If you mean models like the Nautilus 802 it's mainly aesthetics and practical considerations - with the curved baffle mounting it on the front or rear panels would not be easy nor would it look very good in relation to the overall cabinet design.
Front mounted ports can emit a lot of spurious midrange resonances from the port tubes - not so much of a problem with a 3 way system however there could still be significant output at the tube resonance when the woofer is crossed over at 350Hz as it is on the 802.
The curved downwards facing port similar to a rear facing port would reduce any spurious midrange output, although I personally favour and use rear facing ports as they reduce any port tube resonances to inaudibility and if anything improve bass response by moving the apparent source of the ports bass closer to the wall behind the speaker pushing the frequencies where destructive cancellation from that wall occurs up higher out of the range of the port.
Having the exit of a port close to a boundary like a floor changes the tuning of the box slightly by adding some "mass loading". The end result is that for the same diameter and length tube the box tuning will be slightly lower in frequency with the down facing port, (maybe by a couple of Hz) however the drawback is that it can also cause turbulence due to the close proximity to the floor, so I wouldn't necessarily say it was an advantage as such, just a difference.
So I think on those particular speakers its mainly about aesthetics, layout/packaging and keeping with the overall cabinet design aesthetic without spoiling it with an ugly front facing port that can also contribute to diffraction problems at midrange/treble frequencies.
Front mounted ports can emit a lot of spurious midrange resonances from the port tubes - not so much of a problem with a 3 way system however there could still be significant output at the tube resonance when the woofer is crossed over at 350Hz as it is on the 802.
The curved downwards facing port similar to a rear facing port would reduce any spurious midrange output, although I personally favour and use rear facing ports as they reduce any port tube resonances to inaudibility and if anything improve bass response by moving the apparent source of the ports bass closer to the wall behind the speaker pushing the frequencies where destructive cancellation from that wall occurs up higher out of the range of the port.
Having the exit of a port close to a boundary like a floor changes the tuning of the box slightly by adding some "mass loading". The end result is that for the same diameter and length tube the box tuning will be slightly lower in frequency with the down facing port, (maybe by a couple of Hz) however the drawback is that it can also cause turbulence due to the close proximity to the floor, so I wouldn't necessarily say it was an advantage as such, just a difference.
So I think on those particular speakers its mainly about aesthetics, layout/packaging and keeping with the overall cabinet design aesthetic without spoiling it with an ugly front facing port that can also contribute to diffraction problems at midrange/treble frequencies.
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Simon said it in a bit difficult way - "I personally favour and use rear facing ports as they reduce any port tube resonances to inaudibility and if anything improve bass response by moving the apparent source of the ports bass closer to the wall behind the speaker pushing the frequencies where destructive cancellation from that wall occurs up higher out of the range of the port."
In other words, when the source (port) is close to the wall, lowest frequencies get boosted! The closer to wall, the higher in frequency this works, until cancellation occurs.
mh-audio.nl - Acoustic
In other words, when the source (port) is close to the wall, lowest frequencies get boosted! The closer to wall, the higher in frequency this works, until cancellation occurs.
mh-audio.nl - Acoustic
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Understood. How about where the port is positioned in the cab. Ie, bottom, middle or top of the cab. Does this have any influence?
Bass waves radiate all over - so yes, most rooms have six boundaries (a cube) of which floor and front-wall are closest to the speaker and port.
But to be serious, port output above 100Hz is usually very small in amplitude. So port location is rather meaningless regarding bass boost. The higher "leaking" frequencies bring another problem which is worst for frontside ports.
Two examples of port signal behaviour, from Home Page | Stereophile.com
Good
Quite bad
But to be serious, port output above 100Hz is usually very small in amplitude. So port location is rather meaningless regarding bass boost. The higher "leaking" frequencies bring another problem which is worst for frontside ports.
Two examples of port signal behaviour, from Home Page | Stereophile.com
Good
Quite bad
Your question is a little bit vague so I'll make my reply a bit more general to hopefully cover what you're asking.
I'm surprised that so many speakers still put ports on the front when there is a long laundry list of technical reasons why not to put them on the front, and only a single reason I can think of not to put ports on the rear.
I suspect it's just for looks, or "because everyone else does it" without ever properly investigating the merits of it.
Here are just a few arguments I can think of to favour rear ports over front, roughly in order of how important I perceive them to be:
1) The elephant in the room is the spurious midrange port resonance. Most speakers these days use a PVC tube for a port, this has a natural pipe resonance frequency typically in the midrange dependent on it's length.
On my 2 way speakers the ports (two of them) are about 20cm long, that puts this sharp, High Q "port resonance" at about 700Hz, right smack in the midrange. Despite the cabinet being well lined inside with underfelt, measured at a distance there is more output from the port at 700Hz than the driver!
This is obviously bad. Because it's a High Q resonance it forms a sharp response peak of it's own, however when summed with the driver output the rapid phase shift on either side of the resonance causes it to constructively add with the drivers output on one side of the resonance and destructively cancel with the drivers output on the other side.
The result is a pointy dip in response followed by a pointy peak in response, in a kind of zig zag. On these speakers it was something like +/- 4dB so very significant. Due to it's frequency the peak around 700Hz may be perceived as "shoutiness" in the midrange, and on certain music with vocal vibrato can add harshness if the vibrato modulates past the peaks and dips.
Stuffing a sock into the front port (at the time the speaker had one on the front and one at the rear) made the midrange both measure and sound smoother and flatter, and once the difference was heard I could not un-hear it so I realised I really ought to move the port to the rear despite the hassle of doing it on an already built (but not finished) cabinet.
Of course this issue theoretically won't be as much of a problem on most 3 way systems, as the woofer in the bass enclosure where the port is shouldn't be producing much output at midrange frequencies, and that is true to some degree, however on a large 3 way system chances are your port tubes are a lot longer, and that just brings down the port tube resonance frequency to somewhere that might cause problems again.
Say the tube on the larger 3 way system was now 40cm long - now it will resonate at 350Hz instead of 700Hz. Will the woofer in a 3 way system have output at 350Hz ? Maybe! Depending on the crossover frequency. A 350Hz port resonance won't cause midrange harshness but it can cause the sound to be "muddy" and possibly a bit muffled and dull sounding, as this is what excess energy around 350Hz does, so in my opinion it should still be avoided.
So I would say even just for this reason alone, no 2 way system should have ports on the front, (*with one exception) and you might or might not get away with it on a 3 way system depending on port lengths and woofer crossover frequency, but I would still avoid it on principle.
* The exception is small bookshelf speakers that are designed to actually sit on shelves or are likely to be pushed right back against a wall. Obviously rear ports won't work if the speaker is pushed right up against a wall - there needs to be a gap equal to at least 2x the port diameter to ensure smooth airflow and avoid mass loading detuning the box resonance. But for any free standing HiFi speaker that will not be pushed right up against a wall, that's not an issue.
In the case of the downward port on the B&W the port is "detuned" by the close proximity of the floor to the port exit, however this is taken into account in the speaker design so the floor becomes part of the design of the speaker as it is always at a constant known distance. (Some subwoofers take the same approach of mass loading from the floor at a fixed distance)
How does putting the port on the rear avoid the pipe resonance ? The pipe still resonates after all whether it's on the front or rear...
Simple really - the baffle width of the box acts as a low pass filter for any signals radiated from the rear which are above the baffle step frequency of the cabinet.
So on my 2 ways the baffle is 39cm wide which is a baffle step frequency of about 300Hz, the resonance is at 700Hz, so when the speaker is free standing very little of that 700Hz will wrap around the cabinet and reach the listener. What little does will be greatly attenuated and significantly delayed in time as well, the end result is there is no measurable or audible effect.
In theory if the speaker was relatively close to the wall behind it some of that 700Hz would reflect off the wall and find its way to the listener after perhaps two bounces, (front wall and side wall) but in practice this doesn't seem to be an issue, probably due to the long time delay and attenuation that occurs.
2) Diffraction. Many speaker designs take care to flush mount the drivers, especially the tweeter, round the baffle edges etc all in the name of minimising diffraction, and then..... go and stick a huge hole in the front panel near the drivers, typically a piece of PVC tube with a sharp right angle where the port tube meets the baffle...
Clearly this sharp right angle from the baffle into the hole made by the port tube will cause diffraction at the edge of the tube just like the edge of a baffle would. And the diffracted wave that travels down inside the tube will diffract off the other end of the tube and/or reflect off the rear cabinet wall again and come back out again further contributing undesirable output.
Why would you want to add an unnecessary source of diffraction on the front panel ? Of course you could gradually curve the entrance of the port as many commercial designs do - this helps reduce turbulence at bass frequencies and is usually the reason it's done, and it would reduce diffraction a little as well, much like rounding the edge of a baffle but it certainly won't solve it.
You still have a big hole in your baffle near the drivers! And most DIY designs don't bother to implement a curved port entrance as its too difficult with DIY tools for the small performance gain it gives.
The answer is obvious - don't put a big hole in the front baffle of your cabinet!
It works just as well for bass on the rear (better actually) and then there are no diffraction effects at all at higher frequencies.The downwards firing port on the B&W is probably partly to avoid unnecessary diffraction on the front "baffle" as they are clearly aiming for minimum diffraction with the overall design of the speaker.
3) Bass performance. I touched on this already, but generally the closer the woofer is to the wall behind it the better the bass performance. When it's too far from the wall the notches caused by destructive interference come down in frequency and start to interfere significantly with the mid/upper bass region.
On the other hand if you put the woofer too close to the wall behind it in a 2 way system there isn't enough time delay for the first reflection in the midrange/treble to give good imaging and midrange clarity. Putting the port on the rear means that one of the two sources of bass from the speaker can be a cabinet depth closer to the wall than the front baffle of the speaker without sacrificing higher frequency performance.
Furthermore, having bass produced at only one point in space (the woofers) can lead to deep notches occurring at very specific frequencies. Move the speaker back and the frequency that previously had a deep notch will be OK but now some other frequency will have a deep notch.
If you have a front port that is right beside or below the woofer, the contribution of the port is essentially at the same spatial location, certainly in the Z axis so the same issue occurs. However if you put a port on the rear there is now a significant Z axis displacement between the two sources of bass.
And at frequencies where both driver and port are contributing to the output - which is from just above the box tuning to about an octave above the port tuning, any frequency which might fall into a notch at the driver or port's physical location is partially filled in by the other.
In short spatial diversity like this significantly lessens the severity of any notches in the bass that are a result of the front wall. This is the same thing that makes the "multi-sub" approach to smoothing bass response work, albeit on a lot smaller more subtle scale.
You can also displace the port in the vertical axis - I have two ports on mine, one is near the top of the rear panel and one near the bottom along the centre line, while the midbass driver is in the middle of the front panel.
In practice you get a modest improvement in bass reinforcement by putting the port on the rear, (1-2dB, especially in the midbass) and you also tend to get a slightly smoother bass response due to the notches not being so deep.
It does depend on the frequency and Q of the box tuning mind you - if the box is tuned to a very low frequency with a high Q the port won't have much output in the midbass frequencies where the spatial separation can be beneficial, but if it is tuned a bit higher and with lower Q (so it's active over a wider range) it can have quite a significant effect.
4) Putting a port on the front panel means yet another cutout in a panel that may already have several cutouts, this could lead to the panel being unnecessarily weakened if there are a lot of drivers and cutouts and/or may have to go somewhere that you'd rather put a panel brace.
Generally the rear panel is devoid of cutouts so one cutout for a port won't significantly weaken it.
5) Aesthetics - personally I don't like the look of a speaker with a big hole/tube on the front, I prefer a cleaner more minimalist look with the port hidden at the back. It's also a less enticing place to put toy cars for my 2 year old!
I think you underestimate how much difference front vs rear ports can make at certain frequencies in the midbass region, due to standing waves and room boundary cancellation effects.
I took the attached measurement many years ago when my speakers still had one port on the front (below the woofer) and one on the rear (above the woofer, in line with the tweeter) which made it easy to block one port or the other and make a direct comparison.
I should point out that the speaker is designed to operate with both ports open (both now on the rear panel) so blocking one port or the other lowers the box tuning frequency below normal.
The normal box tuning frequency was 43 Hz, with one port blocked it dropped to about 32Hz if I recall correctly.
With such a low box tuning frequency the position front or rear shouldn't make much difference to the midbass, right ? Wrong!
The biggest difference is from 70-90Hz, worst at 75Hz where the front port (red) is showing about a 4dB notch relative to the rear port. Also note that sealed (yellow) also shows a significant hole at this frequency, only the rear port shows a significantly flatter response at this frequency, due to the diversity of having the two sources of bass more widely separated.
In fact from every frequency from about 45Hz to 100Hz the rear port gives slightly more bass and suffers less from deep notches.
Below 45Hz being worse with the rear port is interesting, however I believe that is due to the front and rear ports also being at different heights with the rear port being near the top of the cabinet and the front one being at the bottom.
For the low bass frequencies the port closer to the floor will get more floor gain. I think I did flip the speaker upside down and repeat the measurement to confirm this but I don't have the files saved.
So in summary on the rear near the bottom of the cabinet is probably the best port location for bass performance.
Regarding the port resonance itself, you can see the changes in the 700Hz region between the blue and red lines.
It's not very clear which is better or worse as this is an un-windowed measurement that is including all room reflections and effects. If you take a normal 1 metre windowed measurement the result is conclusive - the rear port measures the same as no ports in the 700Hz region while the front port shows a large peak and dip.
Attachments
Your question is a little bit vague so I'll make my reply a bit more general to hopefully cover what you're asking.
I'm surprised that so many speakers still put ports on the front when there is a long laundry list of technical reasons why not to put them on the front, and only a single reason I can think of not to put ports on the rear.
I suspect it's just for looks, or "because everyone else does it" without ever properly investigating the merits of it.
Here are just a few arguments I can think of to favour rear ports over front, roughly in order of how important I perceive them to be:
1) The elephant in the room is the spurious midrange port resonance. Most speakers these days use a PVC tube for a port, this has a natural pipe resonance frequency typically in the midrange dependent on it's length.
On my 2 way speakers the ports (two of them) are about 20cm long, that puts this sharp, High Q "port resonance" at about 700Hz, right smack in the midrange. Despite the cabinet being well lined inside with underfelt, measured at a distance there is more output from the port at 700Hz than the driver!
This is obviously bad. Because it's a High Q resonance it forms a sharp response peak of it's own, however when summed with the driver output the rapid phase shift on either side of the resonance causes it to constructively add with the drivers output on one side of the resonance and destructively cancel with the drivers output on the other side.
The result is a pointy dip in response followed by a pointy peak in response, in a kind of zig zag. On these speakers it was something like +/- 4dB so very significant. Due to it's frequency the peak around 700Hz may be perceived as "shoutiness" in the midrange, and on certain music with vocal vibrato can add harshness if the vibrato modulates past the peaks and dips.
Stuffing a sock into the front port (at the time the speaker had one on the front and one at the rear) made the midrange both measure and sound smoother and flatter, and once the difference was heard I could not un-hear it so I realised I really ought to move the port to the rear despite the hassle of doing it on an already built (but not finished) cabinet.
Of course this issue theoretically won't be as much of a problem on most 3 way systems, as the woofer in the bass enclosure where the port is shouldn't be producing much output at midrange frequencies, and that is true to some degree, however on a large 3 way system chances are your port tubes are a lot longer, and that just brings down the port tube resonance frequency to somewhere that might cause problems again.
Say the tube on the larger 3 way system was now 40cm long - now it will resonate at 350Hz instead of 700Hz. Will the woofer in a 3 way system have output at 350Hz ? Maybe! Depending on the crossover frequency. A 350Hz port resonance won't cause midrange harshness but it can cause the sound to be "muddy" and possibly a bit muffled and dull sounding, as this is what excess energy around 350Hz does, so in my opinion it should still be avoided.
So I would say even just for this reason alone, no 2 way system should have ports on the front, (*with one exception) and you might or might not get away with it on a 3 way system depending on port lengths and woofer crossover frequency, but I would still avoid it on principle.
* The exception is small bookshelf speakers that are designed to actually sit on shelves or are likely to be pushed right back against a wall. Obviously rear ports won't work if the speaker is pushed right up against a wall - there needs to be a gap equal to at least 2x the port diameter to ensure smooth airflow and avoid mass loading detuning the box resonance. But for any free standing HiFi speaker that will not be pushed right up against a wall, that's not an issue.
In the case of the downward port on the B&W the port is "detuned" by the close proximity of the floor to the port exit, however this is taken into account in the speaker design so the floor becomes part of the design of the speaker as it is always at a constant known distance. (Some subwoofers take the same approach of mass loading from the floor at a fixed distance)
How does putting the port on the rear avoid the pipe resonance ? The pipe still resonates after all whether it's on the front or rear...
Simple really - the baffle width of the box acts as a low pass filter for any signals radiated from the rear which are above the baffle step frequency of the cabinet.
So on my 2 ways the baffle is 39cm wide which is a baffle step frequency of about 300Hz, the resonance is at 700Hz, so when the speaker is free standing very little of that 700Hz will wrap around the cabinet and reach the listener. What little does will be greatly attenuated and significantly delayed in time as well, the end result is there is no measurable or audible effect.
In theory if the speaker was relatively close to the wall behind it some of that 700Hz would reflect off the wall and find its way to the listener after perhaps two bounces, (front wall and side wall) but in practice this doesn't seem to be an issue, probably due to the long time delay and attenuation that occurs.
2) Diffraction. Many speaker designs take care to flush mount the drivers, especially the tweeter, round the baffle edges etc all in the name of minimising diffraction, and then..... go and stick a huge hole in the front panel near the drivers, typically a piece of PVC tube with a sharp right angle where the port tube meets the baffle...
Clearly this sharp right angle from the baffle into the hole made by the port tube will cause diffraction at the edge of the tube just like the edge of a baffle would. And the diffracted wave that travels down inside the tube will diffract off the other end of the tube and/or reflect off the rear cabinet wall again and come back out again further contributing undesirable output.
Why would you want to add an unnecessary source of diffraction on the front panel ? Of course you could gradually curve the entrance of the port as many commercial designs do - this helps reduce turbulence at bass frequencies and is usually the reason it's done, and it would reduce diffraction a little as well, much like rounding the edge of a baffle but it certainly won't solve it.
You still have a big hole in your baffle near the drivers! And most DIY designs don't bother to implement a curved port entrance as its too difficult with DIY tools for the small performance gain it gives.
The answer is obvious - don't put a big hole in the front baffle of your cabinet!
The downwards firing port on the B&W is probably partly to avoid unnecessary diffraction on the front "baffle" as they are clearly aiming for minimum diffraction with the overall design of the speaker.
3) Bass performance. I touched on this already, but generally the closer the woofer is to the wall behind it the better the bass performance. When it's too far from the wall the notches caused by destructive interference come down in frequency and start to interfere significantly with the mid/upper bass region.
On the other hand if you put the woofer too close to the wall behind it in a 2 way system there isn't enough time delay for the first reflection in the midrange/treble to give good imaging and midrange clarity. Putting the port on the rear means that one of the two sources of bass from the speaker can be a cabinet depth closer to the wall than the front baffle of the speaker without sacrificing higher frequency performance.
Furthermore, having bass produced at only one point in space (the woofers) can lead to deep notches occurring at very specific frequencies. Move the speaker back and the frequency that previously had a deep notch will be OK but now some other frequency will have a deep notch.
If you have a front port that is right beside or below the woofer, the contribution of the port is essentially at the same spatial location, certainly in the Z axis so the same issue occurs. However if you put a port on the rear there is now a significant Z axis displacement between the two sources of bass.
And at frequencies where both driver and port are contributing to the output - which is from just above the box tuning to about an octave above the port tuning, any frequency which might fall into a notch at the driver or port's physical location is partially filled in by the other.
In short spatial diversity like this significantly lessens the severity of any notches in the bass that are a result of the front wall. This is the same thing that makes the "multi-sub" approach to smoothing bass response work, albeit on a lot smaller more subtle scale.
You can also displace the port in the vertical axis - I have two ports on mine, one is near the top of the rear panel and one near the bottom along the centre line, while the midbass driver is in the middle of the front panel.
In practice you get a modest improvement in bass reinforcement by putting the port on the rear, (1-2dB, especially in the midbass) and you also tend to get a slightly smoother bass response due to the notches not being so deep.
It does depend on the frequency and Q of the box tuning mind you - if the box is tuned to a very low frequency with a high Q the port won't have much output in the midbass frequencies where the spatial separation can be beneficial, but if it is tuned a bit higher and with lower Q (so it's active over a wider range) it can have quite a significant effect.
4) Putting a port on the front panel means yet another cutout in a panel that may already have several cutouts, this could lead to the panel being unnecessarily weakened if there are a lot of drivers and cutouts and/or may have to go somewhere that you'd rather put a panel brace.
Generally the rear panel is devoid of cutouts so one cutout for a port won't significantly weaken it.
5) Aesthetics - personally I don't like the look of a speaker with a big hole/tube on the front, I prefer a cleaner more minimalist look with the port hidden at the back. It's also a less enticing place to put toy cars for my 2 year old!
Simon, Thanks for this and all who have responded.
Location of the port's inside end is critical for midrange leakage, differences are in tens of dB, not just some like in Simon's speaker. The port tube should not "see" the driver, preferred location is close to a lined wall. eg. the top plate. This can be achieved by making a 90¤ bend in the tube (sewer pipes work nice!).
type of port for vented box
Kimmosto told about this at a Finnish forum, the final construction is here
type of port for vented box
Kimmosto told about this at a Finnish forum, the final construction is here
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