A speaker is an electrical device that converts electrical signals to mechanical motion in order to create sound waves. A transducer, which is another name for a speaker, is a device that converts one form of energy to another. The speaker moves in accordance with the variations of an electrical signal and causes sound waves to propagate through a medium such as air or water. The first electrical speaker, patented by Alexander Graham Bell in 1876, was for the earpiece of the telephone. This design was later improved upon by Ernst Siemens and Nicola Tesla in 1877 and 1881 respectively. Siemens and Tesla used a metal horn driven by a membrane attached to a stylus to create the design of what would be the basis for the modern speaker. Thomas Edison was working on a design at this time using compressed air as the amplifying mechanism. He quickly found this was not the most effective way to create the mechanical waves that produce sound. He quickly withdrew his application for a patent and settled on the metal horn design. The metal horn speaker is a speaker which can be found on antique record players.
Metal Horn Speaker Moving Coil Speaker
The modern design of the moving coil driver was established by Oliver Lodge in 1898. Lodge was a British physicist and writer that was involved in many key patents involving wireless telegraphy. In 1915, Magnavox emerged as the first public company to produce a loudspeaker. This design was the first practiced use of the moving coil drivers in a loudspeaker. Magnavox was started in that same year by Edwin Pridham and Peter L. Jensen. The company’s focus was on developing consumer electronics. They would later go on to be the first to develop a phonograph loudspeaker. Today Magnavox is owned by one of the world leaders in electronics, Phillips.
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In 1924, Chester W. Rice and Edward W. Kellogg received the first patent on the moving-coil principle, direct radiator, and loudspeaker. Their patent was different from the previous attempts because of the adjustment of mechanical parameters in their design. The fundamental resonance of the moving system takes place at a lower frequency than that at which the cone’s radiation impedance becomes uniform. In 1926, Rice and Kellogg sold the loudspeaker, ‘Radiola’ which was superior to anything else previously invented because it decreased sound distortion and improved audio quality for the buyer. These speakers used electromagnets instead of large powerful magnets in their design. The electro magnets were used because larger, more powerful magnets were not available at a cheap enough price at the time. In the 1930’s, manufactures began placing two or three band passes worth of drivers in their speakers, which allowed for increased quality, sound pressure levels, and frequency response.
Many of the components involved in the production of modern speakers have been improved upon from their initial designs. The biggest improvements have occurred mainly in the makeup of the materials in the speaker and in the enclosure design. The diaphragm materials and permanent magnet materials are some of the other speaker components which have improved throughout the years. With the advent of computer aided design and increased accuracy in measuring techniques, the development of the speaker and quality of sound have grown exponentially in recent years. The modern loudspeaker has a similar makeup to that of earlier designs, but some of the basic ideas behind the design have changed to give us the speaker we have today.
The Modern Speaker
Modern speakers use a permanent magnet and an electromagnet to induce the reciprocating motion of the diaphragm. The alternating current going through the electromagnet constantly reverses the magnetic polarity of the coil thus reversing the forces between the voice coil and the permanent magnet. This causes a rapid back and forth motion of the coil resembling that of a piston. When the coil moves it causes the diaphragm to vibrate the air in front of the speaker, creating sound waves. The frequency and amplitude of the electrical audio signal dictates the rate and distance that the voice coil moves thus determining the frequency and amplitude of the sound waves produced by the diaphragm. Drivers are only able to create sound in a given range of frequencies, thus many different types of drivers must be manufactured to account for the wide range of possible frequencies.
The main components of the modern speaker are the diaphragm, permanent magnet, suspension, voice coil, and basket with three other important features being coaxial drivers, speaker enclosures, and audio amplifiers. In the following sections we will break down each component and investigate the improvements of each component including those in the material selection and the manufacturing process.
One of the main components of a speaker is the diaphragm, sometimes called a speaker cone. The diaphragm can also be referred to as the diaphragm and its surrounding assembly including the suspension and the basket. However for our purposes the suspension and the basket will be individually discussed in later sections. Movement of the diaphragm causes sound waves to propagate from the speaker thus producing the noise we hear. The ideal properties of a diaphragm are minimal acoustical breakup of the diaphragm, minimal standing wave patterns in the diaphragm, and linearity of the surrounds force-deflection curve. The diaphragm stiffness and damping qualities plus the surround’s linearity and damping play a crucial role in reproducing the voice coil signal waveform.
Eighty five percent of the diaphragms sold worldwide are made of cellulose fibers because they can be easily modified by chemical or mechanical means to giving it a practical manufacturing advantage not found in other common diaphragm materials, although reproducibility can be a problem. The lack of reproducibility can affect imaging, depending on the precision and quality of production. Cellulose is also advantageous over other diaphragm materials because of its low cost to produce. Although Cellulose works well as a diaphragm, new synthetic materials are emerging that are more lightweight, allowing for better audio quality, reduced distortion, and increased vibration and shock durability. These materials include polypropylene, polycarbonate, Mylar, silk, fiberglass, carbon-fiber, titanium, aluminum, aluminum-magnesium alloy, and beryllium.
Polypropylene is the most common plastic material used in a diaphragm. The polypropylene is normally mixed with a filler, such as Kevlar, to reduce the manufacturing costs or it can be to alter the mechanical properties of the diaphragm. Polypropylene diaphragms have been increasingly more popular with the advancements in modern adhesive technology. Although with all plastic materials present, the material tends to have a viscoelastic creep, which is the materials tendency to slowly deform and stretch when under repetitive stresses. However, polypropylene diaphragms are still a popular choice for high performance speakers due to their consistent performance. Research is presently underway in attempts to create new plastic based materials such as TPX, HD-A, HD-I, Neoflex, and Bextrene for diaphragms. These materials generally have the same characteristics as polypropylene so the manufacturing costs cannot be justified for full production.
Another option for low-frequency applications are woven fiber diaphragms. The woven fibers such as carbon fiber, fiberglass, and Kevlar are bonded together with a resin. When the high tensile strength of the woven fibers mixes with the adhesive and bonding characteristics of the resin it results in an incredibly stiff material. This stiffness results in a great low-frequency diaphragm, however the stiffness causes rough high-frequency responses. There have been numerous attempts to improve the high-frequency problems of woven fiber diaphragms such as using two thin layers of Kevlar fabric bonded together with a resin and silica microball combination and another attempt employed a sandwich structure of materials with a honeycomb Nomex core. But again, as with the advanced plastic materials, the cost of manufacturing versus the performance of the material cannot yet be justified.
The final modern practical material for diaphragms is metal. Metals worst downfall is its terrible damping attributes which causes extreme high-frequency distortion. The most common metal of choice are aluminum and magnesium alloys. Due to the lack of technological advances in damping agents to add to these alloys, metal diaphragms are very rarely used in high-frequency applications. However, these alloys have been commonly used in lower end frequencies with great success.
Modern driver magnets have become predominately permanent magnets. Historically this function was filled by the use of electrically powered field coils. When high-strength permanent magnets became available, they eliminated the need for the additional power supply that drove the coils. When this happened, Alnico magnets became popular. Alnico magnets are created from alloying aluminum, nickel, and cobalt. Until about 1980 Alnico magnets were primarily used but because of their tendency to become demagnetized, permanent magnets have since been made of ceramic and ferrite materials. Ferrite magnets are constructed by mixing iron oxide with strontium and then milling the compound into a very fine powder. The powder is then mixed with a ceramic binder and closed in a metal die. The die is then placed in a furnace and sintered to bond the mixture together. Sintering is the process in which the particles of the powder are welded together by applying pressure and heating it to a temperature below its melting point. Although the magnetic strength to weight ratio of ferrite magnets is lower than Alnico, it is considerably less expensive, allowing designers to use larger yet more economical magnets to reach a desired performance.
In manufacturing, the most significant technical innovation of the speaker is due to the use of neodymium magnets. Currently neodymium magnets are the strongest permanent magnets known to man. For this reason neodymium magnets significantly help in producing smaller, lighter devices and improve speaker performance due to their great capacity for generating strong magnetic fields in the air-gap. A neodymium magnet is an alloy of neodymium, iron, and boron to form the molecule Nd2Fe14B. The molecular structure of this molecule is a tetragonal crystalline structure. Important properties in a magnet are the strength of the magnetic field, the material’s resistance to becoming demagnetized, the density of magnetic energy, and the temperature at which the material loses its magnetism. Neodymium magnets have much higher values for all of these properties than other magnetic materials except that it loses its magnetism at low temperatures. For this reason it is sometimes alloyed with terbium and dysprosium in order to maintain its magnetic properties at higher temperatures.
Another critical element in speakers is the suspension. The purpose of a suspension system is to provide lateral stability and make the speaker components return to a neutral point after moving. A typical suspension system includes two major components, the spider and the surround. The spider connects the voice coil to the frame of the speaker and provides the majority of the restoring force. The surround connects the top of the diaphragm to the frame of the speaker and helps center the diaphragm and voice coil with respect to the frame. Both components work together to make sure the diaphragm and coil assembly move strictly linearly and in line with the center of the permanent magnet.
The spider is usually made of a corrugated fabric disk, impregnated with a stiffening resin. The name comes from the shape of early suspensions, which were two concentric rings of Bakelite material, joined by six or eight curved legs. The surround may be resin treated cloth, resin treated non-wovens, polymeric foams, or thermoplastic elastomers that are molded onto the cone body. An ideal surround has sufficient damping to fully absorb vibration transmissions from the cone to surround interface, and the durability to hold out against long term fatigue caused by prolonged vibration.
Advancements in suspension manufacturing have come from innovations in synthetic suspension materials. The use of synthetic materials like kevlar or konex instead of cotton, has made today’s speakers much more stable than those made as recent as ten years ago. A more durable suspension means that a speaker’s sound quality can remain unaltered for a longer period of time. This is especially a concern for speakers that generally operate at low frequencies since lower frequency sounds are created by larger diaphragm travel and larger diaphragm travel must be supported by more suspension travel.
The wire in a voice coil is usually made of copper, though rarely aluminum and silver may be used. Voice coil wire cross sections can be circular, rectangular, or hexagonal, giving varying amounts of wire volume coverage in the magnetic gap space. The coil is oriented co-axially inside the gap; it moves back and forth within a small circular volume (a hole, slot, or groove) in the magnetic structure. The gap establishes a concentrated magnetic field between the two poles of a permanent magnet, the outside of the gap being one pole, and the center post (called the pole piece) being the other. The pole piece and backplate are often a single piece, called the poleplate or yoke. This magnetic field induces a reaction with the permanent magnet causing the diaphragm to move thus producing the sounds we hear. Voice coils can either be overhung, longer than the magnetic gap, or underhung, shorter than the magnetic gap, depending on its application. Most voice coils are overhung thus preventing the coil from being overdriven, a problem that causes the coil to produce significant distortion and removes the heat-sinking benefits of steel causing the speaker to heat rapidly.
The most important characteristic of a voice coil is that it be able to withstand large amounts of mechanical stresses and also be able to dissipate heat to its surroundings without causing damage to the speakers other components. In early loudspeakers the voice coil was wound onto paper bobbins to remove heat from the system. At the time this was enough to cool the system at average power levels but as larger amplifiers became available allowing for higher power levels new technologies had to emerge.
To cope with the increasing power inputs the use of alloy 1145 aluminum foil was widely used as a substitute for the paper bobbins. Aluminum was popular to industry due to its low cost to manufacture, its structural strength, and it was easy to bond to the voice coil. However, problems with the foil emerged over extended use at increased power levels. The first problem was the foil tended to transfer heat from the voice coil into the adhesives used inside the speaker causing them to thermally degrade or even burn. The second problem was the motion of the aluminum foil inside the magnetic gap created currents that actually increased the temperature of the voice coil, thus causing long-term reliability issues.
In 1955 a new material was developed called Kapton, a polyimide plastic film, to replace the aluminum foil. Kapton solved all the problems that were associated with the aluminum foil however Kapton or even its improved cousin Kaneka Apical, were not perfect. Both high-tech materials were costly to manufacture and had a tendency to soften when heated. Although Kapton and Kaneka Apical had their downfalls they became the most widely used coating for voice coils until 1992 when a material called Hisco P450 was developed. Hisco P450 is a thermoset composite created by using a thin film of fiber glass cloth and impregnating it with a polyimide resin. This combination allowed for necessary mechanical strength and endurance of the polyimide and necessary temperature resistance and stiffness of fiberglass. Hisco P450 was able to withstand the grueling temperature requirements of professional speakers while also maintaining enough rigidity to withstand the mechanical stresses associated with long-term, high-frequency motions.
In recent years the copper wire that is almost always used as the voice coil has been replaced sparingly with aluminum wire in extra sensitive, high-frequency applications. The aluminum wire is lighter than the copper wire and has about two thirds of the electrical conductivity allowing the wire to move at higher frequencies inside the magnetic gap. Variations of the aluminum wire include copper-clad aluminum and anodized aluminum. Copper-clad aluminum allows for easier winding along with an even more reduced mass. The anodized aluminum is effectively insulated against shorting which removed the concerns of dielectric breakdown. Aluminum wires are great lightweight, low-inductance choices for voice coils however, they do have their downfalls. The thermal characteristics of aluminum causes power limitations with the coil. If too much power is passed through the aluminum coil it can cause the adhesive bonds between the wire and the bobbin, or the bobbin to the spider and coil to weaken or even burn.
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To cope with the ever increasing power demands on the voice coil in addition to wrapping the coil in some high-tech material to increase its thermal properties, the voice coil has also been submerged in a ferrofluid, an oil that is used to conduct heat away from the voice coil and also creates a small magnetic field thus increasing the power handling capacity of the voice coil.
The basket or frame (as seen below) is the fixture used to hold the diaphragm, voice coil, and magnet in the proper place. The rigidity of this part is extremely important to prevent rubbing of the voice coil and prevent random movements that could cause problems with the permanent magnet. The three most common types of modern baskets are cast metal baskets, rigid baskets made out of stamped steel or aluminum, and cast plastic baskets. Each type of basket offers different advantages and disadvantages; these will be discussed in the flowing paragraphs. The stronger the basket the more power the speaker can handle before failure occurs. A well made basket should have a high power rating, be lightweight, and be able to conduct heat away from the voice coil to prevent physical changes or even possible demagnetization of the permanent magnets.
Cast metal (above right) baskets are the most rigid of the three in all directions, but they are the most expensive to make. Cast metal baskets are made by melting down the desired metal to liquid form. The scorching hot liquid metal is then poured into a mold and once the liquid metal dries inside the mold, the mold is removed revealing a cast metal basket. Cast metal baskets although more expensive than the other two options, usually are more rigid thus preventing motion. They also have better damping characteristics, and they are also more easily manufactured allowing for more intricate shapes. Cast metal baskets are usually the preferred basket choice for higher quality speakers.
A less expensive and yet less rigid basket can be made out of stamped steel. The stamped steel or aluminum sheets arrive to the manufacturer preformed. The sheets are then drilled using a hydraulic press to cut holes in the sheet to allow air flow to and from the diaphragm. The sheet is then pressed using another hydraulic press using a die to form the desired shape. Stamped metal baskets tend to be weaker than their cast metal counterparts. This weakness could cause the basket to flex if the speaker is being used at high volumes.
The final option, which is even less expensive, is a cast plastic basket. Cast plastic baskets are made by using the liquid plastic and pouring it into the desired shaped mold. When the liquid plastic dries the mold is removed revealing a cast plastic basket. Just like cast metal baskets, cast plastic baskets are easily manufactured allowing for intricate shapes. The lightweight characteristics of the plastic would also make the speaker lighter allowing for smaller power consumption. However, as with most engineering decisions, the performance of the part proportionally decreases as the cost to produce the part decreases. The decreased cost of production of the plastic basket means that it is a weaker basket. This weaker, plastic basket will allow for the most flexing as compared to cast metal and stamped steel baskets. The power rating of the speaker would also be less than that of the metal baskets, both cast and stamped, due to the weaker strength characteristics of plastic in comparison with metal.
Coaxial drivers are the components of a speaker that radiates sound from the same point or axis. This is done by placing a high-frequency driver in the center of a low-frequency driver so that they produce sound waves from a single point in a loudspeaker system rather than separate locations. This allows for a more beneficial design over having the low and high frequency drivers separate. There are many different types of drivers and each driver produces sound within a limited frequency range. Subwoofers, woofers, mid-range drivers, and tweeters are all driver types capable of emitting different ranges of sound. A coaxial driver takes one of these higher frequency drivers and places it within a lower frequency driver. For example, a tweeter, the high frequency unit, could be placed in the center of a woofer, the low frequency unit, so that both drivers emit sound from the same point. This example can be seen in the images below. This design, which improves sound quality, was first designed by Altec Lansing in the 1940’s. Although it has many advantages, it is still an uncommon practice in the manufacturing of speakers due to technical and budgetary considerations.
The enclosure of a loudspeaker serves three functions and is made with a specific design that helps improve the quality of the sound produced by the speaker. The first function the enclosure performs is separation of the sound waves. It accomplishes this by preventing sound waves generated at the back of the speaker from interacting destructively with sound waves generated at the front of the speaker. The enclosure is intended to reduce distortion created because the waves that emanate from the front of the speaker are out of phase with the waves emanating from the rear of the speaker. If the front and rear waves were to overlap with one another it would result in wave interference. The second function the enclosure serves is to stop any echo and reverberation that would be created from the two differing sound source locations on the speaker. Because waves are created at the front and rear of the speaker, the two different sets of waves travel through the air differently as a result of their relative locations, and arrive at the person listening at different times. The third function the enclosure serves is to deal with the vibrations produced by the driver and to deal with the heat produced by the electronic components. Enclosures did not always have the fully enclosed container design that they now commonly have. Although present day practices say that enclosures need to have a back, before the 1950’s they lacked one due to the cooling functions of an open container.
Sealed enclosures, the most common type of enclosure, is completely sealed so no air can escape. With this type of enclosure the forward wave travels outward into the surroundings, while the backward wave is limited to only fill the enclosure. With a virtually airtight enclosure, the internal air pressure is constantly changing; when the driver retracts, the pressure increases and when the driver moves out, the pressure decreases. Both movements create pressure differences between the air inside the enclosure and the air outside the enclosure. Because of this, the driver motion always has to fight the pressure differences caused. These enclosures are less efficient than other designs because the amplifier has to boost the electrical signal to overcome the force of air pressure. The force due to air pressure does, however, provide an additional form of driver suspension since it acts like a spring to keep the diaphragm in the neutral position. This makes for tighter, more precise sound production.
Enclosure designs range from very simple, rectangular particle-board boxes (above left) to very complex cabinets made of composite materials (above right). The simplest enclosures are made to prevent destructive interference caused by overlapping of the front and rear sound waves from the speaker. The most complex enclosures contain acoustic insulation and internal baffles, which prevent interference.
Solid materials such as heavy wood, are typically used when building enclosures in order to absorb the vibration caused by the speaker driver. This vibration dampening is extremely important. A speaker’s sound output would be drowned out by the driver’s vibrations if there were not an enclosure incorporated into the design. Since the beginning of the production of enclosures, the most advantageous properties required for minimal energy loss through the enclosure walls have remained unchanged. Different strategies employed to reduce energy losses are to use thicker enclosure walls, denser hardwood plys and sturdier bracing. The downside to these methods is that they all add significant weight to the enclosure. However, with the production of newer materials that possess an increased stiffness-to-mass ratio this is changing. These new materials can improve performance and reduce weight, while also reducing the cabinet’s resonance. The end result is that a greater amount of the speaker’s energy is delivered in the intended direction rather than into mechanical vibrations which are wasted and produce a decrease in sound quality.
A recent alternative to heavy wood construction of enclosures is the use of composite materials. It was for the aerospace industry that composite materials such as carbon-fiber were originally developed. Carbon-fiber was a success because of the high demand for a material with increased strength and rigidity. Speaker applications, such as enclosures use carbon-fiber materials to create a product with a vastly decreased weight and increased strength and rigidity. Enclosures built with carbon-fiber can weigh less than half as much as enclosures built from heavy wood. These enclosures which limit the speaker resonance can provide as much as 3 dB more output than the same speaker would have otherwise had in a heavy wood enclosure. Furthermore, carbon-fiber enclosures are extremely durable adding quality to the final product and they require almost no maintenance. Even though carbon-fiber enclosures cost around twice as much to produce as traditional enclosures, the lighter weight and extra output offer two very advantageous tradeoffs.
An amplifier is any device that increases or decreases the amplitude of a signal. An audio amplifier increases low-power audio signals to a suitable level for loudspeakers. When dealing with a speaker there are a many audio amplifiers involved. These amplifiers are responsible for pre-amplification, equalization, tone control, and mixing effects followed by a higher power amplifier which creates the final amplification for suitable levels of sound output. Amplifiers are found in wireless receivers and transmitters, CD players, acoustic pickups, and hi-fi audio equipment. Amplifiers are used for high-quality sound production, and depending upon the quality of the amplifier, they may cause distortion, which the speaker enclosures are meant to deal with. Distortion in amplifiers is caused by difference in phases of the output waveform and the input waveform. The smaller the difference in between the output and input waveforms the greater the quality of final sound. Audio amplifiers consist of resistors, capacitors, power sources, wires, semiconductors, and stereo jacks all combined on an electronic work board to produce the type of amplifier needed.
Types of Speakers
Woofers are loudspeaker drivers designed to produce sounds of low frequency from around 40 hertz up to around 1000 hertz. The most common design for a woofer is the electro-dynamic driver, using a stiff paper cone driven by a voice coil. Woofers are important to allow for a range of frequency that will hit a low level.
Effective woofer designs efficiently convert low frequency signals to mechanical vibrations. The vibration of the air out from the cone creates concentric sound waves that travel through the air. If this process can be done effectively, many of the other problems speakers run into will be greatly reduced such as linear excursion. For most speakers the enclosure and the woofer must be designed to work hand in hand. Usually the enclosure is designed around the woofer, but in some rarer cases the enclosure design can actually dictate the woofer design. The enclosure is made to reflect the sounds at the right distance, so that they will not be wave cancelling reflections. Below you can see an example of a common woofer.
A subwoofer is a woofer with a diameter between 8′ and 21’s. Subwoofers are made up of one or more woofers. They can be arranged in many different configurations to produce the best quality of sound. Subwoofers usually play frequencies between 20 hertz and 200 hertz, well within the range of human auditory levels. The first subwoofer was created in the 1960’s and added to the home stereo to create bass for sound reinforcement. Up until this point the only form of audio player which contained bass was a phonograph player which was created by Magnavox. This allowed for a more accurate array of music. Subwoofers are used in all sound systems today such as in cinemas, cars, stereos, and for general sound reinforcement.
A mid-range speaker is a loudspeaker driver that produces sound between 300 hertz and 5000 hertz. These are less commonly known as squawkers. Midrange drivers can be found as cone speakers, dome speakers, or compression horn drivers. Mid-range speakers usually resemble small woofers. The most common material the cone is made out of for a mid-range is paper although they can be found to be coated or impregnated with polymers or resins to improve vibration dampening. Much of the rest of the mid-range speaker is made from plastic polymers. Mid-range speakers which employ the dome set up usually only use 90 degrees of the sphere as the radiating surface. These can be made from cloth, metal or plastic film. The voice coil in this design is set at the outer edge of the dome.
Mid-range drivers are most commonly used for professional concerts and are compression drivers coupled with horn drivers. Rarely mid-range speakers can be found as electrostatic drivers. Mid-range speakers handle the most prominent part of the human-audible sound spectrum. This is the region where most sound emitted by musical instruments lie. This is also where the human voice falls in the audible spectrum. Most television sets and small radios only contain a single mid-range driver.
Tweeters are a loudspeaker designed to produce frequencies from 2,000 to 20,000 hertz. Some tweeters on the market today can produce sounds of up to 45000 hertz. The human ear can generally only hear up to about 20000 hertz. The name tweeter comes from the extremely high pitch it can create. Modern tweeters are different from older tweeters because older tweeters were smaller versions of woofers. As tweeter technology has advanced, differen
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