Speaker Impedance & L-Pad Calculator

Two connected jobs: first calculate the nominal impedance of identical drivers in series, parallel or equal series-parallel branches. Then size an ideal constant-resistance L-pad to attenuate that combined load without intentionally changing the nominal input impedance.

Wiring math. A series string is R x drivers; equal parallel branches are branch resistance / branch count. For attenuation A dB, voltage ratio a = 10^(-A/20), series resistor Rs = R(1-a), and shunt resistor Rp = Ra/(1-a).

Derivation. These expressions follow Ohm's and Kirchhoff's laws: choosing the parallel branch so R || Rp = Ra, then adding Rs = R(1-a), makes the ideal input equal R. The circuit identities follow the treatment in Horowitz and Hill, The Art of Electronics, 3rd ed., sections on series/parallel resistance and attenuators.

Worked check. Four nominal 8-ohm drivers arranged as two series drivers in each of two parallel branches return an 8-ohm nominal load. At 3 dB attenuation, the ideal L-pad is about 2.34 ohm series and 19.39 ohm shunt.

The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.

Driver wiring

Series-parallel

Nominal impedance per driver

8 ohm

Drivers in series per branch

2

Parallel branches

2

L-pad attenuation

3 dB

Power entering L-pad

50 W

Primary result: Equivalent nominal load: 8.00 ohm.

• Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
• Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
• When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.

Nominal speaker impedance is not a constant resistance. Real impedance varies strongly with frequency, so a fixed L-pad changes response differently across the band unless the actual driver/network is measured and modeled.

Displayed resistor heat is steady sine-wave math at the entered total power, not a resistor wattage recommendation. Enclosure temperature, music crest factor, crossover bandwidth, ventilation, resistor construction and safety margin must be handled by the designer using component documentation.