RMS, Ohms, Watts

[essexbasscat]

Been pondering the question of RMS, ohms, watts and power recently.

I've noticed that some amps specify watts in terms of RMS, others specify in watts in terms of the ohmage of cabs connected to the amp. i.e.

1. 500w @ 8ohms, 1000w @ 2ohms

2. 500w RMS, 1000w peak. Minimum load = 4ohms.


Example 1 seems pretty straightforward, you get an around x watts at y ohms and the minimum load = 2ohms.

Example 2 is where my question lies. Would it be correct to assume the figures of 500w RMS and 1000w peak would be with a 4 ohm cab connected ?

Assuming the answer to that question is yes, If you use an 8 ohm cab, how many watts would you expect ?

Thanks all

T

Edited June 26, 2012 by essexbasscat

[LiamPodmore]

Yes, it would be correct that the RMS and peak figures are at minimum load, as for the ohmage situation. On the majority of amps you can expect 50-70% of the quoted specs for 4 ohms, so with a 500w it's usuallly around 300w, as far as i know. Some amps are different, like the Orange Terror Bass says it's 500w at both 4 and 8 ohms, but it's 500w at 8 ohms and something in the region of 520-530 at 4 ohms.

In the first example, the figures given are likely RMS as well.

Liam

[essexbasscat]

Thanks Liam. So if an amp of 500w RMS actually gives around 300w @ 4ohms, would it give 150W @ 8ohms ? or am I barking up the wrong tree again ?

[jackers]

I think what Liam meant was that the RMS figure is stated for 4 Ohms, and the figure for 8 Ohms is typically about 50-70% of this. So 500W RMS is 500W @ 4 Ohms, and about 300W @ 8 Ohms.

[warwickhunt]

[quote name='essexbasscat' timestamp='1340710065' post='1708326']
Thanks Liam. So if an amp of 500w RMS actually gives around 300w @ 4ohms, would it give 150W @ 8ohms ? or am I barking up the wrong tree again ?
[/quote]
500w RMS amp will generally be @ 4ohms (very few amps go down to 2 ohms, many Peavey amps being the exception), so 250 - 350w @ 8ohms (no absolute rule/formulae for % of the 4 to 8 ohm equation) and possibly down to 150 - 250 @ 16 ohms (though 16 ohm cabs are pretty rare now).

[gjones]

[quote name='LiamPodmore' timestamp='1340709734' post='1708319']
Yes, it would be correct that the RMS and peak figures are at minimum load, as for the ohmage situation. On the majority of amps you can expect 50-70% of the quoted specs for 4 ohms, so with a 500w it's usuallly around 300w, as far as i know. Some amps are different, like the Orange Terror Bass says it's 500w at both 4 and 8 ohms, but it's 500w at 8 ohms and something in the region of 520-530 at 4 ohms.

In the first example, the figures given are likely RMS as well.

Liam
[/quote]

Bit of confusion here. The higher the resistance (impedance) of the cab in ohms the less you'll get out of your amp. I have a 500w ABM and if I hook it up to a 4 ohm cab (which is the minimum resistance it is designed for) I will get the full 500 watts of power from the amp. If I hook it up with an 8 ohm cab I will get less power - about 350 watts (and less volume).

Seach for posts by Alex Claber who makes barefaced cabs and he will tell you everything you ever wanted to know about impedance of cabs and it's relationship with loudness.

Edited June 26, 2012 by gjones

[discreet]

Just to confuse the issue further, isn't the Orange Terror capable of being switched between 4 and 8 ohms?

Edited June 26, 2012 by discreet

[essexbasscat]

[quote name='gjones' timestamp='1340712578' post='1708388']
Bit of confusion here. The higher the resistance (impedance) of the cab in ohms the less you'll get out of your amp. I have a 500w ABM and if I hook it up to a 4 ohm cab (which is the minimum resistance it is designed for) I will get the full 500 watts of power from the amp. If I hook it up with an 8 ohm cab I will get less power - about 350 watts (and less volume).

Seach for posts by Alex Claber who makes barefaced cabs and he will tell you everything you ever wanted to know about impedance of cabs and it's relationship with loudness.
[/quote]

It is a bit ambiguous, isn't it ?

the whole thing does have implications for a few decisions. i.e. if I have a 300w 8ohm cab (say, a GS112), would that be enough to handle a 500w RMS amp @ 8ohms ? acknowledging that there will be peaks and boosting the lower frequencies may put a GS112 under strain. However, in general, would that be a reasonable idea ?

[LiamPodmore]

[quote name='gjones' timestamp='1340712578' post='1708388']
Bit of confusion here. The higher the resistance (impedance) of the cab in ohms the less you'll get out of your amp. I have a 500w ABM and if I hook it up to a 4 ohm cab (which is the minimum resistance it is designed for) I will get the full 500 watts of power from the amp. If I hook it up with an 8 ohm cab I will get less power - about 350 watts (and less volume).
[/quote]

Exactly. I find a really simple way of visualising the ohms/power relationship is think of the numbers as how far a valve is open on a pipe, and the watts as the water. The more open it is (The lower the number) the higher the flow of water (power) is. That's just my twisted mind though.

[quote name='discreet' timestamp='1340713545' post='1708411']
Just to confuse the issue further, isn't the Orange Terror capable of being switched between 4 and 8 ohms?
[/quote]

Yeah, i mentioned that in my post. It's 500w at ohms and between 520-530 at 4 ohms.

Liam

[LiamPodmore]

[quote name='essexbasscat' timestamp='1340714880' post='1708439']
It is a bit ambiguous, isn't it ?

the whole thing does have implications for a few decisions. i.e. if I have a 300w 8ohm cab (say, a GS112), would that be enough to handle a 500w RMS amp @ 8ohms ? acknowledging that there will be peaks and boosting the lower frequencies may put a GS112 under strain. However, in general, would that be a reasonable idea ?
[/quote]

Most people say to ignore power ratings when it comes to amps and cabs, and just use your ears. It's unlikely that you'll need to put out the full load of your amp into a cab, so it's not really an issue. When the cab starts distorting is when you need to back off the volume/bass frequencies a bit.

Liam

[warwickhunt]

The only think to ensure you get right is not going lower than your amps minimum load (i.e. don't go less than 4ohms on 'most' cabs)... and there is a bit of lee-way with that but you don't even want to go there so far better to stick to rigid rules!

[LawrenceH]

[quote name='essexbasscat' timestamp='1340714880' post='1708439']
the whole thing does have implications for a few decisions. i.e. if I have a 300w 8ohm cab (say, a GS112), would that be enough to handle a 500w RMS amp @ 8ohms ? acknowledging that there will be peaks and boosting the lower frequencies may put a GS112 under strain. However, in general, would that be a reasonable idea ?
[/quote]

This is why speaker 'programme' ratings are usually around double that for the 'rms'. The rms handling is a thermal limit and programme music rarely matches it in terms of duty cycle so there is more cooling time. But using compression (either with a compressor or eg valve saturation) increases the duty cycle and eats into this thermal 'headroom'. Running speakers closer to their thermal limits for extended periods also increases power compression so you lose output efficiency.

An additional factor that's not often mentioned is that the driver voicecoil typically relies on the magnet as a heatsink. If you pump in a lot of bass such that the speaker is being driven beyond xmax the voicecoil starts to leave the gap and cooling gets drastically less efficient, reducing the thermal power handling dramatically. I suspect that thermal failures above xmax account for more driver failures than people realise, as opposed to mechanical overexcursion per se (I've definitely seen thermal failure on woofer PA cones whatever others may say)

If you play with lots of dynamics then an amp with huge clean headroom into lower rated speakers is going to be fine provided you don't overexcurse. If you push closer to limits then it is a problem. At a given volume higher rated speakers should show reduced power compression as well (ie a 500 watt rms speaker driven at 300 watts will behave better than a 300 watt rms speaker). Lower power handling generally means a shorter voicecoil, so a lighter and more efficient driver. But since high excursion drivers necessary for good bass reproduction generally use longer voicecoils, you get the increased thermal handling as a welcome side-effect. There is no harm in driving a woofer that can take 500 watts with only 300 watts, 'underpowering' woofers is a myth.

[gjones]

[quote name='essexbasscat' timestamp='1340714880' post='1708439']
It is a bit ambiguous, isn't it ?

the whole thing does have implications for a few decisions. i.e. if I have a 300w 8ohm cab (say, a GS112), would that be enough to handle a 500w RMS amp @ 8ohms ? acknowledging that there will be peaks and boosting the lower frequencies may put a GS112 under strain. However, in general, would that be a reasonable idea ?
[/quote]

Be careful and use your ears to listen for distortion. At reasonable volume it should be ok.

[flyfisher]

[quote name='essexbasscat' timestamp='1340714880' post='1708439']
It is a bit ambiguous, isn't it ?
[/quote]

Pretty much. Peak power, rms power, watts etc etc are all rigorously defined things. The problem is music - it's just too variable.

Imagine a nice pure sine wave. The peaks are, of course, very easy to measure. Power is a little more difficult. Imagine a 12 volt battery (ie DC) connected to a load that causes 1 amp to flow. The power would therefore be 12 watts. Easy.

Now imagine an AC power supply delivering a sine wave of 12 volts peak-to-peak. Connect that to the same load and it will not cause 1 amp to flow and will therefore deliver less than 12 watts. The reason is not hard to understand if you think about the 12v DC source delivering a constant 12volts but the AC source is up and down all the time so it can't deliver the same 'push'.

So, to deliver 12 watts of power with an AC source (in this example) it turns out that you need more than 12 volts peak-to-peak to deliver a power equivalent to that of a 12 v DC source. That 'equivalent power' is called RMS watts. So, to get 12W RMS it turns out that we need an AC voltage of about 16.8 volts peak-to-peak (I'm skipping all the detail here!).

Another way of imagining this is to think of the power output of an electric heater powered by DC or AC - which in the context of speaker power handling is a reasonable analogy. 12W DC is equivalent to 12W RMS when driven by a sine wave.

Which is all well and good, except that a sine wave is a rather special case in that it is definable and repeatable, whereas a music waveform is most certainly not (well, it is in terms of the music, put not easily in terms of the average power delivered by the driving amplifier).

So this is why all these highly mathematical and scientific principles become a tad vague when we start talking about 'music power' and the like. It's not that these things are not understood, more that you'd need a lab full of fancy test equipment to determine the precise specifications, but even then it would only apply to one particular waveform - or 'song' as we like to call it.

This is just one example why rules-of-thumb can be be a bit vague and very useful at the same time.

So, keep your cabs within the ohmage spec of your amp (definitely) and keep your cab wattage ratings higher than your amp's RMS watts rating (preferably - or be very careful with the volume control) and things will be fine.

[chris_b]

[quote name='essexbasscat' timestamp='1340714880' post='1708439']
....if I have a 300w 8ohm cab (say, a GS112), would that be enough to handle a 500w RMS amp @ 8ohms....
[/quote]

Your 500 watt amp will be so rated at 4 ohms. The maximum that amp will put out into 8 ohms will be approx 300 watts, so your 8 ohm cab will be fine.

You probably won't be running your amp flat out so in real life your 300 watt 8 ohm cab might only be getting 100 watts. That’s ok.

Add another 8 ohm cab and your amp will be putting a maximum of 500 watts into both cabs, which have a nominal rating of 600 watts. That's 250 watts into each cab. Again assuming you're not flat out, your cabs will be able to handle the output from your amp.

So you should have no problem with either rig.

Unless you're playing at very, very loud volumes a 500 watt amp into 2 300 watt 112 cabs will be fine. That is my rig and I can play as loud as I like with no danger to the gear.