# Ports and end correction

> Every port control in 00 Simulator: tuning and port length, shape, size and count, end correction, the lumped and distributed port models and port resonance.

Source: https://simulator.00aud.io/docs/boxes/ports

A port is a tube or slot whose air resonates with the box at the tuning frequency. You choose its shape, size and count; 00 Simulator works out the length that gives your tuning, or the tuning a length gives. This page covers every port control, what end correction does, and when the port starts to resonate on its own.

## Where you find ports

| Enclosure | Ports |
| --- | --- |
| [Vented](https://simulator.00aud.io/docs/boxes/vented) | One chamber and its port. |
| [Bandpass 4](https://simulator.00aud.io/docs/boxes/bandpass-4) | The front chamber’s port. |
| [Bandpass 6](https://simulator.00aud.io/docs/boxes/bandpass-6) | A port on each chamber, each with its own tuning, on the **Front** and **Rear** tabs. |
| [Horns](https://simulator.00aud.io/docs/horns) | The rear chamber of a [front-loaded](https://simulator.00aud.io/docs/horns/front-loaded-horn), [offset-driver](https://simulator.00aud.io/docs/horns/offset-driver-horn) or [stubbed](https://simulator.00aud.io/docs/horns/stubbed-horn) horn, set to **Port** instead of **Sealed**. |

The controls are the same everywhere a port appears, apart from **Port model**, below. The examples on this page use the vented example.

## Tuning and port length

**Tuning** and **Port length** sit under the chamber’s volume, and both are always editable. Whichever you changed last is held, and the other is worked out from it:

- Change **Tuning** (the usual way) and the length follows. A different port size or count, chamber volume or end correction then changes the length, never the tuning. The example’s 10 cm port is 35.6 cm long for 32 Hz.
- Type a **Port length** and it is held instead: the tuning follows, and changing the port’s size moves the tuning. Cut the example’s port to 30 cm and the box tunes to 34.4 Hz.

The length comes from the port’s effective length: the length of air that resonates with the chamber at the tuning, less the end corrections.

```
effective length = port area × c² / ((2π × tuning)² × volume)
port length      = effective length − (inside + outside) × r

c = 343 m/s; port area is all the ports together; r is one port’s radius
```

> **When the length comes out negative** A small port area with a high tuning can need less length than the end corrections already give. A warning then shows beside **Port length**, and the **Port** section opens with “Port volume is 0 L.” Use a wider port, more ports or a lower tuning.

## Size, count and shape

Every port below tunes the 55 L example to 32 Hz, so the SPL is the same for all of them. What changes is how long the port is, how much of the box it takes and how fast its air moves at 200 W.

Ciare 12.00SW in the 55 L vented example tuned to 32 Hz, at 200 W, half space, 1 m. Simulated when this page was built.

| Port | Length | Port volume | Peak air speed | 1st port resonance |
| --- | --- | --- | --- | --- |
| One 7.5 cm | 18.9 cm | 0.8 L | 50.5 m/s | 909 Hz |
| One 10 cm (the example) | 35.6 cm | 2.8 L | 28.4 m/s | 482 Hz |
| One 12.5 cm | 57.4 cm | 7 L | 18.2 m/s | 299 Hz |
| Two 10 cm | 77.1 cm | 12.1 L | 14.2 m/s | 222 Hz |
| One 20 cm × 3.9 cm slot | 35.6 cm | 2.8 L | 28.4 m/s | 482 Hz |

- More port area means slower air and a longer port. Two 10 cm ports take the example from 28.4 m/s to 14.2 m/s, and each is 77.1 cm long; a 7.5 cm port is short but far over the 17 m/s warning line, at 50.5 m/s.
- The port’s air is not part of **Volume**, the net air the driver works against. It shows as **Port volume** under **Results** and adds to the **Gross volume**, the box you build: 67.1 L with the two ports against 57.8 L with one.
- **Shape** has three buttons: a circle for a round port (**Diameter**), a square for a slot (**Width** and **Height**) and a dashed square for a custom **Cross-sectional area** per port. A slot’s end correction uses the radius of a round port with the same area, so the 20 cm × 3.9 cm slot is the same 35.6 cm long as the 10 cm round port.
- **Number of ports** runs from 1 to 10. **Diameter**, **Width** and **Height** take 1 to 100,000 mm, and **Tuning** 15 to 1,200 Hz.

## End correction

Air just beyond each end of a port moves with the air inside it, so the port acts longer than it is. End correction adds that length back: a factor for each end, times the port’s radius. The more correction, the shorter the port you build for the same tuning.

Set it under **Port** › **Advanced** › **End correction**. The menu offers WinISD’s presets, each shown with its average factor, or **Custom**, which opens **Inside** and **Outside** fields from 0 to 3. The default is 0.6 at both ends, as in WinISD.

The 10 cm port of the vented example, held at 32 Hz.

| Preset | Inside | Outside | Shown as | Example port length | 1st port resonance |
| --- | --- | --- | --- | --- | --- |
| WinISD default | 0.6 | 0.6 | 0.6 | 35.6 cm | 482 Hz |
| Two free ends | 0.614 | 0.614 | 0.614 | 35.4 cm | 484 Hz |
| One flanged end | 0.85 | 0.614 | 0.732 | 34.2 cm | 501 Hz |
| Two flanged ends | 0.85 | 0.85 | 0.85 | 33.1 cm | 519 Hz |

Because the tuning is held, the response does not move: only the length does, from 35.6 cm to 33.1 cm. A bandpass box’s two ports share one end correction: change it on either tab and both follow.

## Port model and port resonance

A port is also a pipe, open at both ends, with its own resonance where its length is half a wavelength. **Results** shows it as **1st port resonance**, worked out from the physical length: 482 Hz for the example’s 35.6 cm port. If it sits too low the port lets through sound above the bass and colours the response, so keep it well above where the box plays.

**Port model**, under **Advanced** on a vented box or a horn’s ported rear chamber, decides how the solver treats the port. **Lumped**, the default, moves the port’s air as one mass, so the resonance is only a number in **Results**. **Distributed** treats the port as a duct that sound travels along, so the resonance shows in the graphs, and adds a **Distributed duct resonance** row worked out from the length with its end corrections.

_Figure: The vented example with two 10 cm ports, each 77.1 cm long, with the lumped and the distributed port model. The distributed curve shows the ports' own resonance near 206 Hz._

Ciare 12.00SW in the 55 L vented example tuned to 32 Hz, at 200 W, half space, 1 m. Simulated when this page was built.

| Design | Resonance |
| --- | --- |
| Lumped | 222 Hz (1st port resonance) |
| Distributed | 206 Hz (distributed duct resonance) |

With two 10 cm ports, each 77.1 cm long, the distributed model puts a peak of 10.2 dB over the lumped curve at 210 Hz and a dip of 8.4 dB just above, at 215 Hz, where real ports this long resonate too.

- **Duct loss**, in Np/m, damps the duct. It starts at 0.02 and goes up to 2. At the maximum the ports barely work: the two-port example’s F3 rises from 30.2 Hz to 75.5 Hz.
- Bandpass ports have no **Port model** control.

## Air speed

Fast air chuffs and whistles. The **Port Velocity** tile draws the peak air speed in the port against a warning line, 17 m/s by default; a sixth-order bandpass has a second tile for its rear port. At 200 W the example’s one port peaks at 28.4 m/s.

- [Port Velocity](https://simulator.00aud.io/docs/reference/tiles/port-velocity): How to read the tile and what moves it.
- [Fix a noisy port](https://simulator.00aud.io/docs/guides/noisy-port): Get under the line, and what each fix costs.
- [Bass-reflex port calculator](https://simulator.00aud.io/calculators/bass-reflex-port): Port length for a tuning, outside the simulator.

## Try it

[Open the vented example](https://simulator.00aud.io/app?template=vented&power=200) at 200 W, open **Port** in the enclosure panel and change **Number of ports** to 2: the **Port length** grows from 35.6 cm to 77.1 cm, and the peak on the **Port Velocity** tile falls from 28.4 m/s to 14.2 m/s.

## 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 vented box in my open workspace, try one, two and three ports of the current diameter without saving. Tell me each option's port length, port volume, first port resonance and peak air speed at the saved power.