all about ohms

disneyjoe7

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Must if not all amps you guys & gals are playing with don't use the 14 volts in their amp output sections. Amps use a power supply that takes the 14 volts and brings that up to say + & - 25v or more. You could never get more then say 10watts RMS (1000w PMP) using 14 volts directly.




Quote

>>In a car, you've got assloads of current and jack squat voltage (14 volts, but the sky is the limit for current, however since most people dont like buying alternators, lets say 100 -150 amps... that can be racked up higher with the addition of batteries / alternator replacement)... and hence, low V, high current setups are "gooder"...<<

sntnsupermen131

a ported box--
contrary to what airplay is saying, if you want to talk in terms of graphs, a sealed box is going to be more of a straight line, while a ported box is going to arc in certain frequencies, being louder in those frequencies
a bandpass is going to arc even more in certain frequencies, but cuts a lot of frequencies out all together
some subs are designed for use in ported only, some subs are designed for use in sealed, some can be used in both
a very well designed ported box can give you a 3db gain across almost all frequencies that a sub is played at
a poor designed box will still be louder, but only at certain frequencies, and can be very punchy
but ported boxes are much bigger than sealed boxes...generally
im not talking about like 2 completely difft. subs, cuz im sure i can find a ported box thats smaller for one sub than a sealed box for a completely difft. other sub
but if you look at a sub that can be ported and sealed, the amount of space the sealed needs is less than the ported
which is why i like sealed
its a lot easier to deal with, if i had a trunk, id probably get a ported, cuz i can make a box that doesnt need to fit any type of dimensions, but i have a truck and the box fits under my back seat, so sealed is the way to go for my application
-Cody

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Originally posted by disneyjoe7
Must if not all amps you guys & gals are playing with don't use the 14 volts in their amp output sections. Amps use a power supply that takes the 14 volts and brings that up to say + & - 25v or more. You could never get more then say 10watts RMS (1000w PMP) using 14 volts directly.

the amp output section has a voltage dependant upon the output power...

say you've got a 250 x 2 stereo amp @ 4 ohms (1000 x 1 bridged @ 4) --- assuming a plain jane sine wave input of 1k Hz, you're looking at 250 watts / roughly 4 ohms = 62 = V^2 ... so that's just under 8 volts.

yup.. 8 volts rms roughly.

1000 x 1 at 4 ohms.... 1k / 4 = 250 = V squared.... take the root and you're down to 16 volts. 16 is a bit more than 8. and so we see the great variance, even within an individual amplifier. assuming the speaker stays the same, as you increase volume, you're bumping up voltage, not current. (different frequencies cause the speaker to have a reactance which will change its general impdeance... so a 4 ohm speaker might "act" like 2 ohms at 10 hertz... or "act" like 6 ohms at 20,000 hertz... but in gereral its AROUND 4 ohms at most frequencies.

as far as the power supply goes... that is what i was actually talking about.

in a car, you've got a conditioned 12 - 14 volt power supply from the battery to the amp. the power supply will ramp that up to whatever the output stage needs. the output transistors don't actually supply the JUICE for the speakers.... the power supply does. your output stage is (in its most disgustingly basic sense) a valve through which the energy of hte power supply is channeled, shaped into an AC wave (from its raw DC form) and then spit out the speaker terminals.

in the home environment, you can feesibly have a 25 +++ volt power supply... i haven't ripped apart a home amp in a while... and i'm nowhere near as enlightened with them as car amps, but, i would surmise you can, if you really wnated to, squeak 30 to almost flirting with 40 volts DC power supply out of a 110 volt AC wall outlet with sufficient current to drive an 8 ohm speaker through the output stage.

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oh and as far as the ported box thing -- cody is dead on right -- a sealed box will 99.999999% of the time give you the flattest curve possible with any subwoofer... a great sub should have a dead flat line between say 30 and 90 hertz... trailing off by a bit from there and then a hard line down after.

Airplay355

how can you have a flat curve?

http://www.stryke.com/pics/AV12MKIIvented.gif sorry this is the only website i could think of with graphs. its not as smooth and parabolic as the sealed one. it drops off more rapidly but has higher spl at lower frequencies.


http://www.stryke.com/pics/AV12MKIIsealed.gif this one is sealed and has a more parabolic graph, with a longer and smoother roll off.

you can flatten the response of a ported sub with an equalizer if it really pisses u off that much

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well now i know where you're coming from -- take a look at a more SQ oriented sub....

http://www.imagedynamicsusa.com/website/tech/data_sheet_idq10d2v2.html

between 10 and 90 hertz, for the sealed enclosure - there is only a 4 db "rise" in the middle...

for the subs u were showing us, there's a 39 db difference!!!

yikes!!!!! that's the furthest thing from "flat" i've ever seen.

a good sub will stay around the same sensitivity level throughout its playable frequency range (say 10 to 100 hertz for a sub)

in the case of the IDQ i showed you above -- it is at about 90 when its at 10 hertz.... peaks up at 94 around 50-ish hertz... and hten drops to like 89 or 90 around 100 hertz. that's a whole lot better than the sub you posted with 76 db's at 10 hertz and peaking at 115 db's around 70-ish hertz, and then falling off a bit at 100.

flatter is better.

polk is pretty flat in general... the momo's probably aren't as flat as the IDQ's, but the momo is a goddamn fine sub with a pretty flat frequency response -- that is why is plays so many different kinds of music SO WELL. you can throw it in a sealed box, set your gain, and forget about it-- u dont have to play with it and reset it to a different gain / bass boost for every different type of music BECAUSE it is so flat.

TrappedUnder Ice

The wheels on the bus go round and round

Airplay355

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i agree with everything u said, a sub should have a flat graph, i was just using those subs because i couldnt think of another site with graphs on it.