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The Weakest Link

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The Weakest Link Innovation. Amplified. Chapter 2 The Weakest Link by Hartley Peavey he integral component of the production/repro- consisted of a paper cone driven by voice coil that Tduction of sound involves what engineers call a “reacted” with a fixed magnetic field generated by transducer. This is a device that transforms one kind an electromagnet. The electromagnet consist of of energy into another. A loudspeaker is a device that numerous turns of wire in a magnetic structure that converts electrical energy into a sound pressure wave created a “focused” magnetic field for the voice coil. that is perceived by our ears as what we call sound. This early “loudspeaker” was an instant hit and was Without getting too involved in history, we should be immediately copied by a number of people using a aware that when the telegraph was invented in the number of “variations” on the electromagnetic driver mid 1800’s, the “receiver” was an electromagnetic coil theme. that attracted a metal “armature” that made a “clicking noise” when current flowed through the coils producing What is most interesting about the loudspeaker is the familiar “click/clack” sound of the early telegraph… that while electronics has made vast improvements Even before Morse perfected the telegraph, scientists over the years, the so-called dynamic loudspeaker had discovered that certain materials possessed the has remained very much the same. Today’s speakers ability to produce electricity when they are moved or still use voice coils reacting with a magnetic field bent, and when electricity is applied to them, do the usually including some kind of fiber (paper) cone/ reverse by producing a physical motion. These early diaphragm…. experiments resulted in what is known as the “Piezo (pronounced “pe-a-zo,” not pi-zo) Effect”… Later on, The very first automobile in the latter 1800’s in Germany earphones and microphones using so-called “crystal utilized rubber tires filled with air. Today, more than 100 elements” would come into use with so-called “crystal years later, our cars still utilize air filled rubber tires. It is radio sets” and later on with phonograph pickups… true that the materials and the designs of today’s tires While a Piezo element will produce motion with volt- are vastly better than those used by Benz, but we are age applied or generate a voltage when moved, this still riding on rubber tires filled with air. Likewise, most “motion” is very small and Piezo elements can usually loudspeakers today operate using exactly the same only be used in applications where the motion is small, principles as those early loudspeakers in the 20’s, i.e. either in the pickup mode such as a microphone, or fixed magnet and a diaphragm attached to a voice output mode such as a tweeter (the Motorola Piezo coil… Like the “automobile tire analogy” mentioned tweeter, for example). above, the major difference in today’s speakers using a (versus those of 90 years ago) is the better materials While the “Piezo Effect” has been known for a long time, used in today’s loudspeakers. its limitations re: movement necessitated the search for a better way to move more air, thus producing louder In the early days of audio, as well as, today, the sound. The earliest electromagnetic reproducers were loudspeaker was (and is) the “weakest link” in the audio variations of the telegraph “sounder.” The telephone system. In the early days when a 10-watt amplifier was was made possible by utilizing electromagnetic a “big amp,” power handling really wasn’t a factor, but drivers to activate a metal diaphragm to create the first as electronics became more sophisticated, it soon telephone “receiver.” A similar technique was utilized became apparent there was much work to be done to to make headphones for the early radio receivers. improve loudspeakers. After the invention of the vacuum tube, audio amplifier THE MOVIE BOOM companies attempted to use what were essentially The main driving force in coming up with large speaker “earphones with horns attached” to reproduce the systems was the advent of so called “talking pictures” sound of the (then) new AM radio. The low powered in the late 20’s. By that period of time, Western Electric radio sets of that time drove an electrodynamic driver found it necessary to design audio amplifiers for their coupled to a fairly large horn in order to bring the “repeater” stations used with long distance telephone level up enough to be heard in a small room… This service. Interestingly, Western Electric and AT&T approach was not good, not efficient, and was both invented many of the vacuum tube audio circuits that cumbersome and expensive. are still being used today. Sound for movie theaters The “Dynamic Loudspeaker” was actually invented in was a major market that was served not only by the Germany in the 1890’s. In the early 20’s, a company Westrex Division of Western Electric, but also by RCA called Magnavox introduced the first production who were also involved in the movie business. With “dynamic loudspeaker” to the US marketplace. It sound becoming so important, the race was on to 2 create better sound for movie theaters. Because of multiples of speakers were used such as 2 12’s or 4 the relatively low power of amplifiers then, many of the 10’s. The speakers during that period had relatively theater sound systems were driven by very efficient small diameter voice coils wound on paper bobbins horn loaded transducers. It soon became apparent and speakers were assembled using cellulose based that a single speaker was not adequate to cover the full adhesives, which were notoriously heat sensitive. sound spectrum and high frequency drivers became a Overall, the trend to higher power, coupled with small part of the scene as did ever more efficient and better diameter paper voice coils held together with cellulose performing cone type speakers. adhesive, was a “recipe for disaster”… In 1936, RCA introduced the 6L6, which ushered in A TURNING POINT a whole new level of audio power. When WWII came Makers of premium speakers (initially designed for along, audio development ceased and didn’t began movie theaters) provided the first speakers for high- again until the late 40’s. Using multiple 6L6’s, power level sound reinforcement applications. Altec’s three- amps became available with 50 – 60 watts, which inch voice coil and JBL’s four-inch voice coil were the was a “challenge” for the high frequency drivers and leaders in high power, highly efficient loudspeakers. cone type speakers of the day. Since high frequency Because of their cost and their weight, these premium drivers used smaller and more delicate voice coils, it speakers were limited to only a few “upper end” was these delicate drivers that suffered the most. Most applications both in sound reinforcement and guitar companies supplying high frequency drivers to the amps. Most loudspeakers made in the U.S. utilized theater industry were forced to make the diaphragms relatively small diameter voice coils. One inch, 1½ inch, “field replaceable”… A trend that continues to this and the largest generally available (other than from day. Altec/JBL/EV) were the two inch voice coils offered by the U.S.’s largest speaker manufacturer CTS (Chicago Cone loudspeaker designers had the option of Telephone Supply)... using larger diameter voice coils and powerful alnico magnets to increase power handling and efficiency. A By the early 1960’s, adhesives had improved practical limit seemed to occur in the early 50’s with the significantly with the introduction of epoxies, but voice competing companies selecting slightly different voice coils continued to be wound on paper coil forms. With coil diameters and magnet geometry. Altec settled on the introduction of 100-watt amplifiers, first powered a three-inch coil while Altec’s major competitor JBL by tubes, and later by transistors, loudspeaker failures settled on a four-inch diameter coil. Shortly thereafter, started to “skyrocket.” Previously, most failures were Electro-Voice entered the field with a 2½-inch coil, and in high frequency drivers because of their light weight this basic configuration continues today. (and therefore delicate) construction. CTS responded by offering a laminated aluminum/paper coil form Both before, and after WWII, movies were a very while others such as Jensen and Electro-Voice utilized important part of people’s lives on both sides of a fiberglass coil form which were infinitely better than the Atlantic. Demand for better sound in movies the paper coil forms of their competitors. spilled over into so called” “jukeboxes” which of course generated the desire to have that quality of Bob Gault (the Chief Loudspeaker Engineer at CTS) sound in people’s homes. In the late 40’s, the “hi-fi broke away and formed the Eminence Speaker industry” was born. Interestingly about the same Corporation. From the beginning, Bob utilized time, the so-called “big band” style of music had aluminum coil forms with the latest epoxies and become increasingly expensive to finance both for adhesives. Within just a very few years, Eminence the orchestras themselves, and in the recording of had secured most of the OEM business for guitar large musical groups. After the War, considerable amps and sound systems. Peavey was one of the interest developed in other kinds of popular music first customers of Eminence, and that relationship has such as Hawaiian music, and in the South and the continued until today. While Bob Gault was a brilliant West so called “Country and Western” music. The speaker engineer, he was reluctant to listen to what electric guitar had become a requirement in most the market was asking for.
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