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Posted by CMOS [Email] (more from CMOS) on Wed, 9 Nov 2005 14:34:57 Share Post by Email
In Reply to: Good post explaining the odd OHM # on factory system.., SaabSpeed, Wed, 9 Nov 2005 11:42:06
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There are a few other factors in determining the sound pressure level (SPL) coming from each speaker.

First, let's say that we replace a 2-ohm speaker with a 4-ohm speaker. The current will drop to half the original value. Since the voltage will stay the same, the amount of power will also be cut in half. For otherwise identical speakers, that will translate to a loss of 3 dB. Decibels are logarithmic; for every increase in 3 dB, the sound pressure doubles.

In the example above, we calculate that, at 12V, the HK speaker draws 57.6 watts of power. With a 4-ohm Boston Acoustics speaker, it would draw 36 watts of power. That's about a 2 decibel drop, off the top of my head.

I don't know about the HK speakers' sensitivity (or efficiency), but the Boston Acoustics S35 speakers are rated at 92dB. That means that with one watt of power, these speakers will produce 92dB of sound measured at one meter. If the HK speakers are rated at 90db sensitivity, the sound pressure level will be about the same as the Boston Acoustics. The increase of sensitivity in the BA speakers compensates for the greater impedance. If the HK speakers are rated lower, say 88 or 89 db, the BA speakers would actually be louder. You would need to install a resistor in-line with the speaker to "turn it down". Radio Shack sells (or used to sell) some power resistors in the 1/2 and 1 ohm range that can be used for this purpose.

Ultimately, the only way to really know would be to invest in a sound pressure level meter. You would use a white noise CD and set the volume at a reference level. Then you would connect one speaker at a time and measure it at a given point (maybe sitting upright on the armrest). After you have figured out the relative levels that the Saab and Harmon-Kardon engineers came up with (which I think sounds pretty good), you could then replace some or all speakers, adjusting them to match the reference level.

Or, you can just use what sounds good to you and what makes you happy.

Good luck!

-Chris

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