Get D4 on the App Store

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.

Get D4 on the App Store See how it works

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

D4 Home screen: the ON circle glowing over a night sky, mode Music Boost, 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.

Read the privacy policy

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

What D4 doesn't do (yet)

  • CarPlay: iOS sends sound straight to the car, so D4 isn't used there.
  • Some apps that record voice (Voice Memos, WhatsApp voice notes) can stop recording while D4 is the output. Turning D4 off first avoids it.
  • D4 enhances the built-in speaker. On Bluetooth, AirPlay and headphones it uses a lighter profile, which you can turn off.

Also on iPad.

D4 on iPad: the App Profiles list with Apple Books, Apple Music, Apple TV, Messages, Photos, Podcasts and Safari, each set to Recommended
Same app, same purchase, bigger room.

Why we built D4

We kept turning the phone up and hearing less. Thin voices, no bass, that harsh edge when it gets loud. Headphones aren't always the answer: cooking, a shower, a kid's cartoon on the kitchen table. So we built the thing we wanted: one tap, every app, better sound, nothing sent anywhere.

Carlos Dente, founder, Fort Mill Ventures

Hear your phone the way it was recorded.

Get D4 on the App Store

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