HOW TO UNDERSTAND DIFFERENT IMPEDANCE RATES?!!!

John Fields starship at freeside.fc.net
Thu Jun 19 08:54:13 EST 1997


John Woodgate (jmw at jmwa.demon.co.uk) wrote:
: In article <33b12876.20360271 at news.pacbell.net>, God <god at almighty.com>
: writes
: >The original fallacy:
: >
: >>Consider an amplifier with an 8 ohm output impedance feeding 8 volts into 
: >>a speaker with an 8 ohm impedance.  The speaker will be dissipating 8 
: >>watts of power and will generate a specific sound pressure level.
: >>
: >>Next, consider the same amplifier putting 8 volts into a 16 ohm load.
: >>Since P = E^2 / R, the power being pumped into the load is 8^2 / 16, or 4 
: >>watts, which will yield a lower sound pressure level.
: >>
: >>The assumption, of course, is that both speakers have the same efficiency 
: >>and are, in fact, identical except for their impedances.

Snipped some sniping and prettied up god's conclusions, below...
 
: >1) An amp driving 8 volts into 8 ohms will dissipate 8 W of power and
: >an SPL of  A.

: >2) An amp driving 8 volts into a 16 ohm load will dissipate 4W of
: >power and an SPL of  <A.

: >3) We assume that both "speakers" in 1) and 2) have the same
: >efficiency (and identical except for their impedances.)

: >4) Ergo, a 16 ohm speaker is less sensitive than an 8 ohm because at
: >the same WATTAGE, the former produces a lower SPL ** from 1) and 2)

1. Correct.

2. Correct.

3. Correct. Efficiency was used in my post in order to "level the playing 
   field" and was used in the sense that  E = Pin / Pout, where E = 
   efficiency, Pin = power into the system, and Pout = power out of the 
   system.  

4. Incorrect. If both speakers are dissipating the same amount of power 
   and are identical, they will both generate identical SPL's.
   If the only difference between them is their impedance, then the 
   speaker with the  higher impedance will need to be driven with a higher 
   _voltage_ in order to achieve the same SPL as the lower impedance speaker.
   
   In the example given, the 8 Ohm and 16 Ohm speakers were both driven 
   by the same voltage.  

   Therefore, because of their different impedances they were not each 
   dissipating the same amount of power as the other and could not possibly 
   have generated identical SPL's.

John Fields
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