Here is a simulation for my 10'' woofer project. Tuning frequency: 24Hz. I replaced my regular 10cm port with 4 holes 6mm. Unfortunately, my software does not alow to scale up/down this graph but it might be that the CALCULATED speed of air reaches 100% speed of sound.
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Here is the simulation for my regular port size (5% speed of sound):
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All this happens with the reasonable driver excursion of +/-10mm:
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Simple loudspeaker design simulators use the usual assumptions for acoustics calculations, one of these assumptions is that air is incompressible. The assumption is good at the typical pressures for normal sound waves.
A pressure wave with an oscillating amplitude of 1 Pa (Pascal = N/m^2) gives
94 dB SPL. In comparison, standard sea level atmospheric pressure is 101325 Pa, which means a pressure fluctuating at a mere
[Edit] 0.01% 0.001% of atmospheric pressure already produces a relatively loud sound.
They rule of thumb to start considering compressible effect is when the Mach number exceeds about 0.3. To get to Mach 0.3, using the
compressible flow table, the ratio between the local pressure at the point where M=0.3 and the source pressure needs to be 0.9395, which means a drop in pressure of 6% atm (in absolute scale). It requires a 47% atm drop to reach M=1.0.
What this all means is that it is impossible for a typical bass driver to drive the port velocity to anywhere close to Mach 1. First, it require extreme cone travel to compress the air inside the enclosure to produce the required pressure. Second, the driver motor will not be able to supply the necessary force. Third, the diaphragm will not be able to withstand the stress from the pressure differential between the front (exposed to the room at 1 atm) and back (which needs to be at 1.9 atm) and self-destruct.