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What does duty cycle actually measure?

For a simple repeating on/off pattern, calculate:

The denominator is the whole repetition period, including the pause. For regularly spaced pulses, the same fraction equals pulse duration multiplied by pulse repetition frequency, then multiplied by 100 to express it as a percentage. FDA’s diagnostic ultrasound glossary defines duty factor this way; it is used here for terminology, not to import diagnostic safety limits into cosmetic equipment. [1]

A timing-only illustration: 2 milliseconds on followed by 8 milliseconds off gives 2 ÷ 10 = 20%. A different pattern of 10 milliseconds on and 40 milliseconds off also gives 20%. The fraction is identical, but the pulse length and repetition rate differ. These invented timings explain the arithmetic; they are not settings for a machine or a treatment.

Continuous does not mean “maximum”

In this simple description, continuous emission is 100% duty cycle while output is active. It describes the absence of intentional off intervals, not the amplitude of the wave. A continuous mode can still have an adjustable output level. Stopping a session is also different from the short off intervals within a pulsed waveform.

Ask which timing pattern a percentage describes. A percentage referring to pulses within a burst and a percentage referring to a sequence of whole bursts need different context. “Pulsed” without pulse duration, repetition rate or a mode definition leaves the actual pattern unknown.

Peak and average output answer different questions

Before comparing two output values, check what was measured and how it was averaged. The terms describe different quantities:

• Peak output: a maximum. The document must identify whether it means pressure, intensity or power, and specify the time and location.
• Pulse-average intensity: intensity averaged over a pulse.
• Temporal-average intensity: intensity averaged over the full repetition period, including pauses.

Spatial peak and spatial average describe a separate distinction: a value at a particular location versus an average across an area. These terms are not interchangeable measures of “strength.” [1]

For identical repeating pulses, with no output between them and the same spatial measurement basis:

Health Canada’s diagnostic-output glossary describes the corresponding relationship between pulse-average and temporal-average intensity. Again, only the measurement language is relevant here; its diagnostic limits do not establish a safe cosmetic exposure. [2]

Do not automatically substitute a peak for the pulse average. That shortcut needs a specified waveform for which the chosen on-phase quantity is effectively constant. A maximum pressure reading is not an intensity average, and a screen’s “energy” percentage is not a calibrated acoustic measurement.

Hypothetical output units below are arbitrary, not watts or treatment settings. On a phone, scroll the table sideways to read the final column.

Assumptions: rectangular output envelopes, zero output in each pause, repeated identical patterns and matching measurement conditions. The table demonstrates arithmetic only. Equal averages do not establish equivalent tissue effects, and it is not an instruction to raise output to compensate for a pause.

Why pulsed does not automatically mean cooler, safer or better

Duty cycle describes timing. It does not describe the full exposure. The AIUM Bioeffects Committee’s therapeutic ultrasound review identifies time-average intensity and tissue attenuation among factors influencing heating, and peak negative pressure, frequency and pulse repetition among factors relevant to cavitation. Thermal and mechanical effects can occur together under some exposure conditions. A pulsed label therefore does not establish “no heat,” and continuous output does not automatically identify a thermal treatment. [3]

For a fair mode comparison, first establish what stays fixed. Does switching modes preserve output during each pulse, change it, or change several variables at once? If that information is missing, even the direction of the change in average output is unresolved. Skin warmth or an indicator light cannot supply the missing measurement.

A published protocol is context, not a mode verdict

Tang and colleagues’ 2025 study used a named nonfocused 1 MHz system, MED SmartCool, with a specified pulsed exposure. The prospective, uncontrolled cohort had 17 completers and follow-up through two months after treatment. It did not compare pulsed with continuous modes. Its use of pulsing cannot establish that pulsed delivery is superior, validate a generic 40K/80K machine, or supply home-use instructions. The full methods were checked; no operating settings are reproduced here. [4]

Keep the technology separate: low-frequency nonfocused cosmetic ultrasound, focused mechanical ultrasound and thermal high-intensity focused ultrasound (HIFU) are not one interchangeable treatment. A 1 MHz research device is another specific exposure context. Radiofrequency (RF), vacuum and other attachments need their own documentation; an ultrasound duty cycle does not describe every function on a combined console.

FDA’s guidance for focused aesthetic ultrasound calls for acoustic output and beam characterization, consideration of thermal and mechanical injury, and justification of cooling between exposures. That guidance concerns focused systems. It illustrates why timing belongs within a wider assessment; it neither approves an unnamed nonfocused machine nor creates a universal operating protocol. [5]

What to ask when a machine only says “pulsed”

Use this narrow check to interpret the mode, rather than choosing one:

1. Identify the function. Is the label attached to ultrasound, RF or another accessory? Match the document to the exact model and applicator.
2. Get the timing definition. Request the pulse duration, repetition period or rate, and duty cycle for the relevant mode. Ask what any ratio or letter means.
3. Resolve the output basis. Is the reported value peak, pulse-average or time-average? Is it acoustic output or simply a control level? Request the relevant measurement conditions.
4. Check mode behavior. Ask whether changing the duty cycle also changes on-phase output, frequency or another controlled variable. Do not assume independent controls.
5. Keep the instructions separate. Suitability, permitted sites, coupling, limits and stop criteria come from the matching instructions and professional assessment. A timing calculation cannot replace them.

If the answer is “gentle mode” or “professional pulse technology,” the numerical pattern remains undocumented. Record it as unknown. If the manual and technical sheet disagree, request a clarified document with the correct revision before relying on the mode description.

Common reading errors

• Calling 20% “20% strength.” It is an on-time fraction when explicitly defined as duty cycle, not necessarily an amplitude reduction.
• Confusing carrier frequency with pulse repetition. The acoustic oscillation rate within a pulse and the number of pulses per second are different quantities.
• Comparing averages with different spatial bases. A spatial peak cannot be treated as an area average just because both use W/cm².
• Copying lettered modes. A/B/C/D labels need a device-specific definition; a letter alone supplies neither a waveform nor a percentage.
• Extending the timer to “make up” off-time. Equal calculated output totals do not establish equal effects or authorize a longer exposure.

Read a mode as a description, not a recommendation

A useful duty-cycle specification tells you which pattern is being described and how output is averaged. If timing or output definitions are missing, leave the comparison unresolved. Neither a percentage nor a pulsed/continuous label supplies a universal best mode.