Sag of Moveable (Subwoofer) Driver Parts in Horizontal Mount (Down-Firing)

[STill]

Hello Everyone.

A horizontal mount for a down-firing configuration would greatly enhance the design I intend for a DIY-subwoofer in my living-room.
As of now, I intend a Dayton Audio Reference RSS390HF 15 Inch woofer in a ported enclosure.

One uncertainty for me is the subject gravitational sag of the moving parts (voice coil, diaphragm, etc.) immediately and over time.
I could only find quite unclear and contradictory statements about this subject so far.

One claim I found is that the sag should not exceed 5% of the Xmax, together with the calculation Percentage of Sag = 981,000 / (Xmax × (2 × Pi × Fs)²).
It appears to me as if deriving the moving mass and suspension stiffness out of these parameters is not very reliable.

In my example with the RSS390HF driver, the following parameters are provided:

• Suspension Stiffness Kms = 4.55 N/mm
• Diaphragm Moving Mass Mms = 306 g

That would mean a weight load of ~2.94 N, resulting in a sag of ~0.65 mm, which is ~4.6% of Xmax.

→ First of all, it is unclear to me what parts are counted under "diaphragm moving mass". Are parts like the voice coil included?
→ And of course, can anyone confirm the 5% sag limit of Xmax for horizontal mount suitability?

Another question is the long-term sagging caused by material fatigue of the suspension.
It would be good to know an approximation for this better than the vague statements one can find about "Sag is totally a thing! Just reworked a driver that was already bottoming out.", where no context of timespan/age (month, years, decades) is provided.

Thanks for your insights!

 

[Duke]

→ First of all, it is unclear to me what parts are counted under "diaphragm moving mass". Are parts like the voice coil included?

Yes, the voice coil is included because it is attached to the cone. But the airload on the cone is not included. Mms includes the airload on the cone, and Mmd does not. So, use Mmd instead of Mms. For a 15" woofer the difference between the two is probably about 27 grams, so Mmd would be about 279 grams.

→ And of course, can anyone confirm the 5% sag limit of Xmax for horizontal mount suitability?

That's a rule of thumb. If you woofer has plenty of X-max for your application, I wouldn't worry about going a little bit over.

To combat long-term sagging, you might build your DIY subwoofer so that you can turn it upside-down, with gravity working in the opposite direction. Lowther fullrange drivers tend to sag over time even when mounted normally, possibly causing the voice coil to scrape, so Lowther users routinely rotate their drivers 180 degrees every six months.

 

[Andersonix]

Did you run into this sag calculation from Adire?

 

[EJ3]

Hello Everyone.

A horizontal mount for a down-firing configuration would greatly enhance the design I intend for a DIY-subwoofer in my living-room.
As of now, I intend a Dayton Audio Reference RSS390HF 15 Inch woofer in a ported enclosure.

Another question is the long-term sagging caused by material fatigue of the suspension.
It would be good to know an approximation for this better than the vague statements one can find about "Sag is totally a thing! Just reworked a driver that was already bottoming out.", where no context of timespan/age (month, years, decades) is provided.

Thanks for your insights!

I no longer know the model of the Pioneer 12" Automotive subs (FR 20 HZ-80 HZ) that I have been using for 8 or so years in a pair of old Radio Shack downward firing ported (the larger, BLACK ones that they made). They are supposedly port tuned to 29 HZ and , although I have never actually measured them, they so add some lower bass extension to my main speakers (Dahlquist M-905's of which I have seen measurements wit an FR of 26 Hz-20 KHz +- 2 DB from HiFi Classic).
My Hi Pass is 60 Hz & low pass is 80 Hz.
I have never had any issues with the drivers.