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View Full Version : A 40hz port and an 80hz problem



joe
11-20-2013, 04:06 PM
Hello all I have a pair of jbl en5 cabs loaded with e145 woofers they have been ported , to begin with , with jbls recommendation of using the 4"x4" port . but I have problems at high spl with keeping the woofer cone in control and tweaking around with a variety of ports tuning for that general range doesn't really help . the 80hz slider on my eq is the one that generates the most response from the woofer whereas 25 and 31.5 do almost nothing , and the sliders on either side of 80 hz have much less effect on spl , cone control, everything.

I generally run 80hz about 3 db up because it sounds good , if I play loud I have to cut back to 1.5 db up and if its flat out with the clipping lights blinking on something like "when the levee breaks" I may even have to cut 80hz down a few db .


I would have thought that a 40 hz port would ease the problem by bringing the response down in the frequency range but the only port that seems to help control the woofer at high levels is a small 2" port . This is all I have been able to come up with . I've effectively ported it for control at 80hz by trial and error and I let the lower frequencies fall where they may because no port anywhere in the tuning range of 40 hz ever made much audible difference anyway . 5 cubic foot ...

Do You have any suggestions ? that dont involve using a different woofer? I'm building another set of speakers but I would like to get the most out of these speakers as well . Thank You

Mike Caldwell
11-27-2013, 07:54 AM
Are you using a high pass filter in your system, something in the 40hz range? That would keep un-needed and un-necessary sub low frequencies out of the woofer. From your description what you are seeing is the woofer flopping around trying to reproduce sub low frequencies that it has no chance of doing. It sounds like this is music playback system and if you are using records for a source a high pass filter is even more important to have in place.

What are the speaker cabinet internal dimensions and the port size that is in place now. You said the port is a 4" x 4" does is have any ducting or length that extenders into the cabinet.

Mr. Widget
11-27-2013, 08:12 AM
Check out this thread: http://www.audioheritage.org/vbulletin/showthread.php?34508-JBL-E-145-enclosure

There is some good info there along with links to other threads... a tuned port is not a random item that can be changed with no knowledge of what you are doing.

As Mike pointed out, in a ported system the bass signal being fed to a woofer that is below the tuning frequency will be down in output at 12 dB per octave and therefore very rolled off relatively quickly... boosting this is a terrible idea as the woofer is unloaded below the tuning frequency and will flail about wildly outputting little signal, cause distortion, waste power, and may damage the woofer.

Unless you are using a filter with a fairly narrow Q, a boost at 80Hz is also boosting at 40Hz and below.

Read up on these topics it's fun stuff!


Widget

joe
11-29-2013, 12:40 PM
I have subsonic ultrasonic , and also a filter just for vinyl , apt holman preamp,. this problem is present with any source material at the edge of the speakers acoustic output capability . around 123db sliding the 80 hz fader down is the only thing that will keep it hanging on , and then only with a 2" port. the eq is third octave so its kind of tight but not quite notch filter I dont suppose .

My ports were originally configured using jbl's math to have a 5.5 ? diameter port only 3/4 deep .

at 123db and up only a 2" port minimizes cone travel and distortion .

I dont really want to use a notch filter to cut 80hz out because it sounds good ,sometimes the party gets out of hand though :)

the dimensions are stock en5 "professional series low frequency enclosures"

thanks greatly for the help !