On Sound Machines Posing a Hearing Risk

As a new mother who also happens to be an acoustician, the recent report of infant sleep machines being a hearing risk caught my attention.  How couldn’t it?  Not only did my mother-in-law send the article to me, but even my fellow acousticians were posting it and commenting about it via various social media platforms.  Aside from sensationalizing an issue that is clearly a case of extremes, the report (read the NY Times article here) brings up an important social topic in general: excessive sound.  This is becoming a hot topic, especially for those who work in architectural or environmental acoustics.  Not only can infant sound machines exceed OSHA limits, but so can the daily sounds that we adults expose ourselves to- headphones, traffic, movie theaters, cars or motorcycles with modified exhausts, concerts, bars and restaurants in which excessively loud atmospheres appear to be the trend these days, etc.  Except where intentional (motorcycle exhausts, e.g.), there are usually design flaws to blame.  This means that the problem could have been avoided, but in this business the acoustics of a design are almost never as important as the aesthetics.

This is where an old saying takes on a new meaning: out of sight, out of mind.  Is it possible that  we rely so much on the visual that we’ve come to neglect the aural?  Do we, as a society, simply undervalue our ears and the gift of hearing?

Back to the article for now, why use sleep machines?  Besides the obvious function of drowning out additional noise, Dr. Harvey Karp, pediatrician and author of Happiest Baby on the Block suggests that recreating the womb’s environment is essential in calming a newborn baby.  One aspect of that environment was the way it sounded.  To recreate the aural experience, he recommends shh-ing at your baby’s ear, as loud as your baby is crying.  I watched as he demonstrated, and one of the first thoughts that crossed my mind was whether or not that was safe for the baby’s hearing.

 

This brings us to the real issue: how loud is safe for the baby’s hearing?  One commentor provided the following information:

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Hearing in early infancy is significantly attenuated. The intrauterine environment, itself quite noisy, has been measured at a baseline of 72-88dB (Smith, Satt, Phelan, & Paul 1990). Tharpe & Ashmead 2001 report a longitudinal study in which the 3-month-olds on average require a sound to be 30dB louder than an adult would to reach the threshold of audibility. Results in comparable studies using different methodology are in good agreement, running from approximately 25 to 40dB. At 6 months, Tharpe & Ashmead find a 10-15dB difference between adults and infants; others find 10-25dB, very much in the same range. Even at a year, infants are found to be about 5dB down.

To get a sense of what this means, stick your fingers in your ears to get a 30dB level of attenuation or put your palms over your ears to get 20dB (Holland 1967).It’s surprising that the reported study doesn’t take these findings into consideration. An 85dB noise that irritates and injures an adult should, if perceived at the adult equivalent of 55dB or so, do no harm. Special care should be taken to avoid hurting our infants, and it’s prudent to keep them away from persistent noises that adults perceive as loud, even if we are fairly confident that infants may not. But there’s no reason to spread panic. Parents who wish to monitor closely the loudness of sounds their babies are exposed to can purchase any of several excellent apps for their smart phones which measure Sound Pressure Level quite accurately.

But how safe is that?  There’s something to be said for the difference between perceived sound levels and actual sound levels.  Another poster added:
ZoetMBNew York
Perception of sound is independent of the damage it does to the nerve endings.
And it’s dangerous to propose that if one can “take” a sound that it isn’t dangerous. This is the macho attitude that prevails at most concerts, when the frontman of the rock band says to the audience, “is it loud enough” and the idiots scream “louder”, even though the sound is already beyond the threshold of pain and would violate OSHA regulations if it were a factory (and sounds terrible besides, but few people, including professionals, know what good sound is supposed to sound like anymore.)The nerve endings are not muscles and shouldn’t be treated as such. Once the ears are damaged, it’s over – you’ve lost either threshold or frequencies forever. There’s some research being done now with stem cells to restore hearing, but any practical application is probably at least 20 years away.

 

I think both comments raise legitimate points, so this acoustician is still on the fence.  Here are my thoughts:

  • A grain of salt will go a long way.  Keep in mind that the reported data was from measurements taken less than a foot away from the sound machine at maximum volume.  Additionally, as any architectural acoustic consultant might point out- did they take any room effects into consideration?  I don’t know about others, but my newborn sleeps in a raised moses basket bassinet which includes a fabric-lined barrier that rises several inches above the top of her head when lying down.  Unless a parent is putting the sleep machine IN their baby’s bassinet/crib, the white noise will be affected (filtered and absorbed or reflected) by that fabric and any other physical objects between the sleep machine and baby’s ears.
  • Sound machines are unlikely to pose any more risk than other techniques that desperate parents may turn to (e.g. blow dryer, laundry room, etc.).   However, they need to come with clear directions and warnings regarding the ideal distances, volume levels, and durations to use with your baby, e.g.  At full volume, the sleep machine should be placed no more than X-distance away from baby’s head.  OR Prolonged use (in excess of X-hours) may cause hearing damage.
  • Let’s continue to measure the intrauterine environment to fully describe the aural experience in the womb.  Once the associated sound pressure levels and sound spectra are really understood, a new sleep machine could be created to really mimic that sound.
  • Don’t forget about common sense.  Just as you would test the temperature of your baby’s bathwater or the contents of a warmed bottle, you should also make sure that any sound you subject your baby to is also comfortable to you, regardless of what any sleep expert may tell you.

Aside: 

The comments section of the New York Times article brought out parents of all walks of life, and yet there was one common theme: almost all were as judgmental and critical as the next.  That’s one of the special perks of parenthood- the unsolicited opinions and critique automatically brought on by fellow parents.  A word of advice to new parents: just grin and bear it 🙂

While Sound Advice is not a parenting forum, allow me this one rant:  I wish all these parents who commented on that article would take a step back before they cast judgment on another parent!  Although my little one isn’t sleeping through the night in single stretches, neither does she seem to be particularly fussy.   Other parents, I know, have had it much worse so I consider myself pretty lucky.  Reading the comments on that article was really disappointing.  I can only imagine the desperation some parents are brought to when their newborn won’t sleep whether it’s colic, spitting up, general discomfort, or any other number of possibilities outside a parent’s control.  Combine the heartbreaking cries of an inconsolable baby with a major lack of sleep for the parents, and it’s not so surprising that Mom or Dad will try just about anything to get their baby to sleep.  Need I remind anyone of the terrible thing some parents have been driven to known as Shaken Baby Syndrome?  Even with my mostly-good-natured baby, I’ve still had my share of “WHY WON’T YOU GO TO SLEEP?!” moments.  So by comparison, a white noise sleep machine doesn’t seem like such a bad alternative.  After all, parents have been known to resort to setting their fussy babies on running laundry machines or to use blow dryers to calm their babies.  Neither of which are much safer than a white noise machine.



'On Sound Machines Posing a Hearing Risk' have 1 comment

  1. March 25, 2014 @ 3:00 pm Lindsay

    Very interesting!


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