Open-Baffle & Horn Tuning

A field coil is a tuning knob: excitation current sets Bl, which sets Qes and Qts. This plate matches that knob to the box: an asymmetric open baffle whose dipole cancellation wants a high Qts, or a horn throat that wants a low one.

Parameters

Driver & field motor

Core alloy

Target total Q

Q_{ts}^*

0.65–0.78 open baffle · 0.25–0.35 horn

Field current

I_{fc}

Now Qts = 0.681 · needs 0.1153 AA

Baffle width

W_b

Baffle height

H_b

Side-wing depth

D_{wing}

Folded U-frame wings lengthen the wrap pathcm

Horizontal offset

\Delta X

Off-centre mounting staggers edge diffractioncm

Vertical offset

\Delta Y

Fig. 15Asymmetric baffle, 82 × 115 cm with 22 cm wings. Dashed rays trace the four wrap-around paths; the figure-8 shows the resulting dipole radiation.

Baffle match

Optimal field current

I_{fc}^*

0.1153 A

Field voltage

V_{fc}^*

220 V

Active Qts

Q_{ts}

0.681

Dipole corner

f_{dipole}

142 Hz

System cutoff

f_{-3dB}

70.2 Hz

Path spread

d_{max}/d_{min}

2.39×

Target Bl

Bl^*

14.2 T·m

Suggested foropen baffle

☞ Field matched to baffle: Qts 0.681 ≈ target 0.68. Resonance lifts the dipole roll-off for bass to 70.2 Hz.

Combined driver + baffle response (20 Hz – 1 kHz)

active Qts 0.681target Qts 0.68

How it works

Theory · assumptions · formulas

Dipole cancellation

An open baffle radiates from the front and, out of phase, from the rear of the cone. Once the half-wavelength exceeds the wrap-around distance $D_{eff}$ , front and rear waves cancel and the bass falls at 6 dB/octave below

f_{dipole} = \frac{c}{2 D_{eff}}, \qquad D_{eff} = \frac{8}{1/d_L + 1/d_R + 1/d_T + 1/d_B}

The four paths run from the driver to each baffle edge (plus wing depth); the floor mirrors the bottom edge, stretching $d_B$ by 35 %. Their harmonic mean is the effective diameter.

Field Qts matching

Raising $Q_{ts}$ to 0.60–0.78 plants a resonant lift exactly on the dipole knee, cancelling the 6 dB/octave slope for an octave or more of flat bass. Since $Q_{es} \propto 1/Bl^2$ and $Bl = B_g\,\ell_{vc}$ , the target flux and current follow by inverting the Thiele–Small relations:

Q_{es}^* = \left(\frac{1}{Q_{ts}^*} - \frac{1}{Q_{ms}}\right)^{-1} \;\Longrightarrow\; B_g^* = \frac{1}{\ell_{vc}}\sqrt{\frac{2\pi f_s M_{ms} R_e}{Q_{es}^*}}

The plate then binary-searches the motor's saturating magnetic circuit (Plate V method) for the current that delivers $B_g^*$ . A horn throat wants the opposite: tight damping with $Q_{ts}^* \approx 0.25$ –0.35, i.e. more field current, so the driver follows the horn's acoustic load instead of ringing against it.

Asymmetric staggering

Harry Olson showed that a centred driver piles every edge-diffraction wave into one deep comb notch. Offsetting the driver ( $d_L \ne d_R \ne d_T \ne d_B$ ) spreads the arrivals across an octave (Gilbert Briggs' practice), and the floor reflection smooths the rest. Aim for a path spread above ~1.35.

Working currentsThe presets span 220 V cinema fields (≈ 0.11 A) to a 24 V constant-current Permendur motor (1.65 A). Active: 0.115 A × 1,910 Ω = 220 V.