The Science Of Motorcycle Sound

The roar of a wide-open throttle, the robust, throaty growl of a big V-twin, the thrumpy exhaust notes of a vintage classic—just some of the motorcycle sounds that have entertained petrol heads all around the world for as long as motorcycles have existed. Whether it’s the engine noise, the gears shifting, the tire-pavement interactions, body vibrations, or maybe even some familiar rattling, motorcycles produce sound when they move. Even the pitch and volume of an electric bike’s gear set makes a faint mechanical whine which escalates with speed.

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The particular sound of a bike is very important to riders and motorcycle enthusiasts alike. Anyone who rides knows there’s no comparison for the suite of senses that makes up the experience of riding a motorcycle, including sound. Motorcycle brands know the value of a well-crafted sound and how it affects potential buyers. Entire departments are devoted to precisely tuning engine and exhaust design in order to appeal to an innate emotional response resulting from sound.

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Just consider how different watching a motorcycle race on a screen is as opposed to actually being there, simply because it lacks the auditory bombardment of a racetrack. Imagine being at the racetrack, but in total silence. It doesn’t seem fathomable because we’re so used to the immersive sound that comes along with the intensity and speed of racing.

Different engines sing different songs as they release compressed gas from the cylinders, then through the exhaust valves and exhaust system, crafting their own unique sound signatures. Some designs make such distinctive sounds that they are instantly recognizable long before the bike comes into sight. Whatever your preference may be, whether it is the balanced drone of a horizontally opposed engine or the uneven interval firing of a V-twin, the sound signature is a strong characteristic of our ride.

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Even without a combustion engine, some electric bike makers have added a digital exhaust, allowing you to choose a different sound for your otherwise quiet bike by using an app on your phone.

It is clear that sound isn’t just something that we hear, it is something we feel. Sound is essentially touch at a distance. There is something unmistakable about the powerful auditory and visceral sensation produced by motorcycles which never gets old and enriches our lives every time one fires up.

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We are sensorial creatures, moving about the earth in response to our environment. Human senses have been shaped over time by evolution. Yet we do not see with our eyes nor hear with our ears—we accomplish these things with our brain. The eyes and the ears are simply the receptors of stimuli which are then transported to our brain to be interpreted as a sight or a sound.

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However, the outer ear you see is not the only receptor of sound. Our entire bodies are conductors of sound, including our bones. One of the first examples of sound conduction through bone was from Beethoven, who began losing his hearing at the age of 26. He continued composing music by resting his skull on the piano in order to “hear” the vibrations of the piano strings. He also attached a thin rod to the piano and would clench the rod between his teeth, experiencing the vibrations through his jaw.

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You can easily test this out by completely blocking your ear canals (or just covering your ears) and using your voice. You can still hear yourself because the vibrations coming from your vocal chords are vibrating your jaw, your skull, and your sensitive inner ear. This form of sound perception is known as bone conduction. It is with this same technology that bone conduction headphones have been created.

Rather than sending sound through the auditory canal, bone conduction headphones bypass the eardrum entirely, sending vibrations through the bones in your head to the cochlea, the sensory organ which translates them into nerve impulses for the brain to interpret.

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This leaves your ears available to listen for important sounds while on the road—auditory cues which could save your life, such as oncoming traffic, squealing tires, or emergency sirens.

This technology has already proved helpful for soldiers in war zones, so that the person on the ground can hear communication through bone conduction while still listening to what is happening around them through their ears.

Additionally, this is how elephants communicate over such long distances. Bone is a solid structure and therefore a very fast conductor of vibrations. Elephants stomp on the ground, signaling to other elephants via low-frequency seismic vibrations. The vibrations travel along the earth and up through the bones of another elephant, reaching the cochlea and the brain. The males even touch their trunk to the earth as a way to triangulate the direction in which the female is sending the signal from.

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Water also transmits these vibratory sound waves over great distances and some whale skulls and mandibles (jawbones) have acoustic properties which conduct low frequencies directly to the inner ears.

As you probably know, we’re made up of two-thirds water. However, you may not know that because the water molecule is so small, this quantity translates into 99 percent of our molecules. Water is a great conductor of sound.

Additionally, it has been found that humans also listen through the largest organ in the body, our skin. A type of sensory receptor covering our skin at varying densities known as mechanoreceptors are capable of sensing sound.

Therefore, depending on the bike you’re on, there is a very particular sound that emanates through you as you ride. Not just via the vibrating of your eardrum, but traveling through your entire body—your skin, your organs, and your bones.

It is no wonder we get so attached to our motorcycles.

Interestingly, there is promising research on the healing properties of certain vibrations, such as those discovered by Dr. Lee Bartel in his findings of how sound can stimulate cells in the body and brain to increase blood flow. So far, this treatment has shown significant positive effects for sufferers of Alzheimer’s, dementia, depression, and fibromyalgia.

Of course, there has yet to be any scientific studies done on the physiological ways in which the sounds and vibrations of a motorcycle affect the human body and mind, but if this research were to take place, I would be very intrigued to learn the results. Perhaps, like me, you consider your time on two wheels meditation in motion and feel a sense of relief while out on the road.

Clearly, there are other sensory stimuli at play which likely also attribute to an increase in overall well-being while riding, such as the movement of wind, the experience of velocity, the novelty—whether you’re riding somewhere new or simply noticing changes in your surroundings, perhaps the sun on your face, and/or the views that you travel through (hopefully beautiful land- or urban-scapes).

The majority of us are so used to sound being such a huge part of life that we never stop to wonder what, exactly, sound is. Sound is a pressure wave created by vibration through a medium. Sound is a form of energy, a force which sets surrounding particles in motion, thus transporting this energy through oscillations in air pressure—as a wave—through space and time.

These sound waves need a medium to travel through, such as a solid, liquid, or gas. It is true that there is no sound in a vacuum, however this does not entirely apply to outer space. The majority of space as we know it isn’t an empty void; particles still exist, but they might be so spread out that the sound traveling through them is at a low enough frequency that the human ear cannot detect it.

Ever since I started spending a lot of time out on the road, I’ve met at least a dozen older riders who have felt compelled to express the need for ear protection while riding. It isn’t the bombastic sounds from the exhaust, but rather the damaging effects from the turbulent airflow around your head—the wind.

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Studies have shown that even a full-face helmet cannot completely protect your ears from the permanent damage caused by wind. Just a couple of years ago, research was conducted which revealed some pretty scary results. The studies showed that permanent hearing damage can be caused after only 15 minutes of riding at 62 mph (100 kph) without earplugs. Just as you might expect, the faster you go the less time it takes to cause irreparable harm.

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Filtered earplugs are recommended in order to protect motorcyclists from the severe damage caused by the high-frequency wind, while still allowing them to hear lower-frequency sounds, such as their engine, human speech, and oncoming traffic.

A recent study tested a novel acoustic material designed to significantly reduce noise caused by the turbulent wind around the helmet. It is a porous, natural leather foam set inside the motorcycle helmet with a remarkable ability to decrease the noise caused by a wide range of high frequencies.

Although sound is currently an inextricable feeling of riding motorcycles, we should respect the powerful force of wind regardless of what bike we’re on, and recognize the incredible nature of our sensitive eardrums. Wind, after all, carves canyons and shapes mountains. Losing the ability to hear our loved ones in old age is incredibly sad, especially if it can be prevented via a combination of bone conduction headphones and filtered earplugs while riding—which still applies for quiet electric bikes.

For as long as humans have recognized that sound helps or alters mood and mind states, we’ve been using technology to induce some desirable state of mind. Perhaps this will someday be applied to electric bikes—just imagine restorative binaural beats emanating from your running motorcycle, mechanical vibrations designed to improve brain and other organ functioning or even bone density as you ride. Only time will tell where the unseen waves of sound carry us.

Do you have any experience with filtered earplugs or bone conduction headphones while out on two wheels?

As we transition away from non-renewable fossil fuel sources, do you imagine a quiet future for motorcycles and racing?