The dangers of personal listening devices through volume and time
“If it’s too loud, you’re too old,” goes the venerated rock ‘n roll attitude statement. Sadly, adherents to that philosophy lose their hearing over time. Unlike some health-threatening activities such as smoking, which causes cancer in some, but not others, repeated exposure to music above certain volume thresholds causes hearing loss for anyone.
Headphones and earbuds carry an elevated risk. It’s ironic, since these are hardly high output devices. The threat comes not from the overall loudness, but from a combination of proximity and time. Let’s look at how these combine to create a guaranteed health-damaging condition.
A principle of physics, the Inverse Square law applies to wave phenomenon such as sound waves. Imagine a point source that perfectly radiates sound in all directions. If a listener is 3 feet away, that sound source has a certain volume.
When the listener moves to 6 feet, intuitively we think the sound will now be half as loud, since the distance from the source is doubled.
In fact, only ¼ of the sound energy reaches the listener at 6 feet, because not only has the distance doubled, but the wave’s sphere is now expanded in all directions. Imagine the sound from the source expanding like a balloon inflating. Sound energy falls off very quickly with increases of distance.
This works the same in the other direction as well. Moving from 3 feet to 1.5 feet doesn’t double the sound energy, it quadruples it.
Just as human eyes adjust in sensitivity, allowing us to see in both sunlight and moonlight, so too does out hearing system adapt. While the effects of the inverse square law on sound energy are quite measurable, human ears tend not to agree with that fact because of this adaptive ability. We have a built-in volume control that tells us sounds are louder and softer based on how we hear them, and in what context, rather than based on the real, physical energy of the sound.
So how does this science affect headphones and earbuds? There are two important takeaways:
1. Distance from a sound source is critical to the amount of sound energy arriving at the listener, and;
2. Our perception of sound impedes our ability to judge how much energy arrives at our ears.
Imagine being at an arena concert. There may be several hundred feet between the speakers and your seat. If you were to stand and walk back five rows and then ahead five rows, you’d probably feel as though the sound volume stayed the same. The percentage of distance change in those five rows each way is quite small compared with the overall distance from the speakers.
Wearing a set of headphones puts you much closer to the speakers. Earbuds bring you even closer. Now, changes in millimeters have a significant effect on sound pressure levels. Most listeners have experienced the effect when one earbud moves slightly out of position. The volume in that ear falls off dramatically.
Some designs of headphones and earbuds depend on a seal to achieve the sound response desired by engineers. Circumaural – over the ear – headphones use foam ear cups to create a barrier against the side of the head, blocking outside sound and, when combined with closed-back design, direct all of the headphone’s energy toward the ear canal.
Earbuds can either rest at the opening of the ear canal or sit right in it, usually with rubberized edges to create a seal. Apple earbuds, for example, can be used both ways.
Sound pressure levels have distinct effects on human hearing. It’s not just loudness that creates the problem. It’s the duration of the loudness as well. Through significant amounts of research, 85 dB (decibels) sounds can continue for 8 hours without effect on human hearing. After that, continued exposure causes ear fatigue and starts to damage the mechanisms within our hearing system. High frequencies experience the first effects.
While that sounds like a lot of exposure, 85 dB is equivalent to the noise of a vacuum cleaner. Ordinary conversation is about 65 dB. Because the decibel scale is logarithmic, those 20 points represent a change in sound pressure that’s nearly a magnitude of 7, since power doubles or halves with every 3 dB change.
Rock concerts top 100 dB, usually averaging between 105 and 115 dB. As you can see above, the threshold for damage is less than four minutes of exposure to 106 dB levels. At 115 dB, damage begins in half a minute.
An iPod can generate 115 dB, depending on the listening device connected to it and how it’s worn. Any personal audio listening device has the capability of generating sound damaging volume levels.
What’s worse is that the damage is gradual and not obvious. The person who says that their hearing can handle it, that they aren’t bothered by loud music, is essentially fooling themselves into more advanced hearing loss.
There are some rules of thumb that help keep personal audio at safe levels when using earbuds and headphones. Note that these rules aren’t absolute. Each case depends on the equipment being used. Speakers vary in efficiency, so one set of earbuds may create louder levels than another. Often this is promoted as a positive selling feature. Unfortunately, it’s like comparing cars with maximum speeds of 200 and 250 kilometers per hour for use on a highway with a speed limit of 120.
– Use volume restriction settings on your listening device. Often you’ll hear 60 percent or 70 percent of maximum volume recommended. These are good starting points.
– Time is equally important. The 60/60 rule – 60 percent of max volume for 60 minutes – is also a good rule of thumb to protect your hearing.
– Headphones are generally safer than earbuds, since the increased distance provides more buffer room.
– Choose noise cancelling devices if you frequently listen to music in noisy environments. We tend to raise volume to drown ambient noise. Check out a selection of today’s best wireless headphones – including some with noise cancelling features
– If you boost volume, shorten your listening session.