• WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required. There are many reviews of audio hardware and expert members to help answer your questions. Click here to have your audio equipment measured for free!

Filling/stuffing in Subwoofer?

DanielT

Major Contributor
Joined
Oct 10, 2020
Messages
4,751
Likes
4,633
Location
Sweden - Слава Україні
Hi I am grateful if anyone could sort this out; Is there any point in having filling in a Subwoofer?
The question based on the assumption of an otherwise well-constructed Subwoffer, with , for example,cross braces in the box and with steep LP filter, for example 24 dB, crossover frequency 80 Hz.

See picture for example, filling in Subwoffer.
(ignore that the Subwoofer in the pitcture is not well braced. Assume it had been with cross braces in the box that is)
 

Attachments

  • Picture-1213.jpg
    Picture-1213.jpg
    92.6 KB · Views: 507
Last edited:

RHO

Major Contributor
Joined
Nov 20, 2020
Messages
1,182
Likes
1,087
Location
Belgium
I used to think so, even designed my boxes on the small side to take advantage of isothermization...
I no longer do...
Because of this:
Read Chapter 10 (page 151 and onwards)
Can you summarize what is said for the not so mathematically proficient reader?

As far as I know, the material show as in the OP, is quite useless for subwoofers. Filling the enclosure with BAF (or similar) works better. It should act as an increased volume of the enclosure by slowing down the propagation of sound radiated by the back of the cone. <= not sure how scientific that is.
 

sergeauckland

Major Contributor
Forum Donor
Joined
Mar 16, 2016
Messages
3,440
Likes
9,100
Location
Suffolk UK
Interesting question. My recollection from my early days was that reflex-loaded boxes should not be filled, just have absorbant linings to the box walls with additional damping/bracing as appropriate. Sealed boxes should be filled, as the filling reduces the speed of sound inside the box and thus increases the effective size of the box, thus lowering the LF resonant point.

Thanks to Calleberg for the link, I'll read that in detail later.

S.
 
OP
DanielT

DanielT

Major Contributor
Joined
Oct 10, 2020
Messages
4,751
Likes
4,633
Location
Sweden - Слава Україні
Also when I think about it. Should not filling reduce the efficiency of a Subwoffer? Does it not require more power if more filling is added in the Subwoffer?
 
Last edited:

RHO

Major Contributor
Joined
Nov 20, 2020
Messages
1,182
Likes
1,087
Location
Belgium
Also when I think about. Should not filling reduce the efficiency of a Subwoffer? Does it not require more power if more filling is added in the Subwoffer?
I've seen measurements that showed that not bracing a subwoofer enclosure reduced efficiency and raised F-3, due to mechanical losses.
 

DSJR

Major Contributor
Joined
Jan 27, 2020
Messages
3,314
Likes
4,427
Location
Suffolk Coastal, UK
Interesting question. My recollection from my early days was that reflex-loaded boxes should not be filled, just have absorbant linings to the box walls with additional damping/bracing as appropriate. Sealed boxes should be filled, as the filling reduces the speed of sound inside the box and thus increases the effective size of the box, thus lowering the LF resonant point.

Thanks to Calleberg for the link, I'll read that in detail later.

S.
Now you've done it - my recollection was of a load of lay amateurs removing all the stuffing from speaker boxes regardless of the often careful design (still goes on in one little niche I can think of) and rejoicing in the increased colouration and thumpy bass as 'being better.'
 
Last edited:

Calleberg

Active Member
Joined
Aug 23, 2021
Messages
101
Likes
136
Location
Sweden
Can you summarize what is said for the not so mathematically proficient reader?

As far as I know, the material show as in the OP, is quite useless for subwoofers. Filling the enclosure with BAF (or similar) works better. It should act as an increased volume of the enclosure by slowing down the propagation of sound radiated by the back of the cone. <= not sure how scientific that is.
Even better I will skip the mathematics altogether.

Stuffing a closed box will lower the Q of that box significantly, Fc will also be somewhat lower but not nearly as much as in a larger box with the same Q as the small stuffed box.

This is a given and a well known fact by now.

What is shown in that paper is that most of the mechanisms creating these effects are lossy, and only a small possibly even insignificant part is isothermal.

The isothermal theory is based on the pressure/temperature laws. And the Idea is that when the cone moves inwards the pressure rises but the expected temperature rise is absorbed by the damping material, when the cone moves outward the pressure decreases, but the expected temperature decrease is countered by the damping material releasing the previously absorbed heat. Basically creating the effect of a larger box.
The fact that Fc is much less affected than Q by adding damping material is the first sign of the above not being the whole truth.

If you compare the frequency response of a closed box with a Q of 1.2 and one with a Q of 0.5, there is quite a bit of difference.
if the isothermal thing was real you could stuff the 1.2 box and get close to the FR of the 0.5 box WITHOUT any losses.

What the paper shows is in reality the damping material mostly works trough flow resistive damping, which is lossy, IE Amplifer power to the speaker is lost as heat in the damping material, and the damping material is more or less a Passive EQ of sorts burning away power at the FR peak of the 1.2 Q box. The Conclusion to draw is that it is more efficient to apply similar EQ before the amplifer.
And such approach would allow the sub woofer to play a liiitle bit louder.

There is more but i will stop here for now.
 
Last edited:

alex-z

Addicted to Fun and Learning
Joined
Feb 19, 2021
Messages
910
Likes
1,685
Location
Canada
Porous absorption (including low density foam) reduces air velocity. Meaning it can reduce standing waves inside the cabinet, and emulate a slightly larger air volume. This can be easily verified with an impedance sweep.

However, most subwoofer cabinets have standing waves outside of their operating bandwidth. Running an 80Hz 24db/octave crossover slope means you will never hear them. Porous absorption also works best at the point of highest velocity, meaning in the middle of the box. Lining the walls is the least efficient method.

With ideal subwoofer bracing, any panel resonances should also be pushed outside the audible range. + shaped braces are most effective, as it braces the panels at their weakest central point, effectively creating 4 smaller panels, with correspondingly higher resonances. U shaped braces are effective, but more wasteful of internal volume.
 

Tangband

Major Contributor
Joined
Sep 3, 2019
Messages
2,994
Likes
2,789
Location
Sweden
Do you mean something like this with 90 degree angles or the shape exactly of a: U ?

View attachment 161806
That looks good.:).
I have only made 6 DIY subwoofers with different crossovers, some analog active, two isobarics and later closed boxes with DSP filtering, so Im a novice.
All advice I can give is from my own experience that some damping material is good for the sound, even if the crossover is really steep and the dimensions of the subwoofer is small.* The reason for that I think is that there is no such thing as a perfect driveunit- it gonna distort somewhat, making some sound higher up in frequency. Maybe if the driveunit is of really low distortion type, it makes less sence with damping material ?
Thats also IÖ:s advice in this matter as far as I know.*

The best sounding subwoofer I have done myself was with a low Q driver in a small closed box, with Linkwitz transformer.
The peerless xls 10 inch driver with low Q is from what I have heard, a really good candidate for a small 20-30 L closed box subwoofer. At least does the loudspeaker constructor of Linn Keltik, a big kickass loudspeaker with outstanding bass in the 90:ties , use this in his own modern constructions. GRIMM audio also use this subwoofer.
Peerless xls10 can also be used with two drivers in a 40 L closed box volume with LT, placed on the opposite side of the box, thus cancelling out vibrations.
 
Last edited:
OP
DanielT

DanielT

Major Contributor
Joined
Oct 10, 2020
Messages
4,751
Likes
4,633
Location
Sweden - Слава Україні
That looks good.:).
I have only made 6 DIY subwoofers with different crossovers, some analog active, two isobarics and later closed boxes with DSP filtering, so Im a novice.
All advice I can give is from my own experience that some damping material is good for the sound, even if the crossover is really steep and the dimensions of the subwoofer is small.* The reason for that I think is that there is no such thing as a perfect driveunit- it gonna distort somewhat, making some sound higher up in frequency. Maybe if the driveunit is of really low distortion type, it makes less sence with damping material ?
Thats also IÖ:s advice in this matter as far as I know.*

The best sounding subwoofer I have done myself was with a low Q driver in a small closed box, with Linkwitz transformer.
The peerless xls 10 inch driver with low Q is from what I have heard, a really good candidate for a small 20-30 L closed box subwoofer. At least does the loudspeaker constructor of Linn Keltik, a big kickass loudspeaker with outstanding bass in the 90:ties , use this in his own modern constructions. GRIMM audio also use this subwoofer.
Thanks for the tip Tangband! :)

Here different subwoofers, bass solutions. In fact, I was amazed of Highfive / Björn's sub. That despite thin walls it could sound so good.:)
Built of cardboard pipes, see attached picture.


Häng med nästa gång det blir mässa. Det var jävligt roligt.;)
 

Attachments

  • IMG_4268 (2).jpg
    IMG_4268 (2).jpg
    149 KB · Views: 188

tvrgeek

Major Contributor
Joined
Aug 8, 2020
Messages
1,017
Likes
566
Location
North Carolinia
You can use the stuffing to adjust the Q. I like sealed subs and I target about .5 "critical Q", but reality never quite is what is prototyped, so a bit of stuffing brings me into line.
 
OP
DanielT

DanielT

Major Contributor
Joined
Oct 10, 2020
Messages
4,751
Likes
4,633
Location
Sweden - Слава Україні
Not true in my experience.
Okay, but does it not depend on how much you put in? I mean, if you push in as much as you can, things will probably happen with efficiency?
Although if you know the Q values, you should probably not ... hm ..You should be able to test that with a dB meter? .A lot vs a little moff , stuffing.
Perhaps?
 
Last edited:

Calleberg

Active Member
Joined
Aug 23, 2021
Messages
101
Likes
136
Location
Sweden
Not true in my experience.

I quess you actually do not know how much more efficient a larger 0.5Q box without stuffing would have been, do you? Which is fine, I do not know either, simply because that would mean building both boxes and measure that. Which almost no one does, for obvious reasons.
But, the fact (apparently) is, a small heavilly stuffed box WILL be less efficient than an larger empty true 0.5Q box.

Now as someone mentioned earlier, this DOES NOT mean you should rip the stuffing out of everything that looks like a loudspeaker. If it was there by the original design it serves a purpose and should be left untouched.

However, If you have acces to a PEQ or DSP or LT transform, it will be a "better" choice to build your small box whithout the stuffing and fix the Q thing whith EQ,
 
Last edited:
Top Bottom