gingeRS

regarding subwoofers?

when they are sold as 2ohm, or 4ohm?

the sub i am looking at is a dual 4 ohm? what does this mean

it is also available as a dual 2 ohm model? which one do i want?

someone please explain as i am getting confused

ta muchly

p.s. the amp i will be using is an alpine digital bass amp

Anonymous

dual 4 ohm means 2 coils, of 4 ohms each, connected in parrelel that = 2 ohm, in series its 8 ohm

How many ohms is your amp happiest playing into?

cfoster

I cant really explain it in technical terms (am sure someone will) but wiring a pair of 4 ohms the correct way (cant remember if its serial or parrallel as its been a while!) they will drop to 2 ohms. I'd imagine that your Alpine amp wont be 1ohm stable (unless its a beefy monoblock amp) so the 4 ohms would be the better choice. The dual means it has 2 voice coils.

As an example I ran the below subs all at 2 ohm, but the subs were initially 8ohm subs, 4 wired to each mono amp!!!! Well confusing shit!!



Chris

gingeRS

cheers

apparently the amp is rated to 400Wrms @ 2 ohms

so it seems the 4 ohms are the ones to go for

cfoster

yes mate for sure, and dont forget to run the subs in (keep the gain on the amp right down), it makes a hell of a difference doing so (they say about 48hrs playing time of running in!!).

Chris

Turbocabbie

Ohm's Law defines the relationships between (P) power, (E) voltage, (I) current, and (R) resistance. One ohm is the resistance value through which one volt will maintain a current of one ampere.
( I ) Current is what flows on a wire or conductor like water flowing down a river. Current flows from points of high voltage to points of low voltage on the surface of a conductor. Current is measured in (A) amperes or amps.
( E ) Voltage is the difference in electrical potential between two points in a circuit. It's the push or pressure behind current flow through a circuit, and is measured in (V) volts.
( R ) Resistance determines how much current will flow through a component. Resistors are used to control voltage and current levels. A very high resistance allows a small amount of current to flow. A very low resistance allows a large amount of current to flow. Resistance is measured in ohms.
( P ) Power is the amount of current times the voltage level at a given point measured in wattage or watts.

Typical calculations :-

Power = Watt

1 ) Watt = Volts˛
Ohms

1a) Watts = Amperes˛ X Ohms

you would simply do the calaculations in the same manner as any other expression and adjust depending upon the need to find the value Ohms, Amperes, Volts or Watts...
Its worth remembering however that most AMPs are rated at peak power which is the maximum that amp is capable of producing under ideal situations, what matters with regards to power is the RMS as this informs you of what the amp is reliably capable off

Dave Henshall

1a) I squared x R is only true for pure resistive circuits, a speaker is a big fook off inductor when all said and done so the power factor lags right down,

as a rule of thumb though,

if 2 equal resistive components are connected in series (ie one after the other) the overall resistance of the system doubles,

if 2 are connected in parallel ( ie same pos and neg to both speakers) the resistance will be half that of one of the components,

so as above, if your amp says stable to 2 ohm, then i would connect 2 four ohm speakers in parallel to give it a balanced load of 2 ohms,

and again as above, let them bed in - run them at about 20% for the first week or so

Fudgey

iTrader: (4)

also if a speaker/sub is rated to say 400 Watts, then you need to times this by .707 to find the RMS value

so in esscense a 400W speaker really only needs a 300W amp to run it.

in the ICE for sale bit i have a JL audio 12WO sub for sale, and a 490W amp to run it, but the amp is only running at half power to do it

gingeRS

right then, my amp is rated at 500Wrms @ 2ohms

The sub is rated at 500Wrms

so is this ok?

Turbocabbie

Its not same thing, an AMP is rated at what power it can push out and a speaker what power it can handle effectively... it is wrong to group the two as they are effected by different variables.

The RMS Testing & Measurement Methods for Mobile Audio Amplifiers is nothing more than a "voluntary" standard which advocates a uniform method for determining an amplifier's RMS power and signal-to-noise ratio. Using 14.4 volts, RMS watts are measured into a 4-ohm impedance load at 1 percent Total Harmonic Distortion (THD) plus noise, at a frequency range (for general purpose amplifiers) of 20 Hz to 20,000 Hz. Signal-to-Noise ratio is measured in weighted absolute decibels (dBA) at a reference of 1 watt into 4 ohms.

How to Measure

The fact that different manufacturers, when rating amplifiers, state output power at various supply voltage, load impedance and levels of distortion, makes it impossible to compare the amplifiers according to these measurements.
Primary Output Power should, according to CEA2006, be measured with 14.4V DC supply, a 4-ohm load and with 1% or less total harmonic distortion in the output. Other specifications included in the standard involve different impedances and supply voltages. This is where the manufacturer can indicate the conditions which make their amplifier perform optimally.
Since a loud speaker is much more complex than a resistor, it is not accurate to do measurements only using the rated resistive load (typically 8 ohms). This may result in harmonic distortion and strictly reduced voltage when connecting a real-life loudspeaker to the amplifier.
An 8 ohm speaker can have dipping up to 2 ohms. Therefore a more accurate way of testing an amplifier would be to do it along with shifting impedances. One method of measuring an amplifier’s output power, in an exhaustive and equitable way, is through the use of a Powercube

A PowerCube is a piece of equipment developed to measure an amplifiers output voltage. A PowerCube generates a 3D graph of the measured amplifier’s output voltage relative to the phase and impedance of the connected load. Since the ideal amplifier would deliver a completely constant signal regardless of the load, the graph would be a perfect cube.
A PowerCube can prove that an amplifier is stable regardless of impedance of the load. Practically that would mean that the amplifier could handle any loudspeaker.
More information about the PowerCube and how to do PowerCube measurements is available at http://www.audiograph.se/

Fudgey

iTrader: (4)

yes, but the amp will be running flat out. if used for long periods it may cut out and go into safe mode if it gets too hot.

my old one did anyhow.

its always best to over compensate and run them at slightly less than max power.

IMO anyway (not backed up by any scientific proof )

Fudgey

iTrader: (4)

also, running a speaker at full chat will endorse 'clipping' this is when the waveform hit its maximum threshold, you know when you turn the sterio up too loud and the speakers make farty noises - this is clipping. basically when too much power is proved and the speaker cant cope.

so going back on what i said about the amp, just dont run it at full chat and you should be ok.

Fudgey

iTrader: (4)

one more thing

when i used to set up my subs etc, go some where, where your not going to piss of the neighbors lol

i ususally set the head unit at what ever the loudedst volume i would listen to it at, and then disconnet the speakers so you can only hear the sub.

i then would set the sub by ear to the maximum before i could hear it start to distort. when you find that point, just back the amp off slightly.

on the amp i have for sale it also has an in car variable boost control

this is handy if you have kids in the back etc as you can turn the sub right down easily.

Eagle

iTrader: (2)

never run a big amp with a low powered speaker .. unless you want to burn out the voice coil
fook me how technical can you lot get
dont forget the reactive inductance produced on the speakers inward and outward journeys

the amplifier will at..
2 ohms create 500watts
4 ohms around 250 - 300watts
8 ohms around 150 - 200watts

as you increse the load on the speaker the amplifier will not be able to play as loudly, but decrease the load and it makes more power untill such point as the thermal load on the output stages increases and the amp ...
shuts down or blows up.
only run a low ohm set up if your amp will be able to run safely at this load.
but the more you add on in the way of speakers allows the amplifier to run differently..
you can if you really want to be technical add a mixture of lower powered speakers to the amplifier in a way that they should all survive if the high powered amp is cranked up to full volume...
dont forget tho the initial volume and quality supplied to the amp will be boosted in such a way that if the headunit is distorting then the sound out will be even more distorted.

and i wont even go into the technicalities of putting the speaker into a box
have seen so many setups farting away as they are in silly little boxes that limit the air movement of the speaker.
hth

Dave Henshall

not forgetting , both amp and speaker manufactures live to state peak power, not RMS, so infact a 500W amp will not infact be a 500W, as that will 9.9/10 be its peak value, i think there a a couple of companies that do give them in true RMS, like rockford fosgate IIRC, a 200W amp of thiers is a big heffa lump, but would no doubt be marketed by another frim as 1000W amp