Slo vs Endel (2026): Manual Mixer or Adaptive Audio?

Slo fits if you want a mix you set yourself and can repeat exactly, with 200+ recorded sounds, 89 of them free, a volume for every layer, and a timer from 15 minutes to 24 hours. Endel fits if you’d rather open a Focus, Relax or Sleep soundscape that changes on its own with signals like time, weather and Apple Watch heart rate. Slo is ours, so factor that in.

The short answer: think about whether you want to be in charge of the sound or not. Slo puts you in charge, since you pick the sounds, set their volume, and get that same mix back months later. Endel takes the job off you, with mode-based soundscapes that respond to context such as time, weather, light, movement and, on a supported Apple Watch, your heart rate.

Anything stated here about Endel comes from its official product and technology pages, read on July 31, 2026. The Slo figures come from our own app source, version 3.132, on September 14, 2026. Neither one can promise you better sleep or sharper focus.

Slo vs Endel at a glance

If you care most about…	Better fit	Why
Choosing exact ambient layers	Slo	You get 200+ named sounds, any number of layers, and a volume on each one.
Audio that adapts with minimal input	Endel	It reshapes the soundscape from whatever context signals it can read.
Trying a preset and then editing it	Slo	180 browser sounds and 36 presets open free into the mixer, with no account.
Switching between intent-based modes	Endel	Focus, Relax, and Sleep are how the product is organized.
Apple Watch heart-rate input	Endel	Endel documents it where the device and your permissions allow it.

This page is about how the two products behave. It doesn’t confirm Endel’s wellness claims, and it doesn’t promise that either kind of sound will help you focus, relax or sleep.

What is the main difference?

Slo plays recordings you choose. The app holds 200+ sounds and 89 stay free, among them 22 noise-color recordings across seven colors, 5 of them brown. Add as many layers as you want, give each one its own volume, and the app keeps that balance for the combination, so the same mix plays the same way next week. Export a mix to a file and it opens again on another device. The timer runs from 15 minutes to 24 hours and fades the sound out over up to 2 minutes, so the ending doesn’t wake you. On iPhone and Android it plays as a background service, which is what carries it through the night with the screen off. Each sound downloads on first play and stays on the device, so in the app a mix plays with no connection. You pay for the full library monthly, yearly or once for life, and 180 sounds play free at slonoise.com with no account.

There’s a lot Slo won’t do. Nothing in it adapts, nothing reads the time, the weather or a heart rate, and it has no sleep tracking, no alarm, no recorder and no voice.

Endel builds the sound around you. You choose an intent such as Focus, Relax, or Sleep, and its system assembles the soundscape and keeps changing it. Its technology page says the inputs it can use depend on your device and permissions, and may include time, weather, light, movement, bedtime, and Apple Watch heart rate.

Calling this old audio against AI would miss the point. By Endel’s own account, its sound team writes the musical elements and the rules, and the software decides how those elements unfold while you listen.

Manual control versus adaptive control

Slo hands you the ingredients. If you want one of its 5 brown noise tracks under rain with the rain barely there, you set that relationship yourself and the app holds it. You get the same thing back every time, and it costs you about a minute of setup.

Endel takes the goal instead. You pick a mode, and the system reacts to whatever context it can read. Setup is shorter and the sound has more variety in it, though you don’t get to say which ingredients are in there.

There’s no personality test hiding in this. Try both during the same kind of task. If moving a layer helps you settle the room, manual mixing is worth the minute. If you find yourself adjusting when you should be working, let the app handle it.

What “personalized” means in Endel

Endel’s official technology page lists several possible inputs, and no single device supplies all of them. Heart rate is documented for Apple Watch, while weather or time can feed other experiences, and your permissions decide part of it too.

What we can state is narrow. Endel can use Apple Watch heart rate as an input where that’s supported. Nobody here tested what a change in heart rate does to the sound, and the feature shouldn’t be assumed for other wearables.

Libraries, presets, and modes

In Slo you browse 200+ separate sounds and open 36 browser presets you can edit. That’s useful when you already know the texture you want, or when one recording covers the noise in your room and another leaves it audible.

Endel isn’t a catalog of separate rain, wind and noise files. It’s organized around functional modes and soundscapes that change while they play, which asks less of you and gives you no library of layers to work with.

Platforms and pricing

Endel currently lists iOS, Android, Mac, Apple Watch, Apple TV, Vision Pro, Alexa, and web experiences. Slo runs on iPhone, Android and Mac, with 180 free sounds in a browser as well. Browser timers stop at 60 minutes, and only the app timer reaches 24 hours.

Exact amounts stay off this page, because they move by region. Slo sells the full library as a monthly subscription, a yearly subscription or a one-time lifetime purchase, so it’s the one of the two you can stop paying for. Check the Endel offer shown where you are.

Which should you choose?

Choose Slo if:

• You like naming the sounds and setting how loud each one sits.
• You want tomorrow’s mix to be the same as tonight’s.
• Buying once appeals to you more than subscribing.
• You’d like to try 180 sounds free in a browser before installing.

Choose Endel if:

• You want the soundscape to move without you touching it.
• Focus, Relax and Sleep match how you pick audio in the first place.
• The context inputs it supports, Apple Watch heart rate included, are part of the appeal.

If the subscription is what brought you here, our guide to Endel alternatives covers the wider field, generated audio and plain loops both.

Frequently asked questions

Is Endel just a playlist app?

No. Endel describes a sound team that designs the musical parts and the logic, after which software puts the soundscape together and changes it using whatever inputs a device can offer.

Can Endel use heart-rate data?

Endel documents Apple Watch heart rate as one possible input. Whether you get it depends on your device and the permission you grant, and we don’t extend that claim to other wearables.

Does Endel have a manual mixer like Slo?

Not one we could find. Endel’s official pages are built around modes and adaptive soundscapes, with no documented manual layer mixer like Slo’s, which takes any number of layers and holds a volume for each.

Can I use both in a browser?

Yes, both document web experiences. On our side, 180 sounds play free at slonoise.com with no account, every layer keeps its own volume, and the timer offers 15, 30 or 60 minutes.

Official sources checked July 31, 2026

• Endel: product, modes, and platforms
• Endel: technology and context inputs
• Slo app source, version 3.132 (sound list, timer, fade-out and store products), and the slonoise.com sound catalogue, reviewed September 14, 2026

Related guides

• Endel alternatives in 2026
• Slo vs Brain.fm
• Best focus sound apps in 2026
• How to use sound mixing
• Best sounds for focus

Filip Kowalski is the developer of Slo and writes about sound, sleep, and focus.