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Back Porting & Low Freq Steering

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vertx View Drop Down
Young Croc
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    Posted: 09 December 2022 at 2:01am
I try to always avoid back ports in designs, although many times have seen on this forum people debating the performance of back ported designs, with previous well respected member advising there should be no difference (even advising they have measured no difference) between back ports and front ports.

My 2c after the last year or two of builds I have noticed the following general statements to be often true, based both on measurements and listening tests.

- Rear ports can be used when the port is near a rear boundary, and the number of speakers used in total per passband is low. i.e a pair of floorstanding hifi speakers, or bookshelf speakers.

- Rear ports in PA cabinets can potentially make it hard to phase align the low-mids with the subs. The port gain is also potentially sending the "out of phase" waves rearward. This outdoors (not in a club environment for example) is potentially leaking SPL towards the performers. (Wrong! see video in laster post)

- The port gain from a port at the rear can be difficult to measure from the front of the enclosure, especially in half or full space, rather than indoors

Below shows a back ported 3-way hifi speaker with a 15" woofer designed to be used up against a boundary such as a wall. The orange and blue lines show measurements taken 60cm from the front of the enclosure (in a semi-anechoic/impulse gated half space environment), and the black line shows the measured port response at the rear of the enclosure.

The response (with added port gain) tracks both simulations and listening tests.



Would love to hear about anyone else's testing, measurements, related articles, etc regarding back porting vs front porting :)


Edited by vertx - 03 February 2023 at 11:49pm
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Elliot Thompson View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 09 December 2022 at 1:22pm
Originally posted by vertx vertx wrote:


- Rear ports in PA cabinets can potentially make it hard to phase align the low-mids with the subs. The port gain is also potentially sending the "out of phase" waves rearward. This outdoors (not in a club environment for example) is potentially leaking SPL towards the performers.



Would love to hear about anyone else's testing, measurements, related articles, etc regarding back porting vs front porting :)


Phase alignment is more on how the two non-identical reflex boxes are tuned. In addition to how the loudspeaker is reacting in the box based on its TS Parameters. Many don't like rear ports for they cannot keep the boxes flush against the wall. If you have enough bass, you do not need walls and corners as a means of room gain. All you need is the ground it sits on.

Best Regards, 


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 10 December 2022 at 3:56am
Originally posted by Elliot Thompson Elliot Thompson wrote:

Many don't like rear ports for they cannot keep the boxes flush against the wall. 

Best Regards, 



Just to expand on what I wrote. Based on my measurements I've done 20 - 25 years ago, 1 foot is the bare minimum you would have rear ported reflex near a wall. Anything closer would alter the tuning frequency. 

If the bulk of the sound (based on the tuning frequency of the port) is coming from the port, it would make sense to offer a front firing port. If the port is used for extended frequency response, it does not matter where the port is located. 

I might add, rear firing ports offers minimal (if any) port noise reaching to the listeners ear unlike front firing ports. Rear firing ports are definitely more High End as, the resonance above the tuning frequency of port is not noticeable to the listeners ears like front firing ports. 

I've used subs offering front firing ports for a little over 32 years and, rear firing ports for around 30 years. Personally, I like the sound rear firing ports. However, if you like that rolling type of baseline (which stems from a lot of resonance), front firing ports is the ideal choice.

Best Regards,  


Edited by Elliot Thompson - 10 December 2022 at 3:57am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote vertx Quote  Post ReplyReply Direct Link To This Post Posted: 03 February 2023 at 2:22am
Having thought about this some more. I find it hard to imagine the placement of the port on a speaker not having a measurable effect in the speakers overall response, specifically in relation to the "steering" of low frequency.

As we know it's very easy to steer low frequency with multiple subwoofer enclosures by playing with delay and phase - should this not be also seen in the low frequency coming from an enclosure vent which as we know comes out out-of-phase from the woofer?

I would love to see a Kipple NFS scan of the same enclosure with a front port, back port, and side port, to see if there is a measurable effect on (not the overall SPL) the distribution/directionality/coverage pattern of SPL by frequency (specifically around port tuning frequency)

If anyone has links to papers on phase steering of low freq in enclosures by port placement I'd love to have a read!




Edited by vertx - 03 February 2023 at 2:23am
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Post Options Post Options   Thanks (1) Thanks(1)   Quote DMorison Quote  Post ReplyReply Direct Link To This Post Posted: 03 February 2023 at 1:26pm
Fulcrum Acoustics have a line of passive cardioid bass bins, there's almost certainly some kind of info on their website about how they do it, maybe a decent white paper or patent link if you're lucky.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote vertx Quote  Post ReplyReply Direct Link To This Post Posted: 03 February 2023 at 11:10pm
Very cool stuff.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote vertx Quote  Post ReplyReply Direct Link To This Post Posted: 04 February 2023 at 12:27am
Thinking about this from a HiFi/studio monitor view - maybe some similar concepts going down in the Dutch & Dutch 8C and the Barefoot mons. EDIT: Maybe not Barefoot that looks like just standard dual opposing woofers.

Edited by vertx - 04 February 2023 at 12:41am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bitSmasher Quote  Post ReplyReply Direct Link To This Post Posted: 04 February 2023 at 12:50pm
HornResp can model the difference made by changing the port to driver distance, I wonder if you can model placement too
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 06 February 2023 at 8:41pm
Originally posted by vertx vertx wrote:

Having thought about this some more. I find it hard to imagine the placement of the port on a speaker not having a measurable effect in the speakers overall response, specifically in relation to the "steering" of low frequency.



It depends on the tuning. Here in the states, the majority is not going to tune a reflex cabinet above 35 Hz. Using 35 Hz as a guideline, you would need a boundary of 32 feet to control it's directivity. Controllability will be greater in terms of feet if you are aiming below 35 Hz. 



Best Regards, 


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Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 21 February 2023 at 8:15pm
Originally posted by bitSmasher bitSmasher wrote:

HornResp can model the difference made by changing the port to driver distance, I wonder if you can model placement too


hornresp in standard modes only allows you to set a positive distance between driver and port (which I assume means driver is closer than the port). If it allowed you to set a negative distance (I seem to remember it allowing this years ago) you might be able to get an idea of the amount of rear rejection by comparing with saved response chart.

Or you could design two models that are equivalents of each other but swapping the ported rear chamber for a front chamber with horn section acting as a port. If you did it as a compound horn you make the port in both models a horn section and put filling in it if you wanted.

you can model U-frames with resistive filling if you you choose a Compound Horn and make S1 S2 S5 and S6 all equal in value. This opens up a pattern window in the loudspeaker wizard. Maybe this model could be expanded in future Hornresp versions to model a Fulcrum-type box....
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Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 21 February 2023 at 8:47pm
Originally posted by snowflake snowflake wrote:

[QUOTE=bitSmasher]
Or you could design two models that are equivalents of each other but swapping the ported rear chamber for a front chamber with horn section acting as a port. If you did it as a compound horn you make the port in both models a horn section and put filling in it if you wanted.



this doesn't seem to work - get exactly the same response both ways.
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