14 DAY TRIAL // Last article presented the basics of sound cards and now it is time to analyze the loud speakers - those pieces of equipment that make it possible for you to listen to your favorite music or enjoy high quality sound effects in movies and games. We are going to see how speakers work, a little bit of history and some wide-spread speaker formats.
Pump up that volume A loudspeaker, or speaker, is an electromechanical transducer which converts an electrical signal into sound waves. The term "loudspeaker" is used to refer to both the device itself, and a complete system consisting of one or more loudspeaker drivers (as the individual units are often called) in an enclosure.
The traditional design of a dynamic loudspeaker includes a lightweight semi-rigid cone, a coil of fine wire (usually copper), a circular magnet, and a rigid support structure.
The coil (or voice coil) is attached to the apex of the cone. A "gap" is a small circular hole, slot or groove which allows the voice coil and cone to move back and forth. The coil has a coaxial orientation inside the gap. The gap is established between a permanent magnet and a center post (a.k.a. "pole-piece"). The gap is the place where magnetic fields become the most concentrated. One magnetic pole is outside the voice coil, while the opposite one is inside. The speaker components need to return to a neutral point after moving and in order to achieve this, the speaker structure has to integrate some kind of suspension system. A typical suspension system includes the spider (or "damper"), which is placed at the apex of the cone and the surround (or "bellows"), which is made of rubber and merged with the outer circumference of the cone. All the components are held together by the frame. When an electrical signal is applied, a magnetic field is induced by the electric current in the coil which becomes an electromagnet.
The coil and the permanent magnet interact with magnetic force which causes the coil and a semi-rigid cone (diaphragm) to vibrate and reproduce sound at the frequency of the applied electrical signal. When a multi-frequency signal is applied, the complex vibration results in reproduction of the applied signal as an audio signal.
Driver cones may be built of various materials such as paper, metal, polypropylenes and kevlar alloys. Frames have to be built as rigid as possible and are typically cast or stamped metal, although injection-molded plastic baskets are becoming much more common in recent years. The magnets may come in different sizes and it is said that larger and more powerful magnets are associated with higher quality speakers.
Old stuff The first loudspeaker design was patented by Alexander Graham Bell and it was integrated in the 1876 telephone. Soon, improved versions started to appear. Ernst Siemens in Germany and England introduced one in 1878. Nikola Tesla is believed to have created a similar device in 1881, while the modern design of moving-coil loudspeakers dates back to 1898 and was established by Oliver Lodge in England. The moving coil principle was patented later on, in 1924, by two Americans, Chester W. Rice and Edward W. Kellog.
At first, loudspeakers were built using electromagnets because large, powerful permanent magnets were not freely available at reasonable costs. The quality of loudspeaker systems up until the 1950s was very poor compared to modern hi fi standards. Developments in cabinet technology (e.g. acoustic suspension) and changes in materials used in the actual loudspeaker, led to audible improvements. The most common material used for cones was paper (or doped paper, where the paper is treated with a substance to improve its performance). These materials are still used today in low-end equipments, but they can provide satisfactory results. Materials such as polypropylene and aluminum are much better from this point of view.
The first commercial acoustic suspension loudspeaker was developed by Henry Kloss and Edgar Villchur at Acoustic Research, and further developed by a company named KLH?
The latest most important improvements date back to the 1970s and include the introduction of higher temperature adhesives, more reliable permanent magnet materials and improved thermal management.
Present-day standards The current design allows a series of separate drivers to be included in one speaker or system of speakers. The usual specifications classify speakers as "N-way" models, where N indicates the number of separate frequency bands into which the system splits the sound frequencies. A 2-way system consists of woofer(s) and tweeter(s) sections; a 3-way system is constructed as a combination of woofer(s), tweeter(s) and mid-range drivers, etc. The frequency bands are separated and routed to the correct driver by an N-way crossover step.
We'll now take a closer look at each type of sound driver.
- The woofer - is a loudspeaker driver capable of reproducing low (bass) frequencies. The frequency range varies widely according to design. Whilst some woofers can cover the audio band from the bass to 3 kHz, others only work up to 1 kHz or less.
- The mid-range driver (or squawker) - is designed to handle the middle frequencies of the audio spectrum, typically from about 200 Hz to about 4-5 kHz. The distinction between woofers and mid-ranges is blurred however since many woofers can operate up to 3 kHz. These are used when the bass driver (or woofer) is incapable of covering the mid audio range.
- The tweeter -is a loudspeaker driver capable of reproducing the higher end of the audio spectrum, usually from about 5 kHz to 20 kHz.
- Full-range loudspeaker drivers - are designed to have as wide a frequency response as possible. These often employ an additional cone called a whizzer to extend the high frequency response and broaden the high frequency directivity. A whizzer is a small, light cone attached to the woofer's apex around the dust cap. The use of a whizzer requires that the main cone decouples from the coil at high frequencies such that most or all of the motion at those frequencies is imparted to the whizzer, which then acts like a second smaller coaxial loudspeaker.
- The subwoofer - is a woofer optimized to handle the lowest range of the audio spectrum. Modern speaker systems often include a single speaker enclosure dedicated to reproducing the very lowest bass frequencies. A typical subwoofer usually reproduces sounds below 120 Hz. Because the range of frequencies that must be reproduced is quite limited, the design of the subwoofer is usually quite simple, often consisting of a single, large, down-firing woofer enclosed in a cubical "bass-reflex" wooden cabinet. Subwoofers often contain integrated power amplifiers and all the other speakers connect to the back of the subwoofer. That is why home cinema and gaming sound systems are dubbed 2.1, 4.1, 5.1 or 7.1. The "1" signals the presence of a separate subwoofer that comes equipped with a LFE (low frequency enhancement) output channel. The preceding digit represents the number of channels that come equipped with mid-range and tweeter drivers. Starting with the 4.1 scheme, the sound system may be called a surround sound system, because the speakers (without the subwoofer) can be placed in specific locations around the listener (front, center, back etc.).
The latest developments for subwoofers include the TRW-17 Rotary Woofer, invented by Bruce Thigpen of Eminent Technology. This model is designed to reproduce, for the first time, audio frequencies from DC (zero) to 20 Hz and is aimed at the home theater and professional audio markets. Typical subwoofers using moving cones don't couple well with air below 20Hz, and thus their Sound pressure level (SPL) falls off significantly below 20Hz. This is not to say that traditional cone subwoofers cannot create sound pressure between 0 and 20Hz, because they certainly can. However, the human ear is quite insensitive to acoustics below 20Hz and as sound frequency goes below 20Hz down to 0Hz, the sound pressure level needs to increase in order to remain audible to the human ear. The Rotary Woofer technology has the capability of reaching these audible sound pressure levels at these low frequencies. It was pointed out that such pressure levels below 20 Hz are usually stirred up by seismic waves or atomic explosion fallout waves and can make human organs enter into a state of resonance, damaging tissues.
Specifications
Here's what you should look for when acquiring a new sound system: * Speaker or driver type for the individual units - full-range, subwoofer or woofer, mid-range and tweeter. * Rated Power - Nominal or continuous power and peak or maximum short-term power that the loudspeaker can handle (i.e. maximum allowed output power of the amplifier without destroying the loudspeaker). This is measured in watts. * Impedance - measured in ? (ohm) * Baffle or enclosure type - sealed, bass reflex, etc. * Number of drivers - 2-way, 3-way, etc.
Optionally, you can take these specifications into consideration:
* Crossover frequency - the frequency or frequencies where electrical filtering occurs. * Frequency response - the measured or specified variance in sound pressure level over a range of frequencies.
Bring the sound sources on! Here, we take a look at the various existing technologies for speaker construction.
- The ribbon speaker - consists of a thin metal-film ribbon suspended in a magnetic field. The electrical signal is applied to the ribbon which vibrates creating the sound. The advantage of the ribbon loudspeaker is that the ribbon has very little mass; as such, it can accelerate very quickly, yielding good high-frequency response. Ribbon loudspeakers can be very fragile, thin ones can be torn by a strong puff of air. That is why ribbon technology is usually included only in tweeters.
- Flat panel speakers - there are two, related problems with flat panel technology; firstly, that the flat panel is more flexible than the cone shape and therefore fails to move as a solid unit, and secondly, that resonances in the panels are difficult to control, leading to considerable distortion in the reproduced sound. Some progress has been made using such rigid yet damped material as styrofoam, and there have been several flat panel systems demonstrated in recent years.
- Planar magnetic speakers - consist of a flexible membrane with a voice coil printed on them. The current flowing through the coil interacts with the magnetic field of strategically-placed magnets, causing the membrane to vibrate. The driving force covers a larger percentage of the membrane and reduces the resonant problems inherent in coil-driven planar diaphragms.
- NXT speakers - involves an intentionally flexible panel and an "exciter", mounted off-center in such a location that it excites the panel to vibrate. Speakers using NXT design methods can reproduce sound with a wide directivity pattern (paradoxically somewhat like a point source) with adequate quality.
- Digital speakers - the design of these is disarmingly simple; the least significant bit drives a tiny speaker driver, of whatever physical design seems appropriate; a value of "1" causes this driver to be driven full amplitude, a value of "0" causes it to be completely shut off.
- Electrostatic loudspeakers (ESL) - some speakers are electrostatically driven rather than via the usual electromechanical voice coil, thereby giving a more linear response. Today, modern materials and insulative coatings have allowed engineers to design electrostatic speakers that are safe and reliable. Electrostatic loudspeakers are large by nature. The sole objective of loudspeakers is to move the air analogue to the electric signal applied to them. The amount of air that can be moved is determined by the size of the membrane and the allowed excursion. With electrostatic loudspeakers, the excursion is limited to millimeters, while normal dynamic loudspeakers can move centimeters. This means that the membrane of an electrostatic loudspeaker has to be larger than an equivalent dynamic loudspeaker.
- Isobaric speakers - the term "isobaric" defines the operational characteristics of the use of at least two woofers in a loudspeaker unit. The two bass drivers are coupled to work together as one bass unit: they are mounted one behind the other in a casing to define a chamber of air of constant pressure in between. The two drivers are placed either "cone to magnet" and wired in phase with one another or "cone to magnet" and wired out of phase with one another so that cones move together when driven with an audio signal, keeping the pressure of air in the chamber substantially constant. The loudspeaker which is mounted on the front wall of the cabinet thus operates under substantially "ideal" conditions. The result is that the coupled driver pair (iso-group) can now produce the same frequency response in half the box volume that a single driver of the same type would require.
This is quite enough for a panoramic view over the existing loudspeaker technologies. Next article we'll see how optical drives work.