Assist to review my design, please.

[presscot]

I have a 5.25” mid-woofer driver and a 8” passive radiator. Also, I already have a 0.75 cu.ft. sealed enclosure.

If I build a subwoofer with these existing components, will it work fine?

I mean the sub will be passive radiator system with a 5.25” active driver and a 8” PR driver in a 0.75 cu.ft. closed box.

My concerns are:
1. Will the 5.25” active driver be capable of driving the 8” passive driver? In other words, is the 8” passive driver too big for 5.25” active driver?
2. Is the 0.75 cu.ft. enclosure too big for 5.25” active driver?
3. How much stuffing should be used in PR system?

The parameters of each driver are below:
1. Active driver
fs = 68 Hz
Vas = 0.23 cu.ft.
Qts = 0.45
Xmax = 6 mm.
Imp. = 4 Ohms
Power = 65 W

2. Passive driver: DAYTON DSA-215 PR
https://www.daytonaudio.com/product/1591/dsa215-pr-8-designer-series-aluminum-cone-passive-radiator

 

[presscot]

Anyone?

 

[mattstat]

1. Will the 5.25” active driver be capable of driving the 8” passive driver? In other words, is the 8” passive driver too big for 5.25” active driver?

Not a problem.

2. Is the 0.75 cu.ft. enclosure too big for 5.25” active driver?

This is more of a problem. That's about 4x the standard design volume for your driver in a ported box. As a result, the tuning frequency is too low and excursion on the passive radiator is high (the lower the passive radiator is tuned, the higher its excursion at resonance). Power handling without significantly exceeding the driver and passive radiator Xmax will be around 20 watts.

The frequency response droops through the midbass range.

The resonance is under-damped, so there's a peak in the frequency response there, with somewhat degraded transients.

This doesn't mean you can't do it, but the results won't be optimal. That may not matter to you though.

3. How much stuffing should be used in PR system?

Generally people stuff lightly. Some leave an open channel between the driver and passive radiator/port. Some keep the fill mostly on the walls. In a true subwoofer, some don't stuff at all.

https://www.parts-express.com/Acousta-Stuf-Polyfill-1-lb.-Bag-260-317?quantity=1
"Most sealed and vented enclosures require 1/2 lb. of damping material per cubic foot of enclosure. For best results it is recommended to loosely fill the material throughout the enclosure."

 

[presscot]

Thank you very much, mattstat.

Actually, the 5.25" mid-woofer is car speaker. AFAIK, it might be designed to use in infinite baffle system. So, I conjectured it could be used in a relatively big enclosure. Also, could you assist to simulate the result for the 1.5 cu.ft. (42 liters) enclosure? I'm unsure whether it would help relieve the Xmax issue.

 

[David Morison]

Putting a driver in a bigger enclosure makes excursion worse, not better.
If you want better control of excursion, you need to block put some of the volume of the cab to make it more like 10 litres.
This will also flatten out some of the dip in the 40-50Hz range.
However, it will also restrict how deep the system goes, with f3 closer to 40Hz.
I think you got some good modelling done for you in another thread.

 

[presscot]

To my understanding, going for bigger enclosure will bring resonance frequency lower, although introducing higher cone excursion. This might cause resonance occurs at the outside audible spectrum. And also, most musics contain less deep bass detail in that region, at about lower than 40Hz. So, the lower tuning frequency may eliminate the unwanted effect of extreme excursion, I presume.

 

[David Morison]

Yes, increasing box volume lowers the main resonance, which lowers the frequency at which the PR is helping control the active driver's excursion.
However, excursion above this frequency gets worse for the active driver, and excursion at the resonance frequency is worse for the PR (though in the case of your combination, the PR has enough Xmax to cope with that part).

(Edit PS - green line = 10 litres, didn't realise I'd screwed up the cropping untill after posting, sorry!)

 

[mattstat]

could you assist to simulate the result for the 1.5 cu.ft. (42 liters) enclosure? I'm unsure whether it would help relieve the Xmax issue.

As David Morison stated, the larger box makes most of the problems worse. In 42 liters with 20 watt drive level:

To my understanding, going for bigger enclosure will bring resonance frequency lower, although introducing higher cone excursion. This might cause resonance occurs at the outside audible spectrum. And also, most musics contain less deep bass detail in that region, at about lower than 40Hz. So, the lower tuning frequency may eliminate the unwanted effect of extreme excursion, I presume.

The problem with this approach is that if the signal is not filtered, any music that does contain low frequency content will cause extreme excursion. In the 42 liter scenario at the 20 watt drive level, this is only significantly worse for the passive radiator excursion. The woofer excursion is still reasonable.

If you add 60 grams of mass to the passive radiator in 42 liters, you can push the resonance down more. But again, all of this is making the frequency response worse. If you use the speakers close to the rear wall, some of this can likely be offset.

And, if you are just trying to make do with what you have and aren't concerned with optimizing things, some of these nuances may not matter to you. Most posters here encourage others to do things as technically correct as possible.

But some people like to experiment even if it's not the best way to do something. If you're having fun with it, keep having fun. Once you're outside normal approaches though, sometimes you just have to try things and see if you like the result.

 

[presscot]

That’s sufficient and very useful info. I’m really appreciated. Thank you again, @mattstat.