modifying cabinet/adding ports

[RobbieNuke]

First timer here. I've done a brief search for the subject at hand, but could not find anything. I will continue the search after this is posted.

I have an old PA monitor wedge cabinet originaly set-up up with a 1x12" + HF horn. I got the cabinet w/o any driver elements. I've replaced the baffle board to accomodate 2x10" bass drivers and a Foster HF unit (w/ X-over @ 3.8 kHz). Although currently a sealed box, I would like to add some ports to increase the LF efficiency. Normally taking the the T-S parameters and running them through a formula would give me the proper box size as well as port size. But with two drivers how does that effect the equasion?

I know for optimum results I should build an entirely new box, but I don't have the resources (woodworking tools, spare time, etc.) at this time do that. I am willing to accept a compromise by just making some holes/tubes at the "right" size to achieve any sort of increased LF performance.

My goal for the cabinet is for electronic keyboard in a live band situation as a stage monitor/back-line cabinet.

[sreten]

Hi,

Most sim packages let you input the number of drivers.

What is the 10"ers Fs, Qts and Vas ? what is the box volume ?

/sreten.

[HK26147]

* For 2 identical woofers:

The Fs for 2 will be the same as for 1 driver.
The Qts will be the same.
Vas ( and the associated box volume, Vb ) will be 2X that of a single.
Combined impedance: 1/2 for parallel, 2X for series connection.
Sensitivity will increase +3db for parallel, -3db for series.
Cone excursion will be 1/2 of a single.

* SB 1.91

[BlueWizard]

While I'm no expert, port size is usually a matter of cabinet volume and desired resonance frequency. The drivers matter very little (sort of).

Here are two port calculators -

http://www.linearteam.dk/default.asp...ventcalculator

http://www.carstereo.com/help2/Articles.cfm?id=31

Remember to subtract something from the total volume of the cabinets to account for the space taken up by the drivers.

The necessary parameters are -

Vb = Net Cabinet Volume
Fb = tuning frequency of the box
Number of Vents
Diameter of Vents

This will give you length.

If you don't have the right cabinet volume, or cabinet depth, you may not be able to get the port to work at your selected resonance frequency.

Steve/bluewizard

[HK26147]

I am willing to accept a compromise by just making some holes/tubes at the "right" size to achieve any sort of increased LF performance.

I'm glad you are aware that there are compromises to jamming down an alignment to get LF extension.
An impedance sweep will accurately measure your port tuning.

[Ron E]

Originally posted by HK26147
Sensitivity will increase +3db for parallel, -3db for series.
Cone excursion will be 1/2 of a single.

Sensitivity is +6dB for parallel, no change for series.
Excursion is the same for parallel, half for series.

[RobbieNuke]

Originally posted by HK26147

An impedance sweep will accurately measure your port tuning.

How does one accomplish an impedance sweep? BTW, thanks for all the fantastic responses, especially the links! Great forum, everyone stayed focused on my question.

For those that asked: Impedance is rated 16 Ohms @ driver wired in parallel for a total of 8 Ohms. Fs=48.6 Vas = 67.01 liters Qt=.250 (there was no Qts on my spec sheet).

Vb= 2.11 cubic feet (approx), not counting driver volume. The drivers are currently monted in the box so I'll get more accurate dimensions when I dive into this project in the next week or so.

[HK26147]

Sensitivity is +6dB for parallel, no change for series.
Excursion is the same for parallel, half for series.

See Loudspeaker Design Cookbook ( not SB my err. ) section 1.91
If you add resistance ( series ) Sensitivity HAS to go down.
With 2 pistons of equal SD they each only have to move 1/2 as far to equal the same Vd as 1 piston.

6db IF close coupled.
Close coupled Parallel doubling of drivers = +3 dB due to acoustic efficiency (mutual coupling) and a +3 dB due to lowered impedance. Contrasted with a series/parallel configuration where there is a +3 dB in acoustic efficiency, but a -3 dB due to increased impedance.
The LDC section reflects the electrical/mechanical effect, it doesn't include acoustic efficiency due to mutual coupling. As the 2 sources are separated by distance they decouple, the coupling gain dependent on the wavelength vs distance relationship, and the destructive interference that occurs with separation.

RobbieNuke

You can go old school:
See LDC or:
http://www.epanorama.net/documents/a...impedance.html
Or you can use PC based system.
I use Woofer Tester II

[BlueWizard]

Frequency Sweep -

Well, there is an expensive way to do a frequency sweep and a cheap way to do it.

The expensive way is to have some type of frequency generator, and some measuring equipment. Both of these can be your computer if you have the right software.

The cheap way is with a CD disk that has frequency tones on it and a simple Volt-0hm meter which you can get at any hardward store, plus a small value resistor.

Ideally, it would also be nice to have a SPL meter which can be bought from Radio Shack and other sources for about $50.

You can make a CD of test tones by downloading the tones from Real Traps -

http://realtraps.com/test-cd.htm

This is a series of frequency sweeps starting at 10hz and proceeding in 10hz incriments up to 300hz.

Each 10hz sweep is 100 seconds long, so it sweeps 1hz every 10 second.

Place a 8 to 10 ohm resistor in series with the speaker.

Place one Volt meter across the resistor and another across the speaker.

Start the frequency sweeps starting with roughly 20hz up to about 70hz. That is most likely the range your cabinet is going to be tuned at.

Look for dips and peaks in the voltage measurements. The voltage across the speaker should peak at the resonance of the speaker. Though a tuned cabinet, being an anti-resonance chamber may cause a dip in the resonance. At any rate by charting the peaks and valleys, you should be able to get an impedance graph of your speaker/cabinet combination. And that should also tell you about the resonance of the speakers as a complete system.

Alternately, do the frequency sweeps from the CD and place the SPL meter right in front of the tuned port. When the measured SPL peaks, that is likely the tuning of the port and cabinet.

I'm sure others can point you to a more refined description and explanation than what I've given above. But above, is more or less, the gist of it.

Steve/bluewizard

[sreten]

Hi,

Shown are curves for two drivers. 2 cuft (internal) for a 12" wedge
sounds very large to me, I'd expect about half that if your lucky.

If you cannot measure there is no point being too precious as you
cannot trust specifications completely, just get sensible ballpark.

2 drivers 1cuft needs a 4" diameter x 3" long port.
2 drivers 2cuft needs a 4" diameter x 1.7" long port.

/sreten.

Note that vented boxes at high volumes need protecting
from bass below the port tuning frequency, be careful
of keyboard sub-bass stuff - filter it somewhere.