Do DIY, HT or PA (Pro Audio) Subwoofers Need Capacitors or Interior "Foam"

[nola]

Do DIY, HT or PA (Pro Audio) Subwoofers Need Capacitors or Interior "Foam"



I have a pair of Madison Amps M1 218s; moderate level PA road trip
gear. These are dual 18" drivers in each of 2 cabinets (each cab is 2 x
2 x 4 ft). There are no capacitors - only "box", driver framing,
drivers, and wires (and jack handles and speakons - in only....). Not
even any foam / stuffing inside. The "port" is a rectangular opening
between the drivers and faces forward. I run each cab with a separate
Parts Express 1000W sub amp.



Would caps or adding foam / stuffing add anything?



I think car subs use caps; and my low end PA Behringer B1800X subs -
older, non current pro version - have a crossover network inside, and
some foam, plus 2 ports. But they also have a bi amp vs passive switch,
and the network is prob for biamping. These have speakons for in and
out.



Any comments appreciated.



Come to New Orleans

Its Mardi Gras Time

[Daddy Dee]

The sub amps have the flexibility to dial in the top end frequency, right?

As long as that's the case, don't think you'd need anything else. Otherwise you would need a crossover / filter to deliver only LF signal to the sub.

I'm guessing that is what the caps on car subs would be there for. For pro sound subs, usually an electronic crossover would be used.

[nola]

The LFE output from my preamp-processor (Outlaw 990) is usually set to send all signals below say 40-80 Hz (I can alter this) to the sub amp. So I assume that my pre-pro acts as a sort of electronic xover in this case.The sub amp has all of the knobs I have ever seen on a HT amp. However, I cannot absolutely say that no higher frequencies are sent to the subs. However, with main the main spkr amps turned off, all I hear or feel from the subs is bass "thump. I included DIY subs in the title as both sets of PA subs could be made by skilled DIYers.

I thought car capacitors were used to level off peak battery drain. Maybe the much larger home or PA sub amps use their own built in caps for this purpose.

Any other ideas about need for "foam"??

[michael hurd]

Capacitors used in a speaker reject low frequencies and allow higher frequencies to pass. If you were to use a capacitor to protect a woofer ( as a high pass ) it's value would have to be absolutely HUGE, and you would need one the size of a coke can or multiples.

Foam is generally placed in a cabinet for a few reasons, generally in a vented or ported enclosure to keep higher frequency sounds from the rear of the motor and suspension, as well as higher frequency internal resonances in the cabinet from being emitted through the port.

An inductor does the exact opposite of what a capacitor does, in other words it allows lower frequencies to pass and rejects higher frequncies. The value of both capacitors and inductors are based upon where the designer decides to place a crossover point, as well as the impedance of the driver(s).

That being said, there is no reason for a capacitor in a sub, unless it's being used in conjunction with an inductor and a couple of resistors in a notch filter, a filter tailored to reduce the amplitude of a certain frequency range.

[JohnA]

Subwoofers do not need a capacitor. I really doubt car subs have capacitors; there is no reason for them.

Likewise, subs do not need foam, or fiber stuffing. Stuffing may be used to fine tune the cabinet or make a small one act larger.

[TNRabbit]

He's talking about the 1-5 farad capacitors used by car auidophools to "charge up" their system. They don't do jack; they're eye candy that depletes with the first big bass hit & the alternator is scrambling to catch up & refill the cap. You don't need it.'

As far as the filler (not usually foam, but fiberfill) is concerned, it depends on the specs for your sub. Some DIY subs that are ported don't use any; others do. What did the plans call for? I find it's usually only used with sealed type subs.....

[Guest " "]

what Michael Hurd said....

for example...the kp-480 has 300uf of capacitance in line with the normal woofer and 6mh of inductance in line with the sub woofer.....a basic 6db per octave crossover.

[Guest " "]

as an example...the kp-480 has a 300uf capacitor in line with the normal woofer circut and a 6mh inductor in line with the sub.

[colterphoto1]

as an example...the kp-480 has a 300uf capacitor in line with the normal woofer circut and a 6mh inductor in line with the sub.

My schematic shows a 68uf cap across the full range terminal into the single woofer. There is a choke in line for a 12db/slope LP network into that woofer.

A KP480 has a single woofer and a drone cone.

[Guest " "]

" A KP480 has a single woofer and a drone cone. "

I know...I have a pair...and there are probally different version xovers like every thing else klipsch sells...so as an example...like I said....there is a 6mh inductor between the input connectors and the sub. On the input terminal side of the inductor...there are 3 100uf caps that connect to the output terminals which connect from the 480, to the inputs of your main speakers....a basic 6db sloping xover...no 68uf cap on my xover...

any other questions...feel free to ask

[DrWho]

He's talking about the 1-5 farad capacitors used by car auiophools to "charge up" their system. They don't do jack; they're eye candy that depletes with the first big bass hit & the alternator is scrambling to catch up & refill the cap. You don't need it.'

The cap is there to offset the inductance of the often long power cords from the battery to the amplifier. Trying to pull more RMS power than the alternator/battery can deliver is indeed pointless. However the cap will prevent transient power sags...a very real issue.

That said, 10 Farads of capacitance at 12V has the same energy storage as 0.1 Farad at 120V. Since car amplifiers have to boost up the voltage to get their power handling, it makes a whole heck of a lot more sense to put your bulk capacitance on the higher voltage rails.

To answer the original question....amplifiers need the caps....not the subwoofer itself.

However, capacitors can certainly be used as filters, but there are many reasons why it makes more sense to put the filters into the electronics. A receiver with a lowpass for the subwoofer accomplishes just that...

[djk]

"10 Farads of capacitance at 12V has the same energy storage as 1 Farad at 120V."

Actually, 0.1F at 120V has the same storage as 10F at 12V (someone forgot to square the voltage).

A 10F cap can only supply 100A (typical car stereo load) for 100mS.

These caps do seem to smooth the transition from alternator power to battery power ( a slow chemical reaction), and the headlights don't seem to flash as bad. That's about all they seem to be good for.

[colterphoto1]

sf, now I understand what you were saying. Thanks for clarification

[DrWho]

(someone forgot to square the voltage)

Ah crap...and that was my whole point [:S]
1/2CV^2

[longdrive03]

E=mc2?????

[MikeMilliken]

"10 Farads of capacitance at 12V has the same energy storage as 1 Farad at 120V."

Actually, 0.1F at 120V has the same storage as 10F at 12V (someone forgot to square the voltage).

A 10F cap can only supply 100A (typical car stereo load) for 100mS.

These caps do seem to smooth the transition from alternator power to battery power ( a slow chemical reaction), and the headlights don't seem to flash as bad. That's about all they seem to be good for.

In addition they help take some load off the voltage regultor and help the altrnator last longer, even on dual battery systems. Plenty of cars that i was involved with would literally have the headlight go out on bass transients. They would give the amp better damping factor (control of the woofer cone) and reduced the early onset of distortion. The amplifiers also ran cooler....because if they couldn't get the current they needed (the amp) for a transient, the voltage would also drop, which immediately made the need for current (on the transient, music is AC)increase logarithmically. OHMS Law...

I did professional extreme high end installs for almost 10 years. I also owned 2 car stero stores of my own. The caps made a big difference, one you could see in the headlights, one you could hear, and one you noticed in the longevity of not only your amp, but your vehicles electrical system.

They sell similar power conditioning equipment for home audio, "tweaks" (I mean no offense), but with the enormous (comparitively) amount of current available at the 120 V socket, I think the ones for home use are more suseful for removing hum and possibly power surges.

[MikeMilliken]

To answer your initial question, if the gear is used with active crossovers, it does not need and internal (IE; passive) crossover. Someone had earlier mentioned a capacitor being in there, thats used in addition to the inductor to make a second order(12 db per octave) passive crossover, and can also be used as part of a compensation network in a very critical crossover, but not likely used much for PA gear.

If the woofer, has only an inductor on it, thats a first order crossover, IE: 6 DB/octave

if it has a large value capacitor (over 10 MFD) and an inductor, its likely a 12 DB/octave network

2 capacitors and an inductor, usually indicates a 3rd order, 18 DB/octave

This is a simplification, and a generality...The actual roll off of the network also depends on the driver because a driver with a big rise in the response at/near the crossover point may not perform as it should in theory, and conversly a driver with a fast drop at/near the crossover frequency can dip faster then theory.

Its rather common on commercial designs to use a 6 db passive on the woffer and have it function as a 12/18db would because of the drivers natural roll off being used to advantage/cost savings.

I dont mean to make this overly complicated, but I also used capacitors in my impedance compensation networks (LCR) on my crossovers, but they are usually pretty small values like 1.0-4.7 mfd, and they (simplicification) made the woofer behave closer to theory than it wanted to... They do more than that... But thats a whole 'nother can of worms;)

On your other question.......I have never found a box that doesnt show some improvement from damping being applied to the inside. Not once! However, do NOT fill the inside of a bass reflex cabinet, that generally does not give good results if the cabinet was properly desinged for the woofer you are using.

Foam is not necessarily your answer as there is literally a thousand different kinds/grade/densities. There is a material called "black hole" and others similar to it, that are useful in absorbing the upper back wave radiation from a woofer, while leaving the useful part of the backwave (lowest frequencies we want to retain for re-inforcement) This can clean up the sound considerably, and is one of my favorite cheap tweaks on most any commercial design. The great thing about foam/dampening pads is thats its removable, undercoating is a PITA to remove. However there are adhesive backed ashphaltic pads you can remove before you "stick" them down

Now you need to be judicious in the application and not go as far as to change the internal volume significantly. One thing I used to use regularly is 3M rubberized undercoating, and make sure its had plenty of time to dry before you put a speaker back in the box as I have had delamination in the surrounds and the spiders of the drivers, from the fumes! I have used the rubberized undercoating with good results on the outside of metal horns and the sheet metal speaker baskets, but you got to be careful..........

There are several acoustical treatments available, you are NOT supposed to hear the box making its own music. In theory the only contribution the box should make is in a resonance (tuned) at specific frequencies. Panel resonances can also contribute greatly to the output and anything you can do the break up their frequency should result in improvement you can hear. In many speaker designs, the designer is trying to take advantage of the back wave of the woofer, not the noises the box itself is making.

When I build enclosures for critical listening on subs, as a minimum. I use 1 layer (inside) of 3/4" MDF/HDF and then a barrier material (usually 40# roofing felt) then for the outside I use a multilayer (more is usually better) plywood. These materials do not ring/vibrate at the same frequencies. I brace all my cabinet walls internally and NOT AT THE CENTER to break up the resonant frequencies. When I finish my cabinets I use the 3M rubberized undercoating inside and I get liberal with it. A speaker cabinet should not sound hollow/ring when you "tap it", it should be a dull thud.

Do NOT use regular undercoating inside a cabinet, please! if it gets hot it moves/slides down. It also continues to emit unpleasant/harmful fumes that can be death to your woofers. I know this from experience. Regular undercoating never seems to stop stinking either.

This should cover your initial questions

[MikeMilliken]

An extreme simplification, but useful........

If the woofer cabinet is vibrating externally........The woofer is WASTING its energy by flexing it. Thats sound you could put to good use. A "perfect" IB/sealed cabinet would impart nothing itself to the sound, and would totally absorb the backwave.

Any design that uses the backwave to gain efficiency or increase output below resonance should (perfect again...) not put out anything (harmonics or fundamentals) above the frequencies it is intended to re-inforce. The rolloff of these frequencies is influenced by the type/design of the cabinet.

Alas, perfection does not exist, but we still strive towards it.