# 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
-----------

```