Although there have been significant advances in both speaker design and manufacture, the fundamentals of speaker driver technology haven’t changed in close to 100 years: Edward Kellogg and Chester Rice’s 1925 dynamic driver is still the basis of practically every loudspeaker on the market today, from the one in your phone to those in your home theatre system.
But how do speakers work? Let’s start with the basics.
Editor’s note: this article was updated on March 12, 2021, to include more technical information.
How speakers work: the basics
A speaker driver is the raw electroacoustic component that makes a loudspeaker work. As a transducer, its function is to transform energy from one form to another. Specifically, this transducer transforms the amplified electrical waves from your playback device, be that your phone or the cartridge of your turntable, into sound pressure waves in the air for your ears to detect.
An amplifier feeds a signal to two terminals on the back of a speaker. These terminals pass the current into a cylindrical coil of wire, which is suspended in the circular gap between the poles of a permanent magnet. This coil moves back and forth inside the magnetic field as the current passing through it alternates in direction with the signal applied, per Faraday’s law. The center of the speaker cone is attached to one end, which gets driven back and forth by the moving coil. This cone is held at its edges by an airtight suspension or surround. As the cone moves, it pushes and pulls the surrounding air; by doing so it creates pressure waves in the air, called sound.
So that’s how the speaker driver works, but why do they always seem to be mounted in boxes? If the driver makes sound by itself, what’s the box for? And what about port holes and those other bits and pieces?
Why are speakers mounted in boxes?
As a speaker driver’s cone moves, it creates a pressure wave from both the front and from the back. As it moves towards you, pushing the air and creating a positive pressure, it simultaneously pulls the air behind it, creating negative pressure. If the wavelength that corresponds to the frequency of the reproduced signal is large relative to the size of the driver, the pressure generated by the two sides of the driver will effectively cancel each other out. So at any useful distance, the low frequencies (bass) are rendered inaudible. If you want to try this at home, remove a driver from its enclosure. You’ll notice a “tinny” sound quality compared to how the speaker sounded when assembled.
For a speaker to function well at all frequencies, we must prevent the pressure wave created by the back of the speaker cone from cancelling out the wave created by the front of the cone. If you were to mount the driver in a large, rigid sheet of material (a baffle), you could achieve the same effect. A baffle needs to be large to prevent low-frequency cancellation, so this is impractical in most applications. Closed boxes allow a more practical way of doing this.
A combination of the mechanical properties of the driver and the size of the box define the low-frequency behavior of an assembled closed box loudspeaker system. Without getting super technical, the air in the box acts like a spring that the cone pushes and pulls against, and that system has a resonant frequency below which its output drops off considerably.
Loudspeakers must be airtight: leaks in the box allow the cancellation we want to avoid.
Why do some speakers have holes in them?
You may have noticed lots of speaker boxes have circular holes, or sometimes slots, usually in the front or back. What you are seeing are ports, or vents, and this identifies what is known as a bass reflex enclosure.
A bass reflex enclosure works essentially the same way as when you blow air over an open beer bottle and a note sounds. The note changes with the amount of liquid in the bottle because the volume of air inside the bottle changes. If you were able to stretch the glass neck of the bottle, that would change the note too. It is a resonant system that can be tuned by adjusting the port dimensions (the bottle’s neck) or the enclosure (bottle) volume.
If tuned correctly, what this does is create a resonance just below the point at which the loudspeaker response would normally roll off, effectively extending the bass performance of the system. For this to work correctly, the port tuning is calculated for the specific driver in the specific enclosure. If you swap out the driver for a different type, even if it is the same cone diameter, the box and port tuning will no longer be appropriate and it won’t sound right.
Loudspeakers that use passive radiators work on the same basic principle but with a mass loaded, unpowered speaker cone creating the bass resonance with the enclosed air volume.
Tweeters and woofers
Now, you may have noticed that in most loudspeakers, particularly as they get larger than little portable boom boxes, you can see more than one speaker driver—usually a smaller diameter one atop a larger one.
There are several reasons why speakers use multiple drivers in different sizes. While it is true that a single driver can cover almost all the audible spectrum by itself, there are a number of limitations it runs up against. If the driver is small it can’t move much air and will struggle to generate bass at a useful level.
Learn more: Driver types explained
Larger drivers can move more air, but the problem is that speakers become more directional as the frequencies they are reproducing go up. This is known as beaming.
As frequency increases, the associated wavelength decreases; speaker drivers usually start beaming at a frequency with a wavelength equal to the diameter of the radiating cone. This means you’ll only hear the higher frequencies if you’re right on axis with the speaker. It doesn’t make for a balanced sound or a good speaker. The simple solution is to use different sizes of drivers with each one tailored to reproducing a specific range of frequencies—different parts of the audible spectrum (bass and treble, or bass, middle, treble).
This concept works in tandem with a frequency dividing network in the speaker box called a crossover. A crossover delegates the right frequency range to each driver type: tweeters for the highs and woofers for the lows.
Why it’s worth knowing how speakers work
You don’t need to know the science behind speakers to be able to listen to and enjoy them. But if you intend to spend serious dough on audio equipment, it’s always a good idea to first equip yourself with some knowledge. A little base knowledge will help you understand why certain design decisions were made, how they influence the sound, and to help you identify snake oil salesmen.
Knowing how speakers work can also help you diagnose problems. And if this article has you interested to learn more, there are plenty of resources online where you can learn how to build them yourself from readily available parts.