no bass from vented + floorstander 18w-8531- port problems?

[cersepn]

so.. i built another cabinet for the zaph zrt 2-way kit i bought, the first one being a sealed 20L design which had anemic bass (rolled off too much <100hz). The one i just built is a 38L cabinet and i followed Zaph's enclosure plans dimensions very closely, only difference being in the bracing placement.

Well, this time round there is more bass but definitely not what i expected from a vented 18w-8531 floorstander design. I'm really clueless as to what's wrong, though i'm sure you guys will be able to enlighten me

I went with the default 30Hz tuning recommended, and according to the tuning frequency response graph, it should be at least +/- 3 dB down to 50Hz... but somehow i'm not hearing it. Woofer is verified to be in phase. Test tones revealed that the speaker sounds pretty flat down to 80Hz but starts to roll off until output at 40Hz is almost non-existent. I have heard more than a handful of ported bookshelves with < 7" woofer that has more meaningful output at 40Hz than the speaker i just built. I can't help but think i did something wrong since the 18w-8531 is so highly raved about everywhere

I clamped the front baffle on while adjusting the amount of stuffing.
. With this method, there didn't seem to have much, if any, air leaks though i'm not sure if the amount of bass should be affected by that much?

After trying out different amounts of rockwool and acousta-stuf stuffing, it didn't seem to make much of a difference. If it's not the woofers that are faulty, i'm wondering if it's a bracing/port problem.


Circular cut-outs in 25mm bracing are 6.5" diameter. The only problem i can think of is how close the bracing is to the port, so there's not much air clearance near the port. I'm not sure if it affects anything. But even so, should the bass be affected by this much?


The biggest questions on my mind are:
1) Will the placement of braces relative to the port affect bass extension and output so much?
2) Will excessive stuffing/fill amount alter and shape bass response drastically?

Any suggestions are really appreciated, thank you!

[DrDyna]

Is this the same one that had the a crossover issue in the beginning?

[cersepn]

Yea, it is. There is more bass with this design now, for sure. But really not i expect from a cabinet almost twice the internal volume. Any ideas what could be the problem?

[tuxedocivic]

Room null possibly.

[jerome69]

or anaemic amplifier ?

[DrDyna]

Well, i asked because when we were talking about fixing your crossover, you never did say what method you used to fix it.

If you fixed it in the manner that you mentioned (snipping that jumper) then you have a series capacitor on your woofer, which would make it sound more like a midrange.

If you didn't snip the jumper and instead just hooked the woofer up to the other terminal, then it should be performing as it should.

[5th element]

You'd need a very anaemic amplifier to cause bass issues with steady state test tones @ normal listening levels. My first guess would be the room, have you tried positioning the loudspeaker in a different place, or alternatively, you leave the loudspeaker where it is then you walk around into different locations? If you do this, then the bass at a fixed frequency will change considerably in level as you walk into and out of peaks/nulls created by room modes. It's possible you're in a very bad spot or have awful room acoustics. As a test, set the loudspeaker to produce something like 50hz and then walk around the room. If you're in a null then walking into other areas of the room should produce bass that is considerably louder.

If you want to rule out the crossover as the cause then remove it altogether and try the test tones with the 18W full range. The crossover should have minimal effect on the bass below 200hz, so removing it should ideally do nothing.

[Michael Chua]

Originally Posted by cersepn

Well, this time round there is more bass but definitely not what i expected from a vented 18w-8531 floorstander design. I'm really clueless as to what's wrong, though i'm sure you guys will be able to enlighten me

You're getting too much mids. You'll need to shape the response to boost the bass, the mid-bass in particular (about 100Hz). A crossover is simply to determine the stop band of the woofer and the tweeter. Does nothing for the bass.

[chris661]

When the baffle is on, how close is the port to the lining? If the port is obstructed, you'll lose bass.

Are you sure the rest of the cabinet is sealed?
Are you able to provide impedance measurements? An impedance curve would show up what's happening.

[PLB]

Hi cersepn,

I suspect your simulation is based on a half space response, not full space and will therefore not show the normal -6dB drop in level at low frequencies compared to mid/high frequencies. This drop in level at low frequencies is normally compensated for in the XO and is commonly described as the baffle step compensation or BSC.
Also, check the free air space around the internal end of the port (as chris661 suggests) since this can significantly degrade the port output.

Regards

Peter