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What is Baffle-Step?
11/02
Baffle step, AKA:.Baffle
diffraction, AKA: Diffraction
spreading. In most cases, it’s
effects on frequency response
require some sort of compensation
to achieve a flat FR in the intended
listening environment.
Picture a speaker on a stand, in the center of a room. At
frequencies where the wavelength is much longer than the distance
from the center of the driver to the nearest baffle edge, the sound
travels around the edges of the baffle and fills the room evenly with
sound. The physics guys call this a 4pi environment. I’ll call it
a sphere of sound radiating equally in all directions from the
speaker.

At frequencies where the wavelength is shorter than the distance
from the center of the driver to the shortest edge of the baffle, the
baffle tends to block the sound from radiating around the sides and
rear of the speaker, and redirect it forward, like the reflector on a
flashlight. Picture the sphere cut in half. Since the same amount of
sound is concentrated in only one half of the area. (A 2pi
environment) there is a ‘gain’ of 6db.
Wait a minute! First I called it a loss of 6db, now I called it a gain. Well, it’s all relative! The speaker
manufacturers measure their products on a very large baffle, so when they tell you its rated SPL is
90db/1w/1m, it is essentially measured in a 2pi environment. Most of us listen to our speakers in a 4pi
environment; therefore we perceive the phenomena as a loss at low frequencies, not as a gain at higher
frequencies.
Now if you like the sound of your speakers shoved up against the wall, the wall becomes an extension of the
baffle, and you don’t need to compensate for the loss. Get them out in the room a little, the wall still helps,
maybe you only need to compensate a little. Get them out 3-4 feet, however you probably need to compensate
for all 6db.

For those of you using generic or textbook crossovers, please also buy the generic baffle step compensation
circuit too. It’s PE part number… Oh, wait. They don’t have one yet!
To conclude: Unless you are planning to use a full 4 x 8 sheet of MDF for your front baffle, or build your
speakers into the wall, you cannot copy the FR graph you got off the manufacturer’s website and assume a
similar low frequency response in your design.
If you have measuring software, and you measure your speakers frequency response on the baffles you will be
using them on, the baffle step compensation (and other diffraction effects, as well) will be taken into account.

I have taken some liberties here and ignored or omitted certain other effects for brevity and clarity. Among them:
Edge diffraction effects can have a significant effect on the frequency response, but are very sensitive to baffle
size and speaker position on the baffle. This would be a good topic for another discussion…
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Working definition: (This is somewhat simplified for clarity, but is essentially correct.) Baffle step is exhibited as
a gradual loss of up to 6db of SPL at lower frequencies. On a 6-8� baffle, you would expect these
phenomena to start approximately around 1000 hz, down to about 100-200 hz. where the measured SPL would
be down 6 db.
Where did that 6db go? It’s hiding behind the speaker, of course. 8+)
I modeled a typical baffle response for a
woofer, on an 8� x 20� baffle.
Note that at frequencies below 100 Hz,
the gain is 0. By roughly 800 Hz, it has
risen to a peak of 8 dB, then back to 6 dB
by 2000 Hz and above. Not really a â
€˜step’ is it?
Click on the images for a larger picture.
Until they do, try this circuit: For compensation of baffle
widths of 6-8 inches. Use a 1.5 mh inductor paralleled
with an 8 ohm resistor for 8 ohm impedance systems.
(Use a 4 ohm resistor if you only need 3 db.) If you have
a 4 ohm system, (e.g. MTM configuration) use a 3 mh
paralleled with 4 ohms. This would go in circuit (in
series with the positive speaker lead) before the
crossover. While this circuit may not be perfect, it will
sound better than no compensation at all. I like to use a
variable resistor to ‘dial in’ the response, then
measure it and replace it with a fixed resistor. Why donâ
€™t you see this circuit in every crossover circuit? If
you use crossover design software, it can be included in
the crossover and won’t appear as a separate circuit
as I have described.
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