What "entrainment" actually means

Entrainment is a real word with a real meaning. Two oscillators, coupled, drifting into a stable phase relationship. Pendulum clocks on a shared wall. Fireflies. Circadian rhythm and daylight.

Applied to audio and the brain, the term is doing something more specific than most pages using it suggest — and something considerably narrower than "the audio makes your brain match the frequency".

The mechanism that is genuinely measured

Play an amplitude-modulated tone and record the EEG. You will see activity at the modulation rate. This is the auditory steady-state response, and it is robust, replicable and uncontroversial. It is used clinically to estimate hearing thresholds in people who cannot report what they hear.

That response is strongest around 40 Hz, which is one reason the 40 Hz literature exists at all. It falls off substantially at lower rates.

So: a rhythmic auditory stimulus produces a measurable cortical response at the stimulus rate. That much is settled.

What that finding does not establish

Here is where the term gets stretched.

A measurable response is not a changed brain state. The auditory cortex tracking a 10 Hz modulation is not the same claim as "you are now in an alpha state". The first is a stimulus-locked response in sensory cortex. The second is a description of global cortical dynamics, mood and cognition. The gap between them is enormous, and almost every marketing page in this category walks across it without noticing.

Tracking is not driving. Your cortex following a rhythm during the stimulus tells you very little about what persists after it stops. Effects that outlast the stimulus are the interesting claim and the poorly evidenced one.

The response is not uniform. It varies by band, by individual, by attention, by hearing, by time of day. Group averages hide a lot of people in whom nothing much happened.

Frequency-following response, and why the name misleads

You will see "frequency-following response" used interchangeably with entrainment. It is a distinct thing: a brainstem response that follows the fine structure of a sound, historically studied with speech and musical pitch.

Borrowing it as a synonym for cortical entrainment is a category error that has spread widely enough to be worth naming. If a page uses FFR to justify a claim about your mood or your focus, the page does not know what FFR is.

Where binaural beats sit in this

Awkwardly, and honestly so.

A binaural beat has no acoustic rhythm at all. There is no amplitude modulation in the signal — the beat is constructed inside the auditory system from two steady tones. Whatever it does, it is not doing it by the same route as an isochronic pulse, and the steady-state response to it is weaker and less consistent.

That is exactly why this site defaults to amplitude modulation rather than to binaural delivery. The comparison between the three methods sets out the trade: binaural needs headphones and produces a subtler response, isochronic works on any speaker and drives the response harder.

How to read the word from here

When you meet "entrainment" on a product page, ask which claim is being made:

• A rhythmic stimulus produces a cortical response at its rate. Well supported. Uninteresting on its own.
• That response corresponds to a functional brain state. Plausible in places, contested, band-dependent.
• That state persists and changes cognition or mood. This is the claim that sells, and it is the one with the thinnest support.

Most pages assert the third while citing evidence for the first.

We publish the full signal specification for every session — carrier, modulation rate, depth, envelope — so what is actually being delivered is inspectable rather than a matter of trust. 40 Hz Low-Noise Field is the clearest example: a 40 Hz amplitude-modulated field, described exactly, with the evidence for and against the 40 Hz work set out separately.