diff --git a/src/book/02-system/05-modules/03-input.mdx b/src/book/02-system/05-modules/03-input.mdx
index 0e328de4..66327d35 100644
--- a/src/book/02-system/05-modules/03-input.mdx
+++ b/src/book/02-system/05-modules/03-input.mdx
@@ -248,3 +248,24 @@ The resulting filtered data will then be outputted as `message::input::Sensors`
 - `message::input::ButtonMiddleDown` when button pressed.
 - `message::input::ButtonMiddleUp` when button released.
 - `message::eye::DataPoint` if log_level >= DEBUG, indicating if feet are down.
+
+## Whistle Detector
+
+### Description
+
+Reads from a microphone using ALSA, passes it through a Cooley-Tukey FFT, into a band-pass filter to filter out frequencies that aren't whistles.
+After a certain number of consecutive detections above a threshold, it is assumed a whistle event has triggered; it emits a `message::input::Whistle` message.
+The system also has a cool-down between allowed Whistle events, as well as a timer between module configuration and the first allowed Whistle event - this is because NUsense's beep likes to trigger this module.
+
+### Usage
+
+When installed, it expects a USB microphone connected to the NUgus. Not having this microphone is not fatal, however.
+This device is connected to ALSA as `plughw:0,0`. To find ALSA's list of inputs on the system, run `aplay -l` (this depends on having `alsa-utils`)
+
+### Consumes
+
+- Nothing
+
+### Emits
+
+- `messages::input::Whistle` with it's confidence of the whistle event and timestamp.
diff --git a/src/book/02-system/05-modules/09-purpose.mdx b/src/book/02-system/05-modules/09-purpose.mdx
index e4804be5..27d5657b 100644
--- a/src/book/02-system/05-modules/09-purpose.mdx
+++ b/src/book/02-system/05-modules/09-purpose.mdx
@@ -428,6 +428,7 @@ Control flow:
 - Middle button (`ButtonMiddleDown`) disables idle mode after `disable_idle_delay`
 - On penalisation (`GameEvents::Penalisation` for self), purpose tasks are cancelled and localisation/servo state is reset
 - On unpenalisation, purpose and fall-recovery tasks are restarted (unless idling)
+- If in `GameState::Phase::SET` state and we hear a `Whistle` event, move to `GameState::Phase::PLAYING`.
 
 ### Consumes
 
@@ -440,6 +441,7 @@ Control flow:
 - `message::input::ButtonLeftUp`
 - `message::input::ButtonMiddleDown`
 - `message::input::ButtonMiddleUp`
+- `message::input::Whistle`
 
 ### Emits