# What does the rating on speaker mean?

Discussion in 'General Electronics Chat' started by yaantey, Feb 5, 2012.

1. ### yaantey Thread Starter Member

Oct 7, 2011
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I have a speaker which has 8ohms and 0.5W written at the back. What does that mean? From this can I calculate the max or min voltage or current the speaker can take?

2. ### sabazeshan New Member

Feb 4, 2012
1
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Hi
the speaker that you are using is 0.5 Watt and load resistance is 8 ohms so you can calculate the current using P=I^2R, formula and also voltage i hope i am right.

3. ### bountyhunter Well-Known Member

Sep 7, 2009
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Wattage ratings on speakers means absolutely nothing. The impedance tells you important information.

endolith likes this.
4. ### cumesoftware Senior Member

Apr 27, 2007
1,330
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No, it is completely wrong. Both impedance and wattage rating are important. Impedance tells you the average "resistance" of the speaker when powered by the amplifier. Wattage rating tells you the maximum RMS power that the speaker can accept without getting blown. For sure you don't want to hook up a 10W speaker to a 100W amplifier. It will burn at high volume, and worse, you can short out the output of the amplifier in the process, rendering it unusable (though most amplifiers have short circuit protection).

5. ### holnis Member

Nov 25, 2011
50
4
A speaker's power rating (in general) tells you how much A.C. power can be dissipated in the speaker's voice coil without damaging the

speaker.

Giving the rating as continuous RMS watts is the most honest way to rate a speaker (ex: 150 watts continuous rms power). You may

hear speakers advertised as "150 watt 6 x 9s" or "100 watt 6 x 9s" and you may instantly think that the "150 watt 6 x 9s" are better and

will play louder than the speakers rated at 100 watts. The first thing you should realize is that speaker ratings are often exaggerated.

Then you should ask if the rating is in RMS or peak watts and are the speaker ratings for instantaneous or continuous power.

6. ### PaulEE Member

Dec 23, 2011
423
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The power rating on a speaker expresses the power through the coil upon which full deflection of the speaker cone occurs in one direction or the other. This is directly related to the strength of the magnet inside the speaker that the coil surrounds, as well as the coil inductance/resistance/capacitance (impedance). If too much power is inputed, the coil may overheat or the cone may fracture/rip.

The impedance on a speaker tells the designer of the amplifier circuit the number of rms volts that can be applied that tells him/her the number of amps that will flow. This allows the designer to decide the amplifier specifications such that the power rating is not exceeded.

7. ### Audioguru New Member

Dec 20, 2007
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If the power rating does not say RMS and CONTINUOUS then it might just be a peak instantaneous useless rating. Frequently speakers are rated in Music Power ratings that is the peak power (double the RMS power rating) for a brief moment.

A cheap tiny speaker with a power rating of only 0.5W is a joke anyway. No bass, no highs and no loudness.

endolith likes this.
8. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
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If you think speaker's printed watt ratings are anything besides pure BS, I have a bridge I'll sell you.

endolith likes this.
9. ### MrChips Moderator

Oct 2, 2009
12,624
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OK, let's not go there. You have to have something to go by. I would not install a 1W speaker if I'm looking to pump out 100W. I play bass and my speaker is rated at 400W and I believe it!

At 400W, I don't care if this is average music, RMS or peak power.

Last edited: Feb 6, 2012
10. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,787
973
How much? I'm in the market for a bridge.
So in your opinion is the wattage rating for a resistor BS/useless too?

11. ### Audioguru New Member

Dec 20, 2007
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Cheap speakers have fake power ratings. High quality speakers have detailed and true power ratings.

Look at the datasheet for resistors. They are made to get extremely hot at their rated power when they are not enclosed but have open air all around for convection airflow.
Even a short length of a resistor's lead is spec'd as part of the power rating so it can transfer some of its heat to the copper on the pcb.

You should not operate a resistor at its rated power where it can char a circuit board, melt a nearby plastic capacitor or burn you badly. Use half its power rating when it is in free air and use much less than its power rating when it is enclosed.

12. ### holnis Member

Nov 25, 2011
50
4
BTW, Most car audio speakers (with the exception of some subwoofers) are rated in peak power (peak power = 2*RMS power) or music power. Only a few speakers (generally the higher quality speakers) are rated in RMS watts. While peak power is a legitimate way to rate speakers (as long as the manufacturer tells you that the power rating is in peak watts), it can be deceptive.

13. ### Audioguru New Member

Dec 20, 2007
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Peak Power is also called Maximum Power which is deceptive.
But cheap amplifier ratings are also deceptive or worse so that nothing blows up.

endolith likes this.
14. ### PaulEE Member

Dec 23, 2011
423
32
I actually do have a bridge that I legitimately could sell someone. It is in northwestern Pennsylvania, built by PennDOT in 1993. It is on property that a friend just bought and he wants it gone.

As far as the power ratings, what I said is what it is. Whether the company that makes the speaker is lying is another story.

I over-spec every critical component in a circuit by 10 to 20% to alleviate issues with liars. There are a lot of them.

15. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
The biggest lie about speaker power ratings is that the speaker can be blown to bits at power levels probably 1/20 of the rating, even if the rating is "honest". The ratings typically used (for "true RMS" rated speakers) are the RMS power rating where the voice coil burns out.

FIRST: the speaker has to be installed in a proper enclosure to get anywhere near rated power.... or the cone will be blown right out of the frame. Hence my point above.

SECOND: most speakers can easily be destroyed by "bottoming" using low bass notes, so the speaker can easily be blown up at a fairly low power level.

THIRD: many speaker manufacturers are not honest about the power ratings or test methods used to get it, or the info is simply omitted

Hence my point that the rating stamped on the speaker is essentially meaningless. It certainly is not something that one can read as a "real spec" which can be used to address a design margin derating.

Last edited: Feb 7, 2012
16. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,787
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agree to disagree
Sure there are many aspects to properly designing anything.. But just because something bad can happen even when 1 specific spec is still being met doesn't mean that spec is pointless though..
That's like saying the drain current rating (Id) on a mosfet is useless just because it can cook well before that single rating is met. Just means you need to look at the total picture.

17. ### Audioguru New Member

Dec 20, 2007
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Many speakers have their power rated when they are fed continuously with pink noise that randomly produces all frequencies.

Here is part of the datasheet for a ScanSpeak 8" woofer. It is very high quality and it is not cheap. They recommend the size of its enclosure and size of its vent.
Its power rating is with 100 hours of pink noise:

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19. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,787
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Its all already said right there.. Just like anything that's knocked off/copied,etc.. The ratings printed on the "fakes" are typically just as "fake" as the product itself.. But if you purchase the "real thing" you can usually count on there being some "sound" (no pun intended) reasoning behind the printed/stated ratings.
Just don't claim all power ratings are BS when they aren't

20. ### c0de3 Active Member

May 1, 2009
50
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As people have said these numbers have limited use. I think on the surface it is supposed to be *claiming* that this speaker has 8 Ohms of Impedance. And can *handle* 1/2 watt of power. As everyone is pointing out, that is probably debatable.

Do you have another question? Perhaps some detail about what you are trying to accomplish that perhaps someone can help with?

As far as calculations, I'm not a EE so take with a grain of salt, the formulas you'd need to consider are:
Power = Current * Volts
Ohms = Volts / Current