# Basic baffle diffraction network question

Discussion in 'Home Theater Projects' started by TimForman, Aug 7, 2003.

1. ### TimForman Supporting Actor

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I understand using an inductor to attentuate upper frequencies. I see the use of R1||L1 for baffle step compensation. Why is the resistor there and how to you calculate the effect on the circuit with this arrangement?

2. ### TimForman Supporting Actor

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3. ### TimForman Supporting Actor

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I found half of the answer.

4. ### JohnSer Stunt Coordinator

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Tim,

One of these text book baffle-step correction circuits was used in a crossover that Madisound did for me (LEAP Design). It was a TM design. The coil and resister in || was in series with the rest of the woofer circuit. A resistor of the same value (in my case 4 ohms), was in series of the rest of the tweeter circuit. These circuits work OK if the TM design has a relatively flat and equal impedance.

Mine was designed with a relatively flat 8 ohm impedance. So for the tweeter circuit, the 4 ohm series resistor lowers the output by 3db. The 4 ohm resistor will lower the woofer output by 3db only for the higher frequecies that are blocked by the coil. There is a formula for calculating at what frequency it starts, but don't have it now. For mine it was 3mH, which started around 100Hz, if I remember right. You can easily use values from 1 to 3mH, and just pick which sounds best.

Clear as mud, right?

JohnS

5. ### Patrick Sun Studio Mogul

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Think of the inductor as a variable switch that is frequency-dependent. For high frequencies, the inductor acts like brick wall (open circuit condition). For low frequencies, the inductor will allow signal flow through it (short circuit condition). Depending on the value of the inductor, in some midrange region, the inductor will start to choke off the signal flow (for the higher frequencies) in a downward direction.

Then the signal is diverted to through the resistor for higher frequencies, which causes a downward shelving of the high frequency output.

This help in balancing out the output of the woofer (low frequencies will spread out all around the speaker in 4 pi space, while the midrange and higher frequencies will radiate in 2 pi space in front of the speaker's baffle).

So the baffle step correction lowers just the output of the woofer from around 250Hz to 500Hz, salted to taste in the design phase. Then you match the output of tweeter to to the woofer with the flattened baffle step correction in a typical 2-way design.

If you leave out the baffle step correction, and match the tweeter output to the midrange output of the woofer, the bass will sound thin overall.

Of course, you'll be losing some overall SPL output with baffle step correction, but the speaker will sound smoother.

Here's another webpage that discusses the topic:

http://www.t-linespeakers.org/tech/bafflestep/