Last month, we took a look at amplifier characteristics. Well, I realized I may have put the cart before the horse. You see, I recently visited my audiologist to get fitted for custom IEMs. Being a tech-head, I took the opportunity to grill my doctor about hearing, our ears and perception.

That conversation caused me to do some digging around and it made me realize something important. We have been publishing fancy charts and graphs that we see with our eyes.However, we hear with our ears. The perceptions we form in our mind about sound based upon what we see in a chart can sometimes be significantly different than the perceptions we form from hearing that which was visually depicted in the chart. I am sure that most of you have figured that out. In this column, we will look at why that is so.

Interestingly, (at least to me) the eye is formed from brain cells having approximately 130 million light sensitive receptors. The ear is formed from skin cells having approximately 15,000 receptors. Incidentally, despite this difference in receptor cells, there are about the same number of visual and auditory cortical cells.

The operation of the ear is to transform air molecule vibrations into the neural firing that the brain uses to interpret sound, including frequency and loudness. For purposes of this article, I’d like to run through a refresher on the science of hearing.

With reference to Fig. A, the key parts of our ear are shown. We can refer to the ear as having three parts, including the outer ear, the middle ear and the inner ear. The outer ear includes a pinna and ear canal. The pinna serves as a frequency filter, so that as a sound source moves relative to a listener, different frequencies are filtered, thus providing the brain with localization information.

Once sound waves travel into the ear canal, the waves vibrate our eardrum, which is essentially a thin piece of skin that is stretched taught by a muscle.The middle ear connects our eardrum to an oval window of the cochlea within the inner ear by three bones, the hammer, anvil and stirrup. These three bones are held in place by a pair of muscles. These three bones serve to amplify the vibration of the eardrum, which is necessary because the cochlea conducts sound through fluid (try pushing your arm through the air, compared to pushing your arm through water; the water creates much more resistance, hence the need for the amplifier). When stimulated by high intensity sounds, a muscular contraction stiffens the bones, attenuating low frequencies (below 1kHz) that reach the oval window.

The inner ear includes the cochlea, which is responsible for transforming middle ear fluid vibration into neural firings. The cochlea includes three tubes separated by sensitive membranes. The tubes are coiled, like a snail shell. As the stirrup (third bone of the middle ear) moves back and forth, it creates a wave that moves from the oval window down the length of the cochlea. There are thousands of fibers that extend widthwise within the cochlea, extending from this window. The fibers are short and stiff near the oval window, and are relatively longer and less stiff further from the window. Higher pitches vibrate the fibers closer to the window, while lower frequencies vibrate more intensely further down the length of fibers.

Now, here is where things get interesting and mysterious. A structure on a membrane within the cochlea contains thousands of hair cells.Movement of the hair cells generates the neural impulses. Inner hairs located along the inside curve respond to vibration differently from the outer hairs along the outside curve. The inner hairs are arranged in a single row along the lower membrane. The outer hairs are arranged in three rows.Despite having approximately four times more outer hairs than inner hairs, approximately 90 percent of the vibrations of the auditory nerve connect to the inner hair cells.

When the fibers vibrate at a resonant frequency, a burst of energy is released in that area, which pushes on corresponding hair cells at that area.When each hair cell is distorted, it fires a pulse. However, a hair cell needs to reset before it can fire another pulse.No matter how hard the burst of energy, the hair cell cannot fire again until it resets.

It is believed that the fastest rate that a pulse can be sent by a nerve is about 1kHz. Accordingly, the hair cells transmit a pulse at up to 1kHz. Huh?So how do we hear frequencies above 1kHz? Well, that part is still a mystery.However, one theory is that the hairs encode vibration information above and below 1kHz by controlling the number of cycles per firing. That is, each hair fires a maximum of once per cycle of vibration/burst of energy.However, the hair may skip vibrations based upon certain patterns. The cerebral cortex takes this encoded data and makes sense of it.

Where is all of this going? In order to detect pitch information, these hairs need to fire in a way that encodes sufficient information for the cerebral cortex to decode the correct pitch. Our cerebral cortex has logic built in that attempts to distinguish “false triggers” from real information. As such, a number of vibrations must be detected in order for pitch to be deciphered.Until the pitch is deciphered, we perceive the vibrations as noise. Some research has shown that three cycles of 100Hz tone (about 30 milliseconds of information) is required. It is possible that the number of cycles can get shorter for louder sounds. However, even for louder sounds, the membrane has to build up to equilibrium and cannot instantaneously vibrate to its full extent.

So, now take a look at Fig. B. This is me playing a slap line. Notice that the signal includes a sharp attack at each note. Let’s focus on the first note. The initial attack occurs just before 600 milliseconds, and exhibits a positive peak of just under 1 V.

However, that sharp attack only lasts one cycle. As we have learned, our ear cannot discern pitch from a single cycle. So, we may perceive noise at this point, or our brain may be waiting for more information before deciding whether there was in fact, a sound, or if one or more hairs simply misfired.

By the time our ear gets enough cycles to interpret this as a note, our signal level has dropped dramatically. As each note is played, there is an initial burst of energy, exceeding 1.6 V positive peak at 2.8 seconds into this performance. Despite peaking at 1.6 V, the instrument cannot sustain that level for more than a cycle. Note that my positive peaks varied from under 1 V to over 1.6 V. However, the bass always settled into a range between 400 mV and 600 mV after the first cycle. As such, our ear will filter and average that peak out so that over time, the envelope of our note is what we will perceive.

Am I saying that you cannot perceive those strong peaks? No. You may or may not depending upon your ears, your encoding mechanism and the ability of your cortex to decode the information fired from the hairs in your cochlea. I am suggesting however, that those peaks may not be as important as you think. [I am setting aside artifacts such as distortion that an amplifier may generate attempting to reproduce those peaks – that is the subject of another article.]

Take a look at Fig. C. This is a walking blues line. Note that the initial attack is not as prominent as the case in Fig. B, where I was playing a slap line.However, our ears encode the information that we hear, which requires time/cycles to decipher. As such, we tend to perceive our bass tone based upon its average signal. My slap line may have much larger peaks than

my walking bass line. However, my walking bass line has a longer envelope. Over time, our brains will tell us that the line of either Fig. B or Fig. C will sit in the mix and the listener will perceive each – over time – as being close enough in volume that we will not be reaching for the volume knob to change the level. All this, despite the fact that at certain points in time, the charts tell us that the slap line should be clearly louder, etc.

So, why are we going through all of this? Simple. It is easy to see a chart and allow your eyes to tell you something that seems clearly plausible, if not down-right obvious.However, our ears function differently than our eyes. Look at the reviews, study the charts and learn what you can. But at the end of the process, pay more attention to the in-hand review.Then, go out and try the gear yourself, and pay more attention to your own in-hand review.

We spend hundreds, thousands, tens of thousands, etc on gear in the pursuit of tone. But don’t forget that the single most complicated and sophisticated gear we have is our ears. However, our ears cannot be upgraded or replaced.Moreover, no matter how hard you work for awesome tone, if you damage your ears, you will not be able to appreciate it. Being a musician is more like running a marathon, not a sprint, so you owe it to yourself, in the long run, to take care of your hearing.