How to model a stuffed vented box?

[Amphiprion]

I know the standard mantra is to not stuff a vented box, but SVS and Hsu apparently use a significant amount of fiberglass in their offerings, so there must be some sort of advantage.

I experimented with stuffing a vented Tempest subwoofer several years ago. 270L, 18.5Hz tune. I stuffed it 50% of its volume at about 1-1.5lbs/cf (can't remember exactly). I didn't have a mic at the time, but did take an impedance measurement of the before and after:

https://www.hayenga.com/mark/speakers/tempest/tempestcomparo.gif

You can see the change in the LF impedance (very obvious) as well as the attenuation of internal cabinet standing waves at ~200Hz and above (sub was 23x23x42 IIRC with 1.25" walls, so whatever freqs that gives).

Anyway, it's obvious the stuffing is doing something, but I don't know what it is doing to the response curve. Is there anyway to generate a response curve from an impedance curve knowing driver data, box data etc?

More importantly, I can't get simulations to match the results for this by fiddling with Ql, Qa etc. How do I model the effects of stuffing a vented enclosure?

There's got to be some benefit to this if the big boys do it, and I think the standard and very good idea of not to stuff vented boxes is because there's not information out there (at least that I've found) to predict what it does. Is there a model or is it just all empirically done?

[Amphiprion]

Also witness my mad box building skillz - check out the height of those impedance peaks Now that's what I call Ql - and on a ten cubic foot box too. Ok enough self congratulatory stuff.

[Amphiprion]

OOH! Nevermind. I just played with a program that allows me to change Qp and it looks like it does the trick. My usual one only allows Ql and Qa. The stuffing changes Qp from 100 (typical) down to 3! yeesh. Impedance curve very closely matches now. Qp is for the port, right? How does stuffing effect that? My stuffing was kept away from the ports (can post box diagrams showing where stuffing was located).

Now, how do we correlate %volume taken up/stuffing density/placement within the cab to Qp changes?

Edit: Looks like Qp affects lower impedance peak, Qa and Ql both upper and lower.

[Amphiprion]

Why is the stuffing affecting Qp and not Qa? Software I am using is WinISD Pro alpha.

[MJKing]

Hi Mark,

You could try modeling your bass reflex box in my MathCad ported box worksheet and then set the stuffing density to different values to see the impact. Personally, I think adding some light stuffing to a bass reflex design is a good idea. The stuffing will not attenuate the low frequency response very much and will be very effective at killing higher frequency standing waves inside the enclosure that can lead to peaks and nulls in the driver's output.

https://www.quarter-wave.com

If you read my article "Anatomy of a Transmission Line Loudspeaker" you will find an explanation of the double humped impedance curves that are common for bass reflex and TL speaker systems.

[noah katz]

Looks like you're getting 20% effective volume increase if the upper impedance peak went from 35 to 32 Hz.

What s/w lets you change Qp (yes that's for the port)? I don't see it in WinISD.

I'd expect that change on Qp to reduce output noticeably at Fb.

[Dennis H]

Mark, I was able to get an impedance curve similar to yours by adjusting both Qa and Qp. It does reduce output 2-3dB down low.

Noah, on the Box tab, click Advanced and you can set Ql, Qa and Qp. One gotcha, it wants to keep the same tuning frequency by default so it shortens the port as you decrease Q. After you go to the Vents tab and manually set the port length, it will hold it and change tuning frequency as you change Q.

[ThomasW]

I know the standard mantra is to not stuff a vented box, but SVS and Hsu apparently use a significant amount of fiberglass in their offerings, so there must be some sort of advantage

SVS uses what I'd call heavy fill, but it's more like really thick lining on the walls. Here's a shot of the insides of a SVS tube.

[SteveCallas]

Is the amp blocking the other two ports or is that some strange model? That amp does unfortunately displace a lot of volume.

[Amphiprion]

Wow, THE Martin King! I will read the paper and surely be humbled

Steve,

Didn't they start out with single 4" tubes before going to the 3-3" setup?

[MJKing]

Just a guy who obviously has too much time on his hands and spends too much time thinking about speakers.

[Dennis H]

My stuffing was kept away from the ports (can post box diagrams showing where stuffing was located).

I'd be curious to see that. The only reason I can think of that it would affect Qp is if there wasn't a clear air path between the driver and the port. The stuffing wouldn't necessarily need to be near the port to slow the airflow.

[Amphiprion]

OK, It was definitely in between the driver and the port, but not near the port.

[ThomasW]

Is the amp blocking the other two ports or is that some strange model?

I think the angle of the pic creates the illusion the amp is in the way.

That's a pic of the original SVS tube taken when someone was updating the driver.

Someplace I have a pic of the inside of the big SVS box sub, I'll root around and try to find it.

[---k---]

I know the standard mantra is to not stuff a vented box, but SVS and Hsu apparently use a significant amount of fiberglass in their offerings, so there must be some sort of advantage.

Just an undeducated guess, but is it because of the varible tuning? It the lowest tune configuration, iirc, they plug all the ports so the box becomes sealed and then they add boost for the low end with the amp.