And the next speaker is...!

Children, so the old Victorian dictum goes, should be seen and not heard, so it may be fatuous to point out that the converse is true of loudspeakers. They tend to stay respectfully inconspicuous and then announce their presence by making a great deal of noise. Were they able (and arguments about noise pollution aside) loudspeakers would rightly speak up, and contend that this is what they were made for - so don't complain!

 

The humble (and sometimes not so humble) loudspeaker is everywhere. Indeed, it must be one of the most ubiquitous self-contained electrical devices in the world. Don't mention the proud and oft employed LED flat screen, it is a fallacy that they outnumber everything - it's just that you simply can't avoid noticing them. For every TV or VDU there tends to be one (usually two) loudspeakers. Seldom do we see isolated LED screens as the basis for an electronic appliance, but loudspeakers make domestic and industrial TV's, hi-fi systems, portable radios, combos, intercoms, public address systems, rock bands (and hearing loss) possible - I'm still deaf from listening to Eric Clapton and Cream back in... when was it now?

 

Dulcet Tones

 

It wasn't always so. Loud speaking devices didn't really make an impact until the thermionic valve made signal amplitude (and signal power) amplification a reality (let's say about 1916 when

Related further reading...

Further Reading:

Part 1

Barrie Blake-Coleman

thermionic valve 'repeaters' - amplifiers to you and me - started to be used for improving long distance radio and telephone communication). Before that, unamplified voice communication systems, like the early telephones and radio transmitters, relied on the Bell metal diaphragm transducers, in which the magnetic diaphragm vibrated as a consequence of excitation from a varying magnetic field. This varying field was itself originated through a closely positioned (single or double pole) polarised magnet wrapped with wire - this wire carried voice modulated electric currents. The diaphragm was permanently in tension, being close to the polarised electro-magnet - as the field from the coil varied, so did the diaphragm movement.

 

These receivers converted audio modulation from electro-magnetic or carbon microphones (low level) back into sound - but, unfortunately, not as well as they might. The early electromagnetic microphones were the same as the metal diaphragm receivers and generated their own signals, but over transmission lines had no practicable use in terms of transmission distance. Such receivers (like the microphones) were inefficient, poor in frequency range (bandwidth) and they tended to introduce a good deal of distortion.

 

Thomas Alva Edison had grappled with the development of the loud speaking telephone (electromotograph and chalk receiver - British Patent 2909 for 1877), and although in 1878 his experimental units could be heard at 100 feet he was never able to produce a commercially reliable device.

 

These frictional machines produced sound from the electrically controlled friction created between a rotating, polished chalk drum, and a metal stylus attached to a diaphragm, and were later much refined. Johnson and Rahback in 1918 used silica (agate) instead of chalk, and as late as 1923 a modified version based on the agate drum was marketed under the name Frenophone. Neither Edison, nor anyone else, had much commercial benefit from the chalk receiver, but Edison's carbon granule microphone had much better luck.

As carbon granule microphones and higher battery power came into use, they increased transmission distance - but so too the distortion since, for long distance, the whole operation depended on getting as much current as possible through the microphone. Transmission line losses attenuated the signal, so it was necessary to keep the copper (or iron) telephone line resistance to a minimum. In 1885-86 the Belgian F van Rysselberghe managed to transmit unamplified simultaneous telegraphy/telephony over a 1,000 mile circuit, but all in all, it was very critical.

 

Make Me Deaf?

 

Thermionic (valve) based repeaters improved long distance communication, but louder sounds, which obviated the need for earphones, were much in demand. A great deal was done using acoustic amplification methods to try to strengthen essentially weak sounds.

 

The onset of the craze for music, first from Edison's phonograph and then Berliner's disc recordings, made sound reproduction even more important. The usual and most prevalent solution was the exponential acoustic horn, which was the only way to get more sound from (or to) a recording or telephone receiver.

 

But even this was a compromise. Horns accentuated resonances and selectively amplified parts of the audio spectrum so that voices sounded like something from the spirit world and music (instruments hardly identifiable) came out much the same.

 

Another better way was to use the displacing diaphragm to actuate something more powerful - and this principle was applied by Horace L

Short in a patent of 1898 covering his development of the Auxetophone (British Patent No 22,768 granted 23rd September 1899). This 'wonderful device', which in effect was a pneumatic amplifier, had many applications, and had been originated by T A Edison as the Aerophone (US Pat No 1644, filed March 4th 1878). The principle was much copied down through the years.

 

The Short system used a low mass air valve (sliding comb or pin coupled to a telephone or phonograph diaphragm) to modulate a high pressure stream of compressed air. In smaller units, this modulated stream then fed a horn. So powerful was the method that it was heard all over Paris during the 1900 Paris Exposition when it bellowed out Edison phonograph recordings whilst affixed to the top of the Eiffel Tower.

 

Short (later to co-found Short Brothers the aircraft manufacturers) likened the Auxetophone principle to a relay or repeater, in the sense that a small action simply initiated a larger one. (The relay was named from the electro-magnetic 'relay' used to 'repeat' Morse telegraph signals over long distances). The Auxetophone was not, however, a success even though Short licensed his invention to Sir Charles Parsons (of turbine fame) in 1903 for widespread exploitation.

 

The Auxetophone was used by Parsons in early synchronised phonograph cinema sound systems but was never a complete solution even though the principle was evolved further by Gaydon in England. He developed the idea commercially and the device was subsequently manufactured by Creed of Croydon as the Creed Stentorphone being still available in the mid-1920s. However, the principle was flawed (as always) and technically uncompetitive.

 

The main problem was that it was in reality a complicated arrangement, dependent on a compressed air or high pressure steam supply and apart from its bulk suffered badly from air stream noise and nonlinear distortion. This however was not a problem when in later decades those investigating active noise control saw its immense power as a source for antiphase noise cancellation

 

These loud speaking devices based on modulating a powerful airflow to make sound were given the name 'Syren' (siren), as in the siren of mythology, and eventually this was the term for air-raid warning devices which employed the same principle (though as we know, interrupted airflow sirens were known before the Auxetophone).

 

As for receiving loudspeakers, better technical performance was to come by other means.

Moving A Coil!

 

Most modern loudspeakers employ the moving coil (motor) principle, originated and first patented by E W Siemens in 1877 ( German Patent No: 2355 filed December 14th 1877). The motor principle (ie, the fundamental basis of the electric motor) was first discovered by Faraday in the 1820s and is observed when an electrical current in a conductor creates a magnetic field, and then then experiences a force when in close proximity to yet another magnetic field.

 

Siemens, however, originated the modern moving coil (dynamic) speaker method, that is 'a circular coil located in a radial magnetic field'. In this configuration an annular air gap in a magnetic circuit creates a radial field - that is, radial lines of constant magnetic force - which are orthogonal (transverse) to the coil magnetic field, itself caused by (modulated) electric currents in the coil.

The British scientist Oliver (later Sir Oliver) Lodge realised that the voice coil centre position, in the narrow annular re-entrant between poles forming the radial magnetic field, needed to be equidistant and this could be done by means of tripodal suspension arms fixing the flat diaphragm to the voice coil (British Patent 9712 April 27th 1898). Though Lodge successfully produced sounds by acoustically energising a thin wooden board, this of itself wasn't enough though curiously, it may have predated the current NXT speaker technology.

 

Siemens and Lodge had disclosed a low mass drive system for making improved telephone receivers - but to give it the ability to create a powerful sound over the whole audio spectrum was really something else. Construction and sensitivity problems meant that neither the Siemens nor the Lodge ideas had any immediate future - the more so in a period without electronic amplifier technology.

 

Paper Cones

 

In 1906 Miller Reese Hutchinson and Kelly Turner of the Hutchinson Acoustic Company in New York developed a small hearing aid system based on a composite paper diaphragm/cone speaker, using the moving iron and moving coil principle. Like its predecessors, the transducer converted electrical signals into movement by passing the sound through a moving coil, but in this instance affixed to a light composite paper diaphragm. As noted above, varying magnetic fields from the signal creating a force on

Pridham & Jensen's Magnavox

the voice coil by virtue of a strong permanent magnet in close proximity. The reverse of this, where a small magnet was affixed to the cone, and the coil was stationary, was also tried.  This very simple design was eminently successful, and in 1907 became part of their Dictograph office communication/intercom system. The Hutchinson system was initially unamplified, but used a DC mains supply to energise a powerful electro-magnet as the fixed (stationary) magnet for the loudspeaker.  The loudspeaker was never as sensitive a microphone as it was a loudspeaker, but worked well with a carbon microphone as a speech transducer. Each unit had its own microphone/speaker circuit and switching for a master unit. Later, the Lee De Forest Audion amplifying triode valve was introduced for electronic active amplification.

 

As thermionic amplifiers became more refined, electric (instead of acoustic) sound recording techniques began to creep in. The ability to fully amplify electrical signals meant that the conversion of these signals into a high-quality acoustic output required a better method - one that was self-contained and could take full advantage from higher amplitude, better quality, analogue signals. The problem was, however, that the actual process of making a wide range acoustic transducer was not an easy one.

 

Pridham & Jensen

 

An initial step came from two engineers who followed the earlier work of Siemens and Lodge. Indeed, virtually the same ideas were resurrected by Edwin S Pridham and Peter L Jensen whilst working for Magnavox, in California during the early 1920s. They coupled the coil and diaphragm together in a casing which carried the diaphragm suspension and formed a closed magnetic path through the casing from a central cylindrical D C-operated electro-magnet.

 

Their Patent (US Patent No: 1,448,279 for April 28th 1920) shows a tripodal coil/diaphragm coupling, almost identical to that designed by Lodge. But though all the components were in place, even this was not what we would view as the modern loudspeaker. This came from a slow evolution of the basic design.

 

Though little development took place from the Siemens/Lodge era to the work of Pridham and Jensen, the next few years saw some clear refinements of the basic principle. H J Round developed a refined electrodynamic cone speaker, but this, as was typical of the times, had an efficiency of less than 0.3%, that is, only 0.3% of the electrical energy applied to the coil was radiated as acoustic energy. The voice coil was made very light and bonded directly to a stiff but lightweight paper or (shellac impregnated) fabric diaphragm - this diaphragm was invariably shaped as a cone, supported on a flexible corrugated paper or rubber outer surround, while the voice coil itself was fixed in position by a light flexible surround called a 'spider', all mounted on a pressed steel chassis.

 

Occasionally, in early examples, the coil was positioned on the rear of the cone and activated by a flat ended pole of an electro-magnet. In other designs, an annular magnet and voice coil assembly was placed inside the cone vertex. An idea apparently due to N W McLachlan in 1922.

 

Early Gramaphone with an extraordinarily large acoustic horn speaker

What a blast!

Detail from th Lodge Patent (1898) showing the Voice coil suspension

A detail of the Short compressed air modulated Auxetophone

The Creed of Croydon Stentrophone, a development of the Short Auxetophone

Pridham and Jensen's Magnavox DC energised loudspeaker employing the Lodge suspension

Inverted drive speaker of 1926 by McLachlan using impregnated cloth cone

Gold leaf thermophone - its originator is unknown

Outline of McLachlan's moving coil speaker, circa 1930

Hewlett Induction Type speaker, circa 1923