It is almost impossible to avoid talking about loudspeakers, when we are discussing music. This is especially true, when it comes to music enthusiasts and professional audio engineers alike. In every aspect of music and audio production, you will always come across some form of a loudspeaker. But how much do you actually know about this fascinating device? In other words, what is a loudspeaker?
In today’s article, we will be taking a look into the various components, system design and technology behind loudspeakers. The subject of loudspeakers goes down very deep, therefore it is impossible to cover every single detail in one article. However, I hope by the end of this post, you will have a thorough enough understanding of the fundamentals. Hence, let us begin!
The Loudspeaker – Overview
In a nutshell, a “loudspeaker” is a device comprising of one or more electro-acoustic transducers (which functions as converters of electrical audio signals into mechanical sound waves). The speaker type that almost everyone is using today, is known as “dynamic speaker”. Invented by Edward W. Kellogg and Chester W. Rice in 1925, dynamic speakers are designed based on the same fundamental principle as dynamic microphones, but in the opposite fashion – to produce sounds from electrical audio signals.
When an electrical audio signal (alternating current) makes contact with the “voice coil” (a coil of wire, suspended in a circular gap, between the poles of a permanent magnet), it will cause the coil to vibrate rapidly (back and forth, as a result of Faraday’s law of induction), which then excites a diaphragm (typically cone-shaped, and attached to the coil) and causes it to also move back and forth, creating a disturbance in air particles (sound waves).
However, do take note, other than the method just explained earlier, there are also other technologies that can be implemented, to convert electrical signals into sound. The audio signal (e.g., from a microphone) must first be amplified (with amplifiers) before the signal is sent to the speaker’s input. You will find that speakers are usually housed in rectangular or square enclosures (often made of wood or plastic), and these enclosures are pivotal in affecting the overall quality of the sound.
Another thing, when it comes to “high fidelity” (high quality) sound reproduction, multiple loudspeaker transducers (usually 2 to 3) will be mounted in the same enclosure, where each of them are designed to reproduce a part of the audible frequency range. In this situation, the transducers are called “drivers”, and the entire unit (including the speaker enclosure) is referred to as the loudspeaker. The three common “drivers” are known as “Tweeter”, “Mid-range” and “Woofer”.
Now, let us look at the various aspects of the loudspeaker, that we will be covering today:
- Driver design (Dynamic loudspeakers)
- Driver types
History – The beginning
The first ever electric loudspeaker was invented in 1861, by Johann Philipp Reis, who installed it in his telephone. It was only able to reproduce clear tones, but after revisions, it could also handle muffled speech. Alexander Graham Bell then went on to patent his first electric loudspeaker (able to reproduce intelligible speech) in 1876, to be included in his telephone. Ernst Siemens then improved on it in 1877.
Soon after, companies such as “Victor Talking Machine Company” and “Pathé”, manufactured record players using compressed-air loudspeakers (engineered by Horace Short). These designs however, had poor sound quality and were unable to reproduce sounds at lower volumes. Adaptations of the system were often used for public address applications. Other variants have also been used to test space-equipment’s resistance to loud sounds (caused by rocket-launching).
In 1898, the first moving-coil loudspeaker (dynamic) prototype, was engineered by Oliver Lodge. Danish engineer Peter L. Jensen and Edwin Pridham, then manufactured the first dynamic loudspeakers in 1915, in Napa, California. Similar to previous loudspeaker designs, these used horns to amplify the sound that is generated by a small diaphragm. Jensen was however, denied patents to these products.
The moving-coil technology, that is commonly found in the speakers that everyone is using today, was patented by Chester W. Rice and Edward W. Kellogg in 1924. The primary difference between previous designs and the patent by Rice and Kellogg, is that the “fundamental resonance” of the moving-coil system, falls below the frequency where the impedance (of the cone’s radiation) becomes uniform. This is achieved by making adjustments to certain mechanical parameters in the loudspeaker.
Driver design – Dynamic loudspeaker
A “driver”, also commonly referred to as “dynamic loudspeaker”, is made up of a lightweight diaphragm (cone), attached to a rigid “basket” (frame), via a flexible suspension (known as “spider”), that limits a voice coil to only move axially through a magnetic gap (cylindrical shape).
It starts from an electrical signal flowing through the voice coil, which causes a magnetic field, thus making it a variable electromagnet. The coil interacts with the driver’s magnetic system, causing the coil (with the attached cone) to move back and forth (due to the mechanical force generated). Hence, under the control of the amplified electrical signal (coming from the audio amplifier), sound is being produced.
Let us now look at the various individual components of a dynamic loudspeaker in further detail:
- Chassis (frame)
- Suspension system
- Voice coil
Its about to get a little technical in the preceding sections, so hold on tight!
Usually designed in a “cone” or “dome-shaped” profile, the diaphragm can be made of various materials. However, the most widely used are paper, plastic, and metal. Ideally, the material would have to be rigid (to prevent extraneous motions), have low mass (minimizing initial force requirements and energy storage issues), and well damped (to reduce unwanted vibrations after the signal has stopped, with minimal audible ringing, caused by resonance frequency due to its usage).
From a practical perspective, it is impossible for all these criteria to be met simultaneously. Thus, driver designs often involves trade-offs. For instance, paper is well damped and very light, but not rigid; metal is rigid and pretty light, but typically has poor damping; plastic is light as well, but normally, if it is made stiffer, the damping will suffer. Hence, most cones are made of composite material (consists of several elements).
As an example, a cone may be constructed by cellulose paper, but added with some carbon fiber, kevlar, glass, hemp or bamboo fibers. It may also utilize a “honeycomb sandwich” construction and be applied with a coating to provide enhancement in stiffening or damping.
Also known as “basket”, it is designed to be rigid, reducing deformation that alters critical alignments with the magnet gap, which may cause the voice coil to clash with the sides of the gap. Chassis are normally made of aluminum alloy, or from thin steel sheet. Cast chassis are preferred for drivers with larger magnets, as sheet metal can warp easily if the loudspeaker is subjected to robust handling.
Materials such as molded plastic and damped plastic compound are also in high demand, especially for affordable, low-mass drivers. Metallic chassis also provides the benefit of conducting heat away from the voice coil. This is crucial as heating changes resistance, hence causing physical dimensional changes, that may even demagnetize permanent magnets (if the condition worsens to an extreme state).
This system functions to keep the coil centered in the gap and provides a force (restoring) that returns the cone to its neutral position after vibrating. Most suspension systems are made of only two parts, namely the “spider” (provides most of the “restoring force” and connects the diaphragm or voice coil to the chassis), and the “surround” (helps to center the coil or cone components).
The “spider” is often made of a corrugated fabric disk, impregnated with a stiffening resin. Its unique name “spider”, comes from early suspension designs, where there are two concentric rings of Bakelite material attached with six to eight curved “legs”. Variations of this configuration, included the additional use of a felt disc, in order to construct a barrier to particles that might cause the voice coil to rub.
The cone can be found attached to both the outer diaphragm circumference and also to the frame. The cone surround is often made of rubber or polyester foam, or a ring of fabric (corrugated and coated with resin). These various materials, together with different shapes and treatments, can be a major factor in altering the acoustic response of a driver.
Each type and implementation methods have its advantages and disadvantages. Say for instance, polyester foam, which is very light and cost-effective, but will be degraded by excessive exposure to ozone, UV light, humidity and high temperatures. Thus, this limits its shelve-life to only about 15 years.
Voice coils are usually made of copper wires, although aluminum and silver may be used (rarely). Aluminum has the benefit of being light weight, and that will raise the resonant frequency of the voice coil, allowing it to respond much better to higher frequencies. But the downside of aluminum is that it is very hard to solder, hence connections are instead often crimped together and sealed.
Connections are prone to corrosion and will fail over time. Cross sections of voice coil wires can be circular, rectangular, or hexagonal, which leads to different levels of wire volume coverage within the space of the magnetic gap. The coil is aligned co-axially in the gap, vibrating back and forth within a small circular volume (a hole, slot, or groove) in the magnetic structure.
A concentrated magnetic field is then established within the gap, between the two poles of a permanent magnet – one pole on the outside of the gap, and the center post (known as “pole piece”) being the other. The pole piece (center post) together with the backplate, are typically a single piece, called the “pole plate” or “yoke”.
Driver magnets today, are almost always permanent, and are usually made of ceramic, ferrite, Alnico, or “asneodymium” and “samarium cobalt” (rare earth magnets). Due to rising transportation expenses and demand for smaller, light-weight devices (used for many home theatre system installations), “rare earth magnets” are used instead of the heavier ferrite types.
Electrodynamic loudspeakers that are designed with electrically powered field coils (as compared to using permanent magnets), was common in earlier designs. However, when high field-strength permanent magnets were made available, Alnico (an alloy of aluminum, nickel, and cobalt) became popular with manufacturers, since it negates most of the power supply-related problems, associated with field-coil drivers.
Alnico was the universal standard, up until about 1980. Alnico magnets may be partially degaussed (demagnetized) due to unwanted ‘pops’ or ‘clicks’ caused by loose connections. This is especially so, if the speaker is driven by a high-powered amplifier. However, this damage can be resolved by “recharging” the magnet. After the 80s, manufacturers switch to “ferrite” magnets (a mix of ceramic clay and fine particles of barium) which are more cost-effective.
Here’s a great video explaining how loudspeakers function!
Electrodynamic drivers, on their own, are optimized to only be effective within a limited frequency range. Thus, multiple (normally 2-4) drivers are usually combined (within a single loudspeaker enclosure), to produce a loudspeaker system that performs beyond that limitation.
The various types of drivers that we will look at are:
You bored yet? Hang in there! More important information ahead!
This type of driver is designed to be able to reproduce an audio channel on its own (without help from other drivers), thus it must cover the entire audio frequency range. Full-range drivers are typically small (about 7.6 to 20.3 cm) in diameter to allow considerable high frequency response. It is also optimized for “low-distortion output” at lower frequencies, but at the expense of a higher maximum output level.
You can find many full-range (also called wide-range) drivers being used in public address loudspeaker systems, in televisions (though some models are capable of hi-fi audio), portable radios, intercoms and computer speakers. The use of wide-range drivers are a benefit in hi-fi speaker systems, as they can avoid unwanted interactions between multiple driver types, typically caused by crossover network issues.
An additional smaller cone, known as a “whizzer” (attached to the joint between the voice coil and the primary cone), is often used in full-range drivers. Without the “whizzer”, the driver’s high-frequency response would be limited and its directivity would also be more narrow. This is because, the outer diameter cone will not be able to keep up with the vibration of the central voice coil at higher frequencies.
In a “whizzer” full-range driver, the main cone is manufactured to flex more in the outer diameter as compared to the center. This is to optimize the main cone’s ability to deliver lower-end frequencies while the “whizzer” cone handles all, if not, most of the higher frequencies. Considering that the “whizzer” is smaller than the main diaphragm, output dispersion at high frequencies will be much better, in comparison to a larger diaphragm.
Essentially, a “subwoofer” is a woofer driver, designed for the lowest part of the audio spectrum (generally below 200Hz for home audio systems, below 100Hz for professional live sound, and below 80Hz in THX-approved systems). Since it is only optimized for a limited frequency range, subwoofer systems design is often simpler in many aspects, compared to conventional loudspeakers. It often consists of a single driver in an enclosure.
Subwoofers must be properly braced and have a solid construction, in order to accurately produce low bass notes without unwanted resonances (often from cabinet panels). Many subwoofer systems have integrated power amplifiers and electronic sub-filters, with relevant controls for fine tuning. These types are called “active” or “powered” subwoofers. In contrast, “passive” subwoofers require external amplification.
You will see that in most configurations, subwoofers are physically separated from the other drivers. Due to the effects of propagation delay, their output will be slightly out of phase with the rest of the sound. Hence, a subwoofer’s power amp usually comes with a “phase-delay” adjustment parameter. Its also good to take note that in general, most high quality subwoofers are quite heavy.
Simply put, a woofer driver reproduces low frequencies. The driver works in conjunction with the enclosure design, in order to better produce low frequencies. Some high-end, expensive loudspeaker systems only need a woofer for the lowest frequencies, as it functions well enough that a subwoofer is often not required.
There are also some loudspeakers that only uses the woofer to handle low and middle frequencies, thus eliminating the need for a mid-range driver. The core concept of this design involves the selection of a tweeter driver, that can reproduce frequencies low enough, that if combined with a woofer that responds high enough, both drivers will then add coherently in the middle frequencies.
(Photo on the right: 1-Magnet, 2-Voicecoil, 3-Suspension, 4-Diaphragm)
As you’ve guessed it, mid-range drivers are responsible for reproducing middle frequencies. Mid-range driver diaphragms are usually made of paper or composite materials, and can be designed to be direct radiation drivers (similar to smaller woofers) or compression drivers (similar to some tweeter drivers).
If the mid-range driver happens to be a direct radiator, it will be mounted on a loudspeaker enclosure’s front baffle, and if it is a compression driver, it will then be mounted at the throat of a horn for increased output level and better control of its radiation pattern.
(Photo on the right: 1-Magnet, 2-Cooler, 3-Voicecoil, 4-Suspension, 5-Diaphragm)
The reproduction of the highest frequencies in a loudspeaker system is handled by the tweeter. A crucial issue in tweeter design, is in achieving a wide angular sound coverage (off-axis response). This complication is a result of the characteristic of high frequencies, that tends to leave the speaker in narrow beams.
Home stereo systems often employ soft-dome tweeters, as compared to horn-loaded compression drivers, which are common in professional live sound reinforcement. In recent years, ribbon tweeters have gained popularity, as their output capability has considerably improved to levels ideal for professional live sound applications. Additionally, the output pattern is wide in the horizontal plane, which is convenient for live concert situations.
(Photo on the right: 1-Magnet, 2-Voicecoil, 3-Diaphragm, 4-Suspension)
A coaxial driver is fundamentally, a loudspeaker driver which is made up of two or more combined concentric drivers. Coaxial drivers have been manufactured by many companies, such as Altec, Tannoy, Pioneer, KEF, SEAS, B&C Speakers, BMS, Cabasse and Genelec.
Okay fellas, this is the end of it! I hope you have enjoyed reading this article, and as a result, gained some useful knowledge of loudspeakers. Of course, there are much more that can be discussed about loudspeakers, and I intend to cover those in the near future!
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