What modulation depth changes

Every amplitude-modulated session has a rate and a depth. The rate gets all the attention — it is the number in the band label. Depth is the parameter that decides whether the session feels like a pulse or like a breath, and it is published for every session in our catalog for that reason.

What the number means

Depth is how far the amplitude falls between peaks, expressed as a proportion.

At depth 1.0, the signal drops to silence between pulses. Fully on, fully off. This is the most pronounced form of an isochronic tone.

At depth 0.5, the amplitude falls to half its peak. The tone is continuous; it swells and recedes.

At depth 0.15, there is a rhythm present but you may have to look for it. The session reads as a steady field with a slight motion in it.

Rate unchanged in all three cases. Same 10 Hz, three completely different experiences.

Why deeper is not better

The obvious assumption is that a deeper modulation drives a stronger response, so more depth is more effective. There is something to that — the auditory steady-state response does scale with depth over part of the range — but it runs into two limits fast.

Fatigue. A fully modulated tone is relentless. Over five minutes it is striking; over thirty it is exhausting, and people stop the session. A session nobody finishes has an effect size of zero regardless of what the electrophysiology says.

Masking failure. Deep modulation means the signal drops to near-silence many times a second, and each of those gaps is a window through which the room comes back. For a session that is partly doing masking work, high depth actively undermines it.

So depth is a trade between how strongly the rhythm is delivered and how usable the session is for its intended length. A 13-minute session can carry depth a 34-minute session cannot.

How depth interacts with band

The practical rule that falls out of this: slower rates tolerate more depth, faster rates need less.

At 2 Hz you are hearing two events per second. Deep modulation there reads as a slow pulse, which is comfortable and even soothing at length.

At 40 Hz you are hearing forty events per second, which is fast enough to be perceived as a timbre — a buzz or roughness in the tone rather than a rhythm. Deep modulation at that rate produces a harsh, insistent sound that very few people want for twenty minutes. 40 Hz Low-Noise Field is deliberately restrained for this reason; the name is a description of the design goal.

Between 12 and 30 Hz is the awkward middle, where the modulation is too fast to feel like rhythm and too slow to fully fuse into timbre. High Beta Rapid Processing is short, at 14 minutes, partly because of this.

Depth and envelope together

Depth sets how far the amplitude travels. The envelope sets the shape of the journey. They are separate controls and they interact.

A depth of 1.0 with a square envelope is a hard on-off click, with audible transient content at the edges. The same depth with a sigmoidal envelope is a smooth swell to silence and back, with none of that. Why the shape of a pulse matters covers the envelope side in full.

This is why a session cannot be described by a band alone. "Alpha" tells you almost nothing. Rate, depth, envelope, carrier and noise bed together tell you what you are actually going to hear.

Reading it in the catalog

Every session publishes its depth. Two useful habits:

If a session feels too insistent, look for a lower depth at the same rate before you change band. You may like the band fine.

If a session feels like nothing is happening, a higher depth at the same rate is the first thing to try — particularly in the alpha range, where restrained settings are common. Vipassana Sensory Gating is deliberately gentle, and that is a design choice rather than a limit of the band.