Boxes · Types
TYPESEALED REARPORTED FRONTBandpass 4
How it works#
Behind the cone, the sealed rear chamber is a spring, as in a sealed box. In front, the cone drives the air in the front chamber, and that chamber’s port is the only way out: the chamber and the air in its port resonate at the front tuning, the middle of the band. Below the band the output falls as a sealed box’s does. Above it the air in the port is too heavy to follow, so the box rolls off the top of the band on its own.
SPL
dB
- Bandpass exampleBand 38.9–92.2 Hz · Peak 112.7 dB at 59 Hz
- Passband (−3 dB)
- 38.9–92.2 Hz, 1.2 octaves
- Loudest
- 112.7 dB at 59 Hz, from 100 W
- Chambers
- 70 L sealed rear, 50 L front tuned to 59 Hz
- Front port
- One 15 cm port, 21.3 cm long
The SPL tile marks both edges of a bandpass box’s band, F3L and F3H, measured against its loudest one-octave average as F3 is for any box. Reading a response explains the reference.
What you set#
Choose Bandpass 4 as the Type when you add an enclosure, or switch an existing one: the enclosure panel lists the same type as 4th-order BP. The panel then shows one chamber at a time, picked with the Front and Rear tabs beside Chamber.
| Control | What it sets |
|---|---|
| Front › Loading | Port, or Passive radiator to replace the front port with a radiator, as in a passive radiator box. |
| Front volume | The net air in the front chamber. With the port, it sets how wide and how loud the band is. |
| Tuning | The front chamber’s tuning: where the band sits. |
| Port length | The front port’s length. Change the tuning and the length follows, or type a length and the tuning follows. See Ports and end correction. |
| Port | The front port’s Shape, Number of ports, Diameter and, under Advanced, its End correction. |
| Rear › Loading | Always Sealed in a fourth-order box. |
| Rear volume | The sealed air behind the cone. |
| Results | The front port’s Cross-sectional area and Port volume. On the rear tab, the Gross volume once Box volume adjustments add to it. |
With a driver chosen, the Add enclosure dialog fills in a starting point for Bandpass 4: a sealed rear chamber sized as a sealed box for that driver would be, and a front chamber tuned to the resonance that rear chamber gives the driver. For the example’s B&C 18SW115 that is 32.7 L sealed and 26.3 L in front tuned to 80.1 Hz, which covers 49.3–137 Hz at 100 W. The example trades that for a bigger box and a lower band.
The front chamber sets the band#
The front tuning places the band. Tune it lower and the whole band moves down with it, a little quieter.
SPL
dB
- Front tuned to 50 HzBand 31.8–80.7 Hz · Peak 110.3 dB
- Front tuned to 59 HzBand 38.9–92.2 Hz · Peak 112.7 dB
- Front tuned to 68 HzBand 46.5–103 Hz · Peak 115.0 dB
The front volume sets how wide the band is, and width costs level. Shrinking the front chamber from 80 L to 30 L widens the band from 1.0 to 1.7 octaves, and its loudest octave falls from 114.0 dB to 108.8 dB.
SPL
dB
- 30 L frontBand 33.2–111 Hz · Level 108.8 dB
- 50 L frontBand 38.9–92.2 Hz · Level 111.9 dB
- 80 L frontBand 42.2–83.9 Hz · Level 114.0 dB
The rear chamber holds the cone#
The sealed rear chamber hardly moves the band: from 40 L to 120 L its lower edge moves by only 1.1 Hz. It sets how the bass under the band falls, and how far the cone travels there. A bigger chamber is a softer spring: the output reaches −10 dB lower, at 25.2 Hz instead of 30.3 Hz, and the cone moves further for it.
Excursion
mm
- 40 L rearBand 39.9–90.8 Hz · −10 dB at 30.3 Hz · Cone 3.3 mm
- 70 L rearBand 38.9–92.2 Hz · −10 dB at 27.2 Hz · Cone 4.6 mm
- 120 L rearBand 39–92.5 Hz · −10 dB at 25.2 Hz · Cone 6.1 mm
What to check#
- Port Velocity: the front port carries all the output. At 100 W the example’s port peaks at 11.2 m/s near 48.7 Hz, under the 17 m/s warning line. Air speed grows with the square root of power, and a narrower port speeds it up.
- Excursion: below the band the rear chamber keeps holding the cone, so its travel levels off instead of climbing. The example’s cone peaks at 4.6 mm at 10 Hz, 33% of the driver’s 14.0 mm Xmax.
- Port length: the front port has to fit inside the front chamber. The example’s is 21.3 cm long; a lower tuning or a wider port makes it longer.
When to use it#
Good for#
- Loud bass over about an octave, from a driver hidden inside the box. The example covers 38.9–92.2 Hz.
- A cone held in check below the band: the sealed rear chamber works as a sealed box’s air spring.
- Rolling off the top of the band in the box itself.
Watch for#
- Size for the band you get: the example is 120 L of chambers for 1.2 octaves.
- Bandwidth costs level: a smaller front chamber widens the band and makes it quieter.
- One port carries everything, so check its air speed at the power you will use.
- To set the bottom of the band with a tuning of its own, port the rear chamber too: Bandpass 6.
Try it#
The bandpass example at 100 W in the simulator’s own tiles: the front tuning, then the front volume.
To keep going, open the example in the simulator. It opens as a fourth-order bandpass with its tiles set up.
Ask your AI#
Ask your AI
Using 00 Simulator, for each fourth-order bandpass box in my open workspace, try the front chamber tuned 10 Hz lower and 10 Hz higher, and a front chamber a third smaller, without saving. Tell me the −3 dB band, the peak level and the front port’s peak air speed for each option at its saved power.
Connect your assistant so it can open your designs.