@tektondesign wrote: "once we get into the 5-10 watt range the air pressure jets in and out of them at speeds above Mach1 and they get audible."
I do not dispute the "audible" claim, but am skeptical of the "speeds above Mach 1" claim.
North of Mach 1 might be the airspeed that a modelling program indicates but I don't think that reflects what happens in the real world. Turbulence normally sets in at around .1 (point one) Mach, or a bit higher if the fluid dynamics are favorable (which is not going to be the case with a threaded hole). Turbulence will reduce the speed of the airflow. There will be some air movement and therefore a loss of internal back-pressure on the cone in the enclosure. If it's a ported enclosure, the effective enclosure tuning frequency will be lowered a little. The more pressure the more turbulence, and it would take an enormous amount of pressure inside the enclosure to force the airspeed up to or above Mach 1.
Another mechanism that works against the airspeed reaching or exceeding Mach 1 is boundary effect. Not all of the air in a port moves uniformly (even below the onset of turbulence) - the layer of air molecules right at the surface of the port is sort of "stuck" to the surface and doesn't move at all, then the next layer of air molecules moves only a little bit, and so on until eventually the air movement is no longer affected by "stiction". For this reason flat rectangular ports tends to have a higher real-world tuning frequency than same-area-and-length round ports because more of the air in the port is up against a boundary and therefore slowed by boundary effect, reducing the port's effective cross-sectional area. I have read that the real-world "boundary layer" in a port can be approximately 1/4" thick (at high airspeeds), which would be the entirety of a 1/4" bolt hole.
I think that air's compressibility would also work against the airspeed reaching Mach 1. The "sound barrier" is the buildup of pressure in the air that is being compressed by the aircraft. The "sonic boom" is the wave of air that has reached its compressibility limit because the aircraft has reached or exceeded the speed of sound, which is the speed at which the air molecules can no longer move out of the way fast enough. I think the air in the screw holes would compress long before it goes supersonic.
Note that the air gap annulus between the voice coil and the pole piece on a woofer that has a "phase plug" instead of a dustcap does not cause audible distortion nor a significant reduction in performance. This is because there is so much turbulence and/or other airflow-inhibiting mechanisms in play that there is effectively zero airflow though that gap. Now the dimensions and geometry are of course very different for a screw hole, but I offer this as an example of an opening that would cause problems if not for these real-world mechanisms that inhibit airflow through small openings.
If the airspeed through the screw holes actually did reach or exceed the speed of sound, I think it would sound like a bottle rocket was accompanying the bass notes.
That being said, I do think that well below the speed of sound the turbulent airflow through the screw holes could cause a high-pitched version of port chuffing, which might be like a whistling that accompanies bass notes.
Duke
