Between music, podcasts, gaming, and an unlimited supply of online content, most people spend hours a week wearing headphones. Perhaps you’re considering a new pair for the holidays, but with so many options on the market, it can be difficult to know what to choose. I am a professional musician and professor of music technology who studies acoustics. My work examines the intersection between the scientific, artistic and subjective human elements of sound. Choosing the right headphones involves considering all three of those aspects, so what’s really a good pair?
What exactly is sound?
In physics, sound is made up of air vibrations consisting of a series of high and low pressure regions. These are the cycles of the sound wave. The count of the number of cycles that occur per second determines the frequency, or pitch, of the sound. Higher frequencies mean higher pitch. Scientists describe frequencies in hertz, so a 500 Hz sound goes through 500 complete cycles of low pressure and high pressure per second. The loudness, or amplitude, of sound is determined by the maximum pressure of a wave. The higher the pressure, the louder the sound. To produce sound, headphones convert an electrical audio signal into these cycles of high and low pressure that our ears perceive as sound.
human ear
Human ears are incredible sensors. The average person can hear a huge range of pitches and different levels of sound. So how does the ear work? When sound enters your ear, your eardrum converts the vibrations of the air into mechanical vibrations of the tiny middle ear bones. These mechanical vibrations become fluid vibrations in your inner ear. Sensitive nerves then convert those vibrations into electrical signals that your brain interprets as sound. Although people can hear many pitches ranging 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 high-pitched bird have the same loudness, you will actually feel the rumble as quieter than the bird. Generally speaking, the human ear is more sensitive to medium frequencies than to low or high pitches. Researchers think this may be due to evolutionary factors. Most people do not know that hearing sensitivity varies and, frankly, this phenomenon would not need to be considered, it is simply how people hear. But headphone engineers definitely need to consider how human perception differs from pure physics.
How do headphones work?
Headphones Both the larger varieties that sit on your ears as well as the smaller earbuds, are just small speakers. Simply put, speakers do the opposite to your ear: They convert electrical signals from your phone, record player, or computer into vibrations in the air. Most speakers are made up of four components: a magnet to move back and forth, a coil of wire around that magnet, a diaphragm that pushes air in, and a suspension that holds the diaphragm.
Electromagnetism states that when a wire is wrapped around a magnet and the current inside the wire changes, the magnetic field around the wire changes proportionally. When the electrical signal of a song or podcast is pulsed through the wires in a set of headphones, it diverts the current and moves the magnet. The magnet then moves the diaphragm in and out like a plunger pushes and compresses the air, creating pulses of high pressure and low pressure. This is the music you listen to.
Ideally, a speaker would convert the electrical signals of the input into purely sound representations. However, the real physical world has limits. Things like the shape and material of the magnet and diaphragm prevent the speaker from perfectly matching its output to its input. This causes distortion and some frequencies are louder or softer than the original.
While no one can perfectly recreate a headphone signal, there are an infinite number of ways to choose from to distort that signal. Two equally expensive headphones can sound or feel different because they distort things in different ways. When engineers create new headphones, they have to consider not only how human hearing distorts sound, but also the physical limitations of any given speaker.
audience preference
If all the intricacies of ears and speakers weren’t enough, listeners themselves play a big part in deciding what makes a good pair of headphones. Aspects such as age, experience, culture and musical style preference all affect what kind of frequency distortion one will prefer. Headphones are as much a question of personal taste as anything else. For example, some people prefer bass-heavy headphones to hip-hop music, while classical music listeners may want low-frequency distortion. But listening to music or entertaining aren’t the only things to consider.
Headphones for the hearing impaired can highlight frequencies from about 1,000 Hz to 5,000 Hz, as this helps make speech more understandable. You can certainly play a hip-hop song through headphones designed for the hearing impaired, but most people would agree that the results won’t be 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 makers, and human experience all intersect to create the perception of good headphones. Despite all these on-the-go pieces, there’s an easy way to know when headphones are good: Pick a good song and pick a pair! Because when all the features align, a good pair of headphones can give you the opportunity to be transformed by the sound.
[This article by Timothy Hsu, Assistant Professor of Music and Arts Technology, IUPUI first appeared on The Conversation.]
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