RMS Rating of Speakers

Hello Friends,

I know the full form of PMPO and RMS.. and also the fact that there cant be a general conversiona method for them..

However, i m interested in knwoing, how much would be the approximate ratio of PMPO to RMS for low frequency sounds (typical beats) for a tuned impedance amlifier-speaker system..

Also, here's my main query :

I have 5.5" (cone diameter) speaker with a good solid 3.5 - 4" magnet.. When i purchased the pair for Rs.300 ($7.5) in 1998, the seller stated the power o/p of each to be 40 Watts..

The performance of the speaker is quiet good.. but what could be its rms o/p... I have fed the o/p of a 50+50W stereo amplfier to them..

Looking for some replies..!

Actually, the 40W rating of the speakers is probably the RMS. The RMS rating of a speaker denotes how much power can be put through the speaker before the voice coil melts, i.e. it is a THERMAL rating. PMPO is not a real rating, but is a number made up, usually by dodgy Far Eastern companies, to fool people who know nothing about the subject to buy their speakers. I've seen 800W (yeah right!) PMPO ratings on 3X5" computer speakers.

In fact, "RMS" is not a good way of rating speakers because it does not take distortion into account. In reality, even the beefiest, largest speakers will struggle with 50W.
Yeah.. Thanx for that information guys..

And i have my 50+50W amplfier's o/p connected to them... However it seems that the amplfier isnt taking load to its capacity while these speakers are booming upto some limited volume.. In other words, the amplfier's o/p power capacity is not being fully utilised...

Can you suggest or specify, which speakers (ratings) might best be suited for them to recieve max. power...

It is generally recommended for long speaker life that you use an amp with a higher output rating than the speaker input rating.
The reason being that an amp driven into clipping produces an enormous proportion of high frequency power that destroys speakers very quickly.
Avoid the clipping (all the time) and your speakers will usually survive much longer (decades).
Your 5inch speaker could be rated at 40w rms or 40w peak, I suspect peak since your voice coil will probably be in the range 20mm to 25mm and these could maybe go to 50wrms but more commonly 15wrms to 25wrms.
But based on my introductory para keep using the 50w amp and use the volume control to AVOID distortion.
Like Andrew said, stay away from clipping and you'll be fine. A 25watt amp can theortically produce 50watts at full clipping, assuming the power supply and everything else cam keep up. But, this would also be on the magnitude of 30% THD, which would be quite noticable to say the least.

I'm willing to bet that if you stick with the 50 watts, and keep it from sounding bad, you will fine for a long time... Just be judicous with the volume control....
driver power

Drivers have two basic power ratings, the thermal and the excursion limit.
Domestic system woofers and midwoofers are almost universally linear excursion limited which means that the largest peak input drives the diaphram to and beyond its linear limit a long time before the voice coil reaches its thermal limit, taking the usual 90db. with 20db. headroom figure the average disipation is at most a few Watts.
In professional drivers on the other hand especially those intended for mobile sound reinforcement systems, what matters is the amount of sound per unit of volume, and the relavent power rating is the IEC pink noise rating, and the overall system is designed to use the full voice coil disipation withought exceeding the excursion limit, and the average output is 30-40db. more than that of domestic systems, and the dynamic range condsiderably less.
The power rating game for domestic speakers should be looked at in this context, for the most part they reach the limit of excursion and start to sound unpleasant a long time before the voice coil is in danger.