Make an electromagnetic speaker
There’s a way to do it better - find it. - Thomas Edison
Switch off all the lights in the room and listen to Mozart’s K 448 Sonata for two pianos in D Major, and you cannot help but let out of a silent thank you to all the people who made that moment possible, because they wanted to understand and make things better than they were. Pythagoras, Aristotle, Galileo, Boyle, Graham Bell, Edison, Werner Siemens, in that sequence. But also the lesser known, Marin Mersenne (first calculated the speed of sound), Scott De Martinville (recorded the first sound) , Philip Reiss (invented the dynamic speaker), Charles Tainter (invented the brown wax to record sounds better), Lee de Forest (invented the tube amplifier), Chester Rice and Kellog (put it all together and made the first commercial loudspeaker).
But lets rewind back to the beginning. And ask ourselves the question – how do we hear, what we hear?
It was Pythagoras who discovered almost 2500 years ago that sound is perceived due to a vibration in the air. He came to this conclusion by observing the motion of a plucked string that was tied on both ends. A couple of hundred years later, Aristotle came along and concurred. And added his own converse theory that if there were no air, then no sound could be heard. More than 1500 years later, the great scientist Robert Boyle would prove Aristotle right by creating vacuum inside a bottle and demonstrating that a ringing bell could no longer be heard. Of course, there is nothing special about air when it comes to sound. Just about any medium that is dense enough can carry sound waves – water can also carry sound waves.
How the ear processes the music
This greatly cleared up our understanding of sound. Something causes vibration of the air around the source of the sound. The air particles start vibrating and the energy is propagated all around. Soon all air particles start vibrating, including inside the ear should one happen to be in the vicinity. Inside the ear is a small drum, called un-surprisingly, the ear drum, and soon that starts to vibrate as well. The vibrations are transferred to brain, and voila – we hear sound!
Once we understood that, the next obvious question came along. How can we share this sound with someone else is not present at the time. Everyone cannot be expected to around a nightingale bird to hear her glorious tune, and we simply cannot ship a nightingale bird to everyone out there. As is always the case, the process of doing so required doing one thing, understanding it, then building upon it to do the next thing.
In 1857, Édouard-Léon Scott de Martinville created something he called the “phonautograph”. It was able to “transcribe” a sound into a line drawn on paper or glass. So while you could admire your sound by literally “seeing” how it looked, you could not hear it. Unfortunate.
In 1861, Phillip Reiss, a teacher in Germany, while trying to invent the telephone, created a very crude electronic loudspeaker, which was able to generate some noise, but not very much else. Alexander Graham Bell improved upon this a decade or so later but the physics underlying the basics of the modern day speaker were still not fully understood by the folks at the time.
Edison and his phonograph
Edison’s Phonograph – the talking machine
In 1877, Edison invented the “phonograph”. It was the the first device that had, among other things, a large horn, and it could record and playback sound. A device that could to both, now that was a break-through. You shouted into a horn and on the other end a stylus engraved the variations of sound onto a rotating cylinder covered with tin foil. The cylinder could then the rotated again, only this time the variations in the tin foil caused tiny variations within the horn, which then got amplified due to the shape of the horn – and you could hear the sound back! The first recording was done by Edison himself, and he sang the classic lines: “Mary had a little lamb, It’s fleece was white as snow. Everywhere the child went, the lamb was sure to go”.
Horns and the phonograph remained the popular choice for several decades. But ironically it was also in 1877 when the idea that would kill the phonograph took birth.
It was also in 1877 that the idea of an electromagnetic coil
driven speaker too shape in the minds of Werner Von Siemens (yes, of the Siemens fame). He built a small prototype of the machine and while the results were not impressive due to the weak DC current, he theorised it could be done. If only, he could have coupled his invention with an amplifier! Nevertheless, the concept of an electromagnet pushing out sound waves into the air remains true of almost all speakers we see till today.
Werner Von Siemens
Lee de Forest’s Audion
In 1906, Lee de Forest invented the world’s first amplification device, a three element vacuum tube, called the Audion (but audiotron have been way cooler, right?). He did not quite understand how it worked, but he could demonstrate that a weak electrical signal can be amplified using the device. The field of “amplification” was kicked off.
It took another 25 years or so for Rice and Kellog (no not related to the cereal in any manner!), working in General Electric at the time, to use concepts of electromagnetism, amplification and sound to make the first commercial grade speaker in the city of Schenectady. After patenting it, they sold it as “Radiola Loudspeaker #104” and they were all the hit, across all theatres at the time. The output of these speakers were of better quality of the horns, and had the advantage of being able to really “turn up the volume”.
One of the earliest Radiola speakers
The time for the “horn” was over. The time for the electrodynamic “speakers” had arrived.
In this chapter we will build a simple electrodynamic speaker, which operates, in principle, very similar to that of the first “Radiolas”. But, let’s not just talk about. Let’s start making it!
Resistance of the speaker coil should not be too low. If it is, it can damage your music player. Use a multimeter to check the resistance of the coil once you have made it.
The speaker we will make
Click here to play the above video
Things we need
Couple of plastic cups (the thinner the better)
PAM8403 2 x 3W Audio Amplifier
You will also need the following:
- 5V DC battery source
- Wire stripper
- Glue and Sandpaper
- A music player such as a phone
Step 1: Create the speaker coil.
Simply wrap a few turns of the copper wire around two of your fingers to create the coil. About 20 turns should be enough. Make sure to leave about 3-4 inches of straightened wire on either ends of the coil so they can be connected to the audio amplifier. Using a sandpaper, remove the enamel coating from the ends. Now, cut another strip of the copper wire and wind it all around the coil so that the coil maintains it’s shape and the wires do not come undone. The diameter of the coil should be slightly larger than the size of the magnet.
Step 2: Create the base speaker
Take one of the plastic cups and glue the speaker coil to the base of the plastic cup on the outside. Take the other plastic cup and cut in half. Glue on the magnet to the base of the cup on the inside. Now place this inside the first one so that the coil and magnet are separated by a thin film of plastic. Glue the two cups together so they are fused as one unit.
Make sure the wires come out of the cup by making a couple of holes in the cups.
Step 2: Connect speaker to the amplifier
Using a soldering iron, connect the two ends of the speaker coil to either R or L outputs of amplifier board.
Step 3: Connect a 3.5mm audio jack and DC power to the amplifier board
A 3.5mm audio jack will be used to provide the “music” to the amplifier. First, cut and strip the wire inside the audio cable. There will be three wires – the positive, negative and ground. Often times, the ground wire is in a mesh form, and you will need to twist it together to create the third wire. Solder them on to the amplifier board as shown below.
Also connect the 5V DC supply to the amplifier board.
Step 4: Play music and prepare to be amazed
Here is the final arrangement. Press play and you should start to hear sound from the speakers!
How it works
The journey of the music from the time it was recorded, possibly thousands of miles away and many years ago, to its final destination, your ears is a fascinating story. To go into the every single detail would require a book of its own. But here are some of the important steps along the way.
The artist and his musicians record their song in a studio and carefully positioned microphones pick up the vibrations in the air and convert it into digital signals.
The signals are then made available to you in the form of a digital audio file, that you can download, for example, to your phone.
When you now play the song in your phone, the same digital signals are sent over the 3.5 mm audio cable to the amplifier, so that the signals can be amplified.
The amplified signals now flow through the speaker, and the presence of the magnet within it, causes the coil to move. This is electromagnetism at work. The vibration of the coil causes the thin plastic membrane to vibrate too, and as a result, the air particles inside the cup begin to vibrate too. A similar vibration in the air is now reproduced.
The vibration of air particles hits our ears drums, which have some intricate machinery inside to send signals to the brain…that sound like music!
You don’t need air to hear sound – but you do need some medium. In space, sound could not travel since there would not be any particles to vibrate and thus carry the sound waves. Here’s how sound sounds different as it passes through water.
Click here to play the above video
Add some grains of rice or other small particle to visually notice the vibration of the speaker “membrane” and how its vibration correlates with what we hear.
Click here to play the above video
If you hear nothing from the speaker, bring out the multimeter and test the connections. Ensure that:
- Amplifier is receiving the necessary input voltage (about 5V DC)
- Test all the solder joints to make sure you don’t have a “cold” joint.
- Shake the speaker a bit, sometimes the alignment of the coil and magnet need adjustment
- Turn up the volume!
References and Attributions
This link, with some really fun music to boot!