Summer 2016 References

Here are references for the Summer 2016 issue of Hearing Health magazine.

“Rocker Turned Advocate,” page 12:

“Defining Auditory-Visual Objects,” page 37:

“I Wish I Had Been Wearing Earplugs,” page 22:

WEB EXTRA: How Is Sound Measured?

By Bryan Pollard

The decibel scale is used to measure sound. Sound energy travels in waves that, if strong enough, damage the sensory organ of hearing, the cochlear hair cells.

“Absolute sound intensity” measures how much sound energy passes a given area in a given amount of time. Intensity is dependent upon the amplitude of the sound wave, squared. Sound energy is similar to electrical energy in that we can utilize a unit of watts for describing it.

So why haven’t you heard of sound intensity described as 100 watts per meter2? Long ago it was decided that the main interest in sound energy measurements related solely to the perception capabilities of the human ear. Rather than using a raw “watt” of sound energy measurement, all sounds are compared to the threshold of human hearing, which is 10-12 watts/m2.

A measure of sound pressure is more useful than raw sound intensity, since microphones basically “measure” sound pressure. Sound intensity is proportional to the sound pressure squared, so the threshold of human hearing in pressure units is 2 x 10-5 Newtons/m2 (which is also called Pascals). A sound meter measures the sound pressure and divides it by the threshold of hearing to give a ratio called the decibel. (The suffix is in honor of Alexander Graham Bell.)

Because the human ear can perceive such a broad range of sound intensity—from 20 x 10-6 to 2 x 109 Pascals, or from barely perceptible to the roar of an airplane—the logarithmic scale is used. So 10 decibels (dB) represents a sound that is 10 times louder than the threshold of hearing; 50 dB represents a sound that is 100,000 times (105) louder than the threshold of hearing.

An 80 dB sound is 10 times louder (10 times greater energy) than a 70 dB sound. But the human ear adapts to loud noise, so the 80 dB sound is only perceived as twice as loud as the 70 dB sound—unfortunately, however, hearing damage is based on the real sound energy and not the perceived loudness. So the sound that is 10 times louder can do 10 times as much damage even if we hear it as “only” twice as loud.

To further complicate things, scientists determined that the human ear doesn’t perceive low frequency sounds as loud as higher frequency sounds, so they created a weighted decibel scale. The industry standard became the A-weighted scale (dBA), which significantly reduces the sound pressure levels below 1,000 hertz for the total sound pressure calculation. In measuring sound levels, the A-weighted scale is typically used.

The lower frequencies not weighted heavily in the A-weighting may still cause auditory damage, and may also be among the more annoying sound frequencies that humans encounter because they can penetrate walls and travel farther than higher frequencies. An example is the base tones that seem to shake your whole house when a car with a customized boom box stereo dives by.

So the system we use to measure sound accounts for parameters that are particular to the human ear—the vast range we can hear—but it is also slightly confusing because it is logarithmic-derived number based on a ratio.

Now that you are armed with this knowledge that 80 dB is 10 times as loud as 70 dB, it is up to you as an informed consumer to proactively take steps to protect your hearing.

Bryan Pollard is the founder and president of Hyperacusis Research Ltd. and serves on the board of the American Tinnitus Association. He contributed to the story “I Wish I Had Been Wearing Earplugs” in the Summer 2016 issue of Hearing Health.