Get D4

For iPhone and iPad · iOS 27

Your iPhone speaker, finally heard.

D4 sits between your apps and the speaker. Music, podcasts and video sound fuller, clearer and closer. No headphones, no account, and your audio never leaves the phone.

See how it works

One-time purchase. Works with every app that plays sound.

D4 Home screen: the ON circle over a green-teal night sky while a podcast plays, mode Voice, with a Turn off D4 button

Three things we did differently

It works with every app.

Spotify, YouTube, Apple Music, Netflix, podcasts, voice notes: D4 enhances whatever is playing. Nothing to import, nothing to re-buy.

It listens before it acts.

D4 hears whether it's a voice, a song or a film and picks the right mode on its own. Pick a mode yourself and it stays.

Nothing leaves the phone.

All processing happens on the device. No account, no servers, no analytics, no microphone. The privacy policy is one page because there is nothing to disclose.

How it works

  1. Turn D4 on

    Open D4 and tap Turn on D4.

  2. Choose "D4 sound science"

    Pick it in the output list, the same list you use for AirPlay.

  3. Play anything

    Go back to your app. The green D4 is ON confirms it. Turn it off any time and the sound returns to the plain speaker.

D4 stays in the background while you listen. iOS keeps its checkmark on iPhone Speaker; that is normal.

Six modes. D4 picks, you can override.

Every mode is tuned for the physics of a phone speaker, not copied from a hi-fi preset.

  • Voice

    Podcasts, YouTube, audiobooks. Words come forward.

  • Music Boost

    Apple Music, Spotify. Fuller, wider, warmer.

  • Bass+

    EDM, hip-hop, pop. Bass the small speaker was never meant to have.

  • Balanced

    A safe daily driver for everything.

  • Night

    Quiet listening. Clear at low volume, no sudden peaks.

  • Cinematic

    Apple TV, Netflix. Dialogue up front, room around it.

Small speaker. Serious science.

A phone speaker can't move much air, so bass disappears, voices get thin and loud passages turn harsh. D4 doesn't just turn things up. It rebuilds what the speaker can't physically play from what it can, keeps speech intelligible at low volume, and protects the speaker from itself. All of it runs in real time on the phone, with latency you can't hear.

Read about the science

Privacy is the architecture, not the policy.

D4 has no server to send anything to. It never records, never listens through the microphone, and never knows what you are playing beyond the shape of the sound passing through it for a few milliseconds. We can't see your data because we never built a way to.

  • No account
  • No analytics or trackers
  • No microphone access
  • Audio never leaves the device
  • One-time purchase, no subscription

Also on iPad.

D4 on iPad: the Home screen with the ON circle, Music Boost active, and the Quick Intents grid of Voice, Music Boost, Bass+, Balanced, Night and Cinematic
Same app, same purchase, bigger room.

Why Fort Mill Ventures built D4

Phone speakers have improved far less than phone screens and cameras. Platform makers tune them for a safe average, not for how people actually listen: a podcast while cooking, a film on the kitchen table, a voice note on a noisy street. That gap is real, it affects everyone who owns a phone, and it is not where the big platforms put their attention. Through science and research we found that the same hardware can give much more: the speaker can do far more than it is asked to. D4 is Fort Mill Ventures' answer. It applies measured, device-specific processing to every app's sound, on the phone, with nothing sent anywhere, held to the same standard as everything we make: private by architecture, honest about its limits, built to be kept.

Fort Mill Ventures

Sound as it was meant to be heard.

Get D4

iPhone and iPad, iOS 27 or later. One-time purchase.