level yet. The new land speed record attempt at 1000 miles an hour will seek to achieve that.

PS Global warming isn't a theory; it's a lie!

The speed of sound is relative to the density of the medium it is travelling in. Thus it is faster in water than in air and faster at ground level than at altitude. Although Mach 2 airflight is commonplace nowadays, no vehicle has achieved Mach 1 at ground level yet. The new land speed record attempt at 1000 miles an hour will seek to achieve that.

PS Global warming isn't a theory; it's a lie!
thats not what the OP asked. youre explaining why the speed of sound is related to density.

PS banned topic of discussion

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Please explain difference - speed of sound depends on density of transmitting medium

Please explain difference - speed of sound depends on density of transmitting medium
speed of sound depends on the density means its related to the density.

the OP asked if the speed of sound was relative ie relative to the motion of the observer

If the belief that global warming is a lie is banned, it proves that you have no support for its existence - QED

If the belief that global warming is a lie is banned, it proves that you have no support for its existence - QED
not at all, the argument itself is banned. you would be told off if you posted a tonne of scientific journals supporting global warming. also opinions cant be proven and QED is a pretty ridiculous thing to say there.

Speed of sound is not relative to the motion of the observer - heard sound is only only relative to your ear - ears do not observe. Would we hear a sound behind us - no - consider the Doppler Effect

if you move towards a source of sound at speed v, the speed of the sound waves relative to you is (340+v)m/s
contrast this with light where if you move towards a torch with velocity v, the speed of the light waves relative to you is c m/s, the speed of light.

OK, I am banned - global warming is an opinion, so ban yourself - this is not a scientific site
the reason global warming is a banned topic is because this IS a scientific site and there is so much opinion and emotion involved whilst so little actual science is provided when people talk about it. moderating the discussion to ensure that it stayed scientific would require an expert (which this forum doesnt have) in the field to devote their time to moderating crackpot loony theories. because we dont have an expert here, the topic is banned in order to make sure that this site doesnt become a pseudo-science mess.

rcgldr
Homework Helper
The speed of sound is relative to the density of the medium it is travelling in.
The speed of sound is related to the rate of propagation of a wave in the media, and it's also affected by temperature (speed of sound increases if temperature increases and vice versa).

Speed of sound is relative to the air, so if the air is moving, then the speed of sound is relative to the air. As far a a supersonic jet goes, much of the engine noise heard inside is traveling through the body of the aircraft, not through the air.

No vehicle has achieved Mach 1 at ground level yet.
Rocket sleds routinely acheive high mach speeds. The Thrust SST was the first "official" human driven vehicle to exceed the speed of sound.

Rocket sled going 6453 mph:

Thrust SSC (sonic boom at 2:30 into video):

A bit after 1:00 in this video, you can see the shock wave impact on the ground of the approaching Thrust SSC:

Enhanced photo of the Thrust SSC shock wave in attached image:

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