Audio Management — the AMU and the ACP

Article 1 set the division of labour: the RMP chooses the frequency, the ACP handles transmit/receive. Article 2 covered the frequency half; this one covers the other — the audio integrating system (AIS). It has two parts: the AMU (Audio Management Unit) is the switchboard hidden in the avionics bay; the ACP (Audio Control Panel) is the keyed panel in front of the crew. Press "VHF 1 transmit" or "HF 1 receive" on an ACP and the AMU routes microphone, headset, sidetone and PTT to the right station behind the scenes.

This article makes clear a mechanism the crew use every day but rarely have explained: you can select only one transmitter at a time, but several receivers at once; plus a few life-saving and mistake-proofing designs — AUDIO SWITCHING (the third channel when a side fails), the spring-loaded PA key, the INT/RAD-versus-sidestick priority, and the VOICE filter.

1. The AMU — the switchboard behind the panel

The AMU is the heart of audio integrating. Per AMM 23-51-00:

The Audio Management Unit (AMU) ensures the interface between the user (jack panel, ACP, loudspeakers) and the various radio communication and radio navigation systems.

Its function list contains one item the crew must know. Per AMM 23-51-00:

The AMU ensures the following functions: Transmission, Reception, SELCAL and display of ground crew and Cabin Attendant calls, Flight interphone, Emergency function for the Captain and First Officer stations, Cockpit aural indicating. It also supplies signals to the CVR to record communications.

Three points: the AMU manages reception and transmission independently for the captain, first officer and third occupant (they do not interfere); it carries an emergency function for the captain and first officer stations, so basic transmit/receive survives even a degraded AMU; and it feeds three audio channels (captain, first officer, third occupant) to the CVR — the source of the recorder's four-track capture (article 10). Physically the AMU is a 4-MCU box in the avionics bay.

The full system, per FCOM DSC-23-10-30, is an AMU plus three or four ACPs, station jacks (headset, boomset, hand microphone), three cockpit oxygen-mask microphones, a radio PTT on each sidestick, one SELCAL code panel, two cockpit loudspeakers, and an audio-switching facility.

2. Transmission — one transmitter at a time

The most basic, most taken-for-granted mechanism of the ACP is that transmission is mutually exclusive. Per AMM 23-51-00:

An electronic device inhibits the simultaneous selection of several transmitters. Thus, you cannot select several transmission pushbutton switches at the same time. When a new transmission function is selected, the light of the selected pushbutton switch comes on. At the same time, the previously selected pushbutton switch is disabled and its three green bars go off.

At any instant you talk to exactly one station. Press VHF 1 transmit, then HF 1, and VHF 1 releases automatically — the microphone and PTT always route to the last-selected station. This is an enforced design against inadvertently occupying two frequencies. SATCOM is the one exception: it has a HOLD function, so during a SATCOM call you can press another station (e.g. VHF) to speak, and the SATCOM stays connected with its green bars flashing once per second; select SATCOM again to resume.

The PA (passenger address) key is worth remembering. Per AMM 23-51-00:

The rectangular PA pushbutton switch is unstable and you must held it during the complete transmission time. This avoids unwanted transmission on the PA circuit.

[!warning]- The PA key must be held; the other transmit keys latch VHF/HF transmit keys latch (press once to select; PTT or INT/RAD then actually keys the transmitter). The PA key alone is spring-loaded and must be held throughout — release it and the announcement stops. The design guards against leaving the whole cabin under an open PA. If a cabin announcement cuts off when you release, that is normal, not a fault.

3. Reception — several at once

Reception is the opposite — parallel. Per AMM 23-51-00:

You can select several reception pushbutton switches simultaneously. When you release the pushbutton switch, it engages in its initial position and the receiver is disconnected.

Push a round reception knob out to reveal a white skirt and connect that station; rotate to set volume; keep VHF 1, VHF 2 and a VOR all live together. This is how the cockpit monitors ATC while listening to ATIS while hearing a nav-aid identifier. One practical detail, per FCOM DSC-23-10-50:

Note: For the reception of DME audio navigation signals associated with an ILS station, the LS pb on the FCU must also be selected.

One line: transmission is exclusive (one), reception is parallel (many).

4. INT/RAD and the sidestick PTT — which wins

The boomset and oxygen-mask microphones have no built-in PTT; they key through the INT/RAD selector or the sidestick radio PTT. The selector has three positions: centre (receive only), RAD (unstable — keys the selected transmitter, i.e. radio PTT), and INT (stable — direct flight interphone). The sidestick radio PTT equals the RAD position. When the two conflict, per AMM 23-51-00:

If conflicting orders are given by the side-stick switch (RAD position), and the corresponding ACP switch (INT position), the radio function prioritizes the interphone function.

[!warning]- Remember: RADIO wins over INT Press the sidestick PTT (RAD) and the system obeys it even if the ACP is set to INT — it transmits on the radio, not the interphone. The intent is that at a critical moment (e.g. a go-around, thumbing the sidestick PTT to call ATC) the transmission is never blocked by an interphone selection.

5. Call indication — MECH / ATT / SELCAL

Three "someone is calling you" signals flash amber under a transmit key, and on every ACP. Per AMM 23-51-00:

During a ground crew, cabin attendant or SELCAL call, a CALL amber legend flashes under the green lines of the related transmission pushbutton switch, on all ACPs. MECH: INT pushbutton switch (ground crew call), ATT: CAB pushbutton switch (cabin attendant call).

MECH (under INT) is the ground engineer calling (via the external power panel, article 6); ATT (under CAB) is the cabin crew calling (article 7); SELCAL (under a VHF/HF transmit key) is a ground station calling with the aircraft's SELCAL code (article 4), with a cockpit buzzer. Press RESET or the relevant transmit key to clear. There is an automatic reset. Per AMM 23-51-00:

The MECH and ATT legends that flash in amber go off automatically. This occurs after 60 seconds of operation if the call is not cancelled by the RESET pushbutton switch.

6. VOICE filter and loudspeaker volume

The VOICE filter key, per FCOM DSC-23-10-50:

VOICE filter key allows the flight crew to inhibit the most commonly used audio navigation signals (VOR, ADF).

VOR/ADF stations transmit continuous Morse identifiers. When monitoring those stations while trying to hear speech clearly, pressing VOICE filters out the identifier tones and leaves the voice — hence the name.

The loudspeaker volume knob carries an easily-misread limit. Per FCOM DSC-23-10-50:

Note: This knob does not control the loudness of aural alert and voice message. In the case of acoustic feedback (i.e. Larsen effect) from the cockpit loudspeaker, the flight crew should reduce the volume of the cockpit loudspeaker.

[!warning]- Turning the loudspeaker down does not silence the warnings Crews sometimes assume turning the loudspeaker to OFF quiets the cockpit. It does not — ECAM aural warnings and synthetic voice ("RETARD", "SPEED") do not route through this volume knob and will still sound (a safety design; they may not be turned down). The knob governs only radio-communication loudness. In feedback (a hand microphone or oxygen-mask microphone too close to the loudspeaker producing positive feedback), reduce this knob to suppress it — but not so far that communications become inaudible.

7. AUDIO SWITCHING — the third channel when a side fails

If one side's ACP or AMU channel fails, the captain/first officer have a fallback — borrowing the third occupant's audio channel. Per FCOM DSC-23-10-30:

If audio channel 1 or 2 fails due to a failure either in an ACP or the corresponding AMU, the crew can use the AUDIO SWITCHING selector to select the third audio channel.

The three positions and the cost, per FCOM DSC-23-10-50:

The crew can switch to the third audio channel if ACP1 or ACP2 fails. When the crew does this, it takes away the third occupant's access to the acoustic equipment. NORM : Each crew member uses his dedicated communication equipment. CAPT ON 3: The pilot uses his acoustic equipment and the third occupant's ACP. F/O ON 3 : The copilot uses his acoustic equipment and the third occupant's ACP.

[!warning]- AUDIO SWITCHING is the audio twin of the ADIRS "CAPT ON 3" This selector's logic mirrors the navigation chapter's AIR DATA / ATT HDG switches exactly: your side has failed, borrow the third set's resource, at the cost of the third occupant losing access. Remember "CAPT ON 3 = the captain borrows the third channel" alongside ATA-34's "CAPT ON 3 = use ADIRU 3" (ATA-34-01). Article 12 uses it — but note the contrast there with a stuck PTT, where AUDIO SWITCHING must not be used.

Self-test

[!note]- Q1. Which three users does the AMU manage independently, and which three audio channels does it feed the CVR? Captain, first officer and third occupant, managed independently. It feeds the captain, first-officer and third-occupant audio to the CVR — the source of its four-track capture.

[!note]- Q2. Why can you select only one transmitter but several receivers on the ACP? What is the SATCOM HOLD function? An electronic device inhibits simultaneous transmitter selection; reception knobs can be selected together. HOLD keeps a SATCOM call connected (green bars flashing) while you use another station.

[!note]- Q3. How does the PA key differ from a VHF transmit key, and why? The PA key is spring-loaded and must be held throughout the announcement; the others latch. It guards against leaving an open PA.

[!note]- Q4. Which three amber call legends appear, on how many ACPs, and how long before they self-cancel? MECH (ground crew), ATT (cabin crew), SELCAL — flashing on all ACPs. MECH/ATT self-cancel after 60 s if not reset.

[!note]- Q5. Sidestick PTT at RAD, ACP at INT — does it transmit radio or interphone, and why? Radio: the radio function has priority over interphone, so a critical-moment transmission is never blocked.

[!note]- Q6. Can turning the loudspeaker to minimum quiet ECAM warnings? ACP 1 fails — what does the captain do, at whose cost? No — aural warnings/voice do not route through that knob. ACP 1 failed → AUDIO SWITCHING CAPT ON 3, at the cost of the third occupant's access.

Key takeaways

References

• FCOM DSC-23-10-30 — audio management system contents; AUDIO SWITCHING trigger.
• FCOM DSC-23-10-50 — ACP face, INT/RAD, VOICE filter, loudspeaker Larsen note, DME/LS note, AUDIO SWITCHING positions.
• AMM 23-51-00 — AMU role and functions (captain/F-O emergency, CVR feed), transmit exclusivity, reception parallelism, spring-loaded PA key, MECH/ATT 60 s self-cancel, RADIO-over-INT priority.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.