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Port tuning

J

J42

Jul 25, 2010
234
5
#1
I am tuning the ports on a generic 2x10 cab that I bought used recently.

The cab has inside dimensions of 14.5" X 20.5" x 15.875" and an internal brace that is 3.75" x .5" x 15.875"

I am guesstimating the speaker displacement and the brace to be .1 cu. ft. (I am still waiting on a reply from Eminence for the exact speaker displacement).

This yields a displacement of 2.613 cu. ft.

There are two 3" ports.

Using an online calculator at mh-audio.nl, I came up with a port length of 2.532"

This is my first time attempting this, and don't know if there is anything else to consider, such as cabinet damping material. I am going to replace the fiberglass insulation with Acousti-Stuff. Does damping material affect the equation? I have read that it effectively raises the displacement. Is there anything else I need to consider?

The drivers are Eminence and came from an SWR Workingman's cab. Here are the T/S Specs:

05-RE OHMS 4.16
06-LE MH .59
07-QM 3.35
08-QE .380
09-QT .340
10-XMAX MM 3.50
11-BL TM 10.89
12-EFF % 1.87
13-FS HZ 55.26
14-MMS GMS 31.40
15-CMS mm/N .2641
16-RMS NS/M 3.2585
17-VAS LTRS 44.12
18-SD SCM 344.88
19-EBP 144.4
20-SPL dB 94.7
 
#6
You have to get a few lengths & try it.If its no good you can then shorten it or get a longer tube.The different lengths will give you different tunings.Just to play around you can get them cardboard tubes they have for posting stuff through the mail untill you can get the proper tube.I got mine from the net but you can use plumbing pipes also.
It also depends if the tube is wider on one end then the other.
 
#8
J42 said:
I have calculated for 55.26 Hz, which is the Fs of the drivers.



What is the best way to do that?
Click to expand...


http://******.com/?q=winisd

then download winisd

open it and create a new driver

insert all the parameters that you have available

then select the driver and select a vented cabinet

it will come up with it's recommendation, ignore this and plug in the airspace you have available minus the airspace taken up by the would be port.

select the tuning frequency and it will automatically select the port length. It will then generate a graph based on it.

repeat the steps with a different tuning and compare response graphs.

Robby
 
#9
Robby Hoinsky said:
http://******.com/?q=winisd

then download winisd

open it and create a new driver

insert all the parameters that you have available

then select the driver and select a vented cabinet

it will come up with it's recommendation, ignore this and plug in the airspace you have available minus the airspace taken up by the would be port.

select the tuning frequency and it will automatically select the port length. It will then generate a graph based on it.

repeat the steps with a different tuning and compare response graphs.

Robby
Click to expand...

+1. Also, make sure you use WinISD Pro and check out the plots for cone excursion based on your tuning frequency. I use the Alpha version for the basic design, but Pro to double check everything.

WinISD has a tone generator. I lay the cab on it's back and throw some pieces of rice on the speaker and use the tone generator to see where it stops vibrating. Not the most exact scientific way, but it gets me close enough to see where my actual(?) tuning frequency is.

I really need to get more testing equipment. ;)
 
#10
J42 said:
This yields a displacement of 2.613 cu. ft.

There are two 3" ports.

I have calculated for 55.26 Hz, which is the Fs of the drivers.

Using an online calculator at mh-audio.nl, I came up with a port length of 2.532"
Click to expand...

EDIT:- I just checked with Winisd and it suggested 2.56".

I wouldn't go experimenting with port tunings unless you know what you're after and what else changes when port tuning changes. Stick with 55.26Hz for now then see how the cab sounds before you decide if anything needs changing.
 
#11
^ Thanks Pete. I have Windows 7 and have had some troubles running older software, so didn't even want to try installing it.

I used the online WinISD and got the same result. My ports were supposed to be delivered today, but we had a snow storm yesterday, so I am hoping they come tomorrow.

Thanks to all of you guys for your advice.
 
#12
One of the things a designer juggles when it comes to the port tuning frequency is bass loudness vs protection from fartout. It really doesn't matter what the free air resonance of the driver is; that's not the target.

In my opinion 55 Hz might be a little on the high side from a fartout standpoint; it might be worthwhile lowering the tuning to 50 Hz or so.

I presume you didn't order extra ports so that you can experiment with different lengths, so here is what I suggest: Cut your holes for the port(s) a little bit bigger around than you really need, and use a few wraps of electrical tape around the port to get a good snug friction fit. This way if your ports aren't doing what you want, you can remove them and either trim them (if you feel the bass is weak AND fartout isn't an issue) or replace them with longer ports (if fartout is keeping you from getting the SPLs you need).
 
#14
J42 said:
Duke, thanks for the advise. Am I understanding correctly that tuning the ports to a lower frequency will decrease bass output?
Click to expand...
No, it won't. It will decrease the output above the frequency where you were planning on tuning it, and increase the output below that. Which you'll prefer is an unknown variable.
 
#15
J42 said:
Duke, thanks for the advise. Am I understanding correctly that tuning the ports to a lower frequency will decrease bass output?
Click to expand...

I would think the 55 Hz tuning will sound like "more bass" than the 50 Hz tuning because you'll be getting more output in the region of the first overtones of your lower notes, but as Bill pointed out you'd be getting less energy way down low with the higher tuning frequency.

Assuming your amp has plenty of headroom, the lower tuning frequency's slightly better fartout resistance would allow you to either play a bit louder, or crank in a bit of bass boost and play just as loud but your cab would go slightly deeper.
 
#16
Another issue is excursion. The driver excursion is at a minimum at the box frequency. It rises drastically below Fb, and it also rises drastically above Fb. This means you can tune too low, because there's no advantage to having minimum excursion below where you need it, and you can tune too high, as you don't want excursion limits to suck away your low frequency output capability. Knowing how to balance all of the aspects of cabinet design to arrive at a desired result is only realized through years, if not decades, of actual experience.
 
#19
You really need to consider the box dimensions AND the driver specs when tuning the overall system. You don't really tune ports, you tune the system by changing the dimensions of the ports. Do not get stuck on the idea that the system needs to be tuned to the Fs of the driver.

It was mentioned a couple of times already that unloading of the driver under system tuning is severe (I don't remember the math).

My rule of thumb is that you should not tune a bass cab higher than 50Hz and 40Hz is even better. If you cannot get a system to sound good at these tunings then it is time to consider another driver or box.
 
#20
Thanks again guys! I am still waiting on the delivery of my ports. They are 5" long which will start me out at 40Hz. I'll see how they sound for a while, and then shorten them if I think there is room for improvement.
 

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