I'd like to learn about watts, impedance and sensitivity. I've googled so many times, but never found a clean and nice answer, so it would be really awesome if someone made it simple and clear to me. I'm pretty sure that people are really annoyed by these type of questions, but I'd like to learn about it.

I know, that impedance refers to ohms(Ω), watts to well, watts(W) and sensitivity to dB. But what difference do they make? Is there any way I can calculate sensitivity? I have two speakers, 300W at 8Ω powered by a Philips Audio System.(FC C505). I do not use the default speakers of the audio system, and as you already know about the speaker specs, here are the audio system ones:

  • 2000 watts PMPO
  • 2 x 120 watts Music Power
  • 2 x 60 watts RMS Stereo
  • Also, the default speakers have an impedance of 6Ω marked on them
  • i think you'd have way mode luck finding a good answer, if you'd generalize your question a bit , and transfer it over @ electronics.stackexchange.com
    – frcake
    Sep 28, 2016 at 5:49
  • Well, I got basically 2 good answers, but next time I'll try to post it to a right place. Sep 29, 2016 at 13:53
  • happy you did :) , i just figured it would be better for you , also slightly off-topic :) but all is good :)
    – frcake
    Sep 29, 2016 at 14:01

3 Answers 3


I could not find any documentation for a "Philips FC-C505". I did find information about the Philips FW-C505

Although this would not be pro-audio related, and therefore off-topic on this SE-site, let me put in my 2 Cents...

PMPO stands for "Peak Music Power Output". This means absolutely nothing!! It is a rating invented for Marketing purposes, and makes any speaker looks like it can handle a lot of power

My advice: IGNORE PMPO

The specification of the amplifier is clearly detailed on page-26 of the manual, and lists "2 x 60 Watts RMS" with a little footnote which details "6 Ohm, 1 KHz, 10% THD". This means the following:

The amplifier produces 60 Watts RMS when:

  • with a speaker with 6 Ohm impedance
  • at a constant tone of 1 KHz
  • with 10% distortion

In regards connecting your own speakers to this unit. The specifications do not list a "range" of speaker impedances, on page 9 you find a small warning "not to connect any speaker with an impedance lower than 6 Ohms".

In that regards you are safe connecting your 300-Watts/8-Ohm speakers.

In regards to estimating what this would do for your power output, we can use Ohms law to calculate this. (Note: while the calculation is simple, the reality may be slightly different, based on the internal circuitry of the amplifier, which at this stage is unknown)

Lets start with calculating the Amps going through the default 6 Ohm speakers at 60 Watts:

I = SQRT(P/R) = SQRT(60/6) = 3.16 A

We can now calculate the Output voltage of the amplifier:

E = P/I = 60/3.16 = 19 V

With your new speakers the voltage stays the same, so we can calculate the Power:

P = (V*V)/R = (19*19)/8 = 45 Watts

Now this is only an "estimation" of power, but it should give you an idea.

HTH, Edwin.

  • Thank you for the calculations, really appreciate your effort and advices. You're right on the FW, made a typo. Sep 29, 2016 at 13:52

They are independent features. I'll give a quick overview:


This is the power - either the power output by your system, or the maximum power your speakers can cope with (always make sure the latter number is bigger than the former number) :-)


Impedance is the resistance to variable current, so a combination of resistance and reactance. It is one of the most complex areas of audio circuitry, mathematically speaking, but for your purposes, you want to use speakers that are of the impedance specified on your amp documentation (as if you use speakers of too low an impedance, you will drive them at a higher power...) As your speakers are 300W and 8ohm, you will be fine here.


This is a bit of a red herring here. Usually this refers to the separation between signal and noise, and is a measure of how 'clean' your amplifier is. It is not related to your amp power or your speaker power, and in general you would measure it - monitoring the sound output with no input signal, and then with a signal, to give the difference between output signal level and noise floor

In summary - there is no rating for your system. If it has the volume you need then that's fine, the speakers have sufficient capability to not be damaged when you turn the volume up, but there is no information here about the quality of your amplifier or speakers.

  • 1
    Thanks, I think I got the basics. Would upvote, but can't because rep is under 15. Sep 29, 2016 at 13:48
  • 1
    Up voted for you :-) Sep 30, 2016 at 22:01

PMPO ("Peak Music Power Output" or something like that) is also known as spontaneous combustion power. It is a roughly proportional to the number of "professional" and "studio" words in the advertisement, indicating an unfitness for professional and/or studio use.

The interesting value tends to be "RMS", indicating the power that the amplifier will be able to feed into the speakers in a continuous manner without damage. You can convert this into actual acoustic loudness with the "sensitivity" rating (usually given in dB at 1W and 1m). Instrument and PA speakers tend to have quite higher sensitivity than HiFi speakers, particularly compact ones.

RMS power looks rather lacklustre and does not reflect the typical sound mixture in music, so there is a somewhat higher rating of "music power" which is handwavingly larger, typically about 50%. However, this also is not a regulated term and is often used inflationary: it is not rare to find amps with "music power" rating higher than the mains wattage, meaning it has no sensible relation to the actual power that may be put into speakers.

So for better or worse your best bet is to look for an RMS rating and try to guess the speaker sensitivity from their size: larger woofers tend to be more efficient, larger enclosures also help with bass sensitivity; squawkers and tweeters can be high in efficiency particularly when using compression drivers and horn enclosures.

Sensitivities between HiFi speakers and PA systems can readily span a range from 85dB@1W,1m to 105dB@1W, 1m. At equal electrical power, this can mean a factor of 100 in acoustic power. So the RMS power, while being one number of relevance, still needs to be factored into the context of the speaker and enclosure build.

Amps are rated for driving speakers of a particular impedance. Solid-state amps should not be driven with lower impedance than rated, tube amps should not be driven with either significantly lower or significantly higher impedance than rated.

"Impedance" is sort of like resistance at AC frequencies. It will usually be different for different frequencies and will be more or less given as a minimum rating over the usable frequency range of a speaker. Lower impedances output higher power for the same voltage. Usually you will want to match speaker impedance and amp rating in order to get the most power out without distortion or something going up in flames.

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