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What is a Loudspeaker? – Check it out!

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

Speaker Diagram

Photo by Iain at the English language Wikipedia / CC BY-SA 3.0

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.

Signal Processing

The second transducer is a speaker / Photo by Brews ohare / CC BY-SA 3.0

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:

  • History
  • 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.

Johann Philipp Reis

Johann Philipp Reis telephone

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.

First Commercial Loudspeaker

The first commercial version of the speaker

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:

  • Diaphragm
  • Chassis (frame)
  • Suspension system
  • Cone
  • Voice coil
  • Magnet

Its about to get a little technical in the preceding sections, so hold on tight!

Cone

Photo by Grebe / CC BY-SA 3.0

Diaphragm 

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.

Speaker Frame

Photo by Grebe / CC BY-SA 3.0

Chassis (frame)

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).

Suspension system

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).

Speaker Diagram

Photo by Iain at the English language Wikipedia / CC BY-SA 3.0

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.

Cone

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 coilVoice Coil

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.

Voice Coil

Light grey is soft iron, dark grey is permanent magnetic material and the coil is in red / Photo by Iain / CC BY 2.5

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”.

Magnet

Neodymium Magnet

Photo by Magnequench / CC BY-SA 3.0

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!

Driver types

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:

  • Full-range
  • Subwoofer
  • Woofer
  • Mid-range
  • Tweeter
  • Coaxial

You bored yet? Hang in there! More important information ahead!

Whizzer

Photo by Rohitbd / CC BY-SA 3.0

Full-range driver

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.

Full Range Speaker

Photo by Richard Williams

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.

Subwoofer driver

Photo by JJ Harrison / CC BY-SA 4.0

Subwoofer

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.

Subwoofer

Photo by Swicher~commonswiki / CC BY-SA 3.0

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.

Woofer

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)

Mid-range driver

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)

Tweeter

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)

Coaxial driver

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!

Thanks for reading, do leave a comment below and share this article with your friends if you like!

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Farhan

When I'm not rocking out to great music, I'd prefer to be sleeping on a field on a windy day =)

9 Comments

  1. This has got to be the most thorough article about loudspeakers on the internet. My old fogey friends criticized me for buying a speaker set with a woofer so I could better hear the movies on my laptop.

    None of us knew what a woofer was, only that the salesman said it would make the music in scary movies more dramatic. And it does!

    Now, I know what a woofer really is.

    • Hi there Horton, thanks for your comment!

      Hahaha, yeah, nothing beats watching a movie with a proper sound system configuration. And yes, it DOES make scary movies even SCARIER!

      Sadly, most people nowadays would rather watch their favourite TV shows and movies with headphones, as it is more convenient in most situations that busy people find themselves in every day. And just for your info, this article just covers a few aspects of loudspeakers, and there are definitely much more information that I would like to share in future posts!

      Thanks a lot, and do share this with all your “old fogey” friends if you like!

      Cheers =)

  2. The study of sound and how it is perceived and replicated has always fascinated me, as a music producer I can very well relate to the study of loudspeakers and amplitude and how it all relates. But even as an audio nerd, some of these concepts are even over my head! Thanks for providing this detailed explanation.

    • Hey Christian, great to have your comments here!

      It is always awesome to have a fellow audio practitioner commenting on my articles, and visiting my site. I am happy that the article was a benefit for you in one way or another!

      Thanks and do come back for more =)

  3. Hi,
    Wow that was the most comprehensive explanation I’ve come across of a loudspeaker. Everyone loves music, but people forget that, due to these inventions we are able to enjoy music the way we do. You did a great job explaining the technology behind it but also mentioned the brains behind it. Really loved the article. Keep up the nice work. There were some pictures I couldn’t see, maybe because I am on my smartphone.
    Cheers,
    Abdul.

    • Hello Abdul, thanks for dropping by!

      I am really happy that you have enjoyed this article. I was afraid that it might be too technical, or lengthy for people to understand and have a good reading experience. I will see what I can do about those pictures you have mentioned. I myself, never knew how it would look like on my smartphone.

      Thanks a bunch, and do share this with others if you like!

  4. This is great information on loudspeakers. You may want to incorporate some videos that possibly shows the operation and different sounds of drivers. Your an audio engineer is this for live events? Do you have to design your sound stages based on different drivers and acoustics of a venue? Well laid out and a good use of images.

    • Hi Aaron, thanks for reading!

      Well yes, most of the jobs that I have taken so far are for live events. We do not really get to design our sound stages (at least not in the way you have mentioned). Most engineers will of course take into account the acoustics of the venue and requirements of the live performance (bands, speeches etc.), before requesting a certain set of speaker systems to use.

      However, more often than not, we do not get to choose what we want to use, due to logistical constraints and budgeting issues. Unless, you are doing a big gig for an international artist, you would have to just stick with the equipment that the AV production company has provided before hand.

      And in case if you’re wondering, this article is about loudspeakers in general, not just for live sound applications. In any case, the basic principles of a loudspeaker design applies to any application, whether it is for home use, or professional.

      Thanks again, and do come back for more!

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