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Old 07-07-2007, 05:37 PM   #1
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Question about home speakers in bus.

What is the difference between 8ohm home speakers and 4ohm car speakers. I have some PolkAudio bookshelf speakers that I would like to use in my bus powered by an automotive CD/receiver. I know they work, but what's with the different ohm rating. Is it okay to use them?
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Old 07-08-2007, 02:33 PM   #2
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Re: Question about home speakers in bus.

The Ohm rating is the impedance of the speakers. Higher ohm speakers are less taxing on the amplifier while lower impedance gets you more power from the same amplifier, but at the expense of some heat, instability, and a little more distortion (though not noticeable by the human ear if you remain at an approved impedance).

You can always put speakers with more impedance on a receiver as the general rule. Your 8 ohm speakers will actually keep the amplifier chip in the head unit happy, but you might run into a small problem. 52x4 or whatever your head unit is rated at is peak output. The root mean square (continuous for the sake of simplicity) output is actually only about 10-15 watts per channel at 4 ohms. If you load it at 8 ohms you're going to get half that power. This may or may not work for you. You'll know right away if you turn it up and the speakers get muddy sounding.

As a side note...most head units can handle reasonably sized speaker pairs at 2 ohms. I did this for a long time with my car to run a set of component front speakers I assembled and it was fine. With my beefed up amplifier chip in the head unit and appropriate wiring I was getting 22 watts per channel into 2 ohms so the individual speakers were driven with 11 watts continuous power or about the same as a stock single speaker.
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Old 07-08-2007, 05:00 PM   #3
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Re: Question about home speakers in bus.

The Experience said it well. The only thing I would add is you may want to consider adding an amp. The sound will be "fuller". Try hooking your speakers up to your receiver and see how they sound. If it's okay with the bus running don't worry about adding an amp.
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Old 07-12-2007, 10:15 PM   #4
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Re: Question about home speakers in bus.

tell me if I understand this. The impedance is how much resistance the power unit has to overcome to make sound?
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Old 07-12-2007, 11:57 PM   #5
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Re: Question about home speakers in bus.

Impedance is kind of like resistance, but for AC circuits. It's frequency dependant. I've studied electronics, but I went over to Wikapedia--I couldn't understand their explanation. So I am going to say impedance is the resistance to the flow of current in AC circuits, which includes audio, radio, and what the power company sells.
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Old 07-13-2007, 02:50 AM   #6
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Re: Question about home speakers in bus.

Yes. That's a good way to look at it. In electronics class teachers like to use a garden hose illustration but here's my anology which probably isn't completely accurate:
Let's say you go for a drive (the bus is the amplifier- the power output- and the terrain is your speaker). You start out on level ground at a comfortable speed (your 8 ohm pace). Here comes a hill- now you're going at 16 ohms- the bus is starting to work a little bit so to keep your energy the same you have to slow down (you're at a lower volume). You come down the hill- now you're at 4 ohms- you speed up (higher volume) because it's less work. Now it's getting steeper- 2 ohms- you're feeling a little out of control because you're starting to go faster than you're designed for (more volume). Uh-oh, here comes a cliff (1 ohm) - you can't handle it because you have flown over the side and are about to self-destruct at the bottom... It's a good thing you're in a skoolie because we all know that school buses are the safest vehicles to drive in...

So, what you should really do is get another pair of nice PolkAudio speakers and wire them in parallel to get your 4 ohm load and then everybody will be happier! Like I always tell my wife, "You can never have too many speakers!"
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Old 07-13-2007, 11:18 PM   #7
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Re: Question about home speakers in bus.

I LOVE the hilly bus driving analogy!

As I recall a speaker with a 4 ohm impedance will show about 3.7 ohms of resistance on a DC multimeter.

It's good to know I'm not alone in not fully understanding that AC junk. Anyone else have to use a ton of calculus and other garbage to determine phaser angles and the like? The only plus side to it all is that I kind of sort of not really understand the difference between a volt-amp and a volt*amp (watt).
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