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Re: Sonic boom at pressure



Mathieu: Greetings from Montreal! For some reason, I did not receive the
original post of this so I am replying to it via Brian's reply. The formula
you have seen is doubtless the usual v = [1.4 RT/M]^0.5 . The 1.4 is the
adiabatic constant (the ratio of specific heats at constant pressure and
volume) which is 1.4 for diatomic gases like air. M represents the
molecular weight of air (about 29g for dry air) and R the gas constant.
This expression comes from v = [B/rho]^0.5 where B is the bulk modulus
which is 1.4p for adiabatic changes and the form given above then results
from p/rho = RT/M , a form of the perfect gas law.

Lew

********************************************
At 01:07 PM 4/19/97 -0500, you wrote:
At 01:56 PM 4/19/97 -0400, Mathieu Dubreuil wrote:
Hello,

Students of mine (HS) are exploring the way to simulate a sonic boom at
low atmospheric pressure. The argument behind it is since the speed of
sound decrease with density, a sonic boom might be easier to simulate in
a lower pressure system.

We've look at one formula that gives a relationship between temperature,
density and speed of sound in air but couldn't find one with Pressure,
density and speed.


We've hit a wall...

Can the project be done ?

What is the relationship between Pressure-density-speed ?

What is the meaning of the 1.4 constant of air in the temp-density-speed
equation ?

Thanks,

Mathieu Dubreuil
science teacher
Poite-Claire high school
Quebec


For atmospheric air, speed of sound is a function essentially of temperature
alone. The usual formulae are a fertile source of confusion.

I'm sure the kids at Pointe-Claire would enjoy making a supersonic
duct. The project was described in a long-ago issue of Scientific American.

I realise that this is not a very helpful cite.
But there is a web site run by the Soc of Amateur Scientists that carries a
pretty well complete searchable index of the Am Scientist columns,
especially its heyday in Stong's time.
A web search should turn this up quickly, or email me.

Regards
brian whatcott <inet@intellisys.net>
Altus OK


Lew Haddad
Physics Department
Dawson College
3040 Sherbrooke W.
Montreal QC
H3Z 1A4

(lhaddad@dawsoncollege.qc.ca)