How to tune the Port Length?

endrek · 19th April 2011, 10:48 PM

Hi! today i just finished my model of my first bass reflex design (first speaker ever) with a Fane Colossus 18XB. I did it with agglomerate becaue i knew i was going to make some mystake (like i did) and to check stability and know where to put braces, if necessary. Now i know they are not really necessary.
And Tomorrow i'll build the final cab with plywood 18mm.

Here goes my first question:
you think its possible that i wont need braces?? I standed up on my box, and jumped on it, and it only broke a little bit....But maximum there will be 4 more boxes on top....(30kg * 4 = 120kg).....
Any advice?
=========(sorry for this offtopic question)===================

Well the goal of the topic, is:
Now i have a port length of .240 but i'll make it a little bit longer, or make tomorrow some pieces of different sizes. To compare the difference of sound between different length.
My question is,
wich easy and simple method, without any mesurement equipment, can i use to sintonize and trim the port length??? I tryied with a AC voltimeter, but it's not precision enough, because it was really cheap one(30 euros....)
Any help please??

THX FOR YOUR HELP

johnr66 · 19th April 2011, 11:04 PM

The best way is to get a modeling software, like WinISD (free) and use it to help with the design of the cab. It helps take the guesswork out of the build so less need to tune.

I would definitely brace a cab of that size. It is not only the strength of the cab the bracing improves, it also helps with the sound reproduction by keeping the sides from vibrating and coloring the sound.

Rademakers · 20th April 2011, 05:30 PM

Quote:

I would definitely brace a cab of that size.

+1

Just put a few hundred Watts through the 18XB and feel the cabinet walls move!

Quote:

The best way is to get a modeling software, like WinISD (free)

+1

By the way, you're going to use t-nuts in the end-product?

Best regards Johan

weltersys · 20th April 2011, 06:25 PM

Braces are not put in to keep the box from getting crushed, they keep the walls stiff.

Reusing Old Sub Boxes?

Post # 5 tells how to check for port tuning frequency.

Arty · 20th April 2011, 06:26 PM

"wich easy and simple method, without any mesurement equipment, can i use to sintonize and trim the port length??? I tryied with a AC voltimeter, but it's not precision enough, because it was really cheap one(30 euros....)
Any help please??"

Winisd freqvency generator will help you out on thisone.
The chep multimeter is more than suffiecent to mesure Your tuning freqvency, and help you with the tuning.

If You plot the impedance curve of the driver, now that it is in the box, You will see a peak in the impedance curve, that is where the port has tuned the cabinet.
Voltage divided by current is impedance.

This is a very easy methood, and it is accurate enough.

Brian Steele · 21st April 2011, 02:35 AM

Quote:

Originally Posted by Arty

If You plot the impedance curve of the driver, now that it is in the box, You will see a peak in the impedance curve, that is where the port has tuned the cabinet.

For a vented cabinet, impedance actually hits a minimum at Fb.

DrDyna · 21st April 2011, 12:49 PM

Right, you definitely want bracing. It's not so that you can park a car on the cabinet, it's so that the vibrations produced by the speaker affect the cabinet less. For a cabinet of that size, you should probably have 3 or 4 internal braces.

Top Shelf · 21st April 2011, 02:10 PM

Wow that's very similar to mine, including the recessed shelf port brace.

weltersys · 21st April 2011, 03:04 PM

Quote:

Originally Posted by Brian Steele

For a vented cabinet, impedance actually hits a minimum at Fb.

As does cone excursion, which is quite easy to see in most alignments.

Arty · 21st April 2011, 06:13 PM

I em allways surprised how can i mess it up allways

this one is easier done than told it seems.
Apologies if i mislead anyone.

19th April 2011, 10:48 PM	#1
endrek diyAudio Member Join Date: Sep 2010	How to tune the Port Length? Hi! today i just finished my model of my first bass reflex design (first speaker ever) with a Fane Colossus 18XB. I did it with agglomerate becaue i knew i was going to make some mystake (like i did) and to check stability and know where to put braces, if necessary. Now i know they are not really necessary. And Tomorrow i'll build the final cab with plywood 18mm. Here goes my first question: you think its possible that i wont need braces?? I standed up on my box, and jumped on it, and it only broke a little bit....But maximum there will be 4 more boxes on top....(30kg * 4 = 120kg)..... Any advice? =========(sorry for this offtopic question)=================== Well the goal of the topic, is: Now i have a port length of .240 but i'll make it a little bit longer, or make tomorrow some pieces of different sizes. To compare the difference of sound between different length. My question is, wich easy and simple method, without any mesurement equipment, can i use to sintonize and trim the port length??? I tryied with a AC voltimeter, but it's not precision enough, because it was really cheap one(30 euros....) Any help please?? THX FOR YOUR HELP

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19th April 2011, 11:04 PM	#2
johnr66 diyAudio Member Join Date: Mar 2009	The best way is to get a modeling software, like WinISD (free) and use it to help with the design of the cab. It helps take the guesswork out of the build so less need to tune. I would definitely brace a cab of that size. It is not only the strength of the cab the bracing improves, it also helps with the sound reproduction by keeping the sides from vibrating and coloring the sound.

20th April 2011, 06:25 PM	#4
weltersys diyAudio Member Join Date: Jan 2008	Braces are not put in to keep the box from getting crushed, they keep the walls stiff. Reusing Old Sub Boxes? Post # 5 tells how to check for port tuning frequency.

20th April 2011, 06:26 PM	#5
Arty diyAudio Member Join Date: Feb 2011	"wich easy and simple method, without any mesurement equipment, can i use to sintonize and trim the port length??? I tryied with a AC voltimeter, but it's not precision enough, because it was really cheap one(30 euros....) Any help please??" Winisd freqvency generator will help you out on thisone. The chep multimeter is more than suffiecent to mesure Your tuning freqvency, and help you with the tuning. If You plot the impedance curve of the driver, now that it is in the box, You will see a peak in the impedance curve, that is where the port has tuned the cabinet. Voltage divided by current is impedance. This is a very easy methood, and it is accurate enough.

21st April 2011, 12:49 PM	#7
DrDyna diyAudio Member Join Date: May 2007 Blog Entries: 3	Right, you definitely want bracing. It's not so that you can park a car on the cabinet, it's so that the vibrations produced by the speaker affect the cabinet less. For a cabinet of that size, you should probably have 3 or 4 internal braces.

21st April 2011, 06:13 PM	#10
Arty diyAudio Member Join Date: Feb 2011	I em allways surprised how can i mess it up allways this one is easier done than told it seems. Apologies if i mislead anyone.

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