Let’s go back to 1925. What would you have been hearing recorded music from? Perhaps the Iconic horn of a wind up gramophone playing a disc record? Or the latest innovation: The Victor Orthophonic model where the turntable was built into a credenza; one would open the doors on the front of the piece to hear the sound emanating from the horn built into the cabinet.

In 1926, Bell Labs produced the Vitaphone sound system which was the first device to use the moving coil. With its Western Electric speaker driver, coupled with a one-inch horn, the sound efficiency improved decidedly over older designs. But more was to come.

In 1928 an inventor named Herman Fanger designed a speaker with a diaphragm nested inside and in front of the traditional cone loudspeaker. With this arrangement, fairly accurate high frequency sounds could be produced by the centre cone with a diaphragm made of stiff, light material; and lower pitch sounds could be accurately produced by the outer portion of the cone with a diaphragm made of more flexible material. This speaker would be instantly recognisable today. This was truly the birth of the modern speaker!

A year later, 1929, an inventor called Westinghouse developed a horn type loudspeaker that purposely directed sound toward the audience. There became an appropriate area in which to receive the audio signals from this directional speaker. The sweet spot or sweet area was born!

But we have two ears. How far away was stereo? 1933. A stereo transmission would be broadcast from from the Academy of Music in Philadelphia, USA, and heard by an audience in faraway Washington.  Three channels were used with microphones at left, centre and right of the orchestra stage. It was reported that the mood was both bristling with anticipation, but also tense. Never before had such a prestigious audience been witness to a live performance of an orchestra over a wire system which claimed to produce the sound faithfully, as if one were really present with the musicians.

In Washington, the expectant audience was looking at loudspeakers roughly positioned in the same way that the microphones picking up the sound were positioned in Philadelphia, that is, left, centre and right of the orchestra stage.

The speakers used in Washington were created by Wente and Thuras. The idea of the design was to produce a uniform response over the whole tonal range of an orchestra, then enhance the sound power without noticeable distortion, and finally emit the sound at all frequencies throughout a wide angle.

The reports of the day say that the demonstration was a wild success. The movie industry immediately saw the benefits of stereo speakers simulating not only music, but also environmental sounds and talking.

Surely this was the limit to which audio sound reproduction could be taken? Of course not!

On November 15th, 1974 at the Los Angeles Film Festival, the film Earthquake was screened. It had a revolutionary audio track called Sensurround. Sensurround sound was produced by low frequency horns located behind the screen, two in each corner. One of the speakers in each corner was eight feet long, four feet wide, and four feet high. The other in each corner was a modular unit one foot wide and five feet wide. Two additional horns were positioned on a platform in the rear of the cinema, each horn driven by a 1000 watt amplifier, controlled by inaudible tones on a special optical track, along with the normal four tracks of the Panavision filmstrip. Patrons emerged from that performance totally shook up but wild with enthusiasm. They had experienced an audio earthquake! The sub woofer (sometimes unfairly known as a rumble-box) had been born. A subwoofer (or whatever name the manufacturers choose to call it!) is a feature of most TV surround sound systems, providing the “whoomph” usually in action sequences and explosions.

 

Speaker design continued apace with the result that there are many different types of speakers on the market at the moment, as well as different brands. Here are some of the more common types:

Dynamic loudspeakers

The most common type of speaker driver, commonly called a dynamic loudspeaker, uses a lightweight diaphragm, or cone, connected to a rigid basket, or frame, via a flexible suspension that constrains a coil of fine tinsel wire to move axially through a cylindrical magnetic gap. When an electrical signal is applied to the voice coil, a magnetic field is created by the electric current in the voice coil, making it a variable electromagnet. The coil and the driver’s magnetic system interact, generating a mechanical force that causes the coil (and the attached cone) to move back and forth, thereby reproducing sound under the control of the applied electrical signal coming from the amplifier.

Piezoelectric speakers.

Basically lightweight, cheap and cheerful. They are often used as beepers in watches and other electronic devices, and are sometimes used as tweeters in less-expensive speaker systems, such as computer speakers and portable radios. Their frequency response, in most cases, is inferior to that of other technologies. This is why they are generally used in single frequency or non-critical applications.

Electrostatic loudspeakers.

These use a high voltage electric field (rather than a magnetic field) to drive a thin statically charged membrane. Because they are driven over the entire membrane surface rather than from a small voice coil, they ordinarily provide a more linear and lower distortion motion than dynamic drivers. For many years electrostatic loudspeakers had a reputation as a generally unreliable and occasionally dangerous product. Arcing remains a potential problem with current technologies, especially when the panels are allowed to collect dust or dirt, or when driven with high signal levels. Wharfedale produced a pair of electrostatic headphones in the 1980s, that I swear gave me headaches after more than an hour’s listening (but that may have been down to the construction and fit rather than the electrostatic system).

 

Planar Ribbon magnetic loudspeakers

A ribbon speaker consists of a thin metal-film ribbon suspended in a magnetic field. The electrical signal is applied to the ribbon, which moves with it to create the sound. The advantage of a ribbon driver is that the ribbon has very little mass; thus, it can accelerate very quickly, yielding very good high-frequency response. Ribbon loudspeakers are often very fragile—some can be torn by a strong gust of air. Another downside is that they require strong magnets and are expensive to manufacture. Here is an in-ceiling Planar speaker:

 

Bending wave loudspeakers

Bending wave transducers use a diaphragm that is flexible rather than rigid. The rigidity of the material increases from the centre to the outside. Short wavelengths radiate primarily from the inner area, while longer waves reach the edge of the speaker. To prevent reflections from the outside back into the centre, long waves are absorbed by a surrounding damper. Such transducers can cover a wide frequency range (80 Hz to 35,000 Hz) and have been promoted as being close to a completely ideal point sound source. However this uncommon approach is being taken by only a very few manufacturers.

 

Ohm Walsh loudspeakers.

The Ohm Walsh loudspeakers use a unique driver designed by Lincoln Walsh, who had been a radar development engineer in WWII. He became interested in audio equipment design and his last project was a unique, one-way speaker using a single driver. The cone faced down into a sealed, airtight enclosure. Rather than move back-and-forth as conventional speakers do, the cone rippled and created sound in a manner known in RF electronics as a “transmission line”. The new speaker created a cylindrical sound field. Lincoln Walsh died before his speaker was released to the public. The Ohm Acoustics firm has produced several loudspeaker models using the Walsh driver design since then, such as the one shown here:

 

Flat panel loudspeakers

There have been many attempts to reduce the size of speaker systems, or alternatively to make them less obvious. One such attempt was the development of voice coils mounted to flat panels to act as sound sources. However resonances in the panel are difficult to control, leading to considerable distortion. Some progress has been made using such lightweight, rigid, materials such as Styrofoam, and there have been several flat panel systems commercially produced in recent years, but it remains at the fringe.

Air Motion Transducer speakers

Oskar Heil invented the air motion transducer in the 1960s. In this approach, a pleated diaphragm is mounted in a magnetic field and forced to close and open under control of a music signal. Air is forced from between the pleats in accordance with the imposed signal, generating sound. The drivers are less fragile than ribbons and considerably more efficient than ribbon or electrostatic tweeter designs.

ESS, a California manufacturer, licensed the design, employed Heil, and produced a range of speaker systems using his tweeters during the 1970s and 1980s. Radio Shack, a large US retail store chain, also sold speaker systems using such tweeters for a time. Currently, there are two manufacturers of these drivers in Germany, one of which produces a range of high-end professional speakers using tweeters and mid-range drivers based on the technology.

Plasma speaker

Plasma arc loudspeakers use electrical plasma as a radiating element. Since plasma has minimal mass, but is charged and therefore can be manipulated by an electric field, the result is a very linear output at frequencies far higher than the audible range. Problems of maintenance and reliability for this approach tend to make it unsuitable for mass market use.

Digital speakers

Digital speakers have been the subject of experiments performed by Bell Labs as far back as the 1920s. The design is simple; each bit controls a driver, which is either fully ‘on’ or ‘off’. However problems with this design have led manufacturers to abandon it as impractical for the present.

The term “digital” or “digital-ready” is often used for marketing purposes on speakers or headphones, but these systems are not digital in the sense described above. Rather, they are conventional speakers that can be used with digital sound sources (e.g., optical media, MP3 players, etc.), as can any conventional speaker.