In this article, we’re going to have a close look at the tool we all use every day: the ear. This small organ has quite a few surprises in store for us. We’ll see that it’s literally crammed with equalizers and dynamic compressors, including a multi‑band one. It even includes an extremely efficient filter bank, as well as a highly sophisticated analogue‑to‑digital converter. Armed with this knowledge, sometimes referred to as ‘psychoacoustics’, we’ll discover numerous practical consequences for music production. Those include the choice of monitoring level, ideas for how to deal with bass frequencies in a mix, and a surprising antidote to frequency overlap.
Note that this article won’t attempt to cover psychoacoustics in its entirety. In particular, we’ll be restricting our focus to monaural audition and setting aside the notion of integration time, which is the audio equivalent of ‘persistence of vision’.
We’ll start our study of the ear by looking at Figure 1. This drawing shows the morphology of the ear, as usually represented. This is divided into three sections. The outer ear consists of the auditory canal and the exterior of the tympanic membrane, better known as the eardrum. The malleus, incus and stapes, which are small bones often referred to as ossicles, belong to the middle ear, along with the interior of the tympanic membrane. Then there is the inner ear, which includes the cochlea and the semicircular canals. Last, we find two nerves that connect the ear to the brain. (The semicircular canals and vestibular nerves don’t relay any information relating to hearing; their purpose is to give us a sense of gravity and balance, so we’ll leave them aside.)
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Figure 1: The morphology of the human ear (diagram derived from Chittka L, Brockmann A (2005): Perception Space — The Final Frontier, www.plosbiology.org).
What we call ‘sound’ is in fact a progressive acoustic wave — a series of variations in air pressure, spreading out from whatever source made the sound. When these pressure variations strike the ear, they find their way through the external auditory canal to the tympanic membrane, setting it into vibration. The signal is thus converted to mechanical vibrations in solid matter. These vibrations of the tympanic membrane are transmitted to the ossicles, which in turn transmit them to the cochlea. Here the signal undergoes a second change of nature, being converted into pressure variations within liquid. These are then transformed again by specialized hair cells, which convert the liquid waves into nervous signals.
This extraordinary signal path encompasses four distinct states of information: acoustic, mechanical (solid), mechanical (liquid), and electric, more specifically electro‑chemical. The very nature of the information also changes: from analogue, it becomes digital. Really! Whereas mechanical information propagation is analogous to the original sound wave, the nervous signal is even more decorrelated from the sound wave than AES‑EBU audio signals can be. Put simply, the ear includes a built‑in analogue‑to‑digital converter. Figure 2 summarizes these changes of state and nature in the sound information.
Your Idea Of A Great Noiseless Strat…
Besides such changes in state and nature, the audio signal is also subject to important changes in content. It is, in short, rather heavily equalized and compressed. Let’s begin with the external auditory canal, which has the shape of a shallow tube. Now, as I explained in a previous SOS article, reverb in a very small space is so short that it’s perceived as EQ, rather than as ambience. The reverberation of the auditory canal boosts frequencies around 3 kHz by 15 to 20 dB.
At this point, the signal is transformed into mechanical vibrations by the eardrum, which also acts as an EQ. To understand why, compare it to an actual drum. Strike a timpani and it will resonate at certain frequencies, which are inversely proportional to the size of the instrument. Strike a snare drum and, being much smaller, it will resonate at higher frequencies. Strike an eardrum, and it resonates at even higher frequencies, thus filtering the input signal correspondingly. The tympanic membrane is also attached to a muscle called the tensor tympani. When confronted with high sound pressure levels, this muscle contracts, heavily damping eardrum movements. It is, in other words, a mechanical compressor/limiter, allowing low‑level vibrations through unaltered, but damping larger vibrations.
Behind the eardrum, we find the ossicles. The purpose of these minute bones is to convert the eardrum vibrations into pressure variations in the cochlear fluid. Now, converting acoustic waves into variations in fluid pressure is no easy matter — look at what happens when you’ve got water in your ears. This means that conversion of acoustic waves from air to water is anything but efficient. Put differently, fluids have a high input impedance when receiving acoustic waves.
The Psychoacoustics Of Modulation
The ear’s answer to the problem is simple: give me a lever and I can move the earth! To override this high input impedance, the ossicles form a complex system of levers that drastically increase pressure variations from the eardrum to the entrance of the inner ear. This is made possible physically by the fact that the eardrum is 20 times the size of the cochlear window. It really works as a conventional lever does: low pressure across a wide area is converted into higher pressure on a small area.
Dealing with impedance matching with a system of levers as complex as the ossicles doesn’t come without side‑effects. The ossicles’ frequency response is not flat, turning them into another EQ. In this case, frequency response is decent around 0.5kHz, gets even better near 1-2kHz, and then degrades steadily above this frequency. The ossicles also serve as compressor/limiter, thanks to what’s called the stapedian muscle. Like the tensor tympani in the case of the eardrum, the stapedian muscle stabilizes the ossicles at high levels.
The middle ear also contains the eustachian tube. Now, the purpose of this is simple: seal the opening at the rear of a kick drum, and you suddenly get much less sound! Likewise, if you seal the cavity behind the eardrum, you suddenly have problems hearing properly. This happens regularly; for example, when we’re on an airplane or when we get a cold. In both cases, the eustachian tube gets clogged, and that prevents the tympanic membrane from moving as it should.
Caroline Guitar Company
By now, the audio signal has reached the inner ear, and that means the cochlea. This snail‑shaped organ is filled with liquid. Logically enough, it must be waterproof, in order to prevent any fluid leaking. This explains the purpose of the round window, a small, elastic membrane on the surface of the cochlea. Its purpose is to allow movement of the fluid inside the cochlea. Liquids are incompressible, and without this membrane, the fluid enclosed inside the cochlea would completely block the ossicle movements. Indeed, stiffening of the oval window can lead to hearing losses of about 60 dB.
Inside the cochlea we find the tectorial membrane, which moves along with the pressure variations of the cochlear fluid. As shown in Figure 3, this membrane is in contact with the cilia on the top of the hair cells. There are two kinds of hair cells. The outer hair cells are the actual receptors. When the tectorial membrane moves, so does the hair on the outer cells. This movement is then encoded into electrical digital signals and goes to the brain through the cochlear nerve. The inner cells have a different role: when the audio signal gets louder, they stick themselves to the tectorial membrane in order to limit its movements, playing the role of another dynamic compressor.
This tectorial membrane exhibits a clever design. Its stiffness is variable, and decreases gradually towards the center of the ‘snail’. This is a way of tuning the membrane to different frequencies. In order to understand the phenomenon, consider guitar tuning. When you want pitch of a string to be higher, you stretch it so it gets more tense, and stiffer. Generally speaking, stiffer materials are able to vibrate at higher frequencies. This makes the tectorial membrane a bank of filters, with an important result: outer cells are frequency‑specific, each group of cells being dedicated to particular frequencies. Also consider the inner cells, and their ability to attenuate the tectorial membrane’s movement. They function as a frequency‑specific compressor — in other words, a multi‑band compressor!
Millington, Tn Music School
The tectorial membrane’s decreasing stiffness towards its end serves another important purpose, which is frequency tracking. A particular audio frequency will set the membrane in motion at a particular position, and that vibration will be sensed by a specific set of outer cells. A comparatively lower frequency will set the membrane in motion closer to the center of the ‘snail’, and that vibration will be sensed by another set of outer cells. The brain, by analyzing which set of outer cells was put in motion, will then be able to tell that the second frequency was the lower one. Notice how, during this process, the tectorial membrane really acts in the manner of a filter bank, performing an actual spectral analysis of the input signal. Figure 4 illustrates the rough position of a few key frequencies on
The ear’s answer to the problem is simple: give me a lever and I can move the earth! To override this high input impedance, the ossicles form a complex system of levers that drastically increase pressure variations from the eardrum to the entrance of the inner ear. This is made possible physically by the fact that the eardrum is 20 times the size of the cochlear window. It really works as a conventional lever does: low pressure across a wide area is converted into higher pressure on a small area.
Dealing with impedance matching with a system of levers as complex as the ossicles doesn’t come without side‑effects. The ossicles’ frequency response is not flat, turning them into another EQ. In this case, frequency response is decent around 0.5kHz, gets even better near 1-2kHz, and then degrades steadily above this frequency. The ossicles also serve as compressor/limiter, thanks to what’s called the stapedian muscle. Like the tensor tympani in the case of the eardrum, the stapedian muscle stabilizes the ossicles at high levels.
The middle ear also contains the eustachian tube. Now, the purpose of this is simple: seal the opening at the rear of a kick drum, and you suddenly get much less sound! Likewise, if you seal the cavity behind the eardrum, you suddenly have problems hearing properly. This happens regularly; for example, when we’re on an airplane or when we get a cold. In both cases, the eustachian tube gets clogged, and that prevents the tympanic membrane from moving as it should.
Caroline Guitar Company
By now, the audio signal has reached the inner ear, and that means the cochlea. This snail‑shaped organ is filled with liquid. Logically enough, it must be waterproof, in order to prevent any fluid leaking. This explains the purpose of the round window, a small, elastic membrane on the surface of the cochlea. Its purpose is to allow movement of the fluid inside the cochlea. Liquids are incompressible, and without this membrane, the fluid enclosed inside the cochlea would completely block the ossicle movements. Indeed, stiffening of the oval window can lead to hearing losses of about 60 dB.
Inside the cochlea we find the tectorial membrane, which moves along with the pressure variations of the cochlear fluid. As shown in Figure 3, this membrane is in contact with the cilia on the top of the hair cells. There are two kinds of hair cells. The outer hair cells are the actual receptors. When the tectorial membrane moves, so does the hair on the outer cells. This movement is then encoded into electrical digital signals and goes to the brain through the cochlear nerve. The inner cells have a different role: when the audio signal gets louder, they stick themselves to the tectorial membrane in order to limit its movements, playing the role of another dynamic compressor.
This tectorial membrane exhibits a clever design. Its stiffness is variable, and decreases gradually towards the center of the ‘snail’. This is a way of tuning the membrane to different frequencies. In order to understand the phenomenon, consider guitar tuning. When you want pitch of a string to be higher, you stretch it so it gets more tense, and stiffer. Generally speaking, stiffer materials are able to vibrate at higher frequencies. This makes the tectorial membrane a bank of filters, with an important result: outer cells are frequency‑specific, each group of cells being dedicated to particular frequencies. Also consider the inner cells, and their ability to attenuate the tectorial membrane’s movement. They function as a frequency‑specific compressor — in other words, a multi‑band compressor!
Millington, Tn Music School
The tectorial membrane’s decreasing stiffness towards its end serves another important purpose, which is frequency tracking. A particular audio frequency will set the membrane in motion at a particular position, and that vibration will be sensed by a specific set of outer cells. A comparatively lower frequency will set the membrane in motion closer to the center of the ‘snail’, and that vibration will be sensed by another set of outer cells. The brain, by analyzing which set of outer cells was put in motion, will then be able to tell that the second frequency was the lower one. Notice how, during this process, the tectorial membrane really acts in the manner of a filter bank, performing an actual spectral analysis of the input signal. Figure 4 illustrates the rough position of a few key frequencies on
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