Loudspeakers

by Chris Woodford. Last updated: May 19, 2023.

How many times every day do you hear recorded music on the radios on TV, in stores, in elevators—even in the street? You'd never hear music at all if it weren't for loudspeakers: electric sound-making machines. Most of the music we hear around us is played back with big loudspeakers attached to stereos or tiny earbud headphones. Radios, televisions, computers, cellular phones, intercoms, and talking toys are just some of the electric gadgets that make sounds with loudspeakers. But what exactly are loudspeakers and how do they work?

Photo: These neat little Sony bookshelf speakers have a built-in amplifier and a long jack lead, so you can plug them into a CD player, MP3 player, computer, or anything else with a standard headphone socket. They pack quite a punch for something so tiny because they have a reasonably high sensitivity, which is explained below.

How loudspeakers turn electricity into sound

When things shake about, or vibrate, they make the sounds we can hear in the world around us. Sound is invisible most of the time, but sometimes you can actually see it! If you thump a kettle-drum with a stick, you can see the tight drum skin moving up and down very quickly for some time afterward—pumping sound waves into the air. Loudspeakers work in a similar way.

At the front of a loudspeaker, there is a fabric, plastic, paper, or lightweight metal cone (sometimes called a diaphragm) not unlike a drum skin (colored gray in our picture below). The outer part of the cone is fastened to the outer part of the loudspeaker's circular metal rim. The inner part is fixed to an iron coil (sometimes called the voice coil, colored orange in the diagram) that sits in a hollowed-out ring at the front of a permanent magnet (sometimes called the field magnet, and colored yellow). When you hook up the loudspeaker to a stereo, electrical signals feed through the speaker cables (red) into the coil. This turns the coil into a temporary magnet or electromagnet. As the electricity flows back and forth in the cables, the electromagnet either attracts or repels the permanent magnet. This moves the coil back and forward, pulling and pushing the loudspeaker cone. Like a drum skin vibrating back and forth, the moving cone pumps sounds out into the air. This is why loudspeakers are technically called drivers: they "drive" (move) the air to produce sound.

Animation: How a loudspeaker works. When a fluctuating electric current flows through the coil (orange), it becomes a temporary electromagnet, attracted and repelled by the permanent magnet (blue/red). As the coil moves, it moves the cone (gray) back and forth, pumping sound waves into the air (light blue).

How speakers make sounds of different volume and frequency

Loudspeakers will play loud when the cone vibrates a large amount, or soft when it moves a small amount. Why? Think about drums. Banging a drum skin really hard makes the skin vibrate a greater distance and produce a louder sound. In the same way, sending a bigger pulse of electricity into a loudspeaker makes the cone move further and generates a louder noise. Quieter sounds are made by smaller pulses of electricity.

Photo: Large loudspeakers (like these old Celestions of mine) typically contain two separate speakers: a small tweeter that makes high frequencies (top) and a large woofer that makes low frequencies (bottom).

We can reach the same conclusion by thinking about energy. The laws of physics tell us that we can't make energy out of thin air. So if we want to make a loud sound (one that carries lots of energy), we need to create a vibration with lots of energy in the first place (in other words, hit something really hard).

Some drums have pedals on them that make the skin tighter or looser. If the skin is tight, it vibrates more quickly when you bang the drum and produces a higher-pitched sound; if the skin is loose, the opposite happens and you get a much lower note. A similar thing happens in a loudspeaker. Bigger speakers with large cones (known as woofers) move more slowly than smaller speakers with smaller cones (known as tweeters)—so they are better for producing lower frequencies. Any speaker can produce a wide range of different sound frequencies by moving back and forth quickly (for higher notes) or slowly (for lower notes). A typical home loudspeaker, in a wooden case, will contain both a large woofer and a small tweeter so it can produce a full range of frequencies. It uses a circuit called a crossover to divide the incoming electrical signal into a low-frequency part, which is sent to the woofer, and a high-frequency part, which goes to the tweeter.

Speakers that need to produce high-frequency sounds (or ultrasound, well beyond the range of human hearing) sometimes use piezoelectric transducers, based on vibrating crystals, instead of the traditional magnetic coil setup. Often referred to as crystal loudspeakers or just "buzzers," they're also used to produce piercing sounds in things like telephone ringers and smoke alarms, where the effectiveness of the alert is much more important than its quality.

Photos: Speaker cones are generally made from paper, though plastic and even light metals such as aluminum and titanium are also used. 1) Paper cone: The Sony loudspeaker in our top photo with its protective plastic cover removed. Now you can see the white speaker cone in closeup. The small black dome in the middle is called the dust cap. The black outer rim between the white cone and the frame that supports it is called the surround. The frame itself is called the basket. 2) Metal cone: The tiny titanium speakers in this laptop computer have titanium foil diaphragms less than 1cm in diameter.

How to make your speakers sound better

Photo: As speaker designers know only too well, psychology plays an important part in making us appreciate loudspeakers: it they look good, we're tempted to believe they'll sound good as well. Here's an early example: this old 1920s phonograph has a wonderful Sterling Primax loudspeaker attached. Although the cone is only paper, it's made to appear very striking and attractive with a gold-painted, fluted design. It's powered by a moving magnetic coil in the usual way.

It's not just the moving cone that determines how a speaker sounds. Have you noticed how most speakers are built into wooden or plastic cases? That's not just to make them look nice: it drastically changes the sound. You probably know that a guitar's wooden body amplifies the sound the strings make by a process called sympathetic resonance. As the strings vibrate, they make the air around them vibrate too. That starts the air vibrating inside the guitar body in sympathy—and this is what makes the sound loud enough to hear. A loudspeaker case, which is technically called an enclosure or cabinet, works in exactly the same way. Without the resonance of the case, you'd hardly hear a guitar or a loudspeaker at all. (If you ever take a radio or a loudspeaker apart and switch it on with the case open, you'll notice an immediate absence of rich, deep sound.)

Except for earphones, loudspeakers are usually some distance from our ears. The sound waves produced by the speaker cones have to travel through the air in a room before we can hear them. But sound waves travel out from speakers in all directions. They travel backward from the speaker as well as forward; they travel down to the floor and up to the ceiling as well. In practice, one single push or pull of a speaker cone sends sound waves traveling in all directions. These reflected waves bounce off the walls, floors, and furniture in your room and interact in many different ways, sometimes adding together and sometimes canceling out. With the same set of speakers, an empty room will sound very different to a room full of furniture; a living room with rugs and soft furnishings will sound very different to a kitchen or bathroom with lots of hard surfaces.

You can dramatically alter the quality of the sound your speakers make by putting them in different places. Always arrange them symmetrically (so if one is six inches from a wall, the other needs to be six inches from a wall too). Never fasten speakers directly onto a wall or stand them on the floor. Instead, try to mount them roughly at ear level. Put each speaker nearer to the center of the room so there are unequal distances from the speaker to the walls, ceiling, and floor. That will help to stop reflected sounds from interfering with the main speaker sounds. Speaker stands are a great investment: they usually make speakers sound twice as good, although sound quality is clearly a matter of personal taste—and you might prefer to kave your speakers on a shelf or sitting on the floor.

Changing the position of your speakers in this way will emphasize or demphasize certain sound frequencies, so the type of music you enjoying listening to is also a factor here. Chamber music (such as classical string quartets) tends to benefit from crisp, clear higher frequencies, while dance music and rock is enhanced by plenty of bass. Sitting speakers on the floor might give your Neil Young the thump you're looking for, but it will have a very different effect on your Schubert sonatas.

Listening with both ears: stereo, quad, and binaural

When sound comes from a single loudspeaker, we say it's mono or monaural. Mono is like the sound of one person talking: the sound source is fixed in one place.

Stereo (stereophonic sound) is very different. The first time you hear stereo, it sounds like a miracle. Where are the sounds coming from? How do they move around your head like that? Stereo is a simple trick: two loudspeakers each play slightly different sounds and our ears and brains reassemble the noises into a two-dimensional soundscape. If you listen to music with headphones, you'll be used to the way stereo sound jumps back and forth between your ears. You might hear a drum beating in one ear and a guitar playing in the other, for example.

Photo: Most old audio systems had only one speaker, like this, and reproduced sound in mono. This is a wonderfully preserved Amplion speaker made in 1928. It's an exhibit at Think Tank, the science museum in Birmingham, England.

Although stereo is a big improvement on mono, it's still only two-dimensional sound. It is possible to make loudspeakers sound three-dimensional, but you need more speakers to do it. Quad (quadrophonic) sound is like double stereo: you have two speakers in front of you and two behind. Now the sound can move behind you or in front as well as from side to side. Surround sound used in movie theaters (cinemas) works in a similar way.

Binaural is a way of making a sound recording seem three-dimensional with only two speakers. Our ears are more than just holes through which sounds enter. The hills and valleys in our outer ears (pinnae) help us to work out where sounds are coming from and give the sounds we hear in the world their three-dimensional quality. Normal stereo recordings don't pick up this directional information, because ordinary microphones don't have the hills and valleys that our ears have. Binaural recordings are made a different way using a dummy's head with plastic ears shaped like a human's. Two microphones are placed inside the ear holes so they pick up noises like human ears would. When sound is recorded binaurally, in this way, and then played back with conventional headphones, it sounds strikingly different to stereo—and almost lifelike. A binaural recording of a jet plane taking off sounds scarily like it's moving through your head.

How do electrostatic speakers work?

Unless you're a real audiophile, you'll probably never come across electrostatic speakers (sometimes known as condenser or capacitor speakers). Unlike the speakers we've considered so far, which generally have rounded cones mounted in square boxes pumped back and forth by electromagnetism, electrostatic speakers often look more like closets or radiators and resemble capacitors. A capacitor is a device for storing electricity using two parallel metal plates separated by some sort of an insulator (usually air or plastic). Suppose you take a huge capacitor and rapidly change the electric charge on the plates. Since unlike charges attract and like charges repel, if the plates can move, the changing charge is going to make them vibrate, sending sound into the air.

Artwork: Inside an electrostatic loudspeaker: In this design, two large pieces of slate (gray, 1 and 3) are hollowed out to make an air space in between (21). Two conducting metal plates (red, 17 and 19) are fixed to the sides of the slate and a third, low-mass aluminum foil plate (orange, 23) is suspended between them to act as the moving loudspeaker diaphragm. A transformer (green) at the bottom applies a varying voltage, causing the central plate to wobble back and forth. As the diaphragm vibrates, it pushes air in and out through holes (white, 25) in the two outer plates, sending sound into the room. From US Patent: 1,930,518: Electrostatic loudspeaker by Jurjen S. High, Westinghouse Electric and Manufacturing Company, granted October 17, 1933, courtesy of US Patent and Trademark Office.

In practice, electrostatic speakers have two fixed plates on either side and a very thin moving plate (the diaphragm) that vibrates in between them, which is analogous to the moving cone in a traditional magnetic loudspeaker. Although they can produce excellent sound, they have a number of drawbacks: they generally don't work well at low frequencies unless they are very large (to compensate for having a thin diaphragm that doesn't move much); they need to use high voltages to work effectively, which means they need large transformers that consume a lot of power; and they're extremely expensive compared to traditional speakers. The best known maker of electrostatic speakers is English company Quad Electroacoustics.

Photo: A classic Quad ESL57 electrostatic loudspeaker. Photo by Manffred Stein, QUAD Musikwiedergabe, published on Wikimedia Commons under a Creative Commons (CC BY-SA 4.0) licence.

What about headphones?

Essentially, they're just loudspeakers in miniature. To see what headphones look like when you take them apart, take a look at our separate article on headphones.

Who invented loudspeakers?

As with many inventions, it's a little problematic to credit loudspeakers to a single inventor...

Today's speakers are recognizable descendants of the very first moving-coil loudspeaker developed between 1921 and 1925 by Edward W. Kellogg and Chester W. Rice of General Electric (where Rice's father, Edwin was President). [1] The first commercial speaker to use their invention was the RCA Radiola Model 104, a home radio loudspeaker released in 1925. Kellogg later pioneered the electrostatic speaker, and was granted US Patent 1,983,377: Production of Sound for that idea on December 4, 1934.

So were Kellogg and Rice the inventors? It's a bit more complex than that. There were clearly earlier speakers than theirs. How about Alexander Graham Bell's 1876 telephone, for example, or the earlier version of the telephone developed by Johann Philipp Reis in 1861? Sir Oliver Lodge, the British physicist and radio pioneer, was granted a patent for a crude magnetic speaker on April 27, 1898. [2] Danishman Peter Jensen and American Edwin Pridham came up with another design in 1915, which they sold as the Magnavox ("Big Voice"). So others have a claim to the invention too. Even so, it was Kellogg and Rice who perfected the simple, effective, and practical magnetic-coil technology that we still use to this day. [3]

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Find out more

On this website

• Directional loudspeakers
• Headphones
• Microphones
• Noise-canceling headphones
• Sound

On other websites

• Introduction to loudspeaker drivers by Geoff Martin of Bang & Olufsen from his book Introduction to Sound Recording.
• How to Customize Your Sound System for Your Living Room: Wired, February 9, 2016. A good collection of tips on how to position speakers in your room (and rearrange the furniture to make them sound better).

Technical books

• Handbook for Sound Engineers by Glen Ballou (ed). Focal Press/Elsevier, 2015. Detailed reference for sound engineers, covering acoustics and sound recording in theory and practice.
• Loudspeaker Handbook by John Eargle. Springer, 2013. Detailed technical guide for speaker designers and students.
• Loudspeakers: For Music Recording and Reproduction by Philip Newell and Keith Holland. Focal, 2007. A more theoretical reference for home audiophiles, though still with plenty of practical advice (underpinned by science).
• Sound and Recording: An Introduction by Francis Rumsey, Tim Mccormick, Elsevier, 2012. A clear, illustrated guide to all aspects of sound recording and reproduction. "Chapter 4: Loudspeakers" is a clear overview that leads on nicely from my own simple introduction, with more detailed coverage and extra topics like transmission lines and how crossover networks split signals.
• Home Audio: Choosing, Maintaining, and Repairing Your Audio System by Andrew Yoder. McGraw-Hill, 1998. A practical guide to designing and buying a home audio system.

History

• Sound Recording: The Life Story of a Technology by David Morton. Johns Hopkins University Press, 2006. Although this book is mostly about recording technologies, there's also plenty of fascinating information about loudspeakers.
• Perfecting Sound Forever: An Aural History of Recorded Music by Greg Milner. Faber, 2009. A much more off-beat, off-the-cuff, cultural history of recorded sound.

Articles

• Flexible Loudspeaker Made of Nanowires Will Stick to Your Skin and Play Music by Dexter Johnson. IEEE Spectrum, August 3, 2018. How South Korean scientists made one of the world's smallest speakers.
• First 3D Printed Loudspeaker Hints at Future of Consumer Electronics by Jeremy Hsu. IEEE Spectrum, December 20, 2013. Making plastic loudspeaker components with a 3D printer is a proof of concept: we could make all kinds of electronic devices this way in future.
• Nanoparticles Enable Novel Loudspeaker Design by Dexter Johnson. IEEE Spectrum, November 21, 2013. Audio scientists at KTH Royal Institute of Technology have found how to do away with the bulky permanent magnet in a loudspeaker.
• Bang & Olufsen's Geoff Martin Pursues the Perfect Sound by Jean Kumagai. IEEE Spectrum, January 30, 2013. An interview with a leading audio engineer and loudspeaker designer.
• Prick Up Your Ears by Robert Sullivan. The New York Times. March 28, 2012. Explores the work of Jonathan Weiss, of Oswaldsmill Audio, and his quest for great sound reproduction.
• High-end Hemp Speakers are All the Buzz by Chuck Squatriglia. Wired, March 8, 2010. What are the advantages of using hemp in place of paper for a speaker cone?
• Thin speaker offers 'crisp sound': BBC News, 1 April 2009. How the flat, flexible loudspeaker (FFL) was designed by Warwick University professor Dr Duncan Billson.

References

1. ↑   "Notes on the Development of a New Type of Hornless Loudspeaker" by Chester W. Rice and Edward W. Kellogg. Transactions of the American Institute of Electrical Engineers 44, 1925, p. 461–475. There's a good account and a photo of the inventors in Schenectady's General Electric Realty Plot by Chris Leonard, Arcadia Books, 2019, p.39.
2. ↑    Annular gap (cylindrical body) electro-magnet No. 3, c.1898, Science Museum, London is a catalog entry for Lodge's "microphone amplifier or relay, 1898 (British patent 9712 of 1898), a precursor of the moving-coil loudspeaker."
3. ↑    Classic Keys: Keyboard Sounds That Launched Rock Music by Alan S. Lenhoff and David E. Robertson, University of North Texas Press, 2019, p.19. The authors argue that the history of sound technology is "cluttered with names" who deserve credit for contributing to modern speaker design.