4ohm or 8ohm?

[monroe55]

I want to use two 500 watt cabinets for my rig. I currently have a power amp(QSC PLX1804) that puts out @900watts perchannel into 4ohms or @550 watts per channel into 8 ohms. Would it be better to use 4ohm cabs to get more power from the amp or will I risk damaging soething?

 

[billfitzmaurice]

Would it be better to use 4ohm cabs to get more power from the amp

The vast majority of the time no, as most drivers are displacement limited to actually being able to make use of less than half their rated power. In some cases as little as 20%.

 

[UncleBalsamic]

The vast majority of the time no, as most drivers are displacement limited to actually being able to make use of less than half their rated power. In some cases as little as 20%.

+1

 

[monroe55]

uh...displacement limited???? who's that?

 

[BassmanPaul]

It means that the cone cannot physically move any further no matter how much power you apply. If your speaker is as loud as it can get with 50W input, giving it 500W won't make it any louder and will destroy the speaker.

Paul

 

[JustDavid]

It means that the cone cannot physically move any further no matter how much power you apply. If your speaker is as loud as it can get with 50W input, giving it 500W won't make it any louder and will destroy the speaker.

Paul

So, like, it's displacement (movement) is limited?

 

[BassmanPaul]

Well .....Yeah!

Obviously! Anything mechanical can only move as far as it's designed to move. Exceeding that can only cause damage!

Paul

 

[Korladis]

I want to use two 500 watt cabinets for my rig. I currently have a power amp(QSC PLX1804) that puts out @900watts perchannel into 4ohms or @550 watts per channel into 8 ohms. Would it be better to use 4ohm cabs to get more power from the amp or will I risk damaging soething?

Only if your amp can handle a 2 ohm load.

 

[unclejam]

its a power amp bro its 2 seperate channels

 

[Korladis]

its a power amp bro its 2 seperate channels

my bad.

 

[KJung]

The vast majority of the time no, as most drivers are displacement limited to actually being able to make use of less than half their rated power. In some cases as little as 20%.

-1 Sometimes, yes in a BIG way (if you have a relatively low power amp... maybe a couple of hundred watts into 8ohms..... driving a large, multi driver and/or low SPL cabs). Sometimes, no (if you have small, single driver cabs and/or a power amp that puts out high power.... 500 watts+ into 8ohms... and/or very high SPL or mid voiced cabs).

In the OP's case, I would say, as always, it depends. If he was running Acme410's on each side (high power handling cabs, hi xmax, low SPL), I would recommend 4ohm cabs. On the other hand, high SPL, mid voiced 212's on each side... probably doesn't matter at all.

 

[DukeLeJeune]

uh...displacement limited???? who's that?

The rated power handling is usually based on the thermal limits - that is, how much wattage the voice coil can take before something overheats and fails.

Displacement is related to how far the voice coil can physically travel. There will be a "linear" limit ("x-max") based on how far the voice coil can travel before distortion becomes severe (leading to fart-out), and a physical or "damage" limit based on how far the voice coil can travel before something may break.

Now what starts to make all of this complicated is that the cone has to move four times as far for each octave lower we go in order to play at the same volume level (for a sealed box at least). So if the cone has to move 4 mm at 100 Hz, in order to play 50 Hz just as loud it would have to move 16 mm.

Let's say that our woofer can handle 250 watts thermally, and has 4 mm of linear excursion and 8 mm of damage-limited excursion, and at 100 Hz we get 4 mm of cone movement with 250 watts input. But it might take only 40 watts (or less) at 50 Hz for the cone to reach its linear limit of 4 mm!

The above is an over-simplification, as I've left out a lot of factors, but it illustrates why the rated power-handling and actual real-world power handling can be very different.