Hear that? No? The sound may be too low of a frequency. But according to a new study published in Royal Society Open Science, that doesn’t mean your inner ear is safe. In fact, listening to just 90 seconds of low-frequency sound changes how your inner ear works for minutes after it quiets.

It used to be people needed to only protect their ears from high-frequency sound, or any sound that's a pitch of 2,000 Hertz (Hz) and higher. (Low-frequency sound is anything 250 Hz and below.) Overexposure increases risk for hearing loss, a condition that can't be reversed. But the present study takes a closer look at the barely-audible sounds on the lower end of the spectrum.

Researchers recruited 21 volunteers without any hearing problems to sit in a soundbooth where a low-frequency sound (30 Hz) was played for 90 seconds. Once exposed to the sound, researchers used a probe to record spontaneous otoacoustic emissions (SOAE). Defined by the American Speech-Language-Hearing Association:

SOAEs are sounds given off by the inner ear when the cochlea is stimulated by a sound. When sound stimulates the cochlea, the outer hair cells vibrate. The vibration produces a nearly inaudible sound that echoes back into the middle ear. The sound can be measured with a small probe inserted into the ear canal.

And the results showed a volunteer's SOAEs became alternately stronger and weaker, meaning their inner ear was temporarily more prone to damage after low-frequency sound exposure. “Even though we haven’t shown it yet, there’s a definite possibility that if you’re exposed to low-frequency sounds for a longer time, it might have a permanent effect,” Markus Drexl, lead study author and neurobiologist, told the journal Science.

This is all good and well, but how can a person protect their ears if they usually don't even know when they're being exposed? A study published in Noise & Health suggests there are symptoms associated with lower pitch sounds, including "annoyance, stress, sleep disturbance, headaches, difficulty concentrating, irritability, fatigue, dizziness or vertigo, tinnitus, heart ailments anxiety, stitch and beating palpitation."

Source: Drexl M et al. Royal Society Open Science. 2014.