General discussion

75 Watts RMS.. what does it actually mean

I came across a 5.1 Home Theater system mentioning 75 Watts RMS.. what is it supposed to mean in terms of speaker output.

Also could anyone let me know whats the minimum output required for a average Home Theater System



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The definition...

Literally "root mean square." A DC voltage that will produce the same heating effect (power output in Watts) as the AC voltage. For a sine wave, the RMS value is equal to 0.707 times the peak value of an AC voltage. Example: divide Peak-to-Peak by 2 (or in half) and multiple by 0.707 = RMS voltage.

If 75w.RMS is not the total power of the system but rather the individual outputs then that should be plenty for the HTS.

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Thanks for your reply

75 Watts output is the total output of the system, Will it be good enough for a 350-400 Sqft room.

How much is 75 Watts RMS in terms of PMPO, can it be
calculated that way.

What other things should i look for in the music system.


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don't even bother with PMPO. there is no IEEE standard or anything for that... PMPO is mostly a marketing ploy to make your 75 watt speaker system sound REALLY powerful. stick with the RMS.

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I don't believe there is a direct relationship between RMS and PMPO. Acutally the PMPO rating is problaly just as useless as RMS. PMPO sure look like it is LOT more powerful than if it was rated in RMS. and I think the idea was to make you think you got more for your money.

Is 75w. enough? Well..maybe; it depends on the speaker system and how loud a person like his music.

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PMPO rating of audio equipments

this is an advertisement stunt as u said meaningless.

no user can measure the peak music power output

earlier practice of rating in RMS power is more meaningfull

really the acoustic power output of the speakers is what actually matters

then they will have to rate equipments in terms of
decimals of a watt to few watts

as this also is not measureablr to end user this also will not give an idea about the sound output requirements of user

from experience i have seen an equipment rated 10 Watta RMS into 8 or 4 ohms speaker is more than ample for any home use.

finally more depends on the speaker quality and its effeciancy is more important for real sound output

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PMPO stands for "Peak Maximum Power Output" and it is purely a marketing term used to fool people into thinking that small crappy 4 watt RMS speakers can withstand 200watts (PMPO) of power. I'm pretty sure that PMPO is the wattage at which the speakers will actually catch fire.
Always use RMS for speaker and amplifier ratings, if given a peak power rating (and remember that "peak" is NOT PMPO) take the peak wattage and multiply by .707 to get the RMS wattage.
Very few, and only the top of the line amps and speakers, can deliver clean audio anywhere close to their peak output, there is always noise and distortion, you probably won't hear it or notice it, but its there. This "noise" can easily damage your speakers at high outputs, especially if you are anywhere close to your speakers peak power handling (high end speakers handle it better, but they are very expensive).
As a rule of thumb make sure your speakers RMS rating equals your amps PEAK rating. this will keep your equipment safer, and produce better sound.

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75 Watts RMS

Hello Prashanth3004. I am a musician and 75 watt output will peel the paint off of the wall if the room is only 350-400 sq ft. If you know music equipment at all, a Fender Superreverb amp has 50 watt RMS and it will fill up a 2,000 seat auditorium.

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75 Watts RMS.. what does it actually mean

If it 75 watts RMS per channel it shoud be fine. If it is 75 total system power forget it. Only the RMS figure has ANY MEANING, the others are usless. John

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know this....

The relation between power (watts) & loudness (decibels) is logarithmic, so increasing the power output of your speaker from 75w to 100w only gives you a 6.7% increase in decibels. Happy That doesn't make the 75w sound so bad now does it? Rule of thumb, if you want it to sound TWICE as loud, you have supply TEN times the power. Happy

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75 Watts RMS IS GOOD

Hi There,

Its hard for me to explain what exactly 75 Watts RMS means, but it refers to clean power/wattage. Forget about what you hear of read in stores regarding 1000 watt receiver/amp/speaker. 75 watts rms is good for a home speaker, 100 watts rms is even greater. You dont really need anything more powerfull unless you are installing a boom box in your car. My back speaker in my car which are 6.5 inch round push 120 watts rms and they are very sufficient and capable of doing a great job. You will be satisfied with 75 watts RMS. Maybe you should consider something like 100 watts RMS. If your a audiophile like myself, you may see yourself returning the 75 watt speakers for something like 100. Hope this helps a bit.


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power conversion

So how do I convert the peak power to RMS power. Lets say my system can produce 1000 W PEAK power- how do i convert it to RMS power. Lets assume R as 8 ohms.I do know how to convert peak voltage to RMS voltage. so going by what I know:
1) 1000 W corresponds to 89.4 V peak power aprrox
2) 89.4 V corresponds to 63.25 Vrms
3) Doing Vrms^2/R the RMS power is close to 500 W.

is this calculation right? let me know.

thanks guys

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RMS Power

I used to design audio output transformers. A very fine
am-fm radio could generate 6 to 8 watts rms power across the frequency spectrum. 8 watts will blast you out of your office and be objectionable to others around you.

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question remains

so how do you convert the peak power to RMS power. do you calculate the peak voltage - convert that to rms voltage and then calculate the RMS power based on the RMS voltage?

is that right

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Not that simple!

In engineering terms, the Peak Power is double the RMS power. But, this applies for a single frequency.

When you consider peak power over a range of frequencies, like in an audio system, its not that simple. At any instant, the output signal has a band of frequencies and the instantaneous power is the sum of powers of all the frequencies.

There is no standard for relating the RMS power to the so called Peak Music Power Output (PMPO), a term created by the Japanese as an advt. gimmick. I have done some hands on measurements and have noticed that the PMPO can be anywhere from 4 to 12 times the RMS!

The real specification of an audio system will mention the output in terms of RMS / per channel into a specified load at a specified Total Harmonic Distortion (THD). It will also mention the frequency bandwidth at which the power will drop to half the rated value( 3 dB).

I would say that for a High End home theatre system, 30 Watts RMS for the Sub Woofer and about 15 Watts RMS per channel is good.

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power conversion

I'm not sure what you did there, but its easy to convert from Peak power to RMS, all you have to do is multiply by .707 (i'm not going to go into the mathematical proof of why this is correct but it is. lol) So your 1000W peak system outputs 707 watts RMS.

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What about the impedence??

This is one thing that gets neglected in most discussions. The total power output as stated is 75 watts RMS. Now if this is a 5.0 system, there are more things left unsaid. Watts per channel needs to be spelled out as most rear channels are at a reduced wattage. Most likely divided as thirty watts per channel for the front and five watts for the rear pair and the center. This could be any combination that totals 75 watts. Power is voltage times current but the resistance of the speakers causes changes in the voltage as a lower resistance will give you a higher current but lower wattage. Putting speakers in parallel reduces the impedence and could cause much smoke in the amplifier final stages.

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