Messing about with mongrel floor-standers . . . I found the bass response to be a little boxy. The ports were tuned to 60Hz - a little high in my experience.
I extended the ports (42Hz) using the bodies of washing-up liquid bottles. These are made from 1mm, flexible plastic. Does the flexibility of the plastic have a detrimental effect on their performance?
The boxiness is cured but I'm disappointed in the overall bass response.
The cabinet is 30 litres - an environment in which 98% of 6.5" woofers would thrive.
I extended the ports (42Hz) using the bodies of washing-up liquid bottles. These are made from 1mm, flexible plastic. Does the flexibility of the plastic have a detrimental effect on their performance?
The boxiness is cured but I'm disappointed in the overall bass response.
The cabinet is 30 litres - an environment in which 98% of 6.5" woofers would thrive.
What are the values of Fs and Qts of the driver? In general if Qts is equal or close to 0.4 the optimum tuning frequency will be ~equal to Fs. If Qts is greater than 0.4, the tuning frequency should be lower than Fs, and if Qts is lower than 0.4, the tuning frequency should be higher than Fs. It sounds like the box was at first tuned too high, then tuned too low after you increased the port length. I can't address the flexibility question. I've attached a document I created a number of years ago showing this characteristic. I happens to be bass response modeling for TLs, but the characteristics apply to other types of enclosures. [The red line in each graph is the modeled system bass response].
Paul
Paul
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Yes. I've no doubt the original manufacturers tuned the cabinets for SPL. The cabinets are well constructed with plenty of damping material.
Dropping from 60Hz to a 42Hz Fb (2/3 octave) will reduce the output of your 6.5" to a disappointing level, even if the ports were stoutly constructed.
It’s just air, woofer cones are just paper, most ports are cardboard or plastic. I don’t think the forces are very high except for pro sound. The corners of the box are the strongest area of the whole box.
I’ve been wondering for probably 50 years now why ports aren’t made of fiberglass or carbon fiber. At least for marketing reasons.
The tuning should be whatever is optimum for the box and driver, that could easily be 60hz or higher. If it isn't designed properly or optimally though, then arbitrarily changing the tuning could make it either worse or better. From your results I'd guess it was originally tuned far too high for itself giving a louder peak in SPL that sounded like 'more bass' at the cost of quality, you have likely now reduced that peak for better response but inevitably lost some SPL where the peak had been.
Tuning too high for the speaker was a typical commercial technique to make smaller domestic speakers subjectively seem like they have more bass; it has been referred to as saleable, for the masses. But it is at the cost of good frequency response and gives a peak of louder bass, not lower. Many people will perceive louder as better or lower though, until the boxiness is noticed anyway, so correcting it could subjectively then seem like less bass, in comparison.
My initial feeling about the thin soft bottles is that they aren't anywhere near rigid enough for the changing pressures involved; maybe okay as a quick test but not for a proper quality build. However I suppose they wouldn't in and of themselves ring or resonate very much so might at least be inoffensive. I haven't actually tried it myself though, so can't say exactly how they would 'sound'; better bits of card or plastic tube can be got reasonably cheaply though, so if you have doubts do a comparison with some PVC pipe or thick cardboard tubes and let us know.
Tuning too high for the speaker was a typical commercial technique to make smaller domestic speakers subjectively seem like they have more bass; it has been referred to as saleable, for the masses. But it is at the cost of good frequency response and gives a peak of louder bass, not lower. Many people will perceive louder as better or lower though, until the boxiness is noticed anyway, so correcting it could subjectively then seem like less bass, in comparison.
My initial feeling about the thin soft bottles is that they aren't anywhere near rigid enough for the changing pressures involved; maybe okay as a quick test but not for a proper quality build. However I suppose they wouldn't in and of themselves ring or resonate very much so might at least be inoffensive. I haven't actually tried it myself though, so can't say exactly how they would 'sound'; better bits of card or plastic tube can be got reasonably cheaply though, so if you have doubts do a comparison with some PVC pipe or thick cardboard tubes and let us know.
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Beat me to it! I believe that there is much more to ports than meets the eye (ear?). A small pipe shoved into a hole in the enclosure is about as bad as it gets, but still what so many manufacturers and DIYers employ. It is very refreshing to see the recent post about the effects a port can have - good and bad. There are also a few very good port white papers on line which are very revealing.
My two main recommendations would be: 1) Make it big. 2) Make the airflow conditions similar at both ends of the port.
My two main recommendations would be: 1) Make it big. 2) Make the airflow conditions similar at both ends of the port.
I'd personally also include making both ends rounded/flared rather than sharp, especially if the port needs to be on the small side of what is acceptable. But yes, it is interesting to think that a less rigid port could have some advantages even though I struggle to find it appealing overall.
Not just KEF. Tenson gives some measurements of his 3d printed ports here. Adding structural damping to the port significantly reduced the unwanted port resonances with minimal change to the wanted resonance. The high frequency air motion is performing work on the port (moving it around) and the added damping layer is then converting some of this work to heat preventing it leaving the port as unwanted sound.
Commercial manufacturers like KEF (and likely others) will have designed to achieve this in a controlled manner with a soft inside where it is needed and a harder outside to maintain structural integrity.
A port that just rattles around without damping to absorb the motion is not going to achieve much that is positive. Damping that is effective at the unwanted port frequencies is desirable. Many damping materials are most effective at a few Hz and not the few hundred Hz wanted.
Commercial manufacturers like KEF (and likely others) will have designed to achieve this in a controlled manner with a soft inside where it is needed and a harder outside to maintain structural integrity.
A port that just rattles around without damping to absorb the motion is not going to achieve much that is positive. Damping that is effective at the unwanted port frequencies is desirable. Many damping materials are most effective at a few Hz and not the few hundred Hz wanted.
I suspect due to the stiffness and comparatively low damping of glass compared with plastic.
The very idea of deliberately adding a poorly damped resonance to a speaker enclosure is something I abandoned many years ago!
Though something being more robust doesn't inherently mean that it will ring; it is possible to have the best of both worlds with good construction, materials, damping and/or moving any resonances to frequencies that don't get excited by those in question.
So a floppy port need not necessarily ring any less (than a more rigid one), it might just be a cheaper answer. A better/further question might then be: what does the floppiness do to port performance? A bass reflex cabinet is 'intended' to be a controlled resonant design; is the control or predictability of a floppy port better or worse than a dimensionally more stable/rigid one?
So a floppy port need not necessarily ring any less (than a more rigid one), it might just be a cheaper answer. A better/further question might then be: what does the floppiness do to port performance? A bass reflex cabinet is 'intended' to be a controlled resonant design; is the control or predictability of a floppy port better or worse than a dimensionally more stable/rigid one?
The resonance created by the port length will be very high, up in the tweeter range and shouldn’t be a concern. The resonances of the woofer/enclosure/port system will be in harmonics of the system, for example the tuning is at 60hz the system will have resonances at 120hz and 240hz. I believe these are audible and contribute to the ‘boxy sound’. The best way to get rid of them is to damp the box with fiberglass or wool, and check the dimensions of the box for lengths that match 1/4 wavelength of the tuning and harmonics.
One thing you can do to increase port stiffness is make it out of fiberglass tube, this is readily available as rocket tubing (LOC Precision) or high pressure pipe (MacMaster Carr). You can also wrap the cardboard pipe with fiberglass. As far as pipe goes, use regular iron pipe. It’s stiff, rigid, and resonates at super high frequencies. You can brace the port end as some people brace woofer motors to keep them from flopping around, but be careful to not extend the mouth of the port tube with a brace. You can use open cell foam to line the port tube, this is sold as air filters. Heck, cut up an air filter and line the surface of the port with that. But, until we see some data it’s all conjecture. I’m just throwing mashed potatoes at the wall hoping some will stick. You can try to dimple the inside of the port tube to lower friction, like a golf ball. It might be easier to cut holes in a tube then cover that tube with another solid tube.
One thing you can do to increase port stiffness is make it out of fiberglass tube, this is readily available as rocket tubing (LOC Precision) or high pressure pipe (MacMaster Carr). You can also wrap the cardboard pipe with fiberglass. As far as pipe goes, use regular iron pipe. It’s stiff, rigid, and resonates at super high frequencies. You can brace the port end as some people brace woofer motors to keep them from flopping around, but be careful to not extend the mouth of the port tube with a brace. You can use open cell foam to line the port tube, this is sold as air filters. Heck, cut up an air filter and line the surface of the port with that. But, until we see some data it’s all conjecture. I’m just throwing mashed potatoes at the wall hoping some will stick. You can try to dimple the inside of the port tube to lower friction, like a golf ball. It might be easier to cut holes in a tube then cover that tube with another solid tube.
Yeah, maybe for home audio stuff lightweight constructions work.,
Everything i build is pro sound grade, and lord knows I keep finding ports better be stout.
!2" Sonotubes on a recent large bass reflex project were an utter failure, even with added bracing.
Seems to me port velocity from Hornresp and other programs, often begs for stouter construction.... (and bigger ports)
I mean look at the bad reputation ports have with so many folks....I always wonder why??
Because if done right, ports work great, ime.
Sonotube is cardboard. Use a stiffer material or reinforce the tube. Use two layers (thicknesses) of sonotube.
Where was the failure? Was it from the audio vibration or from transportation? I’m very curious as to why the port failed. Maybe the front baffle needs more bracing. Pro sound loudspeakers are notorious for being made light for transportation, this often makes for a weak resonant enclosure.
If you want a better pro sound port why not make a square or rectangular port from 3/4” plywood?
Where was the failure? Was it from the audio vibration or from transportation? I’m very curious as to why the port failed. Maybe the front baffle needs more bracing. Pro sound loudspeakers are notorious for being made light for transportation, this often makes for a weak resonant enclosure.
If you want a better pro sound port why not make a square or rectangular port from 3/4” plywood?
I think you answered your own question... Many folks don't do their ports right.