Choosing the Cabinet Height and Bass Reflex Port

Accuton C168-6-890 + Mundorf AMT25CS2.1-R

During the design process of my next loudspeaker project I revisited the cabinet proportions and the low-frequency alignment. The goal was to remain close to proven “Troels-style” dimensions while optimising driver placement, listening axis height and bass tuning.

Cabinet Height Considerations

An external cabinet height of 120 cm initially looked attractive from a visual standpoint, but with the tweeter placed close to the top (as is common in stepped-baffle designs), this results in the tweeter’s acoustic centre being too high relative to a seated listening position.

Given:

• tweeter mounted approx. 20 mm below the top

• AMT front plate height ≈ 130 mm

• listening ear height typically around 90–100 cm

a cabinet height of 105 cm (external) places the tweeter’s acoustic centre very close to ear level. This ensures:

• correct tonal balance on axis

• optimal imaging and soundstage stability

• less sensitivity to vertical listening position

Conclusion:
An external cabinet height of 105 cm is a much more appropriate choice than 120 cm for a seated listener.

Final Internal Cabinet Dimensions

Based on Troels-inspired proportions and 22 mm MDF construction:

• External height: 105 cm
  → Internal height: 101.6 cm

• External width: 22 cm
  → Internal width: 17.6 cm

• External depth: 280 mm (excluding front baffle)
  → Internal depth: 258 mm

Gross internal volume

V = 1.016 × 0.176 × 0.258 ≈ 0.046 m³ ≈ 46.1 litres

After subtracting:

• bracing

• driver displacement

• port volume

• damping material

the net usable volume is realistically around:

👉 42–44 litres

This volume is very well suited to a single Accuton C168-6-890 in a bass-reflex alignment.

Bass Reflex Port Design (Single Ø80 mm Port)

Instead of passive radiators, a single rear-firing bass reflex port was chosen for its simplicity, predictability and clean appearance.

Port specification

• Diameter: 80 mm (straight or lightly flared tube)

• Placement: rear panel

• End correction: approx. 1.46 × radius
  (realistic assumption for one internal free end and one external termination)

Calculated port lengths

Assuming 42–44 litres net internal volume, the following physical port lengths apply:

Net volume ≈ 43 litres

• Fb ≈ 42 Hz → 13.9 cm

• Fb ≈ 40 Hz → 15.9 cm

• Fb ≈ 38 Hz → 18.3 cm

Net volume ≈ 42 litres

• Fb ≈ 42 Hz → 14.4 cm

• Fb ≈ 40 Hz → 16.5 cm

• Fb ≈ 38 Hz → 18.9 cm

Recommended starting point

For a tight, musically balanced low end:

👉 Ø80 mm port, ~14–14.5 cm physical length (Fb ≈ 42 Hz)

Practical Build Advice

To account for real-world effects (flares, proximity to braces, cabinet stuffing):

1. Build the port 1–2 cm longer than calculated

2. Measure impedance using CLIO

3. Identify Fb as the dip between the two impedance peaks

4. Trim the port length in 5–10 mm steps until the target tuning is reached

This method guarantees an accurate final alignment regardless of small construction variables.

Summary

• 105 cm external cabinet height is acoustically preferable to 120 cm

• Internal volume of ~42–44 litres is ideal for a single Accuton C168-6-890

• A single Ø80 mm rear bass reflex port is fully sufficient

• Target tuning around 42 Hz offers the best balance of depth, control and punch

• Final tuning should always be confirmed by impedance measurement

This approach combines proven proportions with precise, measurement-based optimisation — a solid foundation for a high-performance DIY loudspeaker.