Are there any pros or cons to placing a port on the front as opposed to the rear or vice versa? More flexibility in placement? Aesthetics?
I'm trying to figure out whether it'd be a good (or bad) idea to place the port in the front, making the speaker taller and taking advantage of the increase in volume to tune lower. Seems like there's nothing wrong with that, let me know if it's not.
Thanks.
edit: While we're at it, do you add an inch in length to the calculated port length when using flared ports? So if you need a 3" port, your port will end up being 4" long including the flared portion? Is this correct?
I'm trying to figure out whether it'd be a good (or bad) idea to place the port in the front, making the speaker taller and taking advantage of the increase in volume to tune lower. Seems like there's nothing wrong with that, let me know if it's not.
Thanks.
edit: While we're at it, do you add an inch in length to the calculated port length when using flared ports? So if you need a 3" port, your port will end up being 4" long including the flared portion? Is this correct?
Port measurements: http://www.psp-inc.com/psp-inc.com/public_html/..\cgi-bin\port_length_calculator.cgi
Peter
Peter
I use rear porting and variable-length ports (Precision Ports' modular system).
The "mud" referred to by Geek is caused by excess boundary reinforcement of the port's output. The answer, in my opinion, is to tune the port lower so that the port's output peaks at a lower frequency, were the additional boundary reinforcement is welcome. When done right the result, instead of mud, is deeper bass extension than you'd have had otherwise.
I readily admit borrowing the technique of taking advantage of boundary reinforcement of the port's output from Audio Note UK.
In addition to optimizing for boundary reinforcement, changes to port length can also optimize for amplifier output impedance.
Note however that the longer the port, the more likely you are to get midrange coloration from pipe resonance within the port itself (imagine talking through a cardboard tube). The possible presence of such coloration is another argument for placing the port on the back rather than the front of an enclosure.
All that being said, I think that most professional designers opt for placing the port on the front of a speaker. If I wasn't using an adjustable-length port system, I probably would too.
Duke
The "mud" referred to by Geek is caused by excess boundary reinforcement of the port's output. The answer, in my opinion, is to tune the port lower so that the port's output peaks at a lower frequency, were the additional boundary reinforcement is welcome. When done right the result, instead of mud, is deeper bass extension than you'd have had otherwise.
I readily admit borrowing the technique of taking advantage of boundary reinforcement of the port's output from Audio Note UK.
In addition to optimizing for boundary reinforcement, changes to port length can also optimize for amplifier output impedance.
Note however that the longer the port, the more likely you are to get midrange coloration from pipe resonance within the port itself (imagine talking through a cardboard tube). The possible presence of such coloration is another argument for placing the port on the back rather than the front of an enclosure.
All that being said, I think that most professional designers opt for placing the port on the front of a speaker. If I wasn't using an adjustable-length port system, I probably would too.
Duke
Sounds like a good idea to me, Bastek!
Multiple ports, where you can plug one or more, is another way of changing the port tuning. In theory you would lose bass linearity at a lower maximum SPL with one or more of the ports plugged because you'll run into turbulence sooner, but if you're not pushing the speakers that hard (or if you had more than enough port area to begin with) then it won't be an issue.
Duke
Multiple ports, where you can plug one or more, is another way of changing the port tuning. In theory you would lose bass linearity at a lower maximum SPL with one or more of the ports plugged because you'll run into turbulence sooner, but if you're not pushing the speakers that hard (or if you had more than enough port area to begin with) then it won't be an issue.
Duke
I much prefer the ports in the rear or on the bottom with the speaker raised; works best on carpeted floors. Neither should make much difference to the LF response, but any unintended higher freq info that comes out of the port will not mess with the direct sound.
I tune ports once the box is completed and ant calculations are only intended to get me close.
I tune ports once the box is completed and ant calculations are only intended to get me close.
I follow the B&W science and put the port on the bottom of the cabinet, using legs and a base plate to create a horizontal "V" shape bottom opening - closed in the back - that pushes the bass sound forward.
This port design
1) gives max floor gain
2) gives most uniform gain in different placements & floor material
3) avoids any direct path to your ear from rear-cone waves
4) typically allows the longest port tube inside the box
5) the rear cavity in base can hold Xover compoments
6) hides ugly big port holes with massive radius flare
This port design
1) gives max floor gain
2) gives most uniform gain in different placements & floor material
3) avoids any direct path to your ear from rear-cone waves
4) typically allows the longest port tube inside the box
5) the rear cavity in base can hold Xover compoments
6) hides ugly big port holes with massive radius flare
Attachments
You didn't ask about TL's...but that's where this answer is coming from.
I've done a lot of simulation using Martin King's worksheets in what is referred to as a Mass Loaded Quarter Wave alignment.
On first glance, the elements of a bass reflex and MLQW enclosure are identical...but wait a minute, Mongo...there's more happening here.
Martin's worksheets consider the interrelationships of ALL the elements within the casework. The distances between driver, port and their positions within the cabinet (and above the floor) produce unique ripple characteristics which may cancel or reinforce one another in the output. I have only found through the use of Martin's worksheets the ability to select the distances and dimensions for the ripple to cancel.
This results in a minimum ripple in the output.
This in turn allows a reduction in stuffing and a slight increase in output.
The output from the port can be shaped..increased or decreased... depending on its length and diameter.
I choose a gradual roll-off of 3-5 dB from ~120 hz to my f3. This roll-off provides a lower f3. I expect this roll-off to be compensated with room gain.
So, with that said, I place my port to the front. Final tuning of port output is accomplished with stuffing, if needed.
edit: A rule of thumb for flares is to include the equivalent length of the flare. That is 1/2 of its curved length. If the flared section is 2" long, figure the equivalent length to be 1".
I've done a lot of simulation using Martin King's worksheets in what is referred to as a Mass Loaded Quarter Wave alignment.
On first glance, the elements of a bass reflex and MLQW enclosure are identical...but wait a minute, Mongo...there's more happening here.
Martin's worksheets consider the interrelationships of ALL the elements within the casework. The distances between driver, port and their positions within the cabinet (and above the floor) produce unique ripple characteristics which may cancel or reinforce one another in the output. I have only found through the use of Martin's worksheets the ability to select the distances and dimensions for the ripple to cancel.
This results in a minimum ripple in the output.
This in turn allows a reduction in stuffing and a slight increase in output.
The output from the port can be shaped..increased or decreased... depending on its length and diameter.
I choose a gradual roll-off of 3-5 dB from ~120 hz to my f3. This roll-off provides a lower f3. I expect this roll-off to be compensated with room gain.
So, with that said, I place my port to the front. Final tuning of port output is accomplished with stuffing, if needed.
edit: A rule of thumb for flares is to include the equivalent length of the flare. That is 1/2 of its curved length. If the flared section is 2" long, figure the equivalent length to be 1".
Some clever ideas here. I'll be building a traditional bookshelf speaker with the rear port and then I'll build another cabinet with the front port mounting as well to see how it sounds. I have enough mdf/ports to do that easily. I have adjustable ports so I'll be able to tweak the tuning pretty quickly.
I can't really do a bottom mounted port since it'll be a bookshelf sized speaker. I won't be able to realize any boundary reinforcement gains there. Good idea though.
I can't really do a bottom mounted port since it'll be a bookshelf sized speaker. I won't be able to realize any boundary reinforcement gains there. Good idea though.
A port on the back will need free space on the back. Not much, but still. One should not put such a speaker in a bookshelf.
A port on the front will expose the high frequencies from the port more directly to the listener. High frequencies can come from the port as chuffing sounds and port (pipe) resonances.
If either end of the is port placed close to the driver (inside or outside) pipe resonances tends to become more excited. This sort of speaks for putting the port on the back side.
For low frequencies the difference is very small.
A port on the front will expose the high frequencies from the port more directly to the listener. High frequencies can come from the port as chuffing sounds and port (pipe) resonances.
If either end of the is port placed close to the driver (inside or outside) pipe resonances tends to become more excited. This sort of speaks for putting the port on the back side.
For low frequencies the difference is very small.
Port chuffing isn't an issue unless the ports are too small. Dual ports are super at minimizing chuffing.
The HF bleedthrough also isn't an issue with proper stuffing. Cramped boxes can be stuffed like a sealed, but with the ports free. Density of fill depends on the design and speakers.
Cheers!
The HF bleedthrough also isn't an issue with proper stuffing. Cramped boxes can be stuffed like a sealed, but with the ports free. Density of fill depends on the design and speakers.
Cheers!
In deciding where to put a port (front or rear), given a Bassbox prediction that certain frequencies will invert 180 degrees relative to the front wave, is it correct to say that if you put the port in the front of the speaker, that the 180 degree flip from the rear of the driver will add to the front wave of the driver? and would it subtract (cancel) from the front wave of the driver if placed in the rear of the speaker?
Thanks for your response.
The question of using the rear sound waves is important however, for TL speakers since the speaker is designed specifically to use the rear sound waves to reinforce the sound waves coming from the front of the speaker. What is the difference between the two designs? Clearly a TL design is a taller box, yet both have ports. How much larger must a box be to use its rear waves to reinforce its front waves?
The question of using the rear sound waves is important however, for TL speakers since the speaker is designed specifically to use the rear sound waves to reinforce the sound waves coming from the front of the speaker. What is the difference between the two designs? Clearly a TL design is a taller box, yet both have ports. How much larger must a box be to use its rear waves to reinforce its front waves?
Ports are about a thing called Helmholtz Resonance. There is even some evidence that two ancient stone constructions like "Newgrange" in Ireland and it's twin or conjugate somewhere else hundred of miles away couple hugely. Enough for people at Stonehenge to hear the people at Newgrange. Albeit in a slightly ghostly way. Maybe "Stonehenge" in England might be linked just like two tin cans down a piece of string to "Newgrange".
An alternative theory has Stonehenge as an ancient Eclipse calculator. More bizarre theories have ancient stone circles as a way of warding off "Evil Spirits" through use of even numbers of stones.
What happens with a port is that at a certain frequency, the driver stops moving and the air in the port does all the displacement.
Amazingly, you get much deeper bass from a smallish box. There is however, a price to pay. The bass is slower, and the bass rhythm section of, say, a Jazz band sounds like it is slightly behind the beat.
With a speaker in the middle of the room, it possibly doesn't matter if the port is on the front or back. Close to the wall, the port can only go on the front.
An alternative theory has Stonehenge as an ancient Eclipse calculator. More bizarre theories have ancient stone circles as a way of warding off "Evil Spirits" through use of even numbers of stones.
What happens with a port is that at a certain frequency, the driver stops moving and the air in the port does all the displacement.
Amazingly, you get much deeper bass from a smallish box. There is however, a price to pay. The bass is slower, and the bass rhythm section of, say, a Jazz band sounds like it is slightly behind the beat.
With a speaker in the middle of the room, it possibly doesn't matter if the port is on the front or back. Close to the wall, the port can only go on the front.
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