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Speaker Asylum General speaker questions for audio and home theater. |
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Speaker Asylum General speaker questions for audio and home theater. |
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In Reply to: 1 port or 2 ports posted by jazz1 on January 21, 2008 at 12:04:13:
Inside the port, the 'piston' sliding back and forth is only a mass of air. Therefore, its front and rear 'surfaces' are neither completely rigid nor well-defined. Those become even less so as the drive level is increased, pushing and pulling that air-piston even faster, and/or as the frequency is decreased, push/pulling that air-piston even farther.
Turbulence at either end of a port also decreases its performance very noticeably. This turbulence is a result of the rapid change between the 60 foot/sec piston-velocity (20m/sec) inside the port, to the speed of sound (344m/s) out in the room. Flaring the port's end prevents the edge of the piston from suddenly turning into a turbulent 'smoke-ring' traveling with (by constraining) the air-piston still within it, until it parts your hair at 10 feet away- not good for the actual sound.
The opposite should be happening, which is the velocity of the individual air-molecules traveling as one 'piston', are allowed to disperse their momentum into the room's air molecules; in other words, expanding away at the speed of sound. I hope that's a clear-enough explanation!
An air-piston becomes more rigid as a port's diameter is increased, because its stroke is then decreased while producing the same loudness. As its stroke is decreased, the stress/square inch on the air-piston is lessened and the piston thus undergoes less deformation.
One could split that increased surface area between two small ports, but with more losses. Those come from the air-piston's interaction with the greater amount of sidewall surface area relative to the size of that piston. This could be called frictional loss, but it is really the outer circumference of the air-piston undergoing a more intense shearing-off of its back and forth velocity-- right next to the port's wall, the air's velocity goes to zero. Why? Because there is always a microscopic layer of air molecules stuck to the walls.
An air-piston reaches its full velocity only several millimeters away from the side wall, and that 'depth of shear' is increased by the higher piston-velocities produced in a smaller-diameter port. However, two smaller ports can sometimes avoid picking up internal resonances, ones which a larger-diameter port could pick up from the inside a given shape/size of an enclosure.
One may not like the bass of a particular ported speaker, but in my experience, this is mostly because a) other resonances were inadequately suppressed during the design of the enclosure, and/or b) the port is not tuned to the best frequency for that woofer (perhaps to a designer's own room environment?).
Test for proper port operation, by ear, using a jazz string bass running the scale, such as from Christian McBride on his "Gettin' To It" album. Then listen to the tightness of a steady kickdrum, such as from Pick Withers of Dire Straits, or Stewart Copeland of The Police. Make certain the amplifier is up to snuff for this, with a decent power supply first and foremost.
Best regards,
Roy Johnson
Designer
Green Mountain Audio
"The more you learn about time-coherent sound, the more you'll want it in your life!"
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Follow Ups
- Two, or one larger port. - RoyJ 20:50:56 01/25/08 (1)
- Very informative, Roy. Thank you for taking the time. - Duke 00:28:01 01/26/08 (0)
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