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port location on ported speakers

C

Cesare

pfm Member
I'm in the process of planning some tannoy cabinets for 15 inch golds, and rather than recreating an existing design, i'm doing something a little different. It'll be in the 120 litre or so size, so no bass monster, but it'll be a ported cabinet, and i'm wondering about the pros and cons of where you locate the port.

The planned cabinets will be on castors - this is a decision based on how we use speakers at home, and it'll make it much easier to position and move them around, and this then gives an interesting option, to put downward firing ports. Due to my limited woodworking skills, this would make it much easier to get a decent fit and finish on the ports, since they won't be visible.

So, if I were to port the speakers on the bottom, what would the effect be? Should I consider the path length from the port to the edge of the cabinet when choosing the port length, or will it be irrelevant? I'm thinking that various subs do this successfully, but it's not clear to me if it affects the design.

Any insights welcome!
 
Bottom porting is certainly a design used by some major manufacturers... including my own Kudos speakers
I see no reason you should not be successful in using this technique ...but sadly cannot offer more
 
Have a look at the standard Lockwood Major design. It’s aperiodic loading rather than a conventional port, so has rather better bass to my ears (I’m not a huge fan of tuned ports). That exits downwards. My Universals are the same thing but with the large aperiodic vent of the front face to enable soffit-mounting or flying from the ceiling in a studio. Obviously these are larger cabs, but there may well be things to learn.
 
Cesare said:
So, if I were to port the speakers on the bottom, what would the effect be?
Click to expand...

It will be reinforced by the floor. The internal port mouth may or may not be getting close to the driver making it difficult to place stuffing to suppress driving the port modes and midrange leakage through the port. Similarly the location of the internal port mouth, frequency and shapes of cavity modes and frequency of port resonances may or may not be coinciding.

Cesare said:
Should I consider the path length from the port to the edge of the cabinet when choosing the port length, or will it be irrelevant?
Click to expand...

Yes. It will be like the flaring end of a port. The height between the base and the underside of the cabinet will need to be chosen so that the area seen by the slug of air turning through 90 degrees roughly matches. The drawing at the end of this Revel manual illustrates a competently designed base port.
 
So it sounds like i'm not totally mad, and that this is an option. Tony, i'll need to read up more on aperiodic loading to see whether that's an option. I'm maybe less sensitive to this sort of thing than you appear to be, since i've spent time with ported, transmission line, and sealed cabinets, and don't generally have a preference for one or the other, just hearing well done vs badly done.

And it does sound like i'm likely to iterate on the port length as i'm at this point unsure how high the speakers will be from the ground, or how large a port to include. I may just go with a rectangular aperture as I can then make this large and hence have a short port to avoid any interaction with the driver which isn't *that* far from the port.

The initial plan will be to mount everything on the bottom panel, which will be screwed into the cabinet from below - this will include the port, the speaker connections as well as the crossover and associated controls. If I find that this works well, success! It'll also mean i'm able to change out the bottom panel to try out some other options and leave the rest of the cabinet intact.

Here's the current state of affairs, a happy day in the garage with some 18mm ply, and i've got I think the panels for one cabinet complete, and still have all my fingers - success! I've now got to decide how to join the panels, as i've not got tools for biscuits or splices.

PXL_20201221_172828200 by Cesare Ferrari, on Flickr
 
Aperiodic loading is useful when the desired cabinet volume available is less than that which is optimal for a ideal sealed enclosure.
 
Update - here's the cabinet test assembled. I've included a number of rectangular braces in the design which can be screwed into the cabinet sides and allows me to mock up the final cabinet. I've got a top second baffle that the driver screws to:

PXL_20201222_113456068-1 by Cesare Ferrari, on Flickr

Inside view:

PXL_20201222_113428544 by Cesare Ferrari, on Flickr

Here's the bottom panel, with the speaker terminals, crossover, and a pair of port holes for 6.5mm downpipe. The panel is set 20mm down from the bottom surface, hopefully enough room to add castors and for the gap to be suitable around the bottom edge. This panel was cut without any templates, so it's a bit of a shocker, but it's hidden, and i'll make a proper one when i've decided if the design approach works:

PXL_20201222_220644324 by Cesare Ferrari, on Flickr


I've now got the cabinet glued, but it turns out that one of my sides wasn't very parallel, so i've got a 2-3mm gap at the top corner which is annoying (it'll fill i'm sure), and you can see n the above photo that one corner isn't flush as that side isn't square. Saying that, this build is with run of the mill hardwood ply, so it's compromised stuff, and probably will have voids that'll annoying show on the edges, but i'm ok with that, it's to prove the concept, and if i'm happy with things i'll probably get a proper pair made when I can get access to the right tooling.
 
@Cesare, thanks for sharing this very interesting build with us. Was there a particular reason for locating the crossover Energy and Rolloff controls on the bottom of the cabinet instead of the back panel? I imagine it's not going to be very handy if you want to experiment with different HF settings.
 
ToTo Man said:
@Cesare, thanks for sharing this very interesting build with us. Was there a particular reason for locating the crossover Energy and Rolloff controls on the bottom of the cabinet instead of the back panel? I imagine it's not going to be very handy if you want to experiment with different HF settings.
Click to expand...

Yes, a number of reasons.

I don't think playing with these controls is that common, once you've decided what values make sense you pretty much leave them alone, it's more for room tuning than changing depending on source material.

Second, cutting nice apertures is hard for me as I don't have a router template for these controls (either the crossover or the speaker terminals) so i've just cut out a rough shape with a jigsaw, which doesn't matter on the bottom, but would be much more important to do well on the front or back.

Most important though, i'm actually toying with the idea of making the speakers active. I could mount a plate amp on the bottom panel, and so would loose all of the crossover and go active instead. By keeping everything on the one panel it would make such a swap in the future possible without leaving behind any signs of the previous passive life of the cabinets.

The cabinets have had their edges chamfered, so the shahinian-ness of the design is growing. I've not got any suitable castors so i'll cut some wooden blocks initially to get them up and running. Looks like tomorrow I may be able to get this one complete, although i've not even started the second cabinet, i'm sort of seeing how this one goes and measures in case I want to alter the design. I'm actually wondering whether I should have made them deeper still, but hey, they are around the 110-120 litre size, which isn't too compromised.

PXL_20201223_112942570 by Cesare Ferrari, on Flickr
 
How did you come about the port dimensions (diameter, length)? Not sure if it's an issue with home hifi sound levels, but I know with bass guitar speakers, a larger diameter port is recommended rather than two smaller ones - this drops air velocity and reduces chuffing. A design I am building at the moment with a single 12" driver uses 125mm ducting from Screwfix.
 
RichardH said:
How did you come about the port dimensions (diameter, length)? Not sure if it's an issue with home hifi sound levels, but I know with bass guitar speakers, a larger diameter port is recommended rather than two smaller ones - this drops air velocity and reduces chuffing. A design I am building at the moment with a single 12" driver uses 125mm ducting from Screwfix.
Click to expand...

I got the figures from VituixCad, although there are lots of assumptions. I've chucked some generic T&S figures for Tannoy Gold drivers in, and the rough cabinet volume, then twiddled around a bit based on the fact that 65mm pipe was easy to come by (as i've got some offcuts having replaced the downpipe on our house over the summer). The program gives some idea of the port velocity and guidelines as to what works without obvious problems. With my 2 65mm pipe setup the velocities were very low, and the port length was short but not too short, so fingers crossed it'll work well. At the moment i've not got a good solution for cutting round holes of those sorts of diameters, since I would want an inside and outside diameter hole to make a nice interface between the port and the cabinet bottom panel.

I can imagine with a bass amp, lots of low end, and serious volume and cone excursion you have your work cut out to avoid problems - it might have been easier to cut a rectangular port using the bottom/sides of the cabinet as three sides of the port, so just an internal board to create the shape.
 
Got the cabinet together and it's playing tunes:

PXL_20201224_145723144 by Cesare Ferrari, on Flickr

Comparing the driver without cabinet (red trace) against in the new cabinet (blue trace). It's not quite like for like as the red trace was with the driver sat on the floor, so different boundaries etc, but I think this demonstrates what the cabinet is adding at the bottom, but also some odd sorts of stuff around 275Hz, which might be a rear reflection not being damped before coming back out the front, not sure, as the internal damping isn't very organised at present (just some loose stuffing I had lying around).

Screenshot 2020-12-24 at 17.38.07 by Cesare Ferrari, on Flickr

The todo list is sort the internal stuffing and panel damping, get some castors, finish the cabinet. After that, decide whether to make a second one, or a different design. I've got a spare 8*4 18mm ply board so I can make a matching cabinet. Alternatively, get some decent veneered ply and make a really nice looking pair. Could do both, lockdown level 4 for us from boxing day, so time to make cabinets :)
 
Bit of an update. The first cabinet has some danish oil on it, cheap castors, and some internal damping material (some heavy rubberised stuff used as sound insulation in studios, held in with some accoustic sealant that I had lying around from putting a drum riser together). It needs more loose stuffing, and an attempt to check the port lengths. The second cabinet has been assembled and is waiting for a bottom panel, so we're getting there.

Here's the first one in situ:

PXL_20201228_152807720 by Cesare Ferrari, on Flickr
 
Because it is low to the floor, I would guess floor covering alters the tuning. Also it's really a double tuning as the ports vent into a semi-confined space, which then vent through the floor clearance. None of this is bad, it might spread the tuning a bit wider and lower port Q which is sort of a Tannoy philosophy. Remember to also try it with just 1 port.
 
They look great Well done. I’m sure the ports will be fine - I’ve always found the room plays much bigger part of bass issues.As above pointed out , you’ve got 2 to play about with to tweak to your taste.
 
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