How do you determine correct port size for accustic suspension?

[robertbartsch]

Is this a factor of the woofer size or box size, both or some other variable?


My woofer is a JBL 2206 - 12" and the cab size is about 1/3 larger than a Model 4312 Control Monitor factory cab (e.g., a factory Altec Santana 2 cab) .

I was thinking of using a 3" diameter port which is similar to the 4312 cab.

Does the length of the port tube impact cab tuning?

Thanks...

[Lee in Montreal]

Does the length of the port tube impact cab tuning?

You bet. It is called Bass Reflex tuning. :D

Basically, the port's diameter is related to the maximum amount of air the woofer can move until you start hearing it in the port...

The port's length is related to the enclosure's volume, the driver's Fs and the tuning frequency you wish for.

[Odd]

Is this a factor of the woofer size or box size, both or some other variable?


My woofer is a JBL 2206 - 12" and the cab size is about 1/3 larger than a Model 4312 Control Monitor factory cab (e.g., a factory Altec Santana 2 cab) .

I was thinking of using a 3" diameter port which is similar to the 4312 cab.

Does the length of the port tube impact cab tuning?

Thanks...


Download WinISD (http://www.linearteam.dk/default.aspx?pageid=winisd) it free.
It has data for most JBL drivers built-in.

[more10]

When you are happy with the frequency response from WinISD, change output power from 1W to whaterver maximum power you are going to use. Then check air speed in port, it should be lower than 17 m/s, otherwise the port will make too much sound.

[Mr. Widget]

Is this a factor of the woofer size or box size, both or some other variable?


My woofer is a JBL 2206 - 12" and the cab size is about 1/3 larger than a Model 4312 Control Monitor factory cab (e.g., a factory Altec Santana 2 cab) .

I was thinking of using a 3" diameter port which is similar to the 4312 cab.

Does the length of the port tube impact cab tuning?

Thanks...First, an acoustic suspension system is a sealed box. A ported system on the other hand has a tuned port that must be made with the specific woofer's TS parameters in mind. The speaker system designer can choose the volume of the box and the port tuning to accomplish the type of bass response that is desired. The tuning of the port is a function of the area of the port opening, the length of the port, and the volume of the box. Basically the woofer's parameters, the box, and the port are all interrelated.


If you don't understand all of this, copy an existing system... exactly. Kinda sorta copying can get you into all manner of problems.


Widget

[duaneage]

Copying another system has it's pitfalls too. The crossover network affects the box tuning a lot so if better or worse inductors are used the box needs to reflect the new Qts and Qes which results. Higher resistance in the coil requires a larger box and different tuning (sometimes) and damping on the driver can change.
The box always suffers a loss which is usually reflected in Vbl and for a stout strong thick cabinet it can be very low, typically 7 is what most boxes have for loss. Really good ones get to 5 or lower.

There is no getting around the requirement to measure the Fb yourself after measuring the driver both in and out of the box. If you want a properly tuned system you have to measure several times and cut once. The driver you have might have different parameters than published, although I've seen drivers with off params actually work in the same box. One difference tends to cancel out another.

Learn to use a sweep generator and make a resistance bridge to find the driver f1 and f2 along with Fb of the system.