Jims 3 way speaker build

[rpb]

There's different reasons for doing different things, in different ways. I agree that 1/4" should work well, but I didn't want Jim to hit his woofer. The gating needs to be very long, or possibly no gating at all. I use HOLM. It's easy to extend the gate a long way, so that's what I do. See my post. 172 Looks like I went about 65 meters on that one. There will be a dip at the box tuning frequency. The roll-off will probably be smooth, like mine is. My box is sealed, so there's no dip. There is no need to process the measurement! Just look at it. It can tell you a lot. Don't use the measurement for any other reason, unless someone points out how to do it. I just want to see the tuning frequency, and woofer roll-off.

[a4eaudio]

T...There is no need to process the measurement! Just look at it. It can tell you a lot. Don't use the measurement for any other reason, unless someone points out how to do it. I just want to see the tuning frequency, and woofer roll-off.

Got it! I hadn't followed exactly what the measurements were for and jumped to the conclusion it was for MERGING nearfield, port and farfield.

[jimbones]

ok no gating, port "correct" I have a 5mh and 82uf on the woofer and it is not enough to get a XO point of about 300hz. Also why does the port look like that? I though it is supposed to be a fairly narrow response centered on the tuning frequency? why whould I be getting high frequency information ??

[rpb]

I don't know. What I would close mic, is the woofer.

For a second measurement, plug the port, and measure again.

[jimbones]

Id really like the get that woofer response to tail off a bit. In an earlier post (i could not find it) Someone suggested a notch filter. I simmed it and it does look like it may work but I will need to order some parts. What do you think??

[DeZZar]

Your real system bass response is the combination of the port and woofer FRD. Can you attach the files and confirm the inner diameter of the port? Set the gate time to 100ms before you export the FRD on both woofer and port.

[jimbones]

yes, in the last measurements i just tuned gating off. Also when I measure the port I assume I put the mic tip flush or just a bit into the opening.(maybe a 1/2 inch)

[rpb]

Id really like the get that woofer response to tail off a bit. In an earlier post (i could not find it) Someone suggested a notch filter. I simmed it and it does look like it may work but I will need to order some parts. What do you think??

I would suggest a close mic on the woofer before deciding on the xo change.

There is another way to reduce the 800hz range. Check with your siim to see how it affects the impedance, etc. This a WAG on the cap value. Add a 2uf cap in parallel with the 5.11 mH coil. This is called a tank filter. A smaller cap moves the notch higher. A bigger cap moves it lower. You may need to add a small amount of resistance in series with the cap to keep impedance amp friendly. Maybe 2 to 4 ohms. If it helps you to get a better measurement for this you can plug the port with a rag. Since the port is close to the woofer, it may be affecting the close mic measurement. Set the gate around 100ms as DeZZar suggests in the previous post. This may be a very narrow notch due to the coil being so large.

[jimbones]

In the Sim a 3uf cap helps. But the bump is really persistent lol. resistance didnt do much

[jimbones]

attached are frd of woofer and port as well as snapshots. Does the impulse show the woofer phase is inverted?

[rpb]

I have a suspicion, and a suggestion, and some questions too.

1 I'd add a cap for a tank filter centered on the peak at 4k. It shows up in both measurements, which leads me to the next suggestion.

2 I think the sound off the back of the woofer is bouncing directly off the back wall, and out the port. If you add an elbow, that should fix it, I think. The manufacturers data shows a peak there. The xo needs to knock it down as far as possible.

3 I would increase L1 to about 7 or 8 mH.

4 What gate setting did you use? It needs to be 100ms or so.

5 Other sims show the response dropping like a rock above 1k, but your measurement indicates that's not what is happening. I would suspect an issue with the files used. A second order filter should only drop at a rate of 12dB per octave, which is about what the close mic measurement shows. Both your sim, and DeZZars show a much steeper roll-off.

[DeZZar]

It can be 24db/oct electronically and still have a lesser of greater effect on the acoustic result. The measurement for the woofer, assuming its fine, shows that its response naturally falls off a cliff after 1k. So with a 7.5mh and 75uf in place its pretty easy to fully suppress everything from about 800hz up without any notch - again, all according to the files provided.

[Chris Roemer]

From what I think I remember, on a vented box the driver and port are in "quadrature" (90*) at the tuning freq?, and the port phase is sliding w/respect to the driver output (rotating more out of phase) as you drop below Fb, which is what creates the -24dB/oct bottom-end rolloff associated w/a vented box. (A closed box rolls off at -12dB/oct going down.)

[DeZZar]

Based on the aligned sum (blue) of these measurements you're currently sitting at an f3 of around 46hz. I've assumed a 4" port diam...
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The measurement really shouldn't be taken with crossover parts in place at this stage - hard to tell what sort of effect they are having.

As you've started before your woofer is near the floor so I have no real way of knowing what sort of effect that's having on the measurements either (maybe none). I would apply 80-100ms gate time to see if there is much difference in what is showing.

I also don't know if you have or haven't included the acoustic stuffing required to try and tame the resonance issues. Just to state this very clearly once again - it will have a sizeable impact on the results!

You mentioned you've cut off some of the port length - is the above before or after that? Have you re-measured impedance now to confirm where the tuning lies? Optimum results for an SB34 in net 70L is around 32hz...


Here's an example of the sorts of results you should expect when measuring a ported system:

First up is the model for this woofer - its an SB WO24P-8 in a 37L (net) enclosure tuned to 28hz. Here is the expected magnitude:


Here's our measured impedance which shows the port tuning is right on the desired 28hz from the model
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And here are the measured frequency results...


In these results you can very clearly see the woofer response has a deep null right around the tuning frequency (28hz) - which is further confirmation of where the tuning has landed.

The ports contribution to the system response also starts to peak around the tuning frequency and its overall contribution is fairly narrow spanning roughly 1 octave overall before trailing off.

The aligned sum is very close to our predicted results. We can confirm an increase of 1db or so around 60-70hz and an f3 of 38hz (a little higher than predicted but this is a brand new woofer without break-in and so we expect the compliance to reduce along with the fs over time which should bring us a lot closer to predicted f3).

I'm just sharing this as an example of what you should be seeing in measurements which in turn gives you confidence your measurements are correct and can be relied on.


I suggest in your case....

- Replace any polyfill you intend to include to tame standing waves and other resonances
- Remove your crossover parts
- Re-measure the impedance with the shorter port
- Re-measure the woofer and port responses without crossover parts, with all stuffing and with new port length

[jimbones]

I have no stuffing in the box. I will add some (I actually have the lambs wool lol!)

[rpb]

Factory measurements of woofer.​ There is a drop above 1k, but it comes back. Jims measurement was likely off axis if he measured on the tweeter axis, so his measurement was probably more like the red trace.

[DeZZar]

I was of the understanding the woofer was on axis.....if the woofer was measured with the mic pointing at the mid point between tweeter and midrange it's little wonder. Perhaps Jim can confirm?

[jimbones]

I measured the woofer on axis nearfield I'll do it again no gating and bypass the XO. If you see the impulse graph it shows the gating at 14 msec. I'll do it at 100msec.

[rpb]

I made the assumption that it was measured on the tweeter axis. My bad. Either way, the breakup still exists. I think it should be notched to potentially improve sound quality.

[jimbones]

here are a set of measurements with the cabinet with stuffing and nearfield measurement and no gating at all coil changed from 5mh to 8.3mh

[Billet]

That looks like a nice FR curve to build on. I'd add the midrange with a single cap (1st order electrical) and see how it looks.

[jimbones]

Billet, I am not that optimistic with the results. I thought that the raw Fr of the woofer would go lower as well as the Port. This set up should be able to get to 35hz

[jimbones]

here are a set of measurements bypassing the XO.

[jimbones]

All, Id be lying if I said i wasn't nervous about having a good outcome here. I have a finished cabinet and not sure of all my options at this point. I calculated the box volume many time to be sure I had enough volume. I made sure the port was sized appropriately and correct length. I appreciate everyones help hopefully we can get past what I thought was the easiest part of the project so so I can move on to the next step.

[DeZZar]

Can we get the latest frd for woofer and port and can I confirm the inner diameter of the port? Run your impedance sweep again to see where the port has landed the tuning.