INVENTOR.HTM --- Part of Manual for Driver Parameter Calculator --- by Claus Futtrup.
Created 19. March 2001, last revised 30. May 2004. Ported to XHTML 1.0 on 2. October 2004. Last modified 25. October 2004.

Table of Contents:

  1. Who invented the electrodynamic loudspeaker?
  2. Peter L. Jensen, the story before the invention
  3. Jensen meets Pridham in California
  4. Invention of the electro-dynamic loudspeaker
  5. The loudspeaker
  6. The vacuum tube
  7. Honouring Peter L. Jensen
  8. End of story

Who invented the electrodynamic loudspeaker?

Many people would like the credits. They are probably all dead by today, but you never know for sure. If you ask an american, then it was an american who invented the electrodynamic loudspeaker. If you ask a dane, then it was a dane who invented the electrodynamic loudspeaker. Probably if you ask a jamaican, then it was somebody from Jamaica. It is the intention / purpose of this document to clear up this dull picture a bit.

When I look in my book about inventions, which was probably originally an american book and then translated to danish, there are 3 patents listed:

The first patent on a loudspeaker with a coil is from Ernst Werner von Siemens. The description was sent december 1877. According to J. Eargle and M. Gander the US Patent 149,797 is from 1874 and quite a prophetic one. It describes a radial magnet structure with a (voice) coil attached to a radiating surface. Siemens had no input signals except for DC transients and other telegraphic signals, but he anyway mentions that it could be used for moving visible and audible signals.

The second patent was sent by Oliver Lodge in 1898. At this time there was still no source to drive the loudspeaker.

The german magazine "Audio" had a special issue in 1998, either in August or in September, which celebrated the 100th year anniversary for the invention of the loudspeaker, as we know it today.

Chester W. Rice and Edward Kellog from General Electrics development laboratory filed for the 3. patent on the electrodynamic loudspeaker in 1924. They built at the same time a 1W amplifier to drive the loudspeaker. The total equipment came on the market the following year, named Radiola Model 104, and was sold for 250 USD.

Edgar Villchur, A Short History of the Dynamic Loudspeaker, Voice Coil, July 2000, pages 27-32 (I believe that it starts on page 27) says the following : The practical dynamic speaker begins with the Rice and Kellogg AIEE paper (1925) and the Radiola loudspeaker based on that paper (1926). The article continues with focus on flexible leads to the moving coil and (possibly) lower resonance frequency (fres).

The people who may be considered as the true inventors - or at least pioneers - of the electrodynamic loudspeaker are Peter Lavrids Jensen and Edwin Pridham. They are not mentioned in the book about inventions, perhaps because no patents had been issued.

Peter L. Jensen, the story before the invention

Peter L. Jensen was born in 1886 on the island Falster, Denmark.

Jensen was an assistant at Valdemar Poulsen (who invented the telegraphone, a predecessor of the modern tape recorder). Jensen became a mechanic at Poulsen and around 1905 he sent amplitude-modulated (AM) signals / radio broadcasting from Bagsvaerd Lake, later to become Lyngby Station (near Copenhagen, Denmark). This is pioneer work. A sender which could transmit at 28 different frequencies was constructed in 1904.

Later Valdemar Poulsen invented "the ticker" (for receiving morse code) and the transmission distance was improved from about 5 km to several hundred kilometres.

Later a crystal detector was connected instead of the ticker and tones could be transmitted instead of just pushes and dots. Later Jensen experimented with a microphone and had one of his friends help. Voiced numbers from 1 - 10 was transmitted (speech), but not until the ticker was disconnected. When Valdemar Poulsen was informed about this experiment his engineers started the development of the cordless phone.

On the 4. of March 1909 Jensen had finished building a system in the basement of the Tuborg brewery and they succeeded to receive the melody "Carnival in Venice," which Jensen was sending from Lyngby Station. The sender station was primarily sending to ships etc. because ordinary people did not have a radio (and were generally not allowed to have one, permissions were under government control).

Valdemar Poulsen delegated the permission to use his patents to two americans, Coburn and Elwell, who established the Poulsen Wireless Telephone and Telegraph Company, later reorganized as Federal Telegraph Company (FTC). Complete equipment for two stations were manufactured and sent to the USA. In 1909 Jensen was sent to California to build these two radio stations on 9. December 1909.

Jensen meets Pridham in California

At the telegraph station in Sacramento, Jensen met Edwin Pridham, an Electrical Engineer, who among other things taught Jensen to speak english. They built a new radio station in San Fransisco in 1910 and made tests to the other two stations. The station could not make money on sending to private amateurs. Income was created by sending telegrams between the three cities.

By todays measure the radio system was not used much. It was primarily used for communication to ships. Most people were using the spark sender because it was cheaper than the arc sender but it could only send around 3-400 km (approx. 200 miles). Besides, the radio was used more in England and Germany, primarily because the american merchant marine was small when compared to eg. Englands.

At the end of 1910 a business man, named Richard O'Connor, shows up. He supports financially. Each of the 3 men owned 1 stock each. Jensen and Pridham were on their own because the reorganisation to FTC left Jensen and Pridham jobless. I believe their common company was named Commercial Wireless and Development Co. and established primarily to take out patents when something was invented. They continued their experiments in 1911 in Napa (Napa Valley), about 35 miles north / northwest of San Fransisco, where they bought a bungalow and some land.

Invention of the electro-dynamic loudspeaker

They built a Poulsen arc sender and started a radio station. A fair amount of the apparatus was from Denmark. Among other things a telegraph writing machine, capable of handling 300 words per minute. This receiver had a fine thread between two poles of a magnet, and the weak incoming signal put the thread to motion. Jensen and Pridham was wondering whether this thread could vibrate to the motion of normal speech, ie. to be used as a receiver of a telephone. A thicker thread was mounted and a match in the middle connected to a diaphragm. They could hear the speech clearly, they called it the electro-dynamic principle. A patent was applied, but denied.

They tried to improve on the instrument, partially by reducing its size, partially by connecting a transformer. A better result was achieved. They tried to sell the apparatus to American Telephone and Telegraph Company, but they did not buy it.

In 1915 a blacksmith by the name Ray Galbrath shows up at the lab. He listens to the phone. He suggests, since it cannot be sold to the telephone companies, that they should make it speak louder. Then it could be used at the football court for everybody to better be able to hear the commentator / speaker. The day after Jensen and Pridham started the development of the loudspeaking telephone.

Another source says that Jensen was inspired by a local phenomena named James (Foghorn-) Murphy, a big irish man, who was riding around the streets of San Fransisco in the baseball season, announcing when the next match was taking place.

The loudspeaker

Powerful microphones, from germany, connected to a common tube to talk into. An Edison fonograf horn was used. A 12 Volt, 10 Ampere accumulator battery was connected, when everything was ready. A loud howling sound appeared. The first acoustical feedback / resonance had been heard. The loudspeaker was mounted on top of their chimney instead. The sound seemed not to have decreased until the people were half a mile away (1 km, well lets just say it was loud).

The next day O'Connor was called in from San Fransisco. They agreed to name the speaker Magnavox, which means "the large voice" in latin.

When playing music for the town Napa (or Napsta) they were among other things playing a female voice, who sang "Holy Night."

A second source says that the speaker was tested on top of the chimney with the words "If you can hear this, turn on a fire."

Even though it was Jensen, who discovered the "high fidelity" sound from a moving coil construction for sound reproduction it could not be patented because the moving coil principle had been patented 2 years earlier, in 1913.

Christmas evening 1915 in San Fransisco, the system was for the first time officially demonstrated for 75000 people (more than 10% of the population in San Fransisco) looking up on the balcony of the city hall, where Thomas W. Hickey spoke. One of the famous opera singers, Luisa Terazzini (Tetrazzini?) sang, and it was possible to hear her voice 1 mile away, in to Napa Valley.

Jensen and Pridham became the first disk-jockeys of the world, when they on several occations were entertaining the people of Sacramento with grammophone records, which could be heard up to 8 miles away.

The big break-through of Magnavox came in 1919 when the president, Woodrow Wilson, spoke on San Diego Stadium, addressing to 50000 people and easilly heard with the aid of 2 Magnavox speakers.

Jensen was for years after the invention of the loudspeaker a part owner of "The Magnavox Company." In 1927 he pulled out and started "Jensen Radio Manufacturing Co." which in the following years became the largest manufacturer of loudspeakers in the world. Jensen aim was to eliminate distortion and improve fidelity in sound reproduction.

In 1943 he pulled out and started Jensen Industries, manufacturing phonograph needles.

According to my second source, Peter L. Jensen never became a wealthy man on his invention or his productivity because the business world did not have his interest. He was not interested in the mass production either. The factories started in Chicago were taken over by his son.

The vacuum tube

The vacuum tube (according to my source) was invented by Lee de Forest around 1915. These amplifiers were installed along telephone lines, eg. between New York and San Fransisco, which meant that the transmission of of speech could be transmitted without decrase of the quality of the speech (well, I guess one could still understand each other quite clearly). It took years before the vacuum tube became normal in radio receivers and took the alternative name "radio-tube."

Honouring Peter L. Jensen

Jensen is not well known in Denmark. Whenever the Jensen brand of loudspeakers are seen in Denmark, we know that it is from the USA, and that the person has danish roots (so much is obvious with that last name). This is probably so because Jensen lived most of his life in America, and he also invented the loudspeaker "overthere." This is probably also the reason why more americans are familiar with the Jensen brand name for speakers.

Peter L. Jensen has recently been honoured for his invention in the american Consumer Electronics Hall of Fame, by an electronics show in Las Vegas (I assume it is the Winter CES, year 2001). With this honour, Jensen is put in the same category as Valdemar Poulsen (who invented the arc generator) and Graham Bell (who invented the telephone). See for more information.

He has also been honoured by the Audio Engineering Society (AES) and the American Institute of Radio Engineers. He was knighted by the danish king in 1956 and he became an honour member of the Danish Engineering Union,

Peter L. Jensen died in Illinois in 1961, 75 years old.

End of story

Rice and Kellog are normally considered the inventor of the electrodynamic loudspeaker, but from the above description I tend to believe that it was up and running 10 years before, by Peter L. Jensen. The patent by Rice and Kellog must either circumvent this original work by Jensen, or include something extra, which is not to be considered the electro-dynamic principle itself.

The first loudspeaker by Jensen and Pridham consisted of a 3 inch diaphragm of nickel-silver alloy, mounted with a coil. The coil was immersed in a permanent magnetic field created by a powerful electro-magnet (another coil, which is supplied with DC-current). The loudspeaker was mounted at the bottom of a long horn. Microphones were, according to my source, connected directly to the voice coil of the loudspeaker (later through a transformer).

Permanent magnet speakers were manufactured from 1930. The first car radio speakers were produced by Paul Galvin in 1931.

The last development under Jensen was the flat piston woofer, introduced in 1960.

If you have information regarding the invention of the electro-dynamic loudspeaker principle, then please inform me. My email address is available in README.HTM.

For more information on Valdemar Poulsen and the development of magnetic recording, see eg. the magazine Studio Sound, December 1998 pages 61-67.

For information on Peter L. Jensen, read his autobiography from 1948, I believe, by the title "Jensen."

A note on Rice and Kellogg. According to J. Eargle and M. Gander (JAES, vol 52, no 4, april 2004, pp 412-432, notably page 416) the key difference between previous attempts and the patent by Rice and Kellogg was the adjustment of mechanical parameters so that the fundamental resonance of the moving system took place at a lower frequency than that at which the cone's radiation impedance had become uniform.

This means that the speaker had an area of (theoretically) flat frequency response, suitable for direct radiators (non-horn-loaded application) as we know it from almost all loudspeakers of today.