It's usually 8 ohms and between 0.25W --- 1W. There is no voltage spec for speakers.

Lower impedance (4ohm or lower) -> not recommended as it'll draw more current
Higher impedance (16ohm or higher) -> safe but lower volume

Voltage is always 5V in case of the PC speaker. That's a perfect fit for 8 ohms at 0.25W.

True, 5v used to be the usual voltage for TTL levels. Some current mainboards (for i7, Ryzen, etc) may use 3.3v now, but that's another story (they come with piezos, anyway).

Sorry for necromancing this old thread of mine but it seems I asked this question in the past, so another (follow up) question would not be completely out of line..

I have found some 57mm speakers that are 4R amd 0.5W - would these be too out of spec to use as PC Speakers? I'm thinking current draw would be too high?

For example: 0.5 W, 8 Ohm.

I think it's possible to add a resistor here (in series), to prevent a short/a too high draw.
I'd start with ~47 Ohms, to be on the safe side (it's one of those standard values).
In theory, an ~4 Ohm resistor may do, but I haven't tried that.

Edit: The PC Speaker circuit is essentially an output pin of the PC system timer, so it makes sense to be cautious.
It's not a real sound output (AF, AC) in the common sense, but a 5v DC signal (digital TTL signal).

appiah4 wrote on 2023-08-14, 07:49:

Sorry for necromancing this old thread of mine but it seems I asked this question in the past, so another (follow up) question would not be completely out of line..

I have found some 57mm speakers that are 4R amd 0.5W - would these be too out of spec to use as PC Speakers? I'm thinking current draw would be too high?

From my personal experience... well... you can theoretically use any speaker as a PC speaker as long as its impedance and output are appropriate.

Recently I installed a relatively large, bass-capable speaker of comparable size to yours, rated 4R/3W, inside a system of mine as PC speaker. It works, the volume feels adequate, and sounds better than a conventional piezo.

Just that with such a large speaker you need to look for a good place inside the case to put it and keep it in place somehow, so that it won't accidentally fall somewhere and hit delicate components.

Still, you probably need programs/games that utilize PC speaker in a somewhat complex way for a better comparison, as under normal circumstances (simple beeps), it may not sound too different.

OK, so if I am getting this right,

P = I x V and V = I x R, therefore substituting V / R for I we get:

P = V^2 / R

That means for any given speaker, the Voltage it will draw is:

V = SQRT ( P x R )

That means for a 4R 0.5W Speaker we will get:

V = SQRT ( 0.5 x 4) = 1.4V

Going back to the original formula:

P = I x V therefore 0.5W = I x 1.4, and I = 0,36A

Is this a high current draw for a speaker?

Because if we do the same for a 8R 0.25W speaker:

V = SQRT ( 0.25 x 😎 = 1.4V

P = I x V therefore 0.25W = I x 1.4, and I = 0,18A

So basically double the current for double the power, which makes sense?

In that case an 8R 0.5W speaker should also work:

V = SQRT ( 0.5 x 😎 = 2V

P = I x V therefore 0.5W = I x 2, and I = 0,25A

Which is more or less half way and looks reasonable as well?

The W is a maximum rating. So how hard you can drive it before you risk damage. Resistance is the number you want to calculate power. The speaker will quite happily sink more than it's W rating (until it fails at least). Similarly it won't care if you push less than it's rating through it.
So P=V^2/R is right
But the V is the voltage supplied by the motherboard. It's a 5V square wave afaik. RMS V is actually what you want, which is 2.5V for a square wave.

So for a 4 ohm speaker you get P=V^2/R = 2.5^2/4 = 6.25/4 = 1.6W
For an 8 ohm speaker you get 6.25/8 = 0.8W

The 4 ohm speaker will draw I=V/R = 2.5/4 = 625mA
The 8 ohm speaker will draw 2.5/8 = 313mA

You can put a 4 ohm resistor in series with the 4 ohm speaker if you are concerned. Use a 0.5W resistor. That will reduce the power to about 0.4W in the speaker though so it may be quiet. 3.9 ohms is the closest standard value.

Ahh, makes sense. By that regard, an 8R 0.5W Speaker will be just fine 😀