Punter
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The ubiquitous moving coil speaker has existed now for one hundred years. There were a few permutations of an electroacoustic transducer that preceded the Kellog-Rice, notably the moving coil speakers manufactured by Peter L Jensen and Edwin Pridham (who founded Magnavox). The Jensen-Pridham design consisted of a coil driven diaphragm mounted in a horn. The Kellog-Rice design, patented in 1925, differed by having a paper cone with the voice coil attached to the apex. The combined cone and coil formed the electroacoustic transducer and dispensed with the transmission line component (the horn).
In these early designs, the magnetic element of the speaker was not a permanent magnet as these were rare and expensive. An electromagnet was used to energise the coil mounted to the rear of the cone. In construction, the electromagnet was a copper coil wound onto a soft iron core the front of which protruded into the rear of the cone. The “voice coil” attached to the rear of the cone was fitted into an annular gap formed by the frame and the end of the core. An adjuster was included to enable the voice coil to be centred in the gap for best efficiency and to preclude “poling” where the voice coil hits either the core or the frame in its travel.
From an electrical perspective, the moving coil loudspeaker is an example of the “motor effect”. This is where the magnetic field generated by the coil attached to the cone interacts with the static field of the fixed magnet mounted to the frame. Subsequently, the coil requires two flexible tails that are usually terminated on the frame so that a connection to an amplifier is possible.
Paper cone loudspeakers existed alongside horn speakers then as they do now but they had very different sonic properties. Cone loudspeakers had far better performance when it came to music reproduction where horns were generally used for public address systems and other applications where the greater throw of the horn was more important than fidelity.
The Kellog-Rice speaker found its first audiences in movie theatres where they were used to reproduce the sound for “the Jazz Singer” in 1927, which was the first moving picture with sound. Kellog-Rice loudspeakers were incorporated into enclosures with a 1 Watt valve amplifier to provide the soundtrack and were actually the first “powered speakers” to exist!
The first permanent magnet speakers were made by Magnavox using iron for the magnet. This made the speakers very heavy and these units were primarily incorporated into public address systems and later theatre sound systems. In domestic equipment the magnetic element in the speaker was primarily the field-coil type where a coductive ciol was wrapped around an iron core and powered by a DC source. After WWII an alloy named “Alnico” was developed and became popular as a permanent magnet in speakers. Alnico had been developed in Japan in the 1930’s and was subsequently improved over the next few decades. Alnico was a major improvement over iron permanent magnets having much higher coercivity but it also suffered from demagnetisation over time. The next big thing in permanent magnets came in the 1950’s with the commercial manufacture of the “Ferrite” magnet. Interestingly, the ferrite magnet was also developed in Japan by a very familiar name, TDK. Ferrite magnets overcame the problem of demagnetisation and also had the advantage of being cheap to produce and easy to form into shapes as the base material was a powder that could be moulded, compressed and then sintered to create a very resilient, corrosion resistant magnet. Ferrite ruled until the development of “rare earth” magnets in the form of the Neodymium magnet which became commercially available in the mid nineteen eighties.
The driven element of the moving coil speaker was almost universally paper with the coil wound onto a short phenolic tube which was glued to the apex of the cone. This combination didn’t change until 1961 when Electro-Voice introduced a 30 inch woofer fitted with a polystyrene cone (don’t think expanded polystyrene here!) following this development, the flood gates opened and a broad range of materials were incorporated into speaker cones including polypropylene, carbon fiber and Kevlar amongst others.
An often overlooked component of a moving coil speaker is the material used as an interface between the cone and the basket. In early speakers, the paper cone was glued to the basket and a corrugated edge formed in concentric rings to allow the cone to flex back and forth in its stroke. As with other aspects of speaker design, a variety of materials and methods were developed for the cone edge or “surround”. Popular materials are foam, rubber, polymer impregnated cloth and polyurethane. All of these materials have their pros and cons as detailed in this article. From a mechanical and performance perspective, the choice of cone surround material might just be the biggest factor as it is directly related to factors like ballistics, sensitivity, damping and resonance. Speaker manufacturers certainly understand this and will favor certain materials for bass versus mid range speakers for example. As a device, a speaker driver has a spread of characteristics that need to be considered when designing an enclosure.
Consider the following:
Impedance:
Measured in ohms (Ω), it's the resistance the driver presents to the electrical signal. A driver's impedance can vary with frequency, so it's important to know its nominal impedance (the impedance at a specific frequency, often 20 kHz) and how it changes over the frequency range.
Sensitivity (Efficiency):
This describes how efficiently the driver converts electrical power into sound pressure. It's usually expressed in decibels (dB) at one meter distance (1M) when driven with one electrical watt of power. A higher sensitivity means the driver can produce louder sounds with less power.
Power Handling:
This specifies the maximum amount of electrical power the driver can safely handle without damage. It's typically given in watts (W) and can be a continuous (RMS) or peak value.
Thiele/Small Parameters:
These are a set of parameters that mathematically describe a speaker's behavior. They include Fs (resonant frequency), Vas (equivalent cabinet volume), Qes (electrical Q), Qms (mechanical Q), and Qts (total Q).
Diaphragm Area (Sd):
This is the effective area of the diaphragm or cone that moves air to produce sound. A larger Sd generally means the driver can move more air and produce lower frequencies with greater output.
Voice Coil Inductance:
The inductance of the voice coil affects the driver's high-frequency response and can be a limiting factor.
Suspension:
The suspension of the driver (e.g., cone surround and spider) affects its compliance and how well it moves at different frequencies.
As a HiFi system component, the actual moving coil speaker itself is only one element of what we all refer to as a “speaker”. The remaining components are the “crossover” and the enclosure. In the world of HiFi these are not trivial elements. Indeed, if there’s a multitude of materials and construction types in the actual drivers, that’s easily matched by the diversity in crossover design and enclosure architecture.
Passive speaker design is the most intricate game of calculation and compromise particularly when it comes to multi driver setups. Arriving at an optimal crossover design is just one element of the process. The purpose of a crossover circuit is to feed the appropriate driver with the appropriate spread of frequencies. To arrive at the optimal component values, speaker resistance and impedance need to be determined. Because speaker drivers are reactive, they have impedance, not resistance over the audio range. This means that the load presented to an amplifier or crossover network is frequency dependent, as shown in any impedance curve you wish to examine. For a passive crossover to work correctly (with the sole exception of a 2-way, first-order series network), the drivers must be made to appear resistive, for a range of at least 1.5 octaves (preferably 2 octaves) either side of each crossover frequency. The truth is that building a properly calculated and engineered passive crossover is complex and expensive from an electronic component perspective. Most speaker manufacturers don’t invest the time and effort into truly optimising crossover networks as the pursuit of “best” is easily trumped by “good-enough”. Some manufacturers resort to “voicing” their designs, in effect applying their own tastes to crossover component choices that produce the kind of performance profile that appeals to them rather than striving for an optimal expression of the drivers themselves.
What this all adds up to is that the speaker represents, not only the last link in the chain of sound reproduction but also the most complex and diverse component. Further, the speaker as an electroacoustic transducer represents a very complicated load to an audio amplifier. The interaction of the crossover and speaker driver is also influenced by the mechanical construction of the speaker component itself. As a result, there is a seemingly limitless array of design approaches that have been pursued with all their advantages and disadvantages.
This leads me to the title of this post, “speakers are the enemy”. What I mean by this is that speakers are the one element of HiFi sound reproduction which appears to have no end point. Regardless of the specification or objective performance of any speaker, the rubber hits the road with the listener. It’s always going to be the subjective opinion of the listener that determines what is good, bad or indifferent.
Just to add to this bottomless pit of opinion regarding the audible properties of a speaker, the aesthetics and cost have weight also.
The choice of an enclosure design is probably far less intensive mathematically than calculating values for a crossover but there are still a bunch of different approaches. These range from a simple sealed box to unusual approaches like isobaric enclosures. So the list of variables pile up, not only do designers have to choose drivers by considering their electrical and mechanical characteristics, they also have to assemble a crossover circuit (if multiple drivers are to be used) and then mount all of this in an enclosure that will compliment the drivers. I suppose this is one of the reasons that a lot of speakers are simple boxes, ported or not, which serves to reduce the dizzying array of variables.
Of course, not all speaker enclosures are simple boxes. As prices rise, it becomes increasingly difficult for manufacturers to sell their products based simply on mechanical and electronic engineering features. To move into the high-end, ever more complex and esoteric looks are needed. It’s tempting to think that much of this elaborate design is there to offer aesthetic appeal without any reference to speaker performance. Surely not!
The images above are speakers that range from $56,000 to $1,000,000. I’m sure if you were to delve into the various features and theories connected with each type you’d be forgiven for thinking that each maker had simply taken a different route to achieve audio perfection. However, based on the form and materials used in these units, ranging from glass to milled aluminum, how could any of them claim to have reached any sort of conclusive quality of sound reproduction?
Simple answer, they can’t. I think anyone with a small amount of practical understanding would recognize “mutton dressed as lamb” in the slew of different shapes, materials and finishes particularly when one accepts that they are all using very similar components. Indeed, at an event called Midwest Audiofest, amateur builders come together to have their home-built designs assessed and judged in the hope of winning a category prize. As the event is sponsored by a company called Parts Express, contestants are required to use components supplied by this operation. Sadly this event seems to have been discontinued in 2022 but there’s an excellent YT playlist showing some of the winners.
https://www.youtube.com/playlist?list=PLduaHatf8xAHyy-UX_8Mdq_LCrFBT-byO
What stands out in the videos is the broad scope of approaches to speaker design. Every contestant is obviously trying to achieve the same thing, subjectively “good” sound reproduction and to a lesser extent, aesthetic appeal. One thing that stands out is the builders detailed understanding of speaker design especially crossovers. The interviews with these DIYers usually reveals what they were striving for in their design and how close they believe got to achieving those goals.
For me, the stand-out feature is the finished cost of the speakers themselves. Being limited in their choice of drivers, the maker has to put in some real effort to create enclosure architectures that extract the best performance from the drivers. Lastly, the cost of the finished devices is very reasonable and even if normal commercial margins were applied to some of the designs, they would come in at a couple of thousand dollars or less.
The title of this post is a quote from a dear friend of mine who has had a lifelong fascination with speakers. Being a practical man, he has constructed a number of his own designs and is happy to illustrate the strengths and weaknesses of them. Being a fan of loaded horns, his latest pair incorporate some high frequency compression drivers and cone mids and lows. The compression drivers are units salvaged from a local movie theatre. Are they perfect? No, they are not but he finds the reproduction pleasant and satisfying. I have heard them and despite their size, they work well, even at low levels driven by a Wiim amp/streamer.
Even though we posses the technology to test speakers with various tools and generate a slew of graphs and figures, there are an almost limitless range of variables, some subtle some obvious, that will affect our individual response to the sound of any given speaker. From a technical standpoint, a speaker that could reproduce an audio signal without any measurable coloration would be “ideal”. Practically, this cannot be achieved because of the imperfection inherent in the hardware. The concept of a “flat response” is a fixture in the world of sound reproduction and can actually be achieved in the hardware upstream of the speakers but thanks to the properties of speakers, they will always add their signature at the end.
I think the statement “speakers are the enemy” describes the frustration of the HiFi enthusiast, knowing that the “perfect” speaker is forever just out of reach. This can mean either an ongoing quest to narrow the gap or a capitulation in the form of accepting that you’ve found something that is good enough based on your circumstances.
In these early designs, the magnetic element of the speaker was not a permanent magnet as these were rare and expensive. An electromagnet was used to energise the coil mounted to the rear of the cone. In construction, the electromagnet was a copper coil wound onto a soft iron core the front of which protruded into the rear of the cone. The “voice coil” attached to the rear of the cone was fitted into an annular gap formed by the frame and the end of the core. An adjuster was included to enable the voice coil to be centred in the gap for best efficiency and to preclude “poling” where the voice coil hits either the core or the frame in its travel.
From an electrical perspective, the moving coil loudspeaker is an example of the “motor effect”. This is where the magnetic field generated by the coil attached to the cone interacts with the static field of the fixed magnet mounted to the frame. Subsequently, the coil requires two flexible tails that are usually terminated on the frame so that a connection to an amplifier is possible.
Paper cone loudspeakers existed alongside horn speakers then as they do now but they had very different sonic properties. Cone loudspeakers had far better performance when it came to music reproduction where horns were generally used for public address systems and other applications where the greater throw of the horn was more important than fidelity.
The Kellog-Rice speaker found its first audiences in movie theatres where they were used to reproduce the sound for “the Jazz Singer” in 1927, which was the first moving picture with sound. Kellog-Rice loudspeakers were incorporated into enclosures with a 1 Watt valve amplifier to provide the soundtrack and were actually the first “powered speakers” to exist!
The first permanent magnet speakers were made by Magnavox using iron for the magnet. This made the speakers very heavy and these units were primarily incorporated into public address systems and later theatre sound systems. In domestic equipment the magnetic element in the speaker was primarily the field-coil type where a coductive ciol was wrapped around an iron core and powered by a DC source. After WWII an alloy named “Alnico” was developed and became popular as a permanent magnet in speakers. Alnico had been developed in Japan in the 1930’s and was subsequently improved over the next few decades. Alnico was a major improvement over iron permanent magnets having much higher coercivity but it also suffered from demagnetisation over time. The next big thing in permanent magnets came in the 1950’s with the commercial manufacture of the “Ferrite” magnet. Interestingly, the ferrite magnet was also developed in Japan by a very familiar name, TDK. Ferrite magnets overcame the problem of demagnetisation and also had the advantage of being cheap to produce and easy to form into shapes as the base material was a powder that could be moulded, compressed and then sintered to create a very resilient, corrosion resistant magnet. Ferrite ruled until the development of “rare earth” magnets in the form of the Neodymium magnet which became commercially available in the mid nineteen eighties.
The driven element of the moving coil speaker was almost universally paper with the coil wound onto a short phenolic tube which was glued to the apex of the cone. This combination didn’t change until 1961 when Electro-Voice introduced a 30 inch woofer fitted with a polystyrene cone (don’t think expanded polystyrene here!) following this development, the flood gates opened and a broad range of materials were incorporated into speaker cones including polypropylene, carbon fiber and Kevlar amongst others.
An often overlooked component of a moving coil speaker is the material used as an interface between the cone and the basket. In early speakers, the paper cone was glued to the basket and a corrugated edge formed in concentric rings to allow the cone to flex back and forth in its stroke. As with other aspects of speaker design, a variety of materials and methods were developed for the cone edge or “surround”. Popular materials are foam, rubber, polymer impregnated cloth and polyurethane. All of these materials have their pros and cons as detailed in this article. From a mechanical and performance perspective, the choice of cone surround material might just be the biggest factor as it is directly related to factors like ballistics, sensitivity, damping and resonance. Speaker manufacturers certainly understand this and will favor certain materials for bass versus mid range speakers for example. As a device, a speaker driver has a spread of characteristics that need to be considered when designing an enclosure.
Consider the following:
Impedance:
Measured in ohms (Ω), it's the resistance the driver presents to the electrical signal. A driver's impedance can vary with frequency, so it's important to know its nominal impedance (the impedance at a specific frequency, often 20 kHz) and how it changes over the frequency range.
Sensitivity (Efficiency):
This describes how efficiently the driver converts electrical power into sound pressure. It's usually expressed in decibels (dB) at one meter distance (1M) when driven with one electrical watt of power. A higher sensitivity means the driver can produce louder sounds with less power.
Power Handling:
This specifies the maximum amount of electrical power the driver can safely handle without damage. It's typically given in watts (W) and can be a continuous (RMS) or peak value.
Thiele/Small Parameters:
These are a set of parameters that mathematically describe a speaker's behavior. They include Fs (resonant frequency), Vas (equivalent cabinet volume), Qes (electrical Q), Qms (mechanical Q), and Qts (total Q).
Diaphragm Area (Sd):
This is the effective area of the diaphragm or cone that moves air to produce sound. A larger Sd generally means the driver can move more air and produce lower frequencies with greater output.
Voice Coil Inductance:
The inductance of the voice coil affects the driver's high-frequency response and can be a limiting factor.
Suspension:
The suspension of the driver (e.g., cone surround and spider) affects its compliance and how well it moves at different frequencies.
As a HiFi system component, the actual moving coil speaker itself is only one element of what we all refer to as a “speaker”. The remaining components are the “crossover” and the enclosure. In the world of HiFi these are not trivial elements. Indeed, if there’s a multitude of materials and construction types in the actual drivers, that’s easily matched by the diversity in crossover design and enclosure architecture.
Passive speaker design is the most intricate game of calculation and compromise particularly when it comes to multi driver setups. Arriving at an optimal crossover design is just one element of the process. The purpose of a crossover circuit is to feed the appropriate driver with the appropriate spread of frequencies. To arrive at the optimal component values, speaker resistance and impedance need to be determined. Because speaker drivers are reactive, they have impedance, not resistance over the audio range. This means that the load presented to an amplifier or crossover network is frequency dependent, as shown in any impedance curve you wish to examine. For a passive crossover to work correctly (with the sole exception of a 2-way, first-order series network), the drivers must be made to appear resistive, for a range of at least 1.5 octaves (preferably 2 octaves) either side of each crossover frequency. The truth is that building a properly calculated and engineered passive crossover is complex and expensive from an electronic component perspective. Most speaker manufacturers don’t invest the time and effort into truly optimising crossover networks as the pursuit of “best” is easily trumped by “good-enough”. Some manufacturers resort to “voicing” their designs, in effect applying their own tastes to crossover component choices that produce the kind of performance profile that appeals to them rather than striving for an optimal expression of the drivers themselves.
What this all adds up to is that the speaker represents, not only the last link in the chain of sound reproduction but also the most complex and diverse component. Further, the speaker as an electroacoustic transducer represents a very complicated load to an audio amplifier. The interaction of the crossover and speaker driver is also influenced by the mechanical construction of the speaker component itself. As a result, there is a seemingly limitless array of design approaches that have been pursued with all their advantages and disadvantages.
This leads me to the title of this post, “speakers are the enemy”. What I mean by this is that speakers are the one element of HiFi sound reproduction which appears to have no end point. Regardless of the specification or objective performance of any speaker, the rubber hits the road with the listener. It’s always going to be the subjective opinion of the listener that determines what is good, bad or indifferent.
Just to add to this bottomless pit of opinion regarding the audible properties of a speaker, the aesthetics and cost have weight also.
The choice of an enclosure design is probably far less intensive mathematically than calculating values for a crossover but there are still a bunch of different approaches. These range from a simple sealed box to unusual approaches like isobaric enclosures. So the list of variables pile up, not only do designers have to choose drivers by considering their electrical and mechanical characteristics, they also have to assemble a crossover circuit (if multiple drivers are to be used) and then mount all of this in an enclosure that will compliment the drivers. I suppose this is one of the reasons that a lot of speakers are simple boxes, ported or not, which serves to reduce the dizzying array of variables.
Of course, not all speaker enclosures are simple boxes. As prices rise, it becomes increasingly difficult for manufacturers to sell their products based simply on mechanical and electronic engineering features. To move into the high-end, ever more complex and esoteric looks are needed. It’s tempting to think that much of this elaborate design is there to offer aesthetic appeal without any reference to speaker performance. Surely not!
The images above are speakers that range from $56,000 to $1,000,000. I’m sure if you were to delve into the various features and theories connected with each type you’d be forgiven for thinking that each maker had simply taken a different route to achieve audio perfection. However, based on the form and materials used in these units, ranging from glass to milled aluminum, how could any of them claim to have reached any sort of conclusive quality of sound reproduction?
Simple answer, they can’t. I think anyone with a small amount of practical understanding would recognize “mutton dressed as lamb” in the slew of different shapes, materials and finishes particularly when one accepts that they are all using very similar components. Indeed, at an event called Midwest Audiofest, amateur builders come together to have their home-built designs assessed and judged in the hope of winning a category prize. As the event is sponsored by a company called Parts Express, contestants are required to use components supplied by this operation. Sadly this event seems to have been discontinued in 2022 but there’s an excellent YT playlist showing some of the winners.
https://www.youtube.com/playlist?list=PLduaHatf8xAHyy-UX_8Mdq_LCrFBT-byO
What stands out in the videos is the broad scope of approaches to speaker design. Every contestant is obviously trying to achieve the same thing, subjectively “good” sound reproduction and to a lesser extent, aesthetic appeal. One thing that stands out is the builders detailed understanding of speaker design especially crossovers. The interviews with these DIYers usually reveals what they were striving for in their design and how close they believe got to achieving those goals.
For me, the stand-out feature is the finished cost of the speakers themselves. Being limited in their choice of drivers, the maker has to put in some real effort to create enclosure architectures that extract the best performance from the drivers. Lastly, the cost of the finished devices is very reasonable and even if normal commercial margins were applied to some of the designs, they would come in at a couple of thousand dollars or less.
The title of this post is a quote from a dear friend of mine who has had a lifelong fascination with speakers. Being a practical man, he has constructed a number of his own designs and is happy to illustrate the strengths and weaknesses of them. Being a fan of loaded horns, his latest pair incorporate some high frequency compression drivers and cone mids and lows. The compression drivers are units salvaged from a local movie theatre. Are they perfect? No, they are not but he finds the reproduction pleasant and satisfying. I have heard them and despite their size, they work well, even at low levels driven by a Wiim amp/streamer.
Even though we posses the technology to test speakers with various tools and generate a slew of graphs and figures, there are an almost limitless range of variables, some subtle some obvious, that will affect our individual response to the sound of any given speaker. From a technical standpoint, a speaker that could reproduce an audio signal without any measurable coloration would be “ideal”. Practically, this cannot be achieved because of the imperfection inherent in the hardware. The concept of a “flat response” is a fixture in the world of sound reproduction and can actually be achieved in the hardware upstream of the speakers but thanks to the properties of speakers, they will always add their signature at the end.
I think the statement “speakers are the enemy” describes the frustration of the HiFi enthusiast, knowing that the “perfect” speaker is forever just out of reach. This can mean either an ongoing quest to narrow the gap or a capitulation in the form of accepting that you’ve found something that is good enough based on your circumstances.
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