Page 1 of Speaker Impedence Question

Ok , heres the deal - many years ago when I was at school , it seems that I was actually in the snooker hall when I should have been in the physics lessons. Enough of my mis-spent youth , but as a result I know little or nothing of electrical impedence (if that is even the right term). I have a set of speakers which I want to attach to my ProLogic TV. The manual for the TV states that the speakers should be 8 ohms , but the speakers appear to be 4 ohms. What are the possible consequences should I attach them - will I fry the speakers , blow the amp in the TV or just get duff sound ?? Advice welcome please.

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"back to the top" umm suspisious posting !!

Anyway - I think !! .... A 4 Ohms speaker will draw more current as it offers less resistance. This in turn will demand more power from your amp. My guess is that your TV will drive these speakers due to the tolerance on modern Amps - However you have been warned - Y not just pick up some nice BiPols from Richers or the like .....

GSM is not correct about A 4 Ohms.

Ohms is the resistance of electrical current.

There is an advantage that comes from using less current to make the same amount of power. The resistance in electrical wires consumes power.
Higher Ohms figures means more power to drive the same size of speakers.

Lower Ohms speakers should therefore sound louder for same power than higher Ohms speakers.

At the risk of appearing to be confrontational towards jush - no!

GSM is nearer the truth.

What is impedance? It is an attempt to reduce a complex interaction between resistance, inductance and capacitance to a figure (expressed in ohms) which may be used to derive an idea of the load presented (in this case) to an amplifier.

Point 1. The impedance figure which manufacturers quote is (whether they say so or not) nominal. That means it is likely to vary quite widely (or even wildly) across the frequency spectrum. So your 4 ohm speakers will, at different frequencies, go from 1 ohm to 10 ohms (or whatever). This is absolutely unavoidable due to the way in which speakers are designed and built.

Point 2. An amplifier acts as a "constant voltage source". This means that for any given setting of the volume control, it will produce (or try to) the same voltage across its speaker outputs. A true constant voltage source has an impedance of zero ohms. Obviously, that can`t be achieved practically, but most amplifiers come close (perhaps, 0.1 ohms). So, let`s say that the amplifier is producing 8V across an "8 ohm" speaker - it`s sinking 1amp (from Ohm`s law V=IR, or R=V/I, or I=V/R) and therefore drawing 8W of power (P=VI). What happens when you use a "4 ohm" speaker instead? It sinks 2 amps and draws 16W. So, GSM is right, the lower the impedance, the higher the current. But only for any given volume setting.

Point 3. Even with all that in mind, you cannot say how loud a speaker will sound. If you look at manufacturer`s speaker specifications, they quote a "sensitivity" figure. This tells you how much power is required to produce a given sound level.

Point 4. How loud is loud? A 3dB increase in loudness is noticeable. 10dB is a doubling in perceived loudness.

Point 5. I must stop sounding like a geek.

Point 6. Unless you wind your volume control right up, you are very unlikely to cause any damage to your TV`s amplifier. I don`t know of a single modern design which doesn`t feature overload protection anyway.

Point 7. I need beer.

This item was edited on Friday, 31st May 2002, 17:34

Cheers Neon -

A word to "Jush" - I have studied and been in the electronics industry for over 7 years - I also started my reply with "I think" meaning I believe the following to be true. Next time you accuse someone of being point blank WRONG make sure YOU have your facts straight.

Thanx guys - I think you have answered my question.
Now , enough of the physics lessons - I`m off , back down the snooker hall