# Could someone explain the relationship between impedance and sensitivity ?

Discussion in 'Archived Threads 2001-2004' started by Samson, Nov 19, 2001.

1. ### Samson Stunt Coordinator

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I just bought a Bohlender Graebner Radia X1 center channel and its rated at 4 ohms - impedance and 93 db - sensitivity.
I thought that 4 ohm speakers are basically inefficient speakers but this speaker has a sensitivity of 93 db.
Most of the 4 ohm speakers (planar ex. Magneplanar, Newform Research ) that I've seen also have a pretty low sensitivity.

2. ### Jagan Seshadri Supporting Actor

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Sensitivity is the sound pressure level (SPL) in decibels measured 1 meter in front of the speaker when 1 Watt of signal is fed to it, typically a 1 kHz sine wave signal.
So that's the meaning of the 93dB figure.
As for the 4-Ohm impedance (which is a 'nominal' purely-resistive value, which actually isn't the case...) it means that to feed 1 Watt of signal power for the above test, you'll need more current. Efficiency (in terms of converting electrical energy to sound energy) has nothing to do with it; it's just a misuse of the word. I'd call it "higher current demand".
Using a constant, purely-resistive impedance measurement:
current = sqrt(Signal_Power / Nominal_Impedance)
4-OHM CASE
sqrt(1W / 4_Ohms) = 0.5 Amperes of current.
8-OHM CASE:
sqrt(1W / 8_Ohms) = 0.353 Amperes of current
So, to drive low-impedance loads you typically need a nice fat transformer (and fat heat-sinked transistors) in your amplifier or receiver to safely provide the kind of current to 4-ohm speakers. If this rating is not met, your receiver/amplifier will just run hot and you may saturate the power transformer resulting in higher total harmonic distortion, or you will smoke your output transistors.
There you have it, pedagogical fallacies and all!
Regards,
-Jagan