20Hz from a 40Hz box?

[robertbartsch]

I have a couple of newly reconed 18" - 2245Hs sitting in my basement and I have begun thinking about acquiring boxes for them. Yeh, I'm married and my preference would be to house them in boxes that are as small as possible, but this is probably a bad idea.

During my research, I came accross this information from the JBL Professional Enclosure Guide that I find interesting:


Do Not Expect 20 Hz From a 40 Hz Box

Bass-reflex systems should not be operated below their tuned frequency. If you try to operate the cabinet below the tuned frequency, you will break woofers. Do not try to reproduce alpha waves or lunar cycles with a speaker not designed for that purpose. The use of a high-pass filter such as the filter in the JBL 5235 Electronic Frequency Dividing Network is highly recommended with vented loudspeaker systems to prevent sub-sonic signal from causing hazardously long cone excursion and using amplifier power at sub-sonic frequencies that make no sound from the tuned system.


I'm not sure what a lunar cycle, alpha wave or sub-sonic signal might be but I assume they are telling box builders not to play program material with a frequency that is lower than the frequency tuned box; right? They are also telling folks to use crossovers that prevent the driver from attempting to create frequencies that are below the frequency of the box; right? They don't specifically mention the design limits of each driver but I suppose that is implied; right?

[Allanvh5150]

The mention of Alpha waves and Lunar cycles is just a poke at fun......basically, a tuned enclosure will controll the speaker down to the limit of the box provided of course the speaker can go down to what the box can produce. Below the tuning frequency of the cabinet the port just becomes a big hole and allows the speaker to over extend. It is always a good idea to put a high pass filter in to limit frequencies below the tuning frequency of the box.

Allan.

[robertbartsch]

Allan:
Thanks.

This makes me wonder about an earlier experience.

I completed a system recently using 15" - 2225H drivers in boxes that are within the recommended size and tuning specs for these drivers. Before I loaded the recommended amount of accustic sound material that I purchased from a national vendor, I tested the system to see if all the drivers were running with the L-pads and x-overs.

When I cranked the system and put my hand over one of the ports I could feel lots of air moving. When I filled the cabs with the recommended stuffing and played the same music at the same volume, I did not feel any air moving, however.

The difference was very significant.

[Mr. Widget]

When I cranked the system and put my hand over one of the ports I could feel lots of air moving. When I filled the cabs with the recommended stuffing and played the same music at the same volume, I did not feel any air moving, however.

The difference was very significant.That really doesn't tell us much, however it would be interesting to run an impedance plot of the stuffed system to see if the tuning frequency has been changed... I'd bet it has... and perhaps you have partially blocked the port creating an aperiodic loading similar to a dynaudio variovent.

Regardless of that speculation, and to continue on with what Allan was saying, as you drive a bass reflex loaded loudspeaker below it's tuning frequency and the woofer becomes unloaded it will continue to produce sound. Therefore you will actually be able to get a 40Hz woofer and box to produce 20Hz bass, just not at anywhere near the level that it will play 40Hz and above. If you try to coax the VLF bass from this system at higher levels you can easily destroy the woofer.


Widget

[duaneage]

Start with WinISD, the free download box design program, and design boxes with proper response and delay times. plug in the TS values and manipulate the box volume until you have something that is workable.

If you use two drivers the output will be +3 db at cutoff, power handling will be doubled for the same sound, and you will get lower bass extension. I built a dual 2214H system that had an f3 of 43 instead of 48 hz because there were two drivers. It normalized the cutoff lower because there were two drivers in the enclosure. Final system cutoff was measured at around 44. Vent tuning frequency was 38 hz and the system was QB3 aligned for best delay and transient response. Oh yeah, it sounds really natural and kicks great.

There is no substitute for design, testing, and tweaking. There are dozens of projects on this site you can duplicate but ultimately you have to invest in a few simple test devices and take the time to test.

[robertbartsch]

Yeh, the design software sounds like the way to go. I bought a couple of empty furniture grade cabs that I am using so I was kinda stuck with certain perametors, however.

Anyway, I was surprised that the slightly expensive commercial stuffing had a big impact. I'm assuming that adding the stuffing resulted in lowering the tuned frequency of the box but that would be a guess, I suppose.

[duaneage]

If the port remains the same, stuffing the box has the effect of enlarging it and that will lower the tuned frequency. But this isn't the answer to everything. The driver needs a certain volume of cabinet to work with, the port acts as a dynamic component, the resonance of the duct at the tuning frequency loads the driver, extending the system (not the driver) to produce bass below where it normally would. At tuning frequency the driver is not moving much at all, almost a dead stop. Acoustic output comes from the duct at this point, the driver is damped by the port's action.

I like to see fairly equal impedance peaks above and below the tuning frequency. And minimum movement of the driver at tuning frequency.

There are several established alignments for vented boxes. The most popular of these alignments are QB3, BB4, SBB4, C4. There are few other alignments as well. These alignments offer response slope, delay, transient response, and cutoff which are acceptable. There are special alignments that use unique driver parameters (passive radiator design for example) and of course there are plenty of misaligned designs. Misaligned boxes usually take the form of too small enclosure and high tuning frequency resulting in punchy bass similar to a driver in a too-small sealed enclosure. Too small of a box and a low tuning frequency is a recipe for disaster.

You are using known drivers with a proven track record in certain designs. I would stick with what works, test the results to be sure it is within specs, and enjoy