Why noise is becoming a humanoid spec

Until recently, few humanoid spec sheets listed a noise figure at all. That changed when home robots arrived. In October 2025, 1X put a number on the front page of its NEO launch: a noise level of 22 dB, which the company says is quieter than a modern refrigerator. It credits the figure to its patented Tendon Drive and a soft, 3D-lattice polymer body. 1X has not published how or at what distance the figure was measured, so it is best read as a design target and a marketing claim, not a lab result.

There is research behind the push. A 2024 study by Aimee Allen and colleagues at Monash University showed 182 participants videos of five robots, with or without their mechanical sounds. Those who heard the sounds reported more negative feelings, felt more distracted and were less willing to share a space with the robots. A follow-up analysis of the same data found loudness, high pitch and abrupt sounds drew the most complaints, while participants still wanted some sound so they could tell where a robot was and what it was doing. For a robot that is supposed to live with people, silence is not the goal. Quiet, predictable and low-pitched is.

The reference points are strict. WHO Europe recommends less than 30 dB(A) in bedrooms at night and less than 35 dB(A) in classrooms. Gearbox maker IMSystems suggests indoor service robots should stay near conversation level, about 60 dB at 1 m. A humanoid made of 20 to 40 geared joints starts far from either number.

Where the sound starts in a joint

A rotary humanoid joint is a chain: a servo drive switches current into a motor, the motor turns a reducer, bearings carry the load, and the output moves a link. Each element makes its own sound, and the robot’s shell and limbs act like a soundboard that radiates vibrations travelling through the structure.

Gear mesh is usually the loudest part. Every time a tooth enters contact, the load shifts and the gear body flexes a little, which produces a tone at the tooth-mesh frequency and its harmonics. The frequency is simply teeth times revolutions per second: a 20-tooth pinion at 3,000 rpm meshes 1,000 times a second, a 1 kHz tone right in the range where hearing is sensitive. IMSystems notes that bearings and motors contribute but the transmission usually dominates, and that conventional spur, helical and worm gearboxes can reach 85 dB and beyond.

Motor drive whine comes from pulse-width modulation. If the drive switches below the edge of hearing, current ripple turns into torque ripple and the motor sings at the switching frequency. Progressive Automations puts the practical fix at 16 to 20 kHz, with the trade-off that faster switching heats the power stage. The low-inductance frameless motors used in many humanoid joints make current ripple a design issue for the joint’s servo drive.

Everything else adds up: bearings, cooling fans in the torso or around hot actuators, the click of backlash when a load reverses, and foot impacts on hard floors. None of these show up on an actuator datasheet, but all of them reach the listener.

What the published figures say

Hard numbers are scarce, and the ones that exist were measured under different conditions. Gearbox catalogues give a running noise in dB(A), usually at a fixed input speed and distance; robot makers rarely publish anything. The chart below puts the available figures on one scale, as orientation rather than a like-for-like ranking.

Two things stand out. First, standard precision reducers, strain-wave and planetary alike, sit in the high 50s to high 60s dB(A) in catalogues, close to conversation level for one gearbox on a test bench. Second, the quietest claims come from designs that change the transmission principle: IMSystems’ Archimedes Drive, a traction-based reducer, reports under 50 dB in its own chamber tests with minimums below 37 dB, and 1X claims 22 dB for a whole robot built around tendons. Both are maker figures without published test conditions.

How a joint gets quieter

The levers fall into three groups: change how torque is transmitted, refine the gears you keep, and stop vibration from reaching the shell. Most joints use some of each.

• Change the transmission. Tendons, cables and traction drives have no teeth to mesh, which removes the dominant tonal source. 1X builds NEO around this idea. Quasi-direct-drive joints, with a single low-ratio stage, at least cut the number of meshes per joint.
• Refine the gears you keep. IMS Gear’s low-noise variants use helical gearing in the first, fastest stage, adapted tooth forms and noise-optimised housings, and test each low-noise gear before dispatch. The company is open that acoustic tuning often costs service life and load capacity.
• Move the drive above hearing. PWM at 16–20 kHz or higher removes switching whine, at the cost of more heat in the drive electronics.
• Stop the soundboard. A soft or damped outer body, like NEO’s lattice polymer, isolates vibration from the panels that would radiate it, and softens footstep and contact noise too.

What it means for buyers and builders

• Ask for the test conditions. A dB figure means little without distance, task, speed and weighting. A robot standing idle is not a robot carrying a basket up stairs.
• Match the number to the room. A warehouse, a hospital ward at night and a child’s bedroom have very different limits. WHO’s 30 dB(A) bedroom guideline is far below any single catalogue gearbox.
• Listen for tones, not just level. Research on human perception suggests high-pitched and abrupt sounds annoy people more than steady, low ones at similar volume.
• Check noise after wear. Bearings, grease and gear surfaces change with hours. Ask whether noise is specified at end of life, not only out of the box.

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