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RF reflections and impedance matching on speaker wires

post #1 of 4
7/15/09 at 9:48pm
Thread Starter 
 So I'm getting my speakers in for my theater setup tomorrow, and before I start prepping to cut my speaker wires to length there was something I was wondering about but noticed was never mentioned.

For anyone who's studied RF of any sort you've encountered wire reflections and impedance matching. For those of you who haven't basically it says that any place where impedance changes on a wire carrying any type of RF wave (for instance at the point where the wire connects to the speaker), if the effective impedances do not match, a reflection can occur reducing the signal magnitude by up to and possibly more than 50% while at the same time causing echoes on the line as well.

Now, it's been a while since I've dealt with this so I don't completely remember how to do it, but I would think that something causing such a large change in signal quality would be rather important, or am I missing something completely? Do speakers have some sort of impedance matching mechanism built in? Or is it just something that requires too much hard math for the average person to care about?

Also, if it is something to consider, does anyone have a short-hand way of calculating it? Back-of-the-envelope style?
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post #2 of 4
7/15/09 at 11:58pm
As far as impedance matching in audio systems goes it's true from a strictly theoretical view that power in a circuit is maximized when impedances are matched.  However this is not practical in loudspeaker-power amplifier situations.  It is actually more important to focus on efficiency rather than maximum power transfer.  A lot of power amplifiers are designed around the fact that they will in fact not have matched loads connected to them.

Also the problem with echo and reflections mainly only affects high frequencies.  In the range of audio if the connection is short (most in-home setups) the reflection would occur so near to the time the signal left the source that it may not even be measured and definitely not heard.  This is however very different with video (shorter wavelength compared to the distance traveled) which is why cables have been designed to maintain the 75 ohm impedance.

Most audio equipment output impedances are made very low in comparison to their input impedance to create high efficiency and reduce power lost to heat, ect.

However there are still hardcore believers that insist on impedance matching and I believe they even made cables that are specialized to have impedance matching or impedance correction.

http://www.monstercable.com/productdisplay.asp?pin=18
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post #3 of 4
7/16/09 at 2:52am
Welcome to the forum, Mike.

Because we're dealing with very low impedances (< 20 Ohms), and hardly any RF waves (even filtered out by the amp's circuits, the problem does not exist in any practical form.

Those unbelievably expensive Monster cables aren't necessary at all. Really not at all!!

To connect spoeakers it suffices to use twin copper wire of the 'power wire' type. 14 gauge is generally enough, if your lengths are > 10', you may consider slightly thicker wire (2 millimeters).

Good luck with your speakers!


Cees
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post #4 of 4
7/16/09 at 8:28am
Thread Starter 
Thanks Cees :) 

And thanks for the replies guys! 

Honestly I'm glad I don't have to deal with this as I hated studying RF signals way back when, I just remember being tortured by impedance matching problems in my seep...
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