what's the point of lining walls in a vented sub?

[Max_Andrews]

In a sub, where the only thing it's producing is low frequencies much larger than the cabinet dimensions, what's the point of lining the internal walls with foam? It seems to me that this would do hardly any absorption or damping to the LF waves, am I missing something? I get that in a sealed box if you stuff the whole thing you can absorb a lot of the energy, but in a vented box does a foam lining do anything?

[curt_c]

[Pete Schumacher ®]

> In a sub, where the only thing it's
> producing is low frequencies much larger
> than the cabinet dimensions, what's the
> point of lining the internal walls with
> foam? It seems to me that this would do
> hardly any absorption or damping to the LF
> waves, am I missing something? I get that in
> a sealed box if you stuff the whole thing
> you can absorb a lot of the energy, but in a
> vented box does a foam lining do anything?

If the rear of the sub is generating any higher frequency noise, say from a vented pole piece, the interior damping would help absorb some of that before it made it out the vent.

[clarkbradshaw]

So, in essence, I just wasted another $30 putting acoustic foam into my downfiring 5 cu. ft. sub. Oh, well. I'll just tell myself it's reducing cabinet resonance.

Clark B.

[philiparcario]

> If the rear of the sub is generating any
> higher frequency noise, say from a vented
> pole piece, the interior damping would help
> absorb some of that before it made it out
> the vent.
It can lower wall vibration. Thats a good thing if the sub acts as a speaker stand for your mtm. foam and fiberglass do not work. cascade will help, tile mastic is good. there is a red rubber underlayment for tiles that paints on. its 40 bucks a gallon.
thicker walls in the first place with good bracing work better.

[billfitzmaurice]

> If the rear of the sub is generating any
> higher frequency noise, say from a vented
> pole piece, the interior damping would help
> absorb some of that before it made it out
> the vent.

All drivers create harmonics via the motion of the cone, and the further the excursion the more they create. Damping any direct radiator sub, vented or not, reduces reflections of those harmonics through the port if it is vented and back to the cone vented or not. The only subs that don't benefit from damping are folded horns, as they have far less excursion for a given SPL output, thus lower harmonic distortion, while harmonics which are created are filtered out by the bends of the horn.

[Pete Schumacher ®]

> So, in essence, I just wasted another $30
> putting acoustic foam into my downfiring 5
> cu. ft. sub. Oh, well. I'll just tell myself
> it's reducing cabinet resonance.

> Clark B.

Besides, it looks nice. Still, as Bill reiterated below, any noise, including harmonic products coming off the rear of the cone, will tend to be damped by the material you have inside, at least, better than a bare interior.

I'd have opted for a few pounds of Pink Panther.

[daryl]

Damping materials will lower Ftc and increase cabinet compliance if the cabinet isn't over stuffed.

In the Loudspeaker Design Coockbook you can see the 50% 1lb/ft^3 fiberglass fill measurement shows considerable increase in compliance and the 50% acoustic foam measurement even more.

In my 4.7ft^3 Lambda subs I put lots of fiberglass and it resulted in considerable mechanical loss and reduction Ftc but only a slight increase compliance (I would imagine more increase in compliance is possible with less fiberglass).

Honestly I could just about cut the power to my sub in half without the fiberglass, but with the fiberglass and three capacitors it produces precisely the response I need to complement my room gain and get flat response to 17hz.

Heres the response with and without fiberglass.
<A HREF="http://img.villagephotos.com/p/2003-4/144711/PB15-5.gif">http://img.villagephotos.com/p/2003-4/144711/PB15-5.gif</A>

And below you see the response with and without fiberglass and three 500uf capacitors in series.

[rudyjakubin]

> Damping materials will lower Ftc and
> increase cabinet compliance if the cabinet
> isn't over stuffed.

> In the Loudspeaker Design Coockbook you can
> see the 50% 1lb/ft^3 fiberglass fill
> measurement shows considerable increase in
> compliance and the 50% acoustic foam
> measurement even more.

> In my 4.7ft^3 Lambda subs I put lots of
> fiberglass and it resulted in considerable
> mechanical loss and reduction Ftc but only a
> slight increase compliance (I would imagine
> more increase in compliance is possible with
> less fiberglass).

> Honestly I could just about cut the power to
> my sub in half without the fiberglass, but
> with the fiberglass and three capacitors it
> produces precisely the response I need to
> complement my room gain and get flat
> response to 17hz.

> Heres the response with and without
> fiberglass.
>
> <A HREF="http://img.villagephotos.com/p/2003-4/144711/PB15-5.gif">http://img.villagephotos.com/p/2003-4/144711/PB15-5.gif</A>
> And below you see the response with and
> without fiberglass and three 500uf
> capacitors in series.

Probably meant parallel caps.
is that blip at 360 hz the secondary port resonance?
The caps & fibreglass eliminated same???

[spencer]

> I've never built a ported cabinet that played clean without a little stuffing or lining. I prefer stuffing over lining for a couple of reasons. It's easier and cheaper for one, and it seems to have a dramatic effect on reduction of port chuffing at high output levels. Again, my testing equipment is just my ears, but to them a little stuffing makes a big improvement.

[daryl]

Hi Rudy,

> Probably meant parallel caps.

The capacitors (3 x 500uf) are wired in parallel to make a single 1500uf capacitor which is wired in series with the driver.

Picture...
<A HREF="http://img.villagephotos.com/p/2003-4/144711/PB15-7.jpg">http://img.villagephotos.com/p/2003-4/144711/PB15-7.jpg</A>

> is that blip at 360 hz the secondary port
> resonance?

It's a third order sealed system (no port) with a lot of fiberglass.

Picture...
<A HREF="http://img.villagephotos.com/p/2003-...11/PB15-41.jpg">http://img.villagephotos.com/p/2003-...11/PB15-41.jpg</A>

The 360hz blip would have to do with the cabinets 18" dimension (internal dimensions 18"H x 27.5"W x 18"D with 1" MDF construction).

Picture...
<A HREF="http://img.villagephotos.com/p/2003-4/144711/Pb15-1.jpg">http://img.villagephotos.com/p/2003-4/144711/Pb15-1.jpg</A>

> The caps & fibreglass eliminated same???

The capacitors transform the bass alignment to third order and extend bass.

100% fiberglass fill eliminates all cabinet modes regardless of cabinet shape.

[aricaudio]

> All drivers create harmonics via the motion
> of the cone, and the further the excursion
> the more they create. Damping any direct
> radiator sub, vented or not, reduces
> reflections of those harmonics through the
> port if it is vented and back to the cone
> vented or not. The only subs that don't
> benefit from damping are folded horns, as
> they have far less excursion for a given SPL
> output, thus lower harmonic distortion,
> while harmonics which are created are
> filtered out by the bends of the horn.

In a horn, does'nt dampening reduce the output of the horn, by converting some of the pressure into heat? I've read this, but was'nt sure if it was fact. Regards, Aric

[Chris Roemer]

> Hi Rudy,

> The capacitors (3 x 500uf) are wired in
> parallel to make a single 1500uf capacitor
> which is wired in series with the driver.

> Picture...
>
> <A HREF="http://img.villagephotos.com/p/2003-4/144711/PB15-7.jpg">http://img.villagephotos.com/p/2003-4/144711/PB15-7.jpg</A>
> It's a third order sealed system (no port)
> with a lot of fiberglass.

> Picture...
>
> <A HREF="http://img.villagephotos.com/p/2003-...11/PB15-41.jpg">http://img.villagephotos.com/p/2003-...11/PB15-41.jpg</A>
> The 360hz blip would have to do with the
> cabinets 18" dimension (internal
> dimensions 18"H x 27.5"W x
> 18"D with 1" MDF construction).

> Picture...
>
> <A HREF="http://img.villagephotos.com/p/2003-4/144711/Pb15-1.jpg">http://img.villagephotos.com/p/2003-4/144711/Pb15-1.jpg</A>
> The capacitors transform the bass alignment
> to third order and extend bass.

> 100% fiberglass fill eliminates all cabinet
> modes regardless of cabinet shape.

Those look like Vifa H26 horn domes to me.

[daryl]

[billfitzmaurice]

> In a horn, does'nt dampening reduce the
> output of the horn, by converting some of
> the pressure into heat? I've read this, but
> was'nt sure if it was fact. Regards, Aric

I've seen more than one place making silly claims about how their products convert pressure into heat, and those claims are just that, silly. The amount of heat involved isn't enough to warm a cup of coffee. But in a world where people will pay $150 for an AC outlet it's not the silliest claim out there.

Any enclosure that's damped will suffer energy losses in both useful and not so useful frequencies. In the case of a direct radiator the benefit gained in supressing unwanted harmonics outweighs the losses in the intended passband. Folded horns don't require damping to remove out of passband harmonics and therefore left unlined don't suffer energy losses.

[aricaudio]

> I've seen more than one place making silly
> claims about how their products convert
> pressure into heat, and those claims are
> just that, silly. The amount of heat
> involved isn't enough to warm a cup of
> coffee. But in a world where people will pay
> $150 for an AC outlet it's not the silliest
> claim out there.

> Any enclosure that's damped will suffer
> energy losses in both useful and not so
> useful frequencies. In the case of a direct
> radiator the benefit gained in supressing
> unwanted harmonics outweighs the losses in
> the intended passband. Folded horns don't
> require damping to remove out of passband
> harmonics and therefore left unlined don't
> suffer energy losses.

Thank you. What would the effect be if you did line the throat of the horn with dampening material? I'm sure it would'nt be a wash, there'd be some frequency abberation directly related to it, no?
Aric

[unknownuser]

> I've seen more than one place making silly
> claims about how their products convert
> pressure into heat, and those claims are
> just that, silly.

I don't think they are claining that wasted energy can become useful heat, like to heat your house...Actually we would really be impressed if someone could harness enough dissipated energy, even from an enormous horn, to warm a cup of coffee even a few degress.

But all dissipated, "practically lost", energy is converted to the heat of entropy. That is: The random increase in kinetic energy of matter.

1st Law: "Energy is never really lost."
2nd Law: "Well...yes it is, pretty much."


(Originally posted by: BFB)

[billfitzmaurice]

> Thank you. What would the effect be if you
> did line the throat of the horn with
> dampening material?

I once lined an entire midbass horn with 1/8" rubber sheet to see what would happen. Response was unaffected. I've also tried various damping schemes in horn throats and throat chambers, none proved worthwhile.

[billfitzmaurice]

> But all dissipated, "practically
> lost", energy is converted to the heat
> of entropy. That is: The random increase in
> kinetic energy of matter.

> 1st Law: "Energy is never really
> lost."
> 2nd Law: "Well...yes it is, pretty
> much."

Of course the acoustic energy absorbed by damping isn't lost and is converted to a different form. In most snake-oil scenarios the lie is based in truth, and those ads that make a big deal out of how their product operates by changing motion into heat do just that. They want you to think that their product does something special and unique, which is just so much piffel.

[daryl]

Rubber is not a damping material for sound the impedance is too high.

Aricaudio is correct.

> I once lined an entire midbass horn with
> 1/8" rubber sheet to see what would
> happen. Response was unaffected. I've also
> tried various damping schemes in horn
> throats and throat chambers, none proved
> worthwhile.