Cabinet Port Sizes.

[B.Flat]

I have been trying to use the WinISD programme to determine my port.
As I have purchased a coupe of telescopic 120mm port tubes I can experiment up to a length of 280mm.
By my calculations a port length of the max. 280mm would tune my 33litre (total volume) Deltalite II 2510 cab to around 55hz.
Can anyone verify this for me?
Also because of the limited box size I will have to rear mount them. As the box is mounted 15mm feet could I mount them underneath ? (or am I loosing my marbles?)

[Balcro]

Here's one I prepared earlier.......... an old winISD file. 2013 to be precise. I can't remember why I did it, but it
saved a bit of time. Back then I was working with an internal volume of 29.5 litres. Fortunately the T/S
parameters for the Eminence Deltalite 2510 II have not changed over time. All I've done is change the volume
and the program re-calculates the ports etc.

Assumptions for the calculation:
1. The panel through which you're going to cut the hole for the port is 18mm thick.
2. The internal diameter of the plastic port is 110mm (11cm) and the external diameter is 120mm (12cm).

Gross Internal volume 33 litres. Minus the volume of the Deltalite - 0.74 litres and minus the net area occupied by
the port - 2.16 litres. This leaves a net volume of 30.1 litres.

This gives an overall port length (from the exterior opening to the end of the port on the inside) of 20.96cm (8.25").
Port tuning works out at 57.06 Hz. With a 90w input, port velocity is about 14 metres per second. With a configuration
of two 1 x 10's that input is a safe but conservative [u]estimate[/u] for general operation

For a graph of how this looks, see the link on Phil Starr's post #10. It's very close to that. Not optimum but
more than workable for a standalone 2 x 10 or more correctly 2x (1 x10) in this case. Good enough to dispense
with the 1x15.

For the best results, consider using the 2 x 10 on it's end i.e. one drive unit above the other. If that is not
practicable for one reason or another, then ignore that last point & use it horizontally. Now decide where
to place your ports. Don't place the port outlets on the bottom of the cabinet; 15mm is nowhere near enough
to allow the ports to "breathe". There is a technical minimum for this; it's probably related to port diameter or
something, but I can't remember which, so play safe and don't place any obstruction within 8" of the port opening.

[B.Flat]

Balcro you are another star, along with Bill Fitz !
I am indebted to you both for your advice, many thanks

[Balcro]

There's a further complication to do with wiring and it bears on my comment about power handling.
Do think about this before you decide to go ahead and bi-amp the 2 x10 cabinet with the 15" cab. See Phil
Starr's post #22. Check the label on the side of the Deltalites and the 15"unit. It may make the mating of the
1x15 and the 2x10 even more impracticable.

See search term "Power handling of a 2 x 10 speaker wired in series". Lots of answers, but as far as I can see
you can only wire (in the case of the 2x10) 2 drive units in series or parallel but not in series/parallel.

If you have any doubts, come back to the forum and double check with an Ohm wizard.

[Bill Fitzmaurice]

[quote name='Balcro' timestamp='1506262963' post='3377399']
Check the label on the side of the Deltalites and the 15"unit. It may make the mating of the
1x15 and the 2x10 even more impracticable.
[/quote]Not when bi-amped.

[B.Flat]

OK. I always thought that a stereo power amp could take differing impedances on each channel. I think this is the point Bill is making.
My setup would be two 10"'s wired parallel, 4 ohms, to channel 1 and the 15", 8 ohms, to channel two.
I would balance the two tens by reducing the gain on channel 1.
Please tell me if I am wrong

[Phil Starr]

Hi sorry I didn't get back to you with the port calculations, I was on holiday in Madrid Let us know how that works out.

You are right about the stereo amp, as Bill was saying.

[B.Flat]

Thanks for your reply Phil.
I am just awaiting a bit of kit to be able to precisely rout the holes for the ports

[Dan Dare]

A few slightly geeky questions that I've been wondering about. Does port shape make any difference - slot/rectangle, circular, triangular (at the corners of the baffle board), etc? Also location - is distance from cabinet walls/drivers relevant. Are several smaller ones better than one large one? Or is it purely down to length and area?

[Phil Starr]

The simple answer is yes, it makes a difference. We dicovered whilst developing our designs here that one big round port is the ideal.

Tust to be clear you can tune the cab to the right frequency with pretty much any port so long as it is the right length. At low to medium sound levels there won't be any difference you'd hear. As the volume increases the air moves more quickly through the ports and turbulence occurs within the ports, effectively caused by the resistance to air flow at the port walls. The ratio of wall to cross sectional area is worse for a lot of smal ports than for one large one so you get more turbulence. This in turn causes port noises and some compression of the ports output.

So, why did I leave the design as four small ports? The reason was ease of construction, it uses standard guttering downpipe and hole saws are readily available for this size pipe. It also allows front mounted ports with less weakening of the baffle.

We had lots of debate about ports, more than almost anything else really. Mainly it revolved around how much output there would actually be and at what frequencies. The science tells you the port velocities but not how loud an actual bass will be at a gig. All I can say is that I've never heard problems in practice.

All speaker design is a bit like wrestling a balloon, you squeeze a little more in one place and something else pops up elsewhere. For example the triangular corner ports in some Genzler designs are a really neat way of stiffening a cab and building within a budget but could be the worse design possible for this sort of port noise. I play with very little deep bass in my eq so it wouldn't be a problem for me, probably not for anyone but designers like to push the envelope.