How hearing works
Our hearing is a wonderful and complex system that allows us to engage with the world around us in an entirely personal and unique way. You may not have considered it before, but just like you have a visual horizon, so too do you have an acoustic one. Everything within your own horizon of sound defines your auditory space, and here’s just how you interpret it.
The genesis of any sound comes from vibration, movement that displaces air particles and sets them into motion. These particles forced into movement, rudely bumping into their surrounding particles, causing a chain-of-events in movement outward from the original source. This results in a wave like motion that travels, taking with it the unique signature of the sound to the listener.
For sound, this wave travels longitudinally, at the speed of 341 meters per second, at standard air temperature and pressure.
For a sound to move, it has to pass through a medium – any space or object that has particles to move. As the transmission of sound relies on particle movement, it will move fastest through mediums of higher density, such as solids before liquids, and finally, gases. If it passes through a vacuum, however, such as space (where it’s estimated there is just one atom per cubic meter!) there simply aren’t enough particles to move. In this environment, no particles means no vibrations, which in turn, means no sound, so don’t bother yelling.
Pitch vs. Volume
So if sound in essence is vibration, how do we hear volume and pitch differences? It turns out volume corresponds directly to the amplitude of the sound wave. If you throw a rock into the ocean, you might notice a small bump of a wave move out from it. Throw a meteor in there, however, and we’re guessing the tidal wave in response shows our point. The more intense the initial vibration, the greater the amplitude or height of the wave. Think of a snare-drum, hit it softly, and the corresponding sound is soft, hit it as hard as you can, and the corresponding sound is much louder than before.
For pitch, we use our ears to decipher the length of the wave, i.e. the distance across between any two peaks. Again coming back to the ocean, you can think of this like the distance between waves in a set. If they come quickly one after another, they’re coming in at higher frequency, if they come further apart, they’re coming in at a lower frequency. For a sound, if the wavelength is long in length (a greater distance between it’s peaks) we get a low frequency sound, if it is a short distance between peaks, a high frequency sound. A guitar is a great example of this; pluck a lower E string (82 Hz), and the wavelength would be roughly 4.15 8 meters long if it could run it’s course, far longer than the high E string (330 Hz), at 1.03 meters in length.
The Human Ear
The full range of frequencies humans can hear runs from 20 to 20, 000 Hz. While we may not have the best hearing in the animal kingdom, our ability to hear pitches is our most finely tuned sense.
After a long journey to our ears, sound goes through a series of energy conversions, from acoustic, to mechanical, to electrical energy. It all starts with our pinna, the outer portion of our ear which acts as a giant funnel for sound. This also has a resonant property, giving a slight boost to frequencies around 6kHz. From there, sound bounces along and down our ear canal before it hits the thin membranous skin of our ear drum (aka Tympanic Membrane). If you were to cup your hands around your mouth and yell, you’d notice certain parts of your voice amplify. Our ear canal works the much the same, however boosting pitches around 2.7 kHz by up to 20 dB (huge!) as they pass through. One theory behind this combined 2.7 and 6 kHz boost from our outer ear is related to it our genetic past, being the right sounds to hear when listening out for potential mates up in the trees.
Once sound batters against the thin Tympanic Membrane, it is converted into mechanical energy, shaking not only it, but the three smallest bones; Malleus, Incus and Stapes (aka hammer, anvil and stirrup). These bones form a small chain called the Ossicles, which hang like a suspension bridge in the Middle Ear cavity. This space is filled with air, allowing the free movement of the bones with sounds as they come through. It’s important for this to stay air-filled, as if it becomes congested or hyper-pressurised (e.g. often with a cold), the movement stiffens, dampening the sound. Keeping this space aerated is the job of the Eustachian Tube, a small opening at the base that continues to the back of the through, opening and closing automatically as needed.
At the end of the ossicular chain, the final bone (stapes), hits like a piston on the small door to our organ of hearing, the oval window. This whole middle ear process acts as a natural amplifier, boosting the volume of sound by up to 57 dB through differences in area, size and placement of each component. And it’s a good thing it does, as we need this to compensate for a later loss of intensity as sound hit’s the thick fluid of our inner ear.
Once sound has traveled mechanically through the middle ear, vibrating on the oval window, fluid moves in our organ of hearing; The Cochlea. Just like dropping a pebble in a pond, the vibrations cause wave like motions that correspond to the frequency and amplitude of the sound. As these waves pass over and through the Cochlea, they cause minute little hair-cells to move with them. Hair-cells can be thought of like sea-weed on the bed of the ocean floor, bending back and forth with the movement of surrounding waves. If the amplitude of the wave is strong enough, a cluster of hair-cells will move together, causing an electrical impulse to be triggered up the auditory nerve, interpreted as – you guessed it – sound. Due to their short length, high pitches move hair-cells at the entrance to the Cochlea (closer to the oval window), while low pitches travel further, stimulating those at the basal end of it’s snail-shell shape.
Tone and Timbre
The center for hearing in our brain is the Auditory Cortex. It along with an intricate chain of centers leading up to it, make up the auditory pathway, and is responsible for processing the complex array of sounds we hear. Music is one of the most complex of all sounds we could interpret, changing in frequency, amplitude and dynamic range quicker than what our ears evolved for; speech. The tone and timbre of music are closely related to our first point, frequency. The pitch of any instrument is never just one defined pitch, but rather hides behind it a shadow of smaller hidden frequencies heard by our mind. This shadow are harmonic frequencies, mathematical multiples of the dominant pitch that create the complex nature of each instruments tone and timbre. It’s the reason why a guitar and a violin playing the same note sound so incredibly different, they are made of different harmonics.
Next time you’re listening to music, take a moment to appreciate the incredible work your ears are doing to interpret what you hear. If you you think sounds worth sharing – share us too. We’re hear to help.
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Last updated June 15, 2017. Conductive Mechanics of Hearing Available from: https://en.wikibooks.org/wiki/Engineering_Acoustics/Conductive_Mechanisms_of_Hearing