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why only 8ohms?
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why only 8ohms?

by whiskey on Sat Mar 20, 2010 2:57 pm

I notice that projects that require a speaker "usually" call for one rated 8ohms, there must be a reason I just can't figure it out :lol: any idea's?
Last edited by whiskey on Sat Mar 20, 2010 7:32 pm, edited 1 time in total.
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Re: why only 8ohms?

by adafruit on Sat Mar 20, 2010 3:39 pm

its kinda standard - speakers are often 4 or 8 ohms. headphones are smaller, and can be as little as 32 ohms.

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Re: why only 8ohms?

by johngineer on Sun Mar 21, 2010 8:54 pm

the short answer is because that's just the way it is...

the long answer is similar to the short answer but...

amplifiers perform at their highest efficiency when driving a known load to which they have been matched. 8 ohms has become the defacto standard for most speakers. this way, you can buy a amplifier/receiver from one company and use it to drive a speaker from another company, and they will be impedance-matched.

However, as adafruit mentioned 8 ohms is not the only "standard". Many car audio speakers are rated at 4 ohms, and some are rated at 16 ohms but are designed to be driven in parallel. If all you're trying to do is make something beep, you can use a 4 ohm or 32 ohm or whatever-ohm speaker in a pinch and it will still beep, provided the rest of the circuit is working properly :)

edit: i just realized i didn't answer your question. d'oh!

second try: it's "only" 8 ohms because speakers need current to operate. The lower the resistance, the more current you can drive through it at a given voltage. If they were rated at, say, 600 ohms and you needed to put 500mA (not uncommon for speakers) through the coils, you'd need an output voltage of 300V. Not difficult to do if you have an output transformer, but pretty much unnecessary, and potentially dangerous too. Besides, a speaker is basically just an inductor and has a low DC resistance anyway. They just decided to standardize it on 8 ohms for the sake of consistency.
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