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Old 3rd March 2006, 04:23 AM   #1
peteS is offline peteS  United States
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Default what is the impact of having a rear firing port

from what i have read it is always assumed that the port is on the same plain as the driver

what impact does having it rearward facing have?
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Old 3rd March 2006, 06:29 AM   #2
owdi is offline owdi  United States
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The main benefit/impact is the higher frequencies that pass through the port have to be reflected before they reach the listener. This is purely my opinion, based only on theory, but a speaker should not have it's port on the front baffle if the driver has any significant output at the first port resonance.

Dan
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Old 3rd March 2006, 06:51 AM   #3
Dan2 is offline Dan2  South Africa
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I have heard that if you have a reasrward facing port, the sound waves stay in phase. then again this suggests that a port on the same plain as your speaker creates out of phase waves so i dont know how true that theory is. i have tried front, rear and sise firing ports, they're pretty much the same if you are using normal- what's more important is the size of the port - and of course the size of the box. if the port is too big you lose compression and the driver distorts easier. you just gona have to mess around.
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Old 3rd March 2006, 09:30 AM   #4
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The wavelength of sound at the frequencies a port works is so long that the position of the port is irrelevant. Front, back, top, bottom, put it where you will.
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Old 3rd March 2006, 09:34 AM   #5
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It said that best result for in-room response is to use several subs at different locations. In line with above logic, how about subwofer with two ports? One port phacing front - the other port at side or rear. I never seen that. Usually subs with two ports have the ports close and in same diection.

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Old 3rd March 2006, 09:41 AM   #6
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Same thing, wavelengths of the frequencies of interest are so long that splitting them around the speaker will make no difference at all. Distributed subs work because they are spaced metres apart, and that is a significant proportion of the wavelength.
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Old 3rd March 2006, 08:53 PM   #7
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Quote:
what impact does having it rearward facing have?
Only one to my knowledge and experience: reducing the amount of midrange escaping from it. Even some manufacturers use rear facing port AND woofer:http://www.verityaudio.com/ :
<<<Our solution is to place the woofer rearward and use a first-order filter. We then benefit from the
room bass reinforcement and benefit from a natural
acoustical filtration of the mid-frequencies that are left over by the electrical filter.<<<

Obviously, its real impact will partly depend of the walls reflection of the listening room.
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Old 4th March 2006, 01:58 AM   #8
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Hi Pete,

Whenever possible, I always locate the port on the bottom of the cabinet and include an angled floor-board that allows the pressure wave out on three sides(F+L+R). This floor board also takes care of carpeting issues. I find this gives the greatest floor gain, gives the most uniform room gain, removes any direct path between higher frequencies off the rear-cone and the listener, and often allows use of the longest straight port tube since most cabinets are higher than deep. For a 3-way, this allows putting the woofer high off the floor and close to the midrange while still getting full floor gain.

I favor stereo subs located near/under the front speakers. I can hear bass speaker positioning at 80Hz crossover. Two woofers also reduces room effects.
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Old 4th March 2006, 03:43 AM   #9
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Quote:
Originally posted by Dan2
.... if the port is too big you lose compression and the driver distorts easier. you just gona have to mess around.
I thought that bigger ports are better excluding pipe resonances? Lower vent velocity and the front and the rear wave(from port) are closer to be in phase and adding coherently?
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