# Bandpass 4

> How a fourth-order bandpass box works in 00 Simulator: a sealed rear chamber, a ported front chamber, and what each one changes.

Source: https://simulator.00aud.io/docs/boxes/bandpass-4

A fourth-order bandpass puts the driver in the wall between two chambers: a sealed one behind the cone and a ported one in front. Only the front port reaches the room, so the box plays one band of bass and rolls off both sides of it. This page shows what each chamber does, on the app’s bandpass example.

_Figure: The bandpass example drawn by the simulator’s schematic: the driver between a sealed rear chamber and a front chamber with one port to the room._

The bandpass example as the **Schematic** tile draws it: 70 L sealed behind the cone, 50 L in front tuned to 59 Hz.

The driver sits between a sealed rear chamber and a front chamber that radiates through a port. Only the chamber outlets reach the room.

## 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.

_Figure: The bandpass example's SPL at 100 W, with the level 3 dB under its loudest octave marked._

B&C 18SW115 in the bandpass example (70 L sealed rear, 50 L front), at 100 W, half space, 1 m. Simulated when this page was built.

| Design | Band | Peak |
| --- | --- | --- |
| Bandpass example | 38.9–92.2 Hz | 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](https://simulator.00aud.io/docs/start/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](https://simulator.00aud.io/docs/boxes/ports#tuning-and-length). |
| **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.

_Figure: The bandpass example's SPL with its front chamber tuned to 50 Hz, 59 Hz, 68 Hz: the passband moves with the tuning._

B&C 18SW115 in the bandpass example (70 L sealed rear, 50 L front), at 100 W, half space, 1 m. Simulated when this page was built.

| Design | Band | Peak |
| --- | --- | --- |
| Front tuned to 50 Hz | 31.8–80.7 Hz | 110.3 dB |
| Front tuned to 59 Hz | 38.9–92.2 Hz | 112.7 dB |
| Front tuned to 68 Hz | 46.5–103 Hz | 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.

_Figure: The bandpass example's SPL with a 30 L, 50 L, 80 L front chamber, all tuned to 59 Hz: the smaller the chamber, the wider and quieter the band._

B&C 18SW115 in the bandpass example (70 L sealed rear, 50 L front), at 100 W, half space, 1 m. Simulated when this page was built. Level is the loudest one-octave average.

| Design | Band | Level |
| --- | --- | --- |
| 30 L front | 33.2–111 Hz | 108.8 dB |
| 50 L front | 38.9–92.2 Hz | 111.9 dB |
| 80 L front | 42.2–83.9 Hz | 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.

_Figure: Cone excursion of the bandpass example with a 40 L, 70 L, 120 L sealed rear chamber, against the driver's 14.0 mm Xmax._

B&C 18SW115 in the bandpass example (70 L sealed rear, 50 L front), at 100 W, half space, 1 m. Simulated when this page was built.

| Design | Band | −10 dB at | Cone |
| --- | --- | --- | --- |
| 40 L rear | 39.9–90.8 Hz | 30.3 Hz | 3.3 mm |
| 70 L rear | 38.9–92.2 Hz | 27.2 Hz | 4.6 mm |
| 120 L rear | 39–92.5 Hz | 25.2 Hz | 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.

- [Port Velocity](https://simulator.00aud.io/docs/reference/tiles/port-velocity): The front port’s air speed against the warning line.
- [Excursion](https://simulator.00aud.io/docs/reference/tiles/excursion): The cone’s travel against Xmax.

## 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](https://simulator.00aud.io/docs/boxes/bandpass-6).

## Try it

_Interactive walkthrough in the page: the simulator's own tiles running a short demo._

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.

- [Open the bandpass example](https://simulator.00aud.io/app?template=bandpass&power=100): 70 L sealed rear and 50 L front at 59 Hz, 100 W.

## Ask your AI

**Ask your AI**

Your assistant runs the same solver on your own designs through the 00 Simulator connector.

> 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.