The truth about amplifier “RMS” power & Rise

As a speaker or subwoofer moves its impedance changes and is affected by everything around it like the cabin, enclosure, humidity, temperature and base wiring load. This is what people call “Box Rise” which is a bit of a misnomer. As a driver has rise whether its in a box or not, the enclosure has an effect on the over dynamic yes but it is not the only cause.

I am going to describe some GENERIC scenarios below for example:

2x Dual 1ohm or Dual 4ohm 15’s wired to a “6000w RMS” Rated amplifier on music will experience varying ohm loads and will average about 30-40% of rated power “clamped” on music. Your amplifier will NEVER see 1ohm while playing musical ranges.

However, at specific frequencies or on test tones this will be a static ohm load and clamp differently as well. (Read: Burps) and can be much higher but isn’t typically sustained unless you’re competing in specific classes. You will see people saying things like “I was playing 32hz at 1ohm rising to 3.6” So at that ohm load you would have to look up the amplifier rating for 2ohm and 4ohm and can work out how much power you should be seeing.

Rise isn’t just a simple multiplier either but it is close enough to use it as an example. So, with that same 3.6x rise mentioned before when wired down to sub ohm loads like .5ohm you would only be rising to 1.8ohms. IF you have an amp capable of this.

Now the next example is my own personal setup 2x 8500w RMS amps that are capable of a half ohm load. Now on an amp dyno they will typically produce up to 50% more power at .5ohm as well. My personal setup sees anywhere from 0.7 – 2.8 ohms on music and I “clamp” close to 18k at 22hz at 1.2ohms on 2x 8500.1 but at 35hz I clamp about 7400w at 2.8ohm

These factors must be taken into consideration when figuring out what your electrical needs are and why there are companies “rating” their batteries for astronomical “RMS” numbers. They are basing it off the factor of you having significant rise and not realizing it. But this leads to another issue when your alternator isn’t able to keep up with the load from your vehicle and system so eventually you will likely experience some voltage drop.

To go back to the previous example of the 6kw amp at 1ohm this is an average load of about 2200w or 166amps of draw before amp efficiency, most amps today are about 80% efficient so this would be about a 210amp load from the amp to produce that power (averaged over time). So, it becomes a race against time if you only have a small amount of reserve. (remember the vehicle also needs anywhere from 40-80amps itself to run as well) so a low capacity (AH) bank combined with a small stock alt will eventually see some voltage drop if you play hard and “long enough”

Now revise this to a 0.5ohm setup and you are now drawing an average of nearly triple at 600amps of draw (amps are also less efficient at lower ohm loads) An impedance chart shows the entire flux and is a bit of different chart but similar enough to show the idea I am describing, I actually plan on doing a full Ohm Graph for my own setup in the near future via manual live charting/testing and will replace the pic once I do.