History and Types of Loudspeakers

The standard dynamic loudspeaker that we know of today was first built in the 1920's and uses a magnetic field to move a coil or magnet which is connected to a diaphragm. There are other kinds of speakers/sound amplification devices besides the standard round speaker, in this article we cover a few of the most important and common speakers.



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1. Horns

Horns were the earliest form of amplification. Horns do not use electricity. Thomas Edison, Magnavox, and Victrola all developed advanced and well-performing horns from 1880 to the 1920's. The problem with horns is that they could not amplify the sound very much. With the use of electrical amplification in the future loud sound could be generated to fill large public spaces. Horns remain a novelty for collectors today. Horn uses: Audio recording for entertainment and record keeping, later on for voice radio Above a straight copper horn used with wax cylinders.

Above: Left: An Edison horn made from strips of tin Right: a small straight brass horn and wax cylinder phonograph, this type was manufactured by Edison's companies until 1913. It was replaced by the record. The smaller horn was used in smaller rooms of the home. These are on display at the Edison Tech Center.

The needle moves up and down and forces the metal device above it to vibrate on the clear diaphragm (similar to a speaker). This vibration pushes air and makes a small amount of sound. This sound is channeled in the brass arm down under the Victrola to a wooden horn inside the box (right photo). The horn amplifies the sound to an impressive level. Opening or closing the wood doors on the front controls loudness by blocking the horn inside.

Video Below: Steve Ainlay speaks into a horn to record onto a wax cylinder:

The horn and loud speaker development were pushed forward by the advent of voice radio:

Left: Pre-1925: The first voice radios used a horn, and very quickly the horn was replaced by the electrodynamic loudspeaker invented in Schenectady at General Electric. (WGY Radio Station)

Right: 1927: Ernst Alexanderson - inventor of broadcast television tests the first television broadcast from the GE plant to his home. To his right you will see a standard radio of the time used for the audio.

Further reading on horns:

Basics of the Acoustic Phonograph from Victor-Victrola.com >

On horn design by Lenard Audio Institute >

2. The Electrodynamic Loudspeaker (The Modern Speaker)

What is an electrodynamic loudspeaker? A: This is a device that uses an electromagnetic coil and diaphragm to create sound. This is the most common type of speaker in the world today.

How does it work?

The modern speaker uses an electromagnet to turn electric signals of varying strength into movement. The coil of copper wire moves as the magnet energizes. This works using induction. The coil is connected to a cardboard/paper/vinyl "cone". The cone is a diaphragm that vibrates along with the coil. Sound is created and amplified by the diaphragm. There are variations on how to build the speaker. A given speaker is designed to produce a specific frequency range. Not all materials and construction designs produce all sounds the same: see tweeter, mid-range, woofer, subwoofer.

Why did it take so long to invent the speaker?

It sounds simple, but to build a speaker one needs to have an understanding of electricity, radio, sound waves, mechanics, chemistry and physics. Today an engineer is trained after a few years of college in these areas if they choose. Back in the early part of the 20th century the basic knowledge of math and frequencies was still being explored. While the electromagnet part of the speaker was invented early on in the 1860s, it took 40 more years to develop a knowledge of acoustics and materials. C.W. Rice and E.W. Kellogg finally invented it by solving the final part of the puzzle. The final work was on how to shape the diaphragm and what materials to use. The then recent developments in vacuum tubes in the 1910s helped to do the job of sophisticated control of frequencies and power regulation and amplification.

2a.) About Sound:

There are two main measurements in sound: frequency and decibels.

Frequency is responsible for the quality of sound in a speaker, decibels measure the 'loudness' of the speaker.

Frequency: Humans can hear audio from 20 - 20,000 Hertz. Hertz is a measure of cycles per second. Sound is is a wave ranging from 0 level of energy to infinity.

A middle 'C' tone on an instrument is not a solid constant level of energy is it sounds to our ears, it is a wave which peaks every 278 times per second.

The human ear does NOT hear all frequencies with the same sensitivity. It is most sensitive to the 2000 - 4000 Hz range. So a 100 dB (loudness level) sound at 20 Hz will not damage the ear as much as a sound at 3000 Hz.

Most sounds fall in the lower range of our audible frequency range, however sounds at 16,000 or 20,000 Hz are important. These upper frequencies give us other information about a sound, such as the environment of the sound. If someone is speaking in a small room or auditorium we can tell thanks to the high frequencies. Developing a speaker which could reproduce the low and very high frequencies was the greatest final challenge in inventing the speaker. Since the 1921 prototype the speaker's abilities to reproduce frequencies got even better. Engineers continue to advance the technology and improve our world by making speakers smaller, more efficient and more durable.

Decibels: The other measure of sound which is important for speaker performance is the potential loudness measured in decibels (dB). Decibels measure sound pressure. The higher the pressure, the more your eardrum pushes inward. Decibels is a logarithmic unit, which means that each unit indicates an increase in power by x10. The 'bel' in decibel originates from telecommunications pioneer Alexander Graham Bell. The need to measure sound efficiency in gave rise to the unit in 1923.

0 dB is set at .0002 microbar (pressure). 120 dB is enough to cause permanent damage to the human ear, however the ear will be more easily damaged at the 2-4kHz range. This aspect of the human body being more sensitive to certain frequencies is related to the species evolution. A baby's cry is located in the 4-5 kHz range. We are programed to be sensitive to a baby's cry and human's scream generated by our naturally limited vocal chords. Similarly when we talk about the electric light, the human eye is more sensitive to frequencies in the green-yellow spectrum. You can read all about the development of the electric light on our pages which start here.



2b.) Timeline of the Modern Speaker:

1861 - A simple type of electronic loudspeaker was developed by Johann Philipp Reis - a teacher at Friedrichsdorf, Germany. The speaker was crudely able to reproduce noise and just an experiment.



1876 - Alexander Graham Bell also tried to produce a speaker based on Reis's work. At this time in history there was not enough base knowledge in physics and material engineering to allow Bell or any other inventor the ability to successfully produce an electrodynamic loudspeaker. The need to amplify sound and telegraph signals over long distance did help spur the development of amplifiers, which is an important component of audio systems later on.



1877 - The idea of the electromagnetic coil driven speaker is formulated by Werner Von Siemens, he used it with input signals of DC transients and telegraphic signals. He had no way to amplify sound to create a useful speaker, but he theorized that this could eventually be done.

1877-1921 - Various inventors and engineers played with the idea of the electrodynamic loudspeaker but could only create rough distorted sounds. There was no way to electrically amplify the signal to create very loud sounds. The industry continued to rely on more advanced horns to create amplification.

1921

The first modern loudspeaker

Above: The first prototype loudspeaker, finished in May 1921. The speaker cone is damaged. Inside the cabinet is a groundbreaking power amplification system. Photos copyright 2010 Edison Tech Center, permission required for use.

C.W. Rice of General Electric and E.W. Kellogg of AT&T worked together in Schenectady, New York to develop the modern speaker and first electric amplification system. They created a working prototype in 1921. Rice and Kellogg solved the final problems which led to a nice crisp sound. Previous attempts to make the loudspeaker created an unacceptable muffled sounding audio. This muffled sound was not good enough to compete with the horn which was well established in the market. Rice and Kellogg were able to fully understand the reproduction of all the frequencies necessary to create an accurate audio sound. Their prototype had enough of a dynamic range in frequencies to be better than the horn, while possessing the ability to greatly increase loudness (dB). In 1925 they filed for patents and made a speech in St. Louis to the AIEE. After several years of work they perfected it as the first commercial product of it's kind called the Radiola Loudspeaker #104. It sold in 1926 for $ 250 (about $3000 today (US dollars)). The speaker was produced under the company name of RCA.

Above: The first speaker. The coil was stationary unlike modern speakers, the moving magnet was inside. Copper wires were insulated with cloth. This prototype was built by C.W. Rice and E.W. Kellogg in 1921. The speaker worked, but needed improvement to make it smaller so it could fit into radio sets. Within a few years they improved the speaker so it could fit into a radio 20 x 12 x 16" high.



An early speaker in a household radio,

it uses a simple cardboard cone and small amplifier (the transformer) RCA: The Radio Corporation of America sold the first speaker instead of General Electric because the RCA was created as a consortium of many US companies. RCA was created under pressure from the US government to have major electric technology companies work together with their various patents in order to more easily create finished products which could compete with state-sponsored European companies. European companies like Philips and Siemens had a leadership which could easily mobilize patents for creation of products. Under the more true 'capitalist' systems in the US at the time, companies fought over patent royalties and kept secrets from each other. During the first era of RCA General Electric Research Center and Bell Labs worked together on projects to produce final products like television and the loudspeaker.

RCA was able to collectivize research in order to create better technology. In addition to having a strong US company to compete internationally in the radio market, having better technology was important for the US during escalating war tensions prior to WW2. World War 1 taught the important lesson not to be left behind, and that radio technology could mean the difference between winning or losing. GE's Ernst Alexanderson was at the epicenter of wartime communication technology. He also assisted C.W. Rice and Kellogg during the creation of the loudspeaker.

Watch the video below to see the first prototype of the first loudspeaker:

The electrodynamic loudspeaker today comes in several types in order to produce a quality sound for a given frequency range:

Tweeter - 2kHz - 20kHz. used to produce all high end frequencies. There are many ways to construct a tweeter today. Most are electrodynamic (magnetic) speakers however there are piezo-electric, electrostatic and plasma tweeters.

Mid-range Speaker - 300 - 5kHz. This speaker covers most human voice along with most instruments.

Woofer - 40 - 1 kHz. low frequencies. The woofer goes back to work done in the 1930s at Bell Labs.

Subwoofer - 20 - 200 Hz. very low frequencies. The human ear can only hear down to 20Hz. low frequency sound is unidirectional. This means that it can be placed anywhere in a room and be heard from any point with the same sound quality. Subwoofers also produce sound waves that penetrate walls easily. Noise from this type of speaker is even known to penetrate vertically through 5+ floors of concrete apartment buildings. Needless to say it is easy to get in trouble with local noise ordinances. If you plan your next loud event you may want to think about frequencies and the ability of objects to absorb/stop or reflect sound. Subwoofers were developed in the 1960s.

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Further reading on standard electrodynamic loudspeakers:

More detail on Loudspeaker History from the Audio Engineering Society >

Electrodynamic Speaker Design Considerations, IEEE publication (purchase or membership required) >

3. Flat Panel Speakers

There are several kinds of flat panel speakers, engineers have been working on flat speakers for many decades as to decrease the size of speaker boxes. The standard flat panel speaker has a exciter attached to a square panel. The flat panel acts as a diaphragm. Below are some examples of different designs. Different materials can be used as a diaphragm, from vinyl to styrofoam.

The standard flat panel electrodynamic loudspeaker has been difficult to make because it is difficult to vibrate the entire flat surface evenly while creating good frequency response. Thus other speaker types have evolved to try to make a speaker in a flat form.

Types of flat panel speakers: ribbon speaker, planar magnetic, electrostatic - read about these below.

Standard flat panel speaker:

Planar magnetic speaker:

This has a conductor attached/embedded in the diaphragm of styrofoam. The diaphragm moves forward and backward to make sound. This type of speaker has traditionally worked better than a standard flat electrodynamic loudspeaker because the entire styrofoam flat surface moves easily, not requiring a large electromagnet to drive it. Further reading on planar speakers:

How they work from manufacturer Magneplanar >



The diaphragm can have paper bonded on both sides of the polystyrene to help sound production. It can also be made of PET foam, polypropylene foam, polypropylene, ABS, glass fiber and carbon fiber.The four corner points of the flat diaphragm are attached to a cushion. The exciter pushes the center of the diaphragm forward causing a bending in the surface, generating sound waves.

Electrostatic flat panel speaker (ESL):

The electrostatic flat panel speaker uses two metal grids with a diaphragm made of a plastic sheet. The sheet is coated with graphite which is electrically conductive. The diaphragm has a constant charge, high voltage audio signal is created by the grids which are really electrodes.

It has poor bass response but the speakers look interesting as a rectangular flat screen. If combined with a regular electrodynamic woofer it can be made into a full sound system.

See a 3d motion graphic of how it works on the video here

Further reading on electrostatic loudspeakers:

ESL speaker basics from wikipeda >

VIDEO: How they manufacture ESL speakers >

VIDEO: about ESL speakers from manufacturer MartinLogan

Ribbon Driven Speakers:

Super light (low mass) aluminum foil film is used as a diaphragm in this type of speaker. The thin strip of aluminum on mylar (like a shiny mylar balloon) is durable and suspended between two bar magnets. This type of speaker does not need a transformer between the amplifier and the speaker.

Further reading on electrostatic loudspeakers:

About the ribbon and quasi ribbon drivers >



Multi-cell Flat Diaphragm Speakers

This speaker is related to the electrodynamic loudspeaker in that it uses magnetic fields to move an element, however its shape is different. This speaker has the coil directly mounted on the diaphragm. Below is an example of a flat panel style diaphragm speaker. The copper coils have been wrapped around the base of the clear plastic bulges in the photos below. The long permanent magnets create separate 'cells' and the whole diaphragm ends up moving uniformly. This is a heavier speaker than the flat panel speakers above. This type of speaker requires a tranformer because each small cell has a lower impedance than a standard speaker with single exciter.

Further reading on multi-cell flat panels:

Article on the Orthophase flat panel of 1961 >



4. Plasma Arc Speakers

Plasma is ionized gas, or current sent through a gas. Plasma is responsive to electrical fields, therefore you can turn electric signal of sound into an electric field which manipulates the plasma. The plasma does have mass and will vibrate creating sound similar to how a diaphragm moves air to make sound. This kind of speaker is visually quite interesting but limited in sound quality. The speaker has reliability problems and therefore remains just a novelty. See the cool videos of them working in our links below and you'll here the sound quality issues.

See one working in the demonstration in the video below:

Further reading on Plasma Arc:

Video playing classical music with arc speaker >

Short video showing plasma arc speaker >

Build your own at home (from instructables) >



5. Piezoelectric Speakers

Piezoelectric speakers use an expanding and contracting crystal to vibrate the air and produce sound. Piezoelectric speakers are limited in frequency response therefore they are only used as tweeters or in small electrical devices like watches/clocks to make simple sounds. It may be possible in the future that this technology may improve, providing a speaker with good sound characteristics and durability, however it remains up to tomorrow's engineers to make this possible.

Piezoelectronics are a solid state technology which makes them durable and good for use as a microphone underwater. These speakers are used as microphones in submarine warfare, they can detect other microphones and hear sounds of other vessels.

Further reading on Piezoelectrics:

Learn by building your own piezoelectric speaker! >

Sonitron, maker of various types of piezoelectric speakers including ceramic and polymer/metal speakers >



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