One can write expression x = A * sin(omega * t). After time derivation:

v = A * omega * sin(omega * t). Therefore, v_max = A * omega. In order to convert dB into A it is necessary to take into account density rho and elasticity c of the media. Thumbnail rule is that for power density 1 Wt/cm^2 displacement in solids is approximately 1 nm. Therefore you can roughly estimate velocity as 1 nm * omega.

Hi

If you are using a loudspeaker, you will generate a sound wave. For free wave propagation, the acoustic particle velocity is u = p/(rho*c) = p/400. Differently put, you wish to create a wave with the pressure p = 400*8 = 3200 Pa, or 20*Log10(3200/2e-5) = 164 dB re 2E-5 Pa. 

This is, to put it mildly, a bit on the high side for loudspeakers. 

To give you an idea, the acoustic power would be 0.5*p2/(rho*c) = 0.5*32002/400 = 12800 W and the loudspeaker efficiency in the creation of sound is 0.5%, i.e. you would need a power amplifier with the power 25600/0.005 = 2560000 = 2.56 MW. 

Within limits, you can use area factors, i.e. a large cone for sound generation and a small tube for modulation but, there are limits as to what you can do. 

If it is flow you wish to modulate, you can use a speaker to modulate the cross section area and thereby the flow. You would then generate flow by other means. 

I would look at the simple acoustic siren for what you wish to do. It gives you a bot of both, i.e. flow and particle velocity modulation.  

http://americanhistory.si.edu/science/sirens.htm

Sincerely

Claes

164 dB are really a lot, but consider also that with a pro woofer speaker (like something from JBL) you would get, say, 100 dB 1W/1m --> 140 dB 1 W / at 1 cm --> 164 dB 250 W / 1 cm: I guess this does not fit your experimental needs, but you'd get that 8 m/s ;-)!  

Hi

As Enrico correctly points out. My numbers apply for a propagating wave, i.e. at a distance from the source. Enrico's apply in the vicinity of the loudspeaker and, I suppose, would be limited by the speaker cone maximum velocity unless area constriction is used to leverage particle velocity.

/Claes

Also, with a 250 W drive cone excursion would make difficult to stay at 1 cm from the cone.. ;-) But, as I wrote, I do not know which experiment we are talking about..