Resistor value

[silverhairbp]

Greetings,

I'm using two 8 ohm drivers in parallel (4 ohms) giving an output of a little over 91dB. What values of resistors do I need to reduce the output by 1dB, 2dB, 3 dB?

Is there a formula I can apply to help narrow down the values as I start on the crossover?

Thanks in advance for your help,

Bill

 

[jan.didden]

Bill, calculate or look up the conversion of a dB value to a fraction, in this case -dB value (attenuation).

I just happen to know that -3dB is ~0.7. So you want a series R that takes 30% of the output voltage while 70% is across the 4 ohms load.
Simple algebra: Rseries/4ohms = 0.3/07 gives Rseries = (0.3/0.7)*4= 1.75 ohms. Use a standard 1.8 ohms.

Jan

 

[nigelwright7557]

Or put speakers in series.

 

[silverhairbp]

Thank you. Enlightening.

 

[jan.didden]

I think I gave you some wrong info: -3dB is 0.7 times the level, but for power -3dB actually is 0.5 times I think.
The concept is the same but the numbers will be different.

For power the equation for the number of dBs is: 10*log(P1/P2). (For level it would be 20*log(L1/L2).

Jan

 

[Galu]

Put 4 ohm and the required dB attenuation in this calculator:

L pad calculator - attenuation dB damping impedance decibel loudspeaker speaker voltage divider - sengpielaudio Sengpiel Berlin

It will give the values for an L pad circuit or for a single resistor.