If you're like the many people that use headphones each and every day, you've likely wondered how they produce sound. In this beginners' guide, we'll discuss the inner workings of headphones simply before diving into the more technical details.
How Do Headphones Make Sound? Headphones act as transducers and convert audio signals (electrical energy) into sound (mechanical wave energy) via a moving diaphragm. Most headphones use magnets and conductive coils (electromagnetic induction) to move these diaphragms back and forth, which, in turn, produce sound.
This quick explanation may seem a bit overly technical. This article aims to dispel the confusion and make it easy to understand how your favourite earbuds or headphones work.
What Is Sound?
Before answering this question, it's important we understand what sound actually is.
Sound is defined as vibrations on a longitudinal wave that travels through a medium (gas, liquid or solid). Sound waves are made of mechanical wave energy and cause localized oscillations in the matter they pass through.
It's important to reiterate that sound waves carry energy, but not matter. They induce localized variations in pressure that cause the particles within a medium (like air) to vibrate in place. They do not carry the air particles along with them.
These waves cause relatively fast vibrations in the medium. The vibrations are measured in cycles per second, otherwise known as Hertz (Hz). The frequency of these vibrations is known as just that: the frequency of the sound.
Higher-pitched sounds have higher frequencies (they cause particles within a medium to vibrate faster), and lower-pitched sounds have lower frequencies (they cause particles within a medium to vibrate slower).
For simplicity, let's continue our discussion on sound by assuming it's travelling through air.
When sound waves reach our ears, they vibrate the air molecules at our eardrums which, in turn, vibrate our eardrums. Our ears are designed to convert eardrum movement into electrical signals for our brains to decipher.
Without getting into riddles about trees falling in the woods, sound can typically be heard when they reach a person's or animal's ear. I say typically because of the different ranges of sound include audible and non-audible frequencies. Frequency ranges of sound are loosely defined as:
- Infrasound (inaudible): <0.1 Hz – 20 Hz
- Audible sound (human range): 20 Hz – 20,000 Hz
- Ultrasound (inaudible): 20 kHz – 1 GHz and beyond
Even in the audible range, hearing damage can limit the response of our hearing even further.
Humans are generally born with the ability to hear across the entire 20 Hz – 20 kHz range. As we age and damage our hearing, this range shortens (particularly in the high-end). On top of this, our ears/brains are not equally sensitive to all frequencies in the spectrum. The Fletcher-Munson curves are a great resource to relate our hearing sensitivity across the audible spectrum of sound.
Sound waves travel at varying speeds depending on the medium and the temperature of that medium. Here are a few examples:
- Air @ 20°C (68°F): 343 m/s (1125 ft/s)
- Air @ 0°C (32°F): 341 m/s (1119 ft/s)
- Water: 1482 m/s (4862 ft/s)
- Steel: 5960 m/s (19554 ft/s)
Depending on the speed of sound, frequencies will have different wavelengths. The relationship between the frequency of a sound and the wavelength of a sound is represented by the following formula:
λ = v/f
• λ = wavelength
• v = velocity of sound
• f = frequency
For more information on frequencies and wavelengths and tables that relate these properties of sound to musical notation, check out My New Microphones article titled: Fundamental Frequencies Of Musical Notes In A=432 & A=440 Hz.
In general, lower frequencies require more energy to create, but their longer wavelengths are steadier, more omnidirectional, and resistant to damping.
On the other hand, higher frequencies require less energy to produce but dissipate faster and a more prone to dampening and cancellation within an acoustic environment.
Vibrating objects within a medium create sound waves. As an object vibrates within the audible range of frequencies (20 Hz – 20,000 Hz), it causes the air particles (or the particles of the particular medium the object is in) to vibrate as well.
Imagine it this way:
The sound source (object) vibrates, which causes the air particles around it to vibrate similarly. These particles then interact with their neighbouring particles, causing them to vibrate as well. These particles that are not touching the original sound source push and pull their neighbouring particles further from the source. This trend continues as the sound waves propagate outward from the sound source.
With this vision in mind, we can envision how sound energy would dissipate as its energy is lost in the friction of air particles rubbing against one another. This is true, and the rate of sound intensity loss can be summed up in the inverse-square law.
The inverse-square law states that sound intensity will be quartered for every doubling of distance from a sound source. In other words, a sound wave will drop 6 dB for every doubling of the distance it travels.
The study of acoustics has to do with the relationship between sound and space and how sound travels within different spaces and mediums.
Sound waves can be reflected and absorbed by surfaces and other mediums. Standing waves can be caused when the wavelengths are equal to a particular dimension of a room. Resonant frequencies cause accentuation in a particular frequency within a solid. Waves from varying sources interact constructively to boost certain frequencies and destructively to reduce other frequencies.
Acoustics are incredibly complex and complicated but worth mentioning briefly in our discussion of sound.
To recap, sound is the vibration of particles in a medium due to mechanical wave energy. Audible sound, which we are most concerned with, has a frequency range of 20 Hz – 20,000 Hz.
Headphones are transducers that convert audio signals into sound waves. For a detailed article on the differences between sound and audio, check out My New Microphone's post titled What Is The Difference Between Sound And Audio?
How Do Moving Diaphragms Produce Sound?
As discussed in the previous section, vibrating objects in a medium will produce sound if they vibrate within the audible range of 20 Hz – 20 kHz.
Moving diaphragms, like those found in headphones and loudspeakers, are designed to vibrate in this range in reaction to the audio signals sent to them. With our knowledge of how sound works, let's explain how moving diaphragms produce sound waves.
The diaphragm is a thin membrane designed to oscillate back and forth.
As the diaphragm moves outward, it pushes against the air molecules it comes in contact with (to the front of the diaphragm) and compresses them, causing positive sound pressure.
As the diaphragm moves backward, it pulls those same molecules (to the front of the diaphragm) backward, causing negative sound pressure.
The vibrating air particles around the diaphragm interact with their neighbouring particles, and the effect of the diaphragm's movement expands out through the medium.
The effect the diaphragm movement has on outward air particles is represented by a sound wave. A sound wave has peaks of maximum compression (positive sound pressure) and peaks of maximum rarefaction (negative sound pressure).
The waves are naturally far-reaching but do lose significant strength as they propagate through the medium.
What Makes Headphone Diaphragms Move?
So we've got a pretty good idea of how the diaphragms in headphones produce sound, but what makes these diaphragms move?
The answer is found in the headphone driver.
The driver is the transducer element of any given headphone. A transducer is a device that converts one form of energy into another form of energy.
In the case of headphone drivers, the conversion is between electrical energy (audio signals) and mechanical wave energy (sound waves).
This article is meant to focus on the basics, so we'll discuss the moving-coil dynamic headphone driver. This driver type is used in the vast majority of headphones on the market today and is worth understanding.
Note that the moving-coil design is also used in most loudspeakers, studio monitors and subwoofers. It is also common in microphones, though wired in reverse to convert sound waves into audio signals.
To learn more about moving-coil dynamic microphones, check out My New Microphones' article The Complete Guide To Moving-Coil Dynamic Microphones.
Before we begin, I must preface the dynamic headphone driver explanation with an explanation of its working principle: electromagnetic induction.
Electromagnetic induction states that when an electrical current passes through a conductor, a magnetizing force and magnetic field are developed around it.
It also states that a voltage will be produced across an electrical conductor in a changing magnetic field.
To understand the dynamic headphone driver, we'll focus on the first definition.
Now let's get into how the dynamic moving-coil headphone driver works.
The Dynamic Moving-Coil Headphone Driver
As the name suggests, this driver design utilizes a coil of conductive wire that is intended to move.
Let's have a look at a simplified and labelled cross-sectional diagram of a moving-coil driver to visualize better:
As we can see above, the moving coil (also known as the voice coil) is attached to the diaphragm and suspended in a cylindrical pocket in an oddly-shaped magnet.
The audio signal is applied to the coil. This effectively sends alternating current through the conductive coil, which produces a varying magnetic field in and around the coil due to electromagnetic induction.
This varying magnetic field interacts with the permanent magnetic field of the oddly-shaped magnet.
As we may know, magnets with the same polarity repel one another while magnets with opposite polarity attract.
With this design, the attraction and repulsion of the two magnetic fields cause the moving coil to oscillate up and down according to the audio signal.
Because the diaphragm is attached to the moving coil, it moves in sync with it. As we've discussed previously, a moving diaphragm produces sound.
And that is about as simple as I can make it! The driver effectively converts audio signals into sound waves.
For a more detailed explanation of dynamic headphone drivers, check out my article What Are Dynamic Headphones And How Do They Work?
To learn more about headphones and magnets, check out my article Why & How Do Headphones Use Magnets?
Other Headphone Types
Though the vast majority of headphones are designed with the moving-coil dynamic drivers we've discussed, there are other designs you should know about.
Though beyond the scope of this article, I'll add links to in-depth articles on each other driver types in the following list. The other headphone driver types are:
What is the headphone hole called? The hole that headphones are designed to connect to is called a headphone jack, while the wired connector tip of the headphones is called a plug. There are different sizes of headphone jacks/plugs, and there are various wiring/connection standards for these connectors.
To learn more about the various headphone jack/plug sizes, check out my article Differences Between 2.5mm, 3.5mm & 6.35mm Headphone Jacks.
How does a headphone jack work? Headphone jacks allow for electrical connections between audio output devices and headphone drivers. They do so with compatible plugs and, typically, cables. A proper headphone-to-headphone jack connection allows the audio signal (alternating current) to flow through the driver, producing sound.
Choosing the right headphones or earphones for your applications and budget can be a challenging task. For this reason, I've created My New Microphone's Comprehensive Headphones/Earphones Buyer's Guide. Check it out for help in determining your next headphones/earphones purchase.