What the baffle step is
At low frequencies a speaker on a small box radiates in every direction, front and back. As frequency rises, the wavelength gets short compared with the baffle (the front panel), and the sound goes mostly forward. A speaker playing forward-only is louder than the same speaker playing in all directions. John Murphy of True Audio puts the gap at 6 dB, with the bass ending up 6 dB below the treble for a speaker out in the open. That gradual shift is the “6 dB baffle step,” also called diffraction loss. Rod Elliott (ESP) adds two markers: the effect starts to show, about 1 dB, when the baffle is around 0.2 of a wavelength wide, and it stops mattering once the baffle is wider than about 3 wavelengths.
Where it happens: a rule of thumb from baffle width
Both True Audio and ESP use the same estimate for the middle of the step, the –3 dB point: f3 ≈ 115 ÷ baffle width in meters (or 380 ÷ width in feet) ESP’s worked example is a 300 mm baffle, which gives about 383 Hz. By the same formula, a 0.25 m baffle (about 10 inches) lands near 460 Hz. A narrower baffle puts the step higher up. A wider one puts it lower. It’s an estimate for a plain box, so treat it as a place to look, not a number to trust blindly.
Driver placement changes the ripples
The step is smooth in theory. Real boxes add ripples where the driver sits the same distance from several edges. True Audio and ESP both point to the same fix: don’t center the driver. ESP suggests making the distances from the driver to the edges different, for example with a ratio such as 1.414:1 or 1.618:1, and rounding the edges, which causes less refraction than square or raised ones.
How much to correct: less than 6 dB in a room
The 6 dB figure is for a speaker in free space. In a room it’s usually less. ESP says that in a normal room the step is unlikely to be greater than 3 dB unless the speakers sit well away from the walls. And with speakers close to a wall, other reinforcements and cancellations often matter more than the step does. Murphy’s own follow-up says 3 or 4 dB of correction may sound closer to flat in some rooms, and that room effects are a separate job from diffraction. So 6 dB is the ceiling, not the target. Start with a partial correction and let your measurements decide.
Three places to deal with it
- Active crossover or DSP. ESP’s shortcut: if the bass-to-midrange crossover sits near the step frequency, raise the bass level a little. His example is a 300 mm baffle with the crossover around 380 Hz, a near-perfect match. See DSP for DIY speakers: where to start.
- Line-level equalizer. ESP offers a simple adjustable circuit that sits between the preamp and the power amp, and advises against putting this kind of network in the speaker lines because of power loss.
- Passive crossover. Murphy notes that designers traditionally lower the tweeter level and adjust the rest of the crossover. He also describes a resistor–inductor network wired in series with the speaker that mirrors the loss. Either way, you’re cutting treble to match the bass, so the speaker plays quieter overall. Keep that in mind when you match the amp.
Measure it, then decide
Run a gated far-field sweep at listening height (REW basics) and look for a gentle rise through the step region. Compare it with the estimate from your baffle width. Don’t EQ from the formula alone. Your room, your wall distance and your driver all move the answer. If the sweep shows ripples near the box edges, revisit driver placement before you reach for a filter, and keep the box basics from enclosure mistakes that kill DIY speakers in mind.
Checklist
Which kind of inductor? See crossover inductors: air core vs iron core.