A Sound of Soul session ends with a four-page report. The first page is a picture — ten labelled points arranged around a centre, with a shaded shape stretched between them. The pages behind it are the numbers from which that picture was drawn. This module is a guide to reading both: what each measurement is, what it is sensitive to, and how the ten-pointed picture is constructed from the measurements behind it.
The report is built from a single raw observation: the interval, in milliseconds, between one heartbeat and the next. Everything else is calculated from those intervals. A healthy heart does not beat metronomically. Each beat arrives a little sooner or a little later than the one before it, and this variability — not the rate — is what reveals the state of the autonomic nervous system that paces the heart from outside.
What follows is the report read in four passes. First, the simple statistics — the time-domain measurements that summarise the raw beat-to-beat intervals. Second, the frequency-domain measurements that decompose the variation into bands governed by different branches of the nervous system. Third, the geometric measurements that read the heartbeat as a pattern. And fourth, the radar chart — the picture on page one — which is a translation of all of the above into ten patient-facing words.
Part 1The Time-Domain Measurements
These are the simplest descriptions of the recording. They take the list of RR intervals — the milliseconds between one R-wave of the ECG and the next — and compute statistics directly on that list. They do not yet ask anything about which nervous system is doing what; they only ask how much variation is present, of what kind, and at what speed.
The average pulse during the recording. In a relaxed adult this should be neither too high nor too low — typically 60–80 bpm. Endurance training pulls it below 60; chronic stress, sedentary life, and excess weight tend to push it higher. Heart rate alone says very little about regulation: a person can have a "normal" pulse and very poor HRV.
The average length of one heartbeat interval. It is the mathematical inverse of pulse — 60 bpm corresponds to 1000 ms between beats, 75 bpm to 800 ms. It is shown alongside Mean HR so the same fact can be read in both directions.
The single most important number on the page. SDNN is the standard deviation of all the RR intervals in the recording — a direct measure of how much the heartbeat varies overall. It reflects the activity of the whole autonomic nervous system: both branches taken together.
In a five-minute measurement, healthy values are typically above 50 ms. Values that stay below 20 ms across multiple recordings are associated with an elevated risk of chronic disease — the regulation has thinned to the point where the body has little reserve left.
Where SDNN looks at how much intervals vary across the whole recording, RMSSD looks at how much they change from one beat to the next. Short-term jitter, in other words. This is the signature of the parasympathetic nervous system — the vagus nerve, the rest-and-digest branch — because only the parasympathetic acts fast enough to alter a single heartbeat.
A practical rule from Rasmus: if during a session the RMSSD rises to meet or exceed the SDNN, the treatment is doing what it should. The parasympathetic has come forward.
A family of related parasympathetic indicators. pNN50 asks: of all the consecutive RR-interval pairs in the recording, what percentage differ by more than 50 ms? pNN20 asks the same with a 20 ms threshold, and so on down to 5 ms.
The cascade is diagnostic. pNN50 falls first, under acute exhaustion or a stressful day; many adults at the end of a working day measure pNN50 near zero. If pNN20 is still healthy at the same time, recovery capacity is preserved. When pNN20 also approaches zero, the deeper reserves are depleted and a medical review is warranted. pNN05 — beats that differ by even five milliseconds — is the last to go; depleting it takes years.
The histogram bins the RR intervals by length and counts how many fall into each bin. A wide, well-spread histogram is healthy; a narrow spike is not. VB is the numerical summary of the histogram's spread — the longest interval minus the shortest.
Rasmus's image is worth keeping: a forest with many tree sizes survives a storm; a monoculture of identical trees falls all at once. A wide variation broadness is a wide forest. The body has many sizes of response available.
A condensation of the histogram into a single number, developed by Baevsky for Russian space-flight medicine. A narrow, concentrated distribution of RR intervals produces a high SI; a broad, well-spread distribution produces a low one. High values indicate sympathetic dominance and acute or chronic stress load.
SDNN expressed relative to the mean — variability scaled by the average interval. Because it is relative rather than absolute, CV compares more cleanly across people with different resting rates. Higher is better; it reflects how flexible the system is, day to day.
Part 2The Frequency-Domain Measurements
The time-domain measurements describe how much the heartbeat varies. The frequency-domain measurements ask at what speed. They take the same series of RR intervals and decompose it into oscillations of different frequencies — slow, medium, and fast waves of variation — and report the energy in each band.
This matters because the two branches of the autonomic nervous system operate at different speeds. The sympathetic — fight, flight, mobilise — works slowly: its signals build over seconds. The parasympathetic, by way of the vagus nerve, acts fast enough to modulate each individual heartbeat. So the slow and fast bands of HRV map, roughly, onto the two branches.
The slowest variations in the heartbeat. In five minutes, only about a dozen oscillations of this frequency can fit, so individual short-term VLF readings should be treated with caution; trends across multiple sessions are more informative than any single value.
VLF reflects the body's slow regulatory layers: thermoregulation, hormonal cycles, inflammatory tone. These processes do not respond from heartbeat to heartbeat — they respond over minutes and hours — and they leave their trace in the slowest band.
The middle band, oscillating around once every ten seconds. LF is dominated by sympathetic activity and is closely associated with the baroreflex — the loop that regulates blood pressure. At very slow breathing rates (around six breaths a minute, or 0.1 Hz) the parasympathetic also enters this band, which is why coherent-breathing exercises drive their effect partly through LF.
In ordinary conditions, however, persistently elevated LF reflects a body in mobilisation mode: short breath, mental load, fight-or-flight tone. Mental stress — even imagined stress — registers here, because the body does not distinguish a remembered threat from a present one.
The fastest band, in the range of normal breathing. HF is almost entirely the work of the vagus nerve. With each inhalation vagal activity drops and the heart speeds up; with each exhalation vagal activity rises and the heart slows. This is respiratory sinus arrhythmia — the most direct, visible sign that the parasympathetic system is engaged.
High HF is what good treatment looks like. Slow breathing, calm attention, and the practitioner's presence all push the body into this band.
The simplest possible summary of nervous-system balance: the energy in the LF band divided by the energy in the HF band. A high ratio indicates the sympathetic is running the show; a low ratio indicates the parasympathetic is. In a relaxed and balanced state the ratio should be around 1.5 or lower.
The standard caveat: the interpretation is only partly correct, since LF contains some parasympathetic contribution. The ratio is useful as a quick read, not as a final word.
The total energy contained in the spectrum from 0 to 0.4 Hz. It is the frequency-domain analogue of SDNN — the overall amount of variation, expressed as power rather than as standard deviation. A healthy five-minute measurement is typically above 2000 ms²; balanced, active adults often show 3000 ms² and above; an active healthy child, or an adult with a long and sustained practice of movement and stillness, may reach 10,000.
A subtle point: Total Power can be high while the parasympathetic share of it is low. A person in chronic mobilisation may have a great deal of variation, all of it sympathetic. This is why the report does not stop at Total Power, and why the radar chart separates Total Energy from Recharge.
Part 3The Non-Linear Measurements
The third pass reads the heartbeat geometrically. Instead of asking how much it varies (time domain) or at what speed (frequency domain), it asks: what shape does the variation make when plotted against itself?
On the Poincaré plot, every heartbeat is shown as a single dot. Its position on the horizontal axis is the length of that beat's interval; its position on the vertical axis is the length of the next interval. A perfectly regular metronome would produce a single point. A heart with healthy variability produces a cloud — and the shape of that cloud is itself diagnostic.
A round cloud means short-term and long-term variation are balanced. A cloud stretched along the diagonal — long and narrow — means there is long-term variation (the cloud is spread out) but little beat-to-beat jitter (it is thin). A small, compact cloud means both are reduced. The two perpendicular widths of the cloud have names: SD1 and SD2.
SD1 is the short axis of the Poincaré cloud — its width measured across the diagonal line. It captures how much each heartbeat differs from the one immediately before it. Because only the vagus nerve acts fast enough to do this on a beat-by-beat basis, SD1 is essentially a geometric reading of parasympathetic activity. It is mathematically related to RMSSD.
SD2 is the long axis of the Poincaré cloud — the spread along the diagonal. It reflects the longer-term variability over the whole recording, which is more strongly shaped by the sympathetic branch and slower regulatory layers.
The ratio of the two axes — the shape of the cloud in a single number. A healthy ratio is roughly 1/2 to 1/3: the cloud is a moderately elongated ellipse, not too round and not too stretched. When the ratio collapses (a long thin cloud) or becomes near-circular without volume, it indicates either chronic mobilisation or a depleted system. A flattened Poincaré plot is one of the clearest visual signs of exhaustion.
Part 4The Radar Chart — Ten Petals from These Numbers
The radar chart on page one is what most patients see first. Each of its ten points is a translation of one or more of the measurements above into a single word that describes the patient's state. The chart was Rasmus Gaupp-Berghausen's attempt to make the report legible to anyone — not just to the practitioner — without losing what the numbers actually say.
What follows is the map between the two. Read the left column as the petal on the chart and the right column as the measurement it comes from, and why that measurement was chosen.
A noteHow to use this report
Three things to keep in mind as you read your own — or your patient's — Sound of Soul report.
One measurement is not a verdict. A bad day, a recent meal, a poor night's sleep, or an artefact from movement during the recording can pull individual numbers around. Rasmus recommends five measurements over time to form a picture. A single recording shows a state, not a trait.
The numbers reflect a population, not an ideal. The "healthy" reference values in the manual come from large samples of mostly modern, mostly stressed adults. They are guidelines, not absolutes. A SDNN of 36 ms is an average in South Korea and nearly twice that in some Chinese samples; an active healthy child can show Total Power above 10,000 ms². The reference ranges are a useful first map, not the territory.
Good treatment is visible. The single most encouraging sign during a session is the moment RMSSD climbs to meet — or surpass — SDNN. It means the parasympathetic has come forward, and the body is letting itself be received. The radar chart's Response-Ability, Regeneration, and Recharge petals all begin to extend in this moment. That is the change worth waiting for, and worth pointing out to the patient.
Where this leadsNext in the series
This module is the first of a planned series on health. The pieces that follow:
Module Two — What the Heartbeat Knows. A step back from the report to the physiology beneath it: the sinoatrial node and the heart's own clock, the vagus nerve, the two branches of the autonomic nervous system, and how a single inhale and exhale carries the signature of the whole system. The module that earns every term used here.
Module Three — The One Dial You Can Turn. The autonomic system runs without your permission, with one exception: the breath. Why slow breathing — and the long exhale especially — raises vagal tone, what the roughly six-breaths-per-minute resonance is, and what coherence really means once you can see it in the trace.
Module Four — A State, Not a Verdict. Reading HRV honestly: why one measurement is a mood and five make a portrait, what good treatment looks like as it happens, and why the heart sometimes reports what the patient will not say.
A closing synthesis — The Body Keeping Its Own Time — draws the series together: HRV as one self-regulating loop made visible, and the place of that loop within the body's wider system of regulation.