I have been looking into the design of loudspeaker drivers and would like to know how to determine the maximum voltage which can be delivered to a given speaker.
From Ohm's law we know that P = V^2 / R. Therefore with for a speaker of any given power rating it would seem that max voltage could be determined by the formula:
V = sqrt(P * R)
Therefore for an 8ohm speaker with a power rating of 10 watts, it would seem that max voltage is around 9 volts. (sqrt(8 * 10 = 80) = 8.944 ~= 9)
What I'd like to know is which voltage measurement this refers to out of the following:
-Peak (where the speaker could handle a range of 0v -> 9v)
-RMS (where the speaker could handle an RMS value of 9v)
-Peak-to-peak (where the speaker could handle a range of -9v -> 9v)
If anyone could clarify this for me I'd greatly appreciate it.
Best Answer
As oldfart commented, use of the word 'peak' in audio is just marketing hype. What is real is true RMS watts into a given load of 4 to 8 ohms, with some amplifiers giving specs for a 1 or 2 ohm load. This is proved by testing with an audio spectrum/THD analyzer and documented.
At no time did I ever do 'peak' testing because it is ambiguous at best, and usually miss-leading the consumer. True 'peak' testing usually involved amplifier clipping distortion and/or speaker thumping or gritty sounds. An amplifier that match's or is a bit stronger than the speakers RMS rating is ok. It means no distortion until the speaker pops or sounds gritty.
This game of 'peak' value is just that. The reason is that you can mathematically play games with time and say things like for 10 mS (1/100th) of a second this amplifier can put out 50% to 100% more power. Heck, for 1 mS the power supply could dump several times its RMS load.
RMS refers to continuous power at a certain THD level into a defined load, and is the only rating to be trusted. The same scenario in terms of peak power has no meaning, as the marketing guru's are talking about very brief periods of time that do not relate to what the continuous volume can be.
Peak power is not to be confused with 'headroom', which is less abstract and means the power supply is over built by 20% or so, so at full volume sudden peaks in music volume from 24 bit or 32 bit audios enormous dynamic range do not overload the amps power supply. It is a built in safety margin when you crank up the volume to loud (but not distorted) levels.
If you hear so called 'peaks' it will be in the form of distortion from the amplifier and/or your speakers, and is fair warning to turn the volume down until it sounds very clean.
@jonk did the math that makes it clear. An 8 ohm speaker driven by a class AB (typical) amplifier with +/- 12 volt power will only give you 5 watts RMS. A 4 ohm speaker would give you 20 watts RMS. Combine them for a 2 ohm load (if the amp is rated for it) and you get 80 watts RMS. At no time was the word 'peak' mentioned, as it represents power into a abnormally low load for a mS or less, which has no meaning in real-world conditions where sustained clean sound is what counts.