# Band energies

> Split audio into log-spaced bands and follow each one's energy as a control lane: the audio-to-visual bridge.

Source: https://docs.tune.toys/nodes/ctl.bands/

Band energies splits audio into log-spaced bands and follows each band as a control lane. It is the audio-to-visual bridge.

## What it does

Each output lane is the level of one frequency band. Bands are spaced evenly on a logarithmic scale between Lowest band and Highest band, which are the centres of the first and last band. Attack and Release shape how fast each band rises and falls, and Gain boosts the audio before the followers so quiet sources show clearly.

## When to use it

Use it to make controls dance to music: one lane per band, ready to cable into a scale or smooth node. For a single overall level use [Envelope follower](https://docs.tune.toys/nodes/ctl.follow/).

## Ports

<!-- ports:start -->

| Port      | Direction | Signal  | Lanes                   |
| --------- | --------- | ------- | ----------------------- |
| `audio`   | Input     | Audio   | Inferred from the cable |
| `control` | Output    | Control | Inferred from the cable |

<!-- ports:end -->

## Parameters

<!-- parameters:start -->

| Parameter    | Label        | Range         | Default | Unit | Scale  | Notes                               |
| ------------ | ------------ | ------------- | ------- | ---- | ------ | ----------------------------------- |
| `attack_ms`  | Attack       | 0 to 2000     | 15      | ms   | linear | How fast each band's level rises.   |
| `release_ms` | Release      | 0 to 5000     | 250     | ms   | linear | How fast each band's level falls.   |
| `low_hz`     | Lowest band  | 20 to 2000    | 60      | Hz   | log    | Centre of the first band.           |
| `high_hz`    | Highest band | 1000 to 20000 | 12000   | Hz   | log    | Centre of the last band.            |
| `gain_db`    | Gain         | -24 to 48     | 12      | dB   | linear | Boost applied before the followers. |

<!-- parameters:end -->

### Attack (`attack_ms`) and Release (`release_ms`)

How fast each band's level rises (0 to 2000 ms) and falls (0 to 5000 ms).

### Lowest band (`low_hz`) and Highest band (`high_hz`)

The centres of the first and last bands, from 20 to 2000 Hz and from 1000 to 20000 Hz, both on logarithmic controls.

### Gain (`gain_db`)

Boost from -24 to 48 dB, applied before the followers.

## Tips

- Narrow the span between Lowest band and Highest band to look closely at part of the spectrum.
- Use [Smooth](https://docs.tune.toys/nodes/ctl.smooth/) afterwards if lanes flicker too much.

## See also

[Envelope follower](https://docs.tune.toys/nodes/ctl.follow/), [Scale](https://docs.tune.toys/nodes/ctl.scale/), [Select lanes](https://docs.tune.toys/nodes/ctl.select/), [Cables](https://docs.tune.toys/concepts/cables/).