Loud Music Sucks!
By now, most folks are aware of the potential for hearing loss by playing music too loud on headphones. I won’t bother you with that yet again. I’m going to try to take another approach. If you are turning the volume up to loud levels, you’re just throwing all that money you spent on great sound out the window. Here’s why.
The Acoustic Reflex
Your listening system has built in protection from loud noise called the acoustic reflex (also called the stapedius reflex). When you are exposed to loud sounds, small muscles (stapedius and tensor tympani) in your middle ear tense to increase the mechanical impedance of the system of small bones (oscicles) that move vibrations from your eardrum to your inner ear. The net result is a reduction in amplitude of sound being transmitted into the inner ear of up to 20dB at low frequencies, with less attenuation as frequency rises up to about 2kHz where no attenuation is seen.
Fig 1. The blue trace shows the attenuation in humans caused by the acoustic reflex. (This graph is from a paper called "Active Control of Ultrasonic Hearing in Frogs".)
The acoustic reflex is actually most active in reducing the sound level of your own voice. (In fact, birds and frogs have a very strong acoustic reflex to reduce the sound of their own calls.) If you shout “HEY!” and you’re very observant, you will notice that you hear a burst of low frequency noise in your ears. This is the sound of the muscles “firing” as they tense to reduce sound transmission. Because the muscles are directly attached to the bones of the middle ear, you can hear them as they do their work. Though this is a reflex, you can consciously tighten these muscles (it’s rather like raising one eyebrow in that it may take some practice) and observe this noise.
I tried to find references of how loud this self-generated muscle noise is to no avail, so I called Mead Killion of Etymotic to get his input. We had a lovely talk, but he also was unaware of any citation that might shed light on this muscle noise, but agreed that it was certainly there and was easily audible. I did a little experiment and played brown noise (like pink noise but less energy in the highs) and tried to level match it with my self-induced acoustic reflex noise. I came up with between 55-60dB SPL. That’s pretty loud.
I also carefully listened for a shift in frequency response, and sure enough I could easily hear a shift in emphasis to the higher frequencies. It made the brown noise quite a bit more pink sounding. Cool.
Generally speaking, the acoustic reflex kicks in at about 85dB SPL for single frequency tones, but may activate as low as 75dB SPL for broadband sounds like pink noise or music. I was surprised to hear the number was that low; 75dB SPL is about what I would consider a solid, but not loud, listening level. My normal listening level is about 70dB SPL, and I think most would consider that a fairly modest level.
Armed with the above information, let’s walk into a demo room at a trade show playing their system at 85dB SPL. Within about 100mSec --- way before you take your seat in the sweet spot --- your acoustic reflex kicks in. With 55dB SPL self-generated noise from the muscles now tensing in your ears, and a 20dB attenuation to the lower half of the audible spectrum, the signal to noise ratio drops to a miserable 10dB in the lows and some mids, and will appear to have a significantly tipped-up frequency response.
Have you ever noticed how audio systems tend to sound bright at a trade shows? I suspect it’s because they’re all being played too loud and your ears are changing things.
Here’s the bottom line: if you want to get your moneys worth out of your headphones, don’t turn them up too loud. I know it may feel like they sound better, but that’s just because your getting stimulated by the sound. Mead Killion said it on the phone to me, “If you listen carefully, you’ll find that loud music just sounds worse.”
Wikipedia’s citation on Acoustic Reflex --- In reading a lot of academic papers for this article I found that the Wiki citation is generally good, but a little misleading in spots.
Here’s a great general information page on Human Hearing.
Lastly, Etymotic makes earplugs that attenuate all frequencies evenly, check out their ETY-Plugs.