Great headphones mix physics, anatomy and psychology

Helmet designers must balance scientific limitations with human preferences.

What you like to listen to is also important when choosing the right pair.

Between music, podcasts, games, and the unlimited supply of online content, most people spend hours a week wearing headphones. You might be considering a new pair for the holidays, but with so many options on the market, it can be hard to know what to choose.

I am a professional musician and music technology teacher who studies acoustics. My work explores the intersection between the scientific, artistic and subjective human elements of sound. Choosing the right helmet involves considering all three aspects, so what makes a really good pair?

Diagram of sound waves

Sound is simply a series of low and high pressure areas where air molecules, represented by the little dots, compress or move apart. Credit: Pluke/WikimediaCommons

What is sound really?

In physics, sound is made up of air vibrations made up of a series of high and low pressure areas. These are the cycles of a sound wave.

Counting the number of cycles that occur per second determines the frequency or pitch of the sound. Higher frequencies mean higher pitches. Scientists describe frequencies in hertz, so a 500 Hz sound goes through 500 complete cycles of low pressure and high pressure per second.

The intensity or amplitude of a sound is determined by the maximum pressure of a wave. The higher the pressure, the louder the sound.

To create sound, headphones transform an electrical audio signal into these cycles of high and low pressure that our ears interpret as sound.

ear anatomy

The human ear is a complex system that transforms vibrations in the air into electrical signals that go to the brain. Credit: Iain/WikimediaCommons, CC BY-SA

The human ear

Human ears are incredible sensors. The average person can hear a wide range of pitches and different volume levels. So how does the ear work?

When sound enters your ear, your eardrum translates the vibrations in the air into mechanical vibrations of the tiny bones in your middle ear. These mechanical vibrations become fluid vibrations in your inner ear. Sensitive nerves then transform these vibrations into electrical signals that your brain interprets as sound.

Although people can hear a range of pitches from about 20 Hz to 20,000 Hz, human hearing does not respond equally well to all frequencies.

For example, if a low-frequency rumble and a higher-pitched bird have the same volume, you will actually perceive the rumble to be quieter than the bird. In general, the human ear is more sensitive to medium frequencies than to low or high frequencies. Researchers believe this may be due to evolutionary factors.

Most people don’t know that hearing sensitivity varies and, frankly, they would never need to take this phenomenon into account – it’s just how people hear. But helmet engineers absolutely have to take into account the difference between human perception and pure physics.

Speaker design

Loudspeakers are basically made up of four components, a magnet (1), a coiled wire (2), a spring or suspension (3) and a diaphragm (4). Credit: Svjo/WkimediaCommons, CC BY-SA

How do the headphones work ?

Headphones – the two larger varieties that sit on your ears as well as small headphones – are just small speakers. Simply put, speakers do the opposite of your ear: they convert electrical signals from your phone, record player, or computer into vibrations in the air.

Most loudspeakers are made up of four components: a stationary magnet, a coil of wire that moves back and forth around that magnet, a diaphragm that pushes air, and a suspension that holds the diaphragm.

Electromagnetism states that when a wire is wrapped around a magnet and the current in the wire changes, the magnetic field around the wire changes proportionally. When the electrical signal from a song or podcast passes through the wires of headphones, it changes the current and moves the magnet. The magnet then moves the diaphragm in and out – much like a piston – pushing and compressing the air, creating high and low pressure pulses. It’s the music you hear.

Ideally, a loudspeaker would perfectly convert the electrical signals from the input into sound representations. However, the real physical world has limits. Things like magnet and diaphragm size and material all prevent a speaker from perfectly matching its output to its input. This leads to distortion and some frequencies are louder or softer than the original.

While no headphone can perfectly recreate the signal, there are endless ways to distort that signal. The reason two headphones this expensive can sound or feel different is that they distort things in different ways. When engineers build new headphones, they not only have to consider how human hearing distorts sound, but also the physical limitations of any speaker.

Listen to headphones

What you like to listen to and how you like the sound of your headphones play a huge role in determining what makes a “good” pair of headphones.

Listener Preference

If all the hassle of ears and speakers weren’t enough, listeners themselves play a huge role in deciding what makes a “good” pair of headphones. Aspects such as age, experience, culture, and musical genre preference all affect the type of frequency distortion someone will prefer. Headphones are as much a matter of personal taste as anything else.

For example, some people prefer bass-heavy headphones for hip-hop music, while classical music listeners may want less frequency distortion. But music or recreational listening aren’t the only things to consider. Headphones for the hearing impaired can highlight frequencies from around 1000 Hz to 5000 Hz, as this helps to make speech more understandable.

You can certainly play a hip-hop song with headphones designed for the hearing impaired, but most people would agree that the results won’t sound great. Making sure the headphones you choose match how you’re going to use them goes a long way in determining what will sound good.

Ultimately, the science of headphone design, the artistry of content creators, and the human experience intersect to form the perception of “good” headphones. Despite all those moving pieces, there’s a surefire way to tell when headphones are good: pick a good song and put on a pair! Because when all the attributes line up, a good pair of headphones can give you the opportunity to be transformed by sound.

Written by Timothy Hsu, Assistant Professor of Music and Arts Technology, IUPUI.

This article first appeared in The Conversation.

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