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Let’s Talk Some Anatomy – No, we wont ask you to memorize any bone or organ names, and there will be no quiz. But, let’s talk some anatomy, we thought it would be fun to round out 2020 by sharing some of the mechanisms that provide human hearing. Of course, none of these other acoustic topics would matter in the least if our bodies were not such marvels of nature!

The detailed diagram above shows a cross section of our ear and the hearing organs. In a nutshell, the auricle (or pinna) helps catch sounds and guide them into our ear canals. The sounds then vibrate the eardrum, which transmits these vibrations to the cochlea and then to the brain.

The Cochlea: Nature’s Spectral Analyzer

Our cochlea is lined with tiny hairs that vibrate proportionally to the intensity of the incoming sound. Various frequencies cause hairs at different positions in the cochlea to vibrate. The graphic below shows that the lowest frequencies are sensed at the deepest position within the cochlea.

Let’s paraphrase one of our favorite acoustic engineers from the 20th century, Harry F. Olson:

Hmm, Bowers and Wilkins. This shape looks a bit familiar. Nice work!

Approximately 4000 nerve fibers run from the cochlea to the brain, with 5 hairs per nerve fiber. Our ears demonstrate lightning-fast response; only a few cycles of most frequencies are needed to bring them to full sensitivity. Recall that the equal temperament scale has about 120 notes across the audible range. Our ears have the ability to distinguish about 1500 separate frequencies. Most, if not all, electrical systems are incapable of resolving this resolution of frequency this rapidly. (Olson, Harry. Music, Physics, and Engineering. Dover, 1967.)

And What About Relative Loudness?

We’re glad you asked! Quite a bit of study has been done on this topic, and several different sets of curves have emerged from various studies. Shown below is the current international standard, updated from the original Fletcher-Munson curves you may be familiar with.

How did they come up with these squiggly red lines? A quick description starts like this: a 1000 Hz tone was played at the threshold of audibility, and the sound pressure level for this tone was measured at about 2 dB SPL. Then tones were played up and down the audible range, once again at the threshold of audibility, and the corresponding sound pressure levels were measured. This test set produced the lowermost red curve. This process of loudness “equalization” was conducted at several levels, extending up to 100 phons (about the loudest that an orchestra plays).

How do I interpret them? The general trend is that the ear is most sensitive to sounds in the upper treble (specifically, 2-5kHz) range, and least sensitive to very low bass. This means that in order to sound equally loud, the measured SPL of bass sounds must be higher (in some cases, a LOT higher!) than most of the mid/treble range. Also worth noting is that our ears are even less sensitive to bass at very low loudness levels.

Ah-ha! This is what the “loudness” button on your old stereo gear was compensating for.

Check out how the ear converts airborne sound into beautiful music!

Have a Great Start to the New Year!

Hopefully you enjoyed this quick lesson in anatomy, physiology, and psychoacoustics. We certainly find these topics interesting and relevant to the services we provide for you, our loyal TubeTrap users. Without the amazing sensory tools we are fortunate enough to possess, life would lack the musical sparkle that we love so much.

Cozy up to your favorite sound system and play your favorite record or review your best mixdown, and just imagine what is going on between your speakers and your brain. No wonder acoustics are so important!