Today I explore the topic of sound machines. Do they boost sleep in babies? What sound levels are considered safe? How should you use them?

This is my favorite white noise playlist on Spotify.

“Are sound machines safe? Can they hurt my little one's ears?”

1       White noise and sleep

Sound machines are often recommended for sleep because certain sounds help babies to fall asleep and to stay asleep. Sound machines can produce white noise or nature sounds, and sometimes even music or a mix of these. White noise resembles the sounds that the fetus heard in the womb and probably because of this, it has a calming or soothing effect on babies, just like shushing. It can  also mask other household noises that might wake the baby up.

So how can you play white noise to your baby? You can use free apps, like the White Noise Generator, where you can mix different sounds, like rain, thunder, wind, water, fire, leaves, cars, et cetera. You can also find white noise tracks on YouTube or Spotify. Or you can buy sound machines that are devices specifically made for producing sounds for sleep.

2       The decibel scale

We measure volume in decibels (dB) on a logarithmic scale. It means that we perceive a 10 dB increase in volume as twice as loud. Here are few examples to help you imagine what different dB numbers mean:

• 0 decibel is imperceptible to the human ear.
• 10 dB: breathing sounds
• 20 dB: rustling leaves
• 30 dB: whisper
• 40 dB: refrigerator (depending on type)
• 50 dB: moderate rainfall
• 60 dB: normal conversation
• 70 dB: cars, city traffic
• 80 dB: trucks
• 90 dB: hairdryers (depending on type)
• 100 dB: helicopter
• 110 dB: trombone
• 120 dB: police siren
• 130 dB: jet engine
• 140 dB: fireworks

3       What level of noise is considered safe?

According to the National Institute for Occupational Safety and Health (NIOSH), a federal agency in the USA, the adult occupational noise limit should be 85 dB (1). It means that if someone works in a noisy environment, eight hours per day, five days per week, then the noise level should not be above 85 dB at their workplace. Anything above that noise level or duration could increase the person’s risk for noise-induced hearing loss throughout their working life.

To assess risk, both the sound level and the duration of the noise is important. With each three dB increase, you have to halve the duration of the noise to get the safe limit. E.g. for 88 dB, it is just four hours, for 91 dB, it's two hours; for 82 decibels, it's 16 hours.

There is not a universal agreement on this topic, e.g. the Occupational Safety and Health Administration (OSHA) is more lenient in its recommendations, see the table below (1). For example, for 85 dB they say that the safe duration is 16 hours instead of eight. Of course, people differ in their personal sensitivities too.

Can you use these recommendations to determine safety levels for infants? If an infant listens to white noise during the whole night and also during their naps, then their daily dose will be something between 12-16 hours, depending on their age. This would put the noise limit to 82 dB according to NIOSH (let’s go with the stricter standards). On the one hand, they won’t listen to white noise for 30-40 years (as long as the entire working life of an adult), which means that we could increase the sound level. On the other hand, we can assume that infants’ ears are more sensitive than adults’ and so we should decrease the sound level. Unfortunately, there isn’t much research on this topic, but as a guideline it is useful to know that the noise limit for infants in hospital nurseries is only 50 decibels!

Noise exposure can have other adverse effects too, apart from hearing loss. Researchers have found (2) that noise exposure is associated with reduced cognitive function, inability to concentrate, increased psychosocial activation, nervousness, feeling of helplessness and increased blood pressure. However, this research was on older children and about noise exposure during recreational time. I didn't find any research on how noise levels during sleep affect infants.

4       Sound levels of sound machines

Manufacturers are not required to limit the maximum sound output of music devices; there are no regulations about baby sound machines either. It sounds unsafe, but if you consider that the effect of noise on people depends very much on how far they are from the source of the sound and also for how long they listen to the sound, it makes sense.

It is true though, that if they are not used with care, these devices can be harmful. For example, music played through headphones at the highest volume is often above 100 dB. According to the occupational noise limits, you can only listen to a 100 dB for 15 minutes daily. I bet many teenagers overstep that limit by far!

Researchers measured the sound levels of 14 infant sleep machines in 2014 (3). They found that at 30 cm (like when the machine is next to the crib, under the crib, or on a crib rail) at maximum sound levels, all devices exceeded 50 decibels, which is the noise limit for infants in hospital nurseries. Some of the machines even produced output levels at, or above 85 decibels, which is the occupational limit for adults.

The conclusion of this study was that "Exposure to these devices may place infants at risk of developing noise-induced hearing loss, or maldevelopment of the auditory system". But it is important to know that they didn't actually study the infants, they only measured the noise levels of the sound machines in this study, which makes the conclusion a bit speculative. The recommendations for families in this study were the following: first, place the sleep machine as far away as possible from the infant and never in the crib or on a crib rail, second play the machine at low volume, and third, operate the machine for a short duration of time. These recommendations sound too vague for me, so let’s dive in further.

5       Harvey Karp’s recommendations in The Happiest Baby On The Block

I looked up the citations of this paper, but I didn't find any research about babies. However, I have found Harvey Karp's website (4). He has a blog post specifically about this research paper (it is actually an excerpt from his book, The happiest baby on the block). Harvey Karp is well-known for the soothing techniques he promotes for infants. One of these is shushing, which can be substituted with white noise, so of course he had his opinion about this paper: "That advice may seem logical, but I believe it is wrong... and even dangerous. By reducing infant crying and boosting a baby's (and mother's) sleep, white noise may prevent many of the terrible problems triggered by these two stressors, including postpartum depression, SIDS and child abuse. But it only works if it is loud enough!"

He continues saying that sound doesn't start boosting sleep until it gets to 60 to 65 decibels, and a baby cry is about 80 decibels. He argues that something that the baby produces himself cannot hurt the baby's ears, right? His recommendations for parents are the following: "When your baby cries boost the sound - for several minutes - to the level of her cries. After she's been asleep for 5 to 10 minutes, reduce the sound level to the level of a soft shower, around 65 decibels".

Most decibel scales say that a soft shower is around 50 dB, not 65 dB. Of course, showers are of different intensities, but just keep in mind that 65 dB is somewhere between the loudness of a normal conversation and city traffic. You also have to know, that there's a slight conflict of interest, because Harvey Karp produces several sound machines, for example, the Snoo Bear, which is a Teddy bear that plays white noise and also the Snoo smart sleeper, which is a kind of crib that also emits white noise.

But all-in-all, I think that Harvey Karp's recommendations are useful, because he emphasizes that time is a very important factor in determining the risk of noise. I also like that he looks at the problem holistically, taking into consideration not only the risk induced by noise, but also the risk associated with loss of sleep.

6       How to use sound machines safely?

What are my recommendations? First of all, decide where you would put the sound machine and then measure the sound level in the crib. Distance from the machine influences the sound level greatly, so try to measure the sound level somewhere where your baby's head would be most of the time. For measuring the sound level, you can use free apps, e.g. the Sound Meter.

Most sound machines do not indicate dB levels, and even if they do that would be the dB level at the machine or close to the machine, but not in the crib. My advice is to mark the different dB levels on your machine with a marker. In this way, when you put the volume at a certain level, you will know what that means in terms of dB in the crib.

When you try to put your baby to bed, start with 50 dB, and see if it helps. If it doesn't help, increase the volume gradually, but I would never go over 85 decibels. When your baby is in deep sleep, decrease the volume back to 50 dB.

To sum up, don’t use sound machines blindly, but measure the sound level in the crib. If you have to increase the volume to higher levels to help your baby fall asleep, just decrease the volume when they are in deep sleep, and don’t worry, because just a few minutes of lauder noise cannot hurt your baby's ears.

7       References

1. http://www.sengpielaudio.com/PermissibleExposureTime.htm
2. Michael Dellarco, S. M. V. (2014). Assessment of Noise Exposure to Children: Considerations for the National Children’s Study. Journal of Pregnancy and Child Health, 01(01), 1–15. https://doi.org/10.4172/2376-127x.1000105.
3. Hugh, S. C., Wolter, N. E., Propst, E. J., Gordon, K. A., Cushing, S. L., & Papsin, B. C. (2014). Infant sleep machines and hazardous sound pressure levels. Pediatrics, 133(4), 677–681. https://doi.org/10.1542/peds.2013-3617
4. https://www.happiestbaby.com/blogs/baby/is-white-noise-bad-for-babies