Hugh et al. 2014: the infant sound machine study that triggered AAP action

Published 2026-05-18 · 9 min read

Informational reference, not medical advice. This page summarises a peer-reviewed audiology study and the AAP guidance that built on it. For clinical questions about your child's hearing or sleep, consult a pediatrician or pediatric audiologist.

The 2014 study by Sarah Hugh, Nikhila Cutler, Ahmed El-Naga and colleagues at the University of Toronto, published in the journal Pediatrics, is the single most cited piece of evidence underlying modern guidance on infant sound machine safety. Before this study, hospital and pediatrician advice on white noise machines was largely intuitive and not grounded in measured data. After it, the conversation became specific: a 50 dB ceiling and a placement recommendation of "as far as possible from the infant," translated by consumer health publishers into the now-familiar 7-foot rule.

This page summarises what the Hugh team actually measured, what they found, how the AAP and consumer publishers used the findings, and where the limits of the study lie.

The full citation

Hugh SC, Wolter NE, Propst EJ, Gordon KA, Cushing SL, Papsin BC.

Infant Sleep Machines and Hazardous Sound Pressure Levels. Pediatrics. April 2014; 133(4): 677-681. DOI: 10.1542/peds.2013-3617. Published by the American Academy of Pediatrics through the Pediatrics journal.

The author list reflects the Department of Otolaryngology, Head and Neck Surgery at the Hospital for Sick Children and the University of Toronto. The lead author is commonly cited as "Hugh" in subsequent literature, hence the shorthand "Hugh 2014" used throughout consumer health writing on infant sound machines.

What the team measured

The researchers obtained 14 commercially available infant sound machines, representing the range of products being sold in the consumer market at the time. They placed each machine in an anechoic chamber (a soundproofed room designed to eliminate reflected sound) and measured the sound pressure level at three distances from the device face: 30 centimetres (representing typical placement on the crib rail or directly beside the crib), 100 centimetres (representing placement on a nearby table or shelf), and 200 centimetres (representing placement across a small to medium room).

For each machine and each distance, they recorded the dB level at the lowest volume setting, at the midpoint, and at the maximum setting. The measurements used A-weighted decibel scaling (dBA), which approximates human hearing sensitivity across frequencies and is the standard used in occupational and public health noise references.

The reference benchmarks used in the analysis were the 50 dB threshold for infant nursery environments (drawn from earlier WHO and pediatric audiology literature) and the 85 dB NIOSH adult occupational ceiling. The 85 dB figure is the level above which sustained exposure produces measurable hearing loss in adult workers, time-weighted over an eight-hour shift.

The headline findings

The study's three central findings were each striking on their own.

First: all 14 machines produced sound pressure levels above 50 dB at 30 cm at their maximum setting. The lowest-output machine at maximum still exceeded the proposed nursery ceiling at typical close-placement distance. The highest-output machine at maximum was measured at around 92 dB at 30 cm, comparable to a power tool or heavy traffic.

Second: three of the 14 machines produced levels above 85 dB at 30 cm at maximum, the NIOSH adult occupational ceiling. The implication was that some commercially marketed infant sleep machines, placed where parents naturally place them (on the crib rail or beside the bassinet), were producing dB levels at which an adult worker would legally be required to wear hearing protection.

Third: distance mattered enormously. At 200 cm, every machine could be kept below 50 dB at typical volume settings. The corollary was that the safest practical guidance was to keep machines as far from the infant as possible, with 200 cm (approximately 6.5 feet) as a workable minimum. Consumer publishers rounded this to the now-familiar 7 feet.

What the study did not show

Honest reading of the paper requires being clear about its limits. The Hugh team did not measure infant hearing outcomes. They did not follow babies through development to test whether sound machine exposure caused hearing loss. They did not establish a dose-response relationship between hours of nightly use and audiometric thresholds. None of these things were the point of the paper.

What the paper did establish was that the devices were physically capable of producing sound levels at which harm in adult occupational contexts is well documented. The argument from there to "use them carefully" is precautionary rather than empirical: we know the machines can be loud, we know loud noise harms hearing in adults, infants are biologically more vulnerable, therefore we should keep the machines quieter and farther away.

There is, to date, no published cohort study showing that AAP-compliant sound machine use (below 50 dB at the head, at adequate distance, with sleep timers) produces measurable hearing damage in infants. The 50 dB rule is built on a margin-of-safety argument, not on documented harm at that level. This is appropriate: precautionary public health guidance for infants typically does not wait for documented harm before recommending mitigations.

How the AAP picked up the findings

The Hugh 2014 paper was cited extensively in the years that followed. The AAP's 2023 policy statement on noise as a health hazard for the fetus, infant, and child cited it among the evidence supporting its updated guidance on infant sound machines. The relevant AAP language, paraphrased: infant sound machines should be placed as far from the infant as possible, kept at low to moderate volume (50 dB or below), and never at full output.

Hospitals and consumer health publishers translated the AAP rule into specific numbers. The 7-foot consumer rule (see the 7-foot placement rule) is the most common rounding of the Hugh team's 200 cm finding. Manufacturers responded in subsequent years by adding volume caps, low-volume "baby" presets, and stepped rather than continuous volume controls to their products.

Modern infant sound machines (Hatch Rest 2nd gen, Yogasleep Hushh, LectroFan Junior, Dreamegg) now ship with explicit AAP-aware features. The change in the consumer market between 2014 (when many devices' maximum output exceeded 85 dB at 30 cm) and the late 2020s (when most major infant-marketed devices cap at much lower outputs by default) is in large part a direct response to the Hugh paper and its downstream AAP guidance.

What the study does and does not change for parents

The practical takeaways are narrow but important.

It does change the case for placement: the machine should not be inside or directly beside the crib. It should be across the room, on a dresser or shelf, at 7 feet or more if the room allows. This is the single most actionable lesson of the study.

It does change the case for volume: most machines do not need to be near maximum to do their masking job. Keeping volume at the lowest level that masks the noise you are trying to mask is the safer default. Many parents reflexively turn machines higher than necessary.

It does not change the case for whether to use a sound machine at all. The study does not say sound machines harm babies. It says they are capable of producing harmful dB levels at close range and high volume, and the precautionary fix is straightforward: distance and modest volume.

Frequently asked

Has anyone repeated the Hugh study with more recent machines?

There have been smaller-scale measurements published in audiology and pediatric otolaryngology journals since 2014, but no large replication. Most subsequent literature cites Hugh 2014 directly and focuses on guidance refinement rather than retesting.

What was the loudest machine the team measured?

The paper reports a maximum near 92 dB at 30 cm at maximum volume for the loudest device. The specific brands are listed in the paper's supplementary material; the AAP and most consumer-facing summaries do not name them individually because the marketplace has changed substantially since.

Does the study address pink noise vs white noise?

Not in the safety analysis. The study measured total sound pressure level rather than spectrum. The AAP guidance treats placement and volume as the dominant safety variables and treats colour selection as a comfort and preference variable.

Is the study available for free?

The full text is paywalled on the AAP Pediatrics portal. The abstract is freely available on PubMed (PMID 24685949). Many medical libraries and university institutional subscriptions provide free access.

Sources

Baby Safety overview →Why 50 dB →Why 7 feet →AAP 2023 statement summary →Hearing damage risk →

Updated 2026-04-27