Designing an enclosure for JBL 8PW8 – Excursion vs SPL peaks issue

[ezemattaldi]

Hi everyone,


I'm working on a 2-way passive speaker build using the JBL 8PW8 8” woofer and I'm running into some trouble designing the right enclosure. I'm simulating everything in WinISD, and would love some guidance from the DIY audio community.

• Fs: 81.4 Hz
• Qts: 0.76
• Xmax: 3.2 mm
• Xlim: 8 mm (from datasheet)
• Sensitivity: ~93 dB (1W/1m)

• A natural and smooth frequency response (flat if possible)
• No DSP or subsonic filtering
• Avoiding over-excursion in the bass region
• Medium-sized cabinet (ideally under 50L)
• Simple, passive design I can build and learn from

In typical reflex alignments (e.g., 45–50L tuned to 50–60 Hz), I'm seeing severe cone excursion at moderate power (30–50W), especially in the 40–50 Hz range, well beyond Xmax.

To mitigate this, I tried:

• Increasing Fb to 90–100 Hz
• Keeping box volume at around 50 L

This significantly improves cone excursion control (stays well under Xlim), and even makes the port extremely short or almost unnecessary. Mechanically, the woofer is much safer.


But a new problem appears:

• The SPL curve now shows a sharp peak near 100 Hz, followed by a dip and slow roll-off.
• The response is no longer flat or natural, and I worry it might sound “boomy” or boxy.

1. Is it valid to tune Fb > Fs with this kind of woofer, especially considering it doesn’t reproduce much below 70 Hz anyway?
2. Is there a known “sweet spot” for tuning high-Qts midwoofers like this?
3. Would going sealed (~12L) be a better compromise? I simulated one and excursion is perfect, but there's little usable output below 100 Hz.
4. Should I accept the SPL ripple and tame it with EQ? Or is this a sign that the design is mismatched?
5. Has anyone here worked with this woofer or similar pro midbass drivers in home reflex enclosures?

Any help, examples, or tuning advice would be amazing. I’d like to keep this build educational and as clean as possible—something I can trust and possibly upgrade later with a dedicated sub.


Thanks so much in advance!

 

[HumbleDeer]

Tuning below your driver's resonant frequency is a good recipe for over-excursion and the likes. There's also no way to avoid the need for a subsonic filter or a high pass filter if your driver needs it. There's no way to acoustically load the driver to stop it overextending when that overextension explicitly happens after the acoustic loading falls away (because the tuning is way below where it needs to be, and the cabinet ill-sized.

On the other hand, since you desire a smooth and fairly natural or flat response, My suggestion is to cut your losses -- this isn't a subwoofer or low-bass woofer, but indeed a midbass -- and just make a smaller sealed cabinet. Trying to tune a reflex cabinet lower than the kind of response you get out of the sealed cabinet is really not going to get you anywhere. The compliance+mass+damping of the driver is just not in line with the sort of response you're trying to achieve by tuning at a low frequency, or a high frequency with a box that's probably too large.

It's good that you noted the excursion peaking way out there, because that's a very very good indicator of your driver's simulated results being outside of what's physically possible. That's in line with what I'd expect from the low tuned large cabinet you have in the first frequency-amplitude graph.

 

[Sonce]

I'm working on a 2-way passive speaker build using the JBL 8PW8 8” woofer ...

• No DSP or subsonic filtering
• Avoiding over-excursion in the bass region ...
  .... In typical reflex alignments (e.g., 45–50L tuned to 50–60 Hz), I'm seeing severe cone excursion at moderate power (30–50W), especially in the 40–50 Hz range, well beyond Xmax.

Unfortunately, it is impossible to escape from over-excursion in vented boxes without subsonic filter.

To mitigate this, I tried:

• Increasing Fb to 90–100 Hz
• Keeping box volume at around 50 L

This significantly improves cone excursion control (stays well under Xlim), and even makes the port extremely short or almost unnecessary. Mechanically, the woofer is much safer.
But a new problem appears:

• The SPL curve now shows a sharp peak near 100 Hz, followed by a dip and slow roll-off.
• The response is no longer flat or natural, and I worry it might sound “boomy” or boxy.

Yes, you just learned it is not a good design. Vb is too large and Fb is too high.

1. Is it valid to tune Fb > Fs with this kind of woofer, especially considering it doesn’t reproduce much below 70 Hz anyway?
2. Is there a known “sweet spot” for tuning high-Qts midwoofers like this?
3. Would going sealed (~12L) be a better compromise? I simulated one and excursion is perfect, but there's little usable output below 100 Hz.
4. Should I accept the SPL ripple and tame it with EQ? Or is this a sign that the design is mismatched?
5. Has anyone here worked with this woofer or similar pro midbass drivers in home reflex enclosures?

1. No.
2. Yes: Fb somewhat lower than Fs, with Vb about the same as Vas.
3. Yes. You need a subwoofer anyway.
4. No. Yes.
5. Many times (with similar drivers). You need to learn what are the shortcomings of different design decisions and to try to find the optimum.