Help with Ampeg cabinet port tuning...

[rustyginn]

Hopefully someone that knows their stuff can answer this...

I have an Ampeg 18E cab, the late model 4 ohm/500w version. I replaced the original eminance woofer with...

"Dayton Audio PA465S-8 18" Pro Subwoofer 4" VC 8 Ohm" from www.parts-express.com!

I was hoping to have better low frequency response with it. I envisioned a flat response down to the low E. But no... I have the same 60hz bump.

This is a large cabinet... 24w x 25h x 22d. When I opened the cabinet to replace the woofer I discovered the two ports were very shallow, 10w x 2h x 4.5d slots. I'm thinking the cab needs to be tuned lower.

So. I have two questions regarding this cab...

1. What tuning would be good to eliminate the 60z bump and make a semi flat curve to 40-ish hz?

2. How deep should the ports be to create that tuning??

Thanks so much for the help!!

 

[DukeLeJeune]

That Dayton woofer's high Qes and high voice coil inductance are what's giving you the 60 Hz bump, and probably not much top end either. Sealing the cab will pretty much eliminate the bump.

Or if you still want a ported cab, I suggest plugging one of the existing ports and making the other as long as you can, like nearly the full depth of the cab. Lower tuning frequency = reduced bumpage at 60 Hz.

 

[AstroSonic]

Seal the cab (with the Dayton), or tune lower... and give the stock driver another try. Also, you could try aperiodic loading: stuff the ports with dacron pillow stuffing (or equivalents). You'll have to experiment with the amount: not tightly packed, but enough so that you can't see more than an inch or two into it.

The Dayton is a decent subwoofer for sealed cab use. It's most significant drawback is its lowish sensitivity. The spec sheet shows response beginning to roll off above 110 Hz, and is down 10 db (relative to 110 Hz) at 2.3 kHz... on axis. Off axis response will almost certainly be considerably worse. Even players who don't like tweeters would likely want a mid crossed over in the mid hundreds. If you are using this for a sub, don't worry about it - you should be pleased.

 

[mmbongo]

That Dayton woofer's high Qes and high voice coil inductance are what's giving you the 60 Hz bump, and probably not much top end either. Sealing the cab will pretty much eliminate the bump.

Or if you still want a ported cab, I suggest plugging one of the existing ports and making the other as long as you can, like nearly the full depth of the cab. Lower tuning frequency = reduced bumpage at 60 Hz.

Which reminds me of a question I have....if the port needs to be as long as possible, how much space should be left in between the end of the port and the inside of the box in order for the port to work properly?

 

[DukeLeJeune]

Seal the cab (with the Dayton), or tune lower... and give the stock driver another try. Also, you could try aperiodic loading: stuff the ports with dacron pillow stuffing (or equivalents). You'll have to experiment with the amount: not tightly packed, but enough so that you can't see more than an inch or two into it.

I really like this idea.

Which reminds me of a question I have....if the port needs to be as long as possible, how much space should be left in between the end of the port and the inside of the box in order for the port to work properly?

If you want your modeling program to do a decent job of predicting the tuning frequency, then I think the conventional wisdom is to keep the inner end of the port something like two diameters away from internal cabinet walls. I routinely ignore the conventional wisdom, as you will see below.

The situation at the inner end of the port is a fluid flow problem: How to get the fluid (air) to flow into the port without becoming turbulent (chuffing). Flared ends help a lot, as they help funnel the fluid into the port. But a nearby wall surface can help too! It will also increase the effective port length and lower the tuning frequency, but you can kinda eyeball it and ballpark guesstimate it and usually be close enough. For example:

Suppose your application calls for a 3" diameter port 13" long, and your cabinet is only 12" deep on the inside, with 1/2-inch-thick walls. We gotta ignore the conventional wisdom mentioned above, because it won't let us come anywhere near the port length we need.

Cutting to the chase, let's run the port all the way to one inch from the back wall. So at first glance, it looks like the port length is 11 1/2 inches (internal cabinet depth - one inch + cabinet wall thickness). But the opening of the port is now in effect the 1" tall annulus in between the end of the port and the cabinet wall. (This 1" annulus height gives us a little bit greater opening area [9.43 square inches] than the port's cross-sectional area [7.07 square inches], so I don't think it will be a "choke point"; on the contrary, I think the cabinet wall just past the end of the port will actually improve the fluid flow relative to just the end of the port all by itself.) So if we eyeball the path that the air takes, including the bend that it has to make, and assume that the nearby cabinet wall extends the effective port length a little bit, we're probably going to come out pretty close to our 13" target. But ideally we measure the tuning frequency and trim the port length if needed... I wouldn't be comfortable with the port tube being any longer than this in this case.

I use this technique in my home audio subwoofer system, actually at both ends of the flared port! The inner end of the port is 3/4" from the top of the cabinet, and the outer end of the port is about the same distance above the floor. I say "about" because the feet are one inch tall, but if the sub is on a carpet, there will be some sink downage. (I use a 2" diameter flared port and the flare diameters ares about 4".) So I end up with an effective port length roughly three inches longer than the measured port length, judging by the measured tuning frequency.

 

[rustyginn]

Thanks for all the good information. I'll extend the ports as far as I can. I'll update with results. Thanks again

I think closing the ports off altogether is probably the best bet. But... I'm running an SVT15 up top with a EVM15BPRO. Also... just one amp feeding both cabs.

The top is so responsive, I'm afraid I'll lose the bottom (volume wise) if its not ported??

The goal is flat response.... Is this even possible?? Fingers crossed...

I'm starting to feel like the 40z/E-string bass cab is a myth... Ugh...