BLS Settings: Speed, Set Length, Tone and Volume
Speed, set length and tone are adjusted continuously during reprocessing, and the adjustments are clinical decisions rather than technical preferences. This is a practical guide to what each parameter does and how experienced clinicians tend to use them.
Speed
Speed is the rate of left–right alternation. In broad terms:
- Faster alternation is typically used during desensitisation, where the dual-task demand is part of what you are after.
- Slower alternation is typical for resource installation, calm-place work and closure, where the aim is consolidation and settling rather than movement.
Within desensitisation, speed is not fixed. If a client's affect rises sharply and they are approaching the edge of their window of tolerance, slowing the rhythm gives them more room. If processing has gone flat and looping, a faster rhythm sometimes restarts movement — though a stall more often means the target needs attention, not the settings.
Set length
Set length is decided between sets, based on what the client reports. The general principle is that you continue a set while material is moving and stop when it settles or when the client needs to speak. Longer sets are common where processing is actively chaining; shorter sets are safer early in a course, with a new client, or when you are calibrating to someone remotely for the first time.
A practical habit: shorten sets when you notice yourself guessing what is happening. Shorter sets mean more check-ins, and more check-ins mean less guessing.
Tone and pitch (auditory BLS)
Pitch gets less attention than speed and often matters more to the client's experience. A bright, high tone is more intrusive; some clients find it aversive enough that they spend part of each set bracing against the sound rather than attending to the target. A lower, softer tone usually recedes into the background, which is where you want it.
Worth asking directly after the first set: how is the sound itself — comfortable, or something you're aware of? Clients frequently do not volunteer that a tone is unpleasant, on the assumption that it is supposed to be that way.
Volume
Loud enough to be clearly localised left and right, quiet enough that it does not dominate. If a client cannot tell which side a tone came from, the bilateral element is not being delivered — check that headphones are stereo, worn the right way round, and that nothing on their device has collapsed audio to mono. In remote sessions this is worth verifying explicitly at the start of every session, not just the first.
A checklist for the first minute of a session
- Confirm the client can hear the alternation distinctly in both ears.
- Set a comfortable volume, then check pitch comfort explicitly.
- Run a short calibration set before the target is brought up.
- Confirm the stop signal.
Thirty seconds spent here prevents the most common avoidable disruption in a reprocessing session: discovering a sound problem after the client is already in contact with the target.
Why therapist-side control changes this
Everything above assumes you can change a parameter without breaking the client's focus. In the consulting room that is trivially true. In a remote session it depends on the tooling: if the client holds the controls, every one of these adjustments becomes an instruction, a search through a settings screen, and a restart. Attention leaves the target and lands on the software — and then has to be brought back.
This is the reason EMDR Mobile puts speed, tone and set control on the therapist's screen and applies changes to the client's device directly. It is not a feature so much as a precondition for making these adjustments the way you would in the room.