Subject: how come it is easy to get ultrasonic air flow under vacuum ?

I don't understand:
According to Bernouilli uncompressible (sonic) : P1-P2 = (rV2^2-rV1^2)/2
where P1 and V1 are the pressure and speed of air (fluid) before a restriction 
and P2 & V2 after, and r the density. If V2 >> V2 (small restriction), we get:
P1-P2 = rV2^2/2, which gives for V2=Mac/3=100m/s and r=29g/22.4L=1300g/m3 :
P1-P2 = 6.5 MPa
So it should be impossible to get ultrasonic conditions (higher speed) with 
P1=1 atm and P2=Vacuum because P1-P2 < 0.1MPa (1 atm)
The fact is that experiments proves it is easy to get ultrasonic condition 
with vacuum (flow does not increase anymore even vacuum rate (P1-P2) increases)

Conclusion: I am for sure missing something...
Thanks for your help !

Re: how come it is easy to get ultrasonic air flow under vacuum ?