What does the rating on speaker mean?

Audioguru

Joined Dec 20, 2007
11,249
I have seen some "fake" car speakers. They are 3-way so they have a woofer, a midrange and a tweeter. When you look closely you will see that the tweeter is just a photo of a tweeter.

My latest set of 2.1 speakers has a sub-woofer and two satellite speakers each with two 2.5" midrange speakers and a "soft-dome tweeter". A close look reveals that the dome tweeters are actually vents with a fabric dome cover.
This powered speaker system was advertised at 150W, the instructions say 75W RMS and the power transformer is stamped 9VAC/1.1A which is 9.9W. The amplifiers get hot so each satellite speaker gets 1.5W to 2W and the sub-woofer gets 4.0W to 5.0W.
I don't care because the system sounds great, looks great and was a bargain price.

I think over-the-ears headphones have tiny speakers rated at 0.5W.
 

Thread Starter

yaantey

Joined Oct 7, 2011
48
So anyway, I want to know what rating of speaker I need to connect to an amplification circuit that can output 5V maximum. More specifically 0V to 5V. The voltage of a 0.5W and 8ohm speaker is 2V, so i guess that means the speaker will be blown?
 

MrChips

Joined Oct 2, 2009
23,078
Actually, no. You are ok with a 0.5W speaker.

Here is the simple calculation. Power = V x V / R.
If we had said V = 5, this would give 5 x 5 / 8 = 3W.
But 5V is the peak-to-peak voltage. The amplitude is 2.5V.
The rms value = 2.5 x 0.7 = 1.75V

Hence the Power = 1.75 x 1.75 / 8 = 0.38W

You're safe.
 

Audioguru

Joined Dec 20, 2007
11,249
You forgot what I said.
"Cheap speakers have fake power ratings."
A "0.5W" speaker is extremely cheap.
Cheap is cheep, cluck cluck, cluck.
 
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