Most machine displays need no audio at all. When you do need a chime or voice message, three routes work: the screen's audio-out jack into self-powered speakers, an HDMI audio extractor at the source, or an interface board with its own amplifier. Below the power the worked example clears, self-powered speakers are the simpler choice; the example shows the arithmetic.
Most machine screens need no sound at all. When one does, a chime an operator hears without looking, or spoken instructions, three routes carry audio out of an HMI or shop-floor display: the screen's audio-out jack into self-powered speakers, an HDMI audio extractor set at the source, or an interface board with its own amplifier driving passive speakers. Below the power the worked example clears, the self-powered route is the simpler build; the extractor works only when the source actually sends audio on the cable; the board covers the rest. This article compares the three, works the loudness arithmetic for a real case, and ends with the decision for each setup.
Does the screen need audio at all
Most shop-floor screens do their job in silence. A status board read at a glance, a process display watched from a desk, and a machine HMI driven by eye and touch need no speaker, and a stack light often carries an alarm message further than any speaker in the same noise. Audio earns its place in three cases: the operator works away from the screen and must hear a fault or andon chime, the message is spoken guidance such as changeover steps, or the screen doubles as a notice board where sound pulls attention. That choice sits one step earlier: whether the screen is a factory status display or operator HMI decides how much audio the job needs.
Set one number before ordering anything: the sound must stand about 10 dB above the noise at the operator's ear, the margin standard alarm design guidance uses so a signal is noticed, not just present. Production floors commonly average 85 dB(A) over a shift, the level where shop floor hearing protection under OSHA 1910.95 requires employers to run a hearing conservation program. That floor puts the target near 95 dB(A) at the ear, and the target decides the route and the power.
The three routes compared
| Route | What you add | Where it wins | Limits to respect |
|---|---|---|---|
| Audio-out jack into self-powered speakers | A powered speaker set and one 3.5 mm or RCA cable | One or two listening positions, total power within the range powered speakers cover, a mains outlet at each speaker | The jack is unbalanced line level: keep the run under about 25 ft and set gain at both ends |
| HDMI audio extractor | A small extractor, its power supply, analog cable to the speakers | The screen has no audio jack, but the source sends sound on the HDMI cable | The source must output stereo PCM; passive HDMI is dependable to about 15 ft, so put the extractor at the source end |
| Audio interface board with its own amplifier | A stereo amplifier board, speaker cable, passive speakers | The source has no audio hardware at all, one board feeds several speaker positions, or the job needs more power than powered speakers cover | You wire the trigger from spare PLC outputs or serial and mount the board in the cabinet |
The power line the example works out below is the practical split. Self-powered speakers carry a matched amplifier inside each cabinet, so there is no separate amplifier to buy, power and gain-stage; below that power the self-powered path is the simpler one to wire. A board with its own amplifier earns its place once the power, the number of speaker positions, or the absence of any audio source pushes past what powered speakers cover.
Where each route wins
Self-powered speakers off the audio-out jack
Many screens, players and panel PCs carry a 3.5 mm audio-out jack, and that jack is all the drive a powered speaker needs. Powered PA cabinets accept a line-level feed on XLR or 1/4-inch combo inputs with their own gain control; desktop powered speakers take the same feed on an aux input. Set the source volume around 70 to 80 percent, trim the speaker gain until the chime sits clear of the noise, and leave both alone. Consumer line level sits near 0.3 V RMS, printed as -10 dBV on spec sheets, and professional gear runs near 1.2 V, printed as +4 dBu. Feed line or aux inputs; a microphone input expects millivolts and will clip. Keep the unbalanced jack-to-speaker cable under about 25 ft.
An HDMI audio extractor at the source end
An extractor splits the audio track out of the HDMI stream, hands it to a pair of analog jacks or an optical output, and passes the video on to the screen. It is the right tool when the screen has no audio path but the source, a player or a headless HMI with an HDMI output, sends sound on the cable. Two conditions decide success. The source must actually output audio over HDMI: a source configured video-only, or set to a multichannel format a basic extractor cannot downmix, produces silence, so set the source to stereo PCM. And cable length matters: the HDMI specification sets no maximum length, but passive cables are dependable to about 15 ft (5 m) in practice, so mount the extractor near the source and run analog line level to the speakers for anything longer. Port direction trips this plan up often enough that it has its own article: why an HDMI input on signage players rarely works.
An audio interface board with its own amplifier
When the display and the controller both lack audio hardware, a dedicated audio interface board closes the gap. The board stores clips, takes a trigger from spare PLC outputs or a serial command, and plays through its own amplifier; stereo boards of the class in the example drive passive speakers directly. This route wins in three cases: no audio exists anywhere in the chain, one board must feed two or more speaker positions, or the loudness sum in the example below needs more than powered speakers cover. It is also the most wiring: the board mounts in the cabinet, the trigger lands on outputs, and every speaker gets a cable run.
Example: sizing a chime speaker for an operator 10 ft away
Inputs. Ambient noise at the ear: 85 dB(A), the 8-hour average where OSHA 1910.95 applies, a common production-floor level. Distance from speaker to ear: 10 ft, about 3.0 m, the case of a board mounted above a workstation. Speaker sensitivity: 90 dB SPL at 1 W and 1 m, a common spec-sheet figure for compact PA cabinets; use the printed value for your own cabinet. Amplifier power: 50 W, the stereo class these interface boards carry. Target: about 10 dB above ambient, from alarm design guidance.
Rule. For a speaker heard directly, every doubling of power adds 3 dB and every doubling of distance removes 6 dB. SPL at the ear equals sensitivity + 10 x log10(power in W) - 20 x log10(distance in m).
Arithmetic. Power term: 10 x log10(50) = 10 x 1.70 = 17 dB. Distance term: 20 x log10(3.0) = 20 x 0.477 = 9.5 dB. Level at the ear: 90 + 17 - 9.5 = 97.5 dB. Margin over the floor: 97.5 - 85 = 12.5 dB, clearing the 10 dB target.
The same cabinet on smaller amps. At 5 W: 10 x log10(5) = 7, so 90 + 7 - 9.5 = 87.5 dB, only 2.5 dB above the floor, too soft to notice. At 1 W: 90 - 9.5 = 80.5 dB, below the floor. At 10 W: 90 + 10 - 9.5 = 90.5 dB, 5.5 dB above, still short. At 30 W: 10 x log10(30) = 14.8, so 90 + 14.8 - 9.5 = 95.3 dB, 10.3 dB above, the first rung of the ladder that clears the target.
Decision and thresholds. At 10 ft against an 85 dB(A) floor, a 90 dB SPL cabinet needs roughly 30 W or more, which is powered-PA territory and exactly the class these interface boards occupy. Distance moves the result by 6 dB per doubling, so every extra doubling costs about four times the power; a cabinet one step more sensitive cuts the power needed in half. A watt-class speaker built into a display fails this case by a wide margin.
Wiring numbers to check before you order
- A source for the sound: an audio-out jack on the screen, or HDMI with audio configured and set to stereo PCM.
- Level and input class: consumer line level near 0.3 V RMS into line or aux inputs; mic inputs clip.
- Distances: unbalanced audio runs under about 25 ft; passive HDMI under about 15 ft, with the extractor at the source end for longer paths.
- Power per position: work the SPL sum of the example for each listening spot, at its own distance and ambient.
- A trigger path if you take the board route: spare discrete outputs or a serial port on the controller.
The decision
- Screen has an audio-out jack and one or two listening positions with outlets: self-powered speakers off that jack. No extractor, no board. This covers every job within the power the example clears.
- Sound exists only inside the HDMI run and the screen side has no audio path: extractor at the source end, source set to stereo PCM, analog cable from there to the speakers, HDMI segment kept within its dependable 15 ft.
- No audio hardware anywhere in the chain, three or more speaker positions, or a loudness sum beyond what powered speakers cover: an audio interface board with its own amplifier into passive speakers, triggered from spare outputs.
- Ambient at the ear is 90 dB(A) or more: audio alone cannot carry a message an operator can rely on at the powers in this article, so pair the chime with a stack light and treat hearing protection as the first subject. The sum behind that line: standing 10 dB above 90 dB(A) needs 100 dB at the ear, and at 10 ft with the 90 dB cabinet that takes 10^((100 - 90 + 9.5) / 10) = 10^1.95, about 89 W.
If the sound belongs to a separate status screen rather than the machine HMI, the wiring is identical: our panel-mount signage displays list audio in and out among their ports, so a powered speaker set connects to the audio-out with no extractor in between. For the screen side of that build, see planning an offline sensor status display for your shop floor.
Recommendation: For one or two listening positions and a chime that must clear an 85 dB(A) floor at 10 ft, run self-powered speakers from the screen's audio-out jack, sized by the example at roughly 30 W at the ear and up. Take the HDMI extractor when the audio lives only on the HDMI cable, placed at the source end with the source set to stereo PCM. Take the interface board when the chain has no audio hardware, when three or more speakers are needed, or when ambient passes 90 dB(A), and there pair the sound with a stack light.
Questions
How does a PLC trigger sound if it has no audio hardware?
Give the controller a trigger path and let a dedicated audio interface board hold the audio. The board stores clips, plays one when a spare discrete output turns on or a serial command arrives, and drives passive speakers from its own amplifier, a stereo stage in the class the example works through. The PLC needs only spare outputs or a serial port, never audio hardware. Check the board's trigger list, clip storage format and amplifier rating against your speaker count before ordering.
Why does my HDMI audio extractor output silence?
An extractor can only split what the source puts on the cable. The two usual causes are a source muted or configured to send no audio over HDMI, and a multichannel bitstream a basic extractor cannot downmix to its analog pair. Set the source to stereo PCM, raise the source volume, and confirm the extractor's supported formats cover what the source sends. If it stays silent with stereo PCM set, take the feed from the screen's audio-out jack instead.
How far can I run the audio and speaker cables?
Unbalanced 3.5 mm or RCA line level stays dependable to about 15 to 25 ft. Balanced XLR line level runs hundreds of feet, which is the fix for long paths on a floor. Speaker-level cable at the powers in this article on ordinary 16 AWG wire is fine to about 50 ft. Passive HDMI is dependable to about 15 ft, so for longer distances place an extractor or a second player near the screen instead of extending the cable.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.