How does speaker peak/RMS and Ohm work?
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OK, this is my sort of thing:
Really simple version:
Peak and RMS power are related, and are two different ways of measuring the same thing. RMS is, speaking as an engineer, a better way of describing the power output of an amplifier or power handling of a loudspeaker. ‘Peak’ is related to RMS, but has bigger numbers, and so the advertising department at the manufacturer likes it more (more watts, more better).
Ohms relates to how much power the loudspeaker can draw from the amplifier when the volume control is set to a given position. Lower impedance loudspeakers (commonly four ohms) will draw more power than higher impedance loudspeakers (commonly eight ohms). For most applications with today’s electronics, neither impedance is ‘better’ than the other.
Much longer, more detailed version:
A loudspeaker amplifier provides a constantly changing voltage (loud is more volts, quiet is less volts, silent is no volts) at its output terminals.
The loudspeaker is connected to those terminals, and current flows through the loudspeaker’s voice coil, approximately proportional to the voltage the amp is putting out (this is simplified, but sufficient for the purposes of this explanation).
A lower-impedance (typically 4 ohm nominal) loudspeaker will pass twice the current than a higher-impedance (typically 8 ohm nominal) loudspeaker for the same voltage input.
Watts (as the term is used in the audio industry to relate to amplifiers and speakers) are volts (signal size coming out of the amplifier) multiplied by amps (current passing through the windings of the loudspeaker’s voice coil.
Amplifiers have a maximum output. It can be limited in one of two ways – either that the signal you’re trying to reproduce is at the maximum voltage that the amplifier can output, or the current that the loudspeaker is trying to draw is at the maximum the amplifier can output.
Typically, amplifiers go into current limiting (clipping) when over-driven. It’s relatively uncommon for an amplifier to go into voltage clipping, because unless it’s some exotic over-the-top hi-fi amplifier they normally run out of current capability when pushing a loudspeaker hard (because, in reality, the loudspeaker’s impedance (ohms) is not constant, and when pushed to large amounts of cone movement a loudspeaker can actually draw significantly more current than its impedance would suggest). This is where the cone and voice coil are moving in one direction, and the amplifier is fighting to get it to change direction – as it must do, many times a second to reproduce sound. Loudspeakers, in real world conditions, are a bit more complicated than just a simple resistance (I warned you earlier that it was a bit over-simplified), but the 4 ohm and 8 ohm nominal figures still stand to give an indication of how much current the loudspeaker will try to draw from the amplifier.
A lower-impedance loudspeaker will draw more current for a given voltage, and hence more wattage.
As far as peak power and RMS power are concerned, here lies much untruth and incorrect numbers in the specifications of amplifiers – manufacturers quote elevated and misleading figures to make their products seem better.
For the simplest signal (a continuous sine wave), the peak voltage is 1.414 times greater than the RMS voltage. It’s just two ways of measuring the same thing. RMS voltage is a better way to describe the power delivered to the loudspeaker, because it more accurately describes how much heat will be generated in the loudspeaker’s voice-coil. Generally, what kills loudspeakers is overheating the voice-coil so that the insulation on the wire fails (or the wire itself breaks).
I see many completely ludicrous adverts for amplifiers, where they claim to be able to deliver many time more watts to a loudspeaker than they can draw from their power source. Much bullshit this way lies.
When developing an amplifier or loudspeaker, noting the maximum RMS level of a continuous sine wave signal is a dependable and repeatable way of measuring its power capability. Engineers like that sort of thing, so that’s what we do. But then the marketing department gets involved, and wants bigger numbers. OK, so peak volts… peak power… I guess that’s derived from the true measurements made in the lab so it’s sort-of legit.
Then, at some point in history, the marketing wank went from ‘peak power’ to ‘peak to peak power’, which is simply doubling the numbers so as to describe the biggest change in voltage (that’s from maximum positive volts to maximum negative volts)… but that’s not a true representation of how power is delivered, and is where we get from ‘massaging the numbers’ to ‘straight-up-bullshit numbers’. Of recent years, the cheap far-eastern rubbish I see on Amazon and AliExpress seems to have just randomly made-up, and completely impossible power ratings.
There’s a bit more to it. With amplifiers, if well constructed (with big heatsinks and, sometimes cooling fans on higher-powered units), they can put out their maximum rated RMS power all day. But the truth is that music – what most people listen to – is a mixture of loud bits and quieter bits, and so a lot of amplifiers are built such that they can briefly produce high signal levels (without voltage or current limitations), but if asked to produce a continuous signal – like that sine wave I mentioned earlier – they would overheat in a few minutes. The thing is that with most music, the ‘peak’ level is somewhere between five times and ten times the ‘average’ level, and so there’s not really any benefit, in a domestic situation, in building amplifiers with huge heatsinks and fans, as real-world signals don’t require them. In general, the sort of gear most consumers buy for home use would be of this type. Manufacturers almost never specify continuous RMS power for domestic stuff, as the numbers would be disappointingly low (and more watts more better).
Source: Am broadcast engineer now, and a former college lecturer teaching this sort of thing in years gone by.
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