Tone-Triggered Offers for Out-of-Home and Transit Audio

Out-of-home media reaches people while they are moving: waiting for a train, riding a bus, walking past a digital screen. The challenge is the same one every billboard has faced. People see the message, but they rarely stop to type in a web address or scan a code. Where an out-of-home placement already plays audio, an inaudible tone can give that moment a direct path to the phone. An opted-in app recognizes the tone and saves the offer for later, so nobody has to stop walking or look down at the wrong moment.

How it works

A short, inaudible tone is mixed into the audio that already plays at the placement. When it plays, a phone running an app with tone detection, and with the user's microphone permission, recognizes the tone and takes an action the brand chose ahead of time:

• Save the offer to the app's wallet or offers tab.
• Match the content to the placement, such as a station-specific message or a route-specific deal.
• Log the exposure so the campaign can count opted-in people who were near the placement.

The tone is only one part of the system. It needs an audio source at the placement, an app the audience already uses, and a plan for what happens next.

Where tones can fit

• Digital screens with audio in stations, concourses, and waiting areas, where a spot already plays with sound.
• In-vehicle media on buses, trains, or shuttles that carry sponsored audio or video segments.
• Sponsored station or route content, such as a branded segment between service information.
• Event and seasonal takeovers, where a brand sponsors a station, line, or shuttle for a limited time.

Not every placement is a fit. Many out-of-home screens are silent by design or by rule, and a tone needs an audio channel to travel on.

Safety and service information come first

Transit and public spaces have a job to do before they carry any advertising. A tone program should be designed around that:

• Never mask or delay service and safety announcements. Branded audio, with or without a tone, should never compete with them.
• Do not prompt people near platform edges, curbs, crosswalks, or doors. Save the offer quietly and show it later, when the person opens the app.
• Skip alerts and pop-ups. No buzzing phones in a crowded car or on a stairway.
• Respect drivers and operators. Nothing in the experience should distract the person running the vehicle.

Media owners and transit agencies set their own rules for what can play over their systems, and local noise rules can apply outdoors. Get written approval from the property owner and review the plan with the brand's legal team before anything goes live.

Test on site, not in the studio

Out-of-home audio is a hard environment. Small speakers, echoing concourses, road and rail noise, wind, and changing crowd sizes all affect how sound reaches a phone. The playback system itself, including the media player, any audio processing, and the speaker, can change the signal as well.

That is why every pilot starts with an on-site test:

• Map the audio chain from the final file to the speaker at each placement type.
• Test at real times of day, including rush hour, with a range of phones held in hands, pockets, and bags.
• Set expectations by placement. A quiet waiting area and a busy platform will not behave the same way.

We share expected results only after that test, never before.

Consent comes first

A phone has to listen to recognize a tone, so the app needs microphone permission, and iOS and Android both ask the user before granting it. For out-of-home campaigns, that usually means partnering with an app the audience already uses, such as a transit app, a brand's loyalty app, or a partner app with tone detection built in.

• Explain the value before the system prompt, such as offers along your commute.
• Make it optional, so the app works fully for people who say no.
• Disclose tone use plainly in the app's privacy notice.

Our consent and permission UX checklist for tone apps walks through the permission flow, and our comparison of inaudible tones vs NFC beacons can help you decide which trigger fits each placement.

Measure what matters

Plan measurement before the first placement goes live. Track how many people have the partner app, how many granted microphone permission, how many tones were detected at each placement, and how many saved or redeemed the offer. Break results down by station, route, time of day, and creative version. Our guide to measuring opt-in rates for tone-triggered campaigns lays out the full funnel.

Frequently asked questions

Can riders hear the tone?

GhostTones tones are designed to be inaudible to most people, so they can sit inside existing placement audio without changing what people hear.

Does every passer-by get the offer?

No. Only people with an app that includes tone detection, who have granted microphone permission, can receive a tone-triggered offer.

Will a tone work on a noisy train platform?

It depends on the site. Speakers, echo, and background noise all affect detection, so every pilot starts with an on-site test at real times of day.

Will tones interfere with transit announcements?

They should not be allowed to. A well-designed program never masks or delays service and safety announcements, and it follows the media owner's and agency's rules for their audio systems.