View Full Version : Measuring Box Resonance (Fb)

12-03-2004, 12:26 PM
If you have two woofers in a single. un-partitioned box, how do you measure Fb? Do you measure each driver, or both drivers at the same time? In parallel? I am using the voltage divider method. Thanks.

12-03-2004, 12:50 PM
Good question!

Measure both in parallel and post your result. This will yield the closest Fb.

Measuring only one will result in an error since the "dead" driver will act as a "tuning" device with losses. Measure just one of them and post your result.

12-03-2004, 04:28 PM
OK. This is the first time I have done this, so I hope I'm doing it correctly. I first used the voltage divider method and got an Fb of 23 Hz for both drivers, and 37 Hz for one. On the other speaker of the pair, I tried the current source method, and got an Fb of 24 Hz for both drivers and 36 Hz for one. My target frequency was 29 Hz, so I'll have to shorten the ports. I used 6 inches of R-19 fiberglass, which is probably more fill than my box program allowed for, so I'm guessing thats what caused the error. The two ports are 16 inches long. How much should I cut off? 2 inches, 4inches?

12-03-2004, 05:26 PM
A rough guess is ~1 inch per Hz.
What are the diameters of the ports?

12-03-2004, 07:32 PM
They are triangular. The area is about 16.5 square inches for each one. I was going to be conservative and start with 2 inches.

12-04-2004, 06:52 AM
I used 6 inches of R-19 fiberglass, which is probably more fill than my box program allowed for, so I'm guessing thats what caused the error.Take all the fibergalss out, measure and post the results. You'll see the effects of the fiberglass. You can figure out the virtual volume added by all that fiberglass. I've never used such a copious amount in any vented system. That's quite a load of stuffing.

12-04-2004, 07:57 AM

Use fiberglass to line 5 or 6 sides of the box interior. Any fiberglass will do, but if you use R-19 or R-25 insulationtype fiberglass, you can ignore the volume of the bracing in the box because thick fiberglass adds virtual volume. If you must use dacron or felt instead of fiberglass, subtract its volume from the box (make the box bigger)."

The above is from JBL's Professional Enclosure Guide, which indicated to me that it was OK to use thick fiberglass. Also, an article written by Drew Daniels in 1990 specifies R-19 unbacked fibergalss for a system he designed. All the R-19 material I have seen is 6 inches thick. You have a very good point however, becasue I've also read that too much fiberglass can hurt performance. Do you recommend a two inch higher density material like pipe wrap insulation? Should I go out and find fiberglass designed for acoustics?

12-04-2004, 08:19 AM
All I'm saying is

A. I've never used that much fiberglass in a vented volume.

B. Take all the fiberglass out and measure the bare enclosure for comparison so you can see the effects of the fiberglass. Put whatever fiberglass you want to use back in after measuring the bare volume. R-6.7, R-13, R-19, whatever.

I'm not arguing with you, or JBL's Enclosure Guide, or D.D.

As for what fiberglass to use - I can't argue with this either:

I use Wrap-on fiberglass insulation. It is virtually identical to the stuff used in the classic JBLs. It is 1" non shedding, and yes it is yellow. You can get a 24" wide by 50' roll in a box (part number 16550) shipped to you for around $30. Contact http://www.wrap-on.com/ and they will give you the name of a near by stocking distributor or as in my case a distributor willing to ship it to you.

I have used this stuff for years and it is by far my favorite. Unlike house insulation it doesn't sag with time and since it is non shedding it is much nicer to work with. If you need two inch, you can double it up.

Egg crate foam is my next favorite choice but it is quite costly.

Thanks, Widget.

For others: The Wrap-On folks will send you to a vendor. B E Atlas in Chicago. 800-305-4393. It is $26.88 plus shipping. Drop shipped from the mfgr. Delivery time about 2 weeks. Mine is ordered!

12-04-2004, 08:27 AM
Sorry, I didn't mean to sound defensive. What I meant was, from what I have been reading, you are probably correct in that R-19 is too much fiberglass. I will take it out, and re-measure as you suggested. I am also going to look for a thinner alternative, like the Wrap-On. Thanks, I really appreciate your help!

12-04-2004, 08:33 AM
I'm just viewing this thread as a great way for everyone to see the effects of fiberglass on the tuned volume and how important it is to do exactly what you are doing and that is actually measuring Fb after solving a formula or running a software package to come up with a theoretical solution.

Thank you for posting:

Fb via voltage method
Fb via current method
Fb with specific fiberglass fill
Fb with no fiberglass fill
Fb with drivers paralleled in a common dual driver volume
Fb with only one driver operating in a common dual driver volume

This is good stuff! - no pun intended.

12-10-2004, 12:50 PM
I had a chance to replace the 6 inch R-19 fiberglass with 2 inch pipe wrap type fiberglass. I was very surprised to find that the resonant frequency (Fb) did not change at all. When I listened to the system, it did sound a bit different. It was less muffled and a little more "resonant" sounding. I then shortened the vents by about 6.5 inches to get to the target frequency of 29 hz (from 24 hz). I still have to do some measuring (this weekend, I hope) to see what happened to the overall system response.