The development of today's conventional loudspeaker started back in 1898 when the basic structure of the dynamic loudspeaker was invented by an English physicist Oliver Lodge[1]. Since then much of the past 40-plus years of loudspeaker development has revolved around identifying and understanding its limitations, such as diaphragm and enclosure resonances, the effect of crossover networks and so on. Yet, in the past few years many new loudspeaker paradigms emerged. When we see how much academic and design effort has been expended on perfecting current technology a question might arise. Are not the conventional speakers good enough, do we need to research new types of sound reproduction systems and if so, what are the current developments?
In order to answer the first question we need to review the basic principles of how conventional loudspeakers operate and identify the fundamental restrictions on performance that they impose.
In Figure 1 we can see the structural outline of a conventional speaker. The aim of such speakers is to create a pistonic motion of the diaphragm for sound reproduction. By pistonic we mean that the diaphragm moves back and forth as a rigid whole.
To achieve this the moving voice coil of the loudspeaker is placed in the air gap of a strong permanent magnet. The moving coil is attached to a conical diaphragm which is supported by flexible suspension to keep the motion axial. The basket attached to the magnet supports the rim of the diaphragm. As electrical current flows through the coil an axial force is generated according to the law of inductance. This force brings the diaphragm into motion thus producing sound waves.[2]
Even though later other methods of transduction were invented - using electromagnetic, electrostatic or piezoelectric forces to set the diaphragm into motion - the basic principle of the operation, pistonic motion, has remained the same.
As we are now more or less familiar with the operation of the conventional loudspeaker let us try to identify the physical limitations of such a system.
Variation in directivity with frequency is one of the great bugbears of loudspeaker design. If we listened to reproduced sound in anechoic environments it would not be a problem: we would hear the diaphragm's on-axis output and nothing else. But the usual listening environment is far from anechoic, so a loudspeaker's output off the listening axis has a significant effect on what we hear. Because of frequency dependent directivity the direct, reflected and reverberant sounds in a room all have different tonal balances. Even if a conventional loudspeaker had an absolutely flat on-axis response and was entirely free of resonance - a high expectation - it would still sound colored and introduce imaging aberrations.
The reason for this phenomenon is the following. At low frequencies, where the wavelength in air is large compared with the diaphragm dimensions the acoustic power output is constant. As frequency continues to rise, though, and the wavelength in air reduces to the point where it becomes comparable with the diaphragm dimensions, a major change occurs. (Figure 2)
Because of various reasons the diaphragm's acoustic power output now begins to fall at a rate of 12dB per octave. This does not mean that the on-axis (in front of the speaker) pressure response falls away: what generally happens is that the diaphragm's acoustic output becomes restricted to progressively narrower solid angles. In other words, it becomes directional; it begins to beam. This is the main reasons why we need to use more than one speaker and a crossover network in better quality speakers. This will reduce the problems related to directivity but at the same time the crossover networks will introduce new disturbances in the signal chain. The crossover networks and their effect will be described later in a little bit more detail.
The second group of problems is resonances. Resonances stem from the fact that we live in a real world and we do not have ideal, completely rigid materials. Resonances appear at certain frequencies, depending on the inherent qualities of the materials. If the source signal contains these frequencies, the resonances of the different parts of a loudspeaker system will distort the sound at those given frequencies.
For starters we have the resonances of the diaphragm itself. An ideal diaphragm would move as a rigid whole. In reality we can never achieve such a pistonic motion. A real diaphragm is never completely rigid, therefore it resonates which results in further colorations in the reproduced sound since the speaker emits such sound components which were not in the original source. In the following table some of the fundamental bending modes of the diaphragm are illustrated.
Furthermore there is the resonance of the enclosure. We need an enclosure because the front and the rear of the diaphragm must be separated in order to avoid acoustical shortcircuit. (The distance between the rear and the front side must be larger than the largest wavelength we would like to reproduce) This is achieved by building the speaker either into the wall or an enclosure. Here we have another source of distortion in sound: the resonance of the enclosure. Similarly to the diaphragm, the sides of the enclosure will also have their own bending modes at different frequencies.
Now let us go back a bit. As it was mentioned earlier when the problem of directivity was discussed, we need to apply multiple loudspeaker systems and crossover networks.
The point of having multiple loudspeakers is that each of them covers just a part of the entire audible range, the part where it can operate without becoming too directional. For this we need an electric network which splits up the incoming signal according to the necessary frequency ranges. This is what a crossover network is. (Usually the signal is split into three parts: low, mid and high range)
A crossover consists of resistive, capacitive and inductive elements. Anyone who has learned a bit of electronics knows that the last two elements are reactive, that is they shift the phase of the signal which passes through them. The result of this will be for example, that the sound coming from the mid range speaker will have a different phase than the sound coming from the low range speaker. This would not be a problem if it were not for a special quality of our hearing. Namely, that we are able to hear phase differences. Therefore a crossover network will introduce phase distortions in the reproduced sound.
In monophonic listening situations this would not be such a significant problem, but in stereophonic situations these phase differences will greatly reduce the stereo image of the sound.
So far we have seen that there are physical limitations on the sound quality that can be achieved by conventional speakers.
Thus we can conclude that because of its physical limitations not even an ideal conventional speaker will reproduce the original signal perfectly, therefore the answer for our first question is that new loudspeaker paradigms are necessary to improve the overall quality of sound reproduction.
Now, we can move on to discuss our second question: what are the current developments in the field of loudspeaker design?
Presently, there are two new designs currently being developed which are worth looking at. The first one is the so called NXT, or Flat speaker, the second is the HSS, HyperSonicSpeaker.
The NXT is based on what we term distributed-mode (DM) operation. Essentially this involves encouraging the diaphragm, in this case a special panel, to produce the maximum number of bending resonances, evenly distributed in frequency. The resulting vibration is so complex that it approximates random motion. This means that each small area of the panel vibrates, in effect, independently of its neighbors, rather than in the fixed, coordinated fashion of a pistonic diaphragm. Think of it as an array of very small drive units, each radiating a different, uncorrelated signal but summing to produce the desired output[3]. (see Fig. 3)
Fig.3
Snapshot of panel motion
Because of this quasi random vibration of the panel there is no need for an enclosure and since we are actually using the resonances of the panel to reproduce sound we have no resonance problems as with the conventional loudspeaker.
Also, the problems related to directivity disappear since, because of the nearly random vibration of the panel, power is transferred into sound through the mechanical resistance of the panel, which is constant with frequency. The radiation resistance is now insignificant because the air close to the panel also moves in a random fashion, reducing the effective air load. This means that diaphragm dimensions no longer control directivity: you can make the radiating area as large as you want without high frequency output becoming confined to a narrow solid angle about the forward axis.
The HSS technology uses ultrasonic (frequency range above 200 kHz) emitters. The process of sound reproduction is based on the non-linear characteristics of air that give way to Tartini Tones, or the frequency differences between two original sounds. Firstly the source signal (e.g. music) is added to an ultrasonic signal, then this is amplified and sent to the emitters. One emitter will radiate this signal, the other one only the ultrasonic signal. Here come the characteristics of air in the game because these two signals will interfere in a special way, such that their sum and their difference (the Tartini Tones) will be present. Because of our limited hearing range we will only hear their difference, which is the original source signal.
We do not have problems with resonances since there is no diaphragm and even if the emitter has resonances, they are way above our hearing range.
As for the problem of directivity, we noted in the earlier discussion of the conventional loudspeaker that if we listened to the reproduced sound in an anechoic environment - this means that we are not disturbed by sound reflected back from the environment - the variation with directivity would not be a problem: we would hear the diaphragm's on-axis output and nothing else. In case of the HSS system its high directivity solves the problem. Even if we are not in a anechoic enviroment we only hear the on axis output.
In summary, it is true to say that the design goals for a conventional loudspeaker have to be a compromise. You are trying to deliver acoustical output across a wide bandwidth, yet when the radiated wavelength becomes smaller than the diaphragm circumference the loudspeaker's power output begins to fall. Because of this, and the need to provide sufficient diaphragm displacement to reproduce frequencies at the lower extreme of its passband, a conventional drive unit's power bandwidth is typically limited to four to five octaves. This is a physical fact that remains a limitation with pistonic speakers even if we could design and make a perfect pistonic radiator. Consequently conventional drive unit design always embodies trade-offs between bandwidth, directivity and smoothness of frequency response. In the finest conventional loudspeakers these engineering compromises are skilfully struck, but they remain compromises. Looking at the new designs the future appears promising. sincethe NXT or the HSS speakers oprerate on a completely different basis, they lack all the physical limitations of the conventional dynamic loudspeakers. Although these systems are still in the development stage, we hope that we will see their implementations and the improvements that they will bring to sound reproduction, in the next couple years.
Bibliography/Web-o-graphy:
- http://www.flatspeaker.com NXT speaker official website
- http://www.atcsd.com/ HSS official website
- Matthias Carstens: Zenei Elektronika, Cser kiado, Bp. 1996.
- Klinger: Hangdoboz építés, Franzis Kiado, München 1989., Marktech kft. Bp. 1991.
- Chronik der Technik, ,Chronic-Verlag, Dortmund 1988.
- Géher Károly: Híradástechnika, Műszaki Könyvkiadó Bp. 2000.
NXT™ is a trademark of New Transducers Ltd 2000
HSS™ is a trademark of the American Technology Corporation
________________________________________
[1]Chronik der Technik, p.364
[2] Géher Károly: Híradástechnika, p. 53
[3] http://www.flatspeaker.com/techology/technology.htm
In order to answer the first question we need to review the basic principles of how conventional loudspeakers operate and identify the fundamental restrictions on performance that they impose.
In Figure 1 we can see the structural outline of a conventional speaker. The aim of such speakers is to create a pistonic motion of the diaphragm for sound reproduction. By pistonic we mean that the diaphragm moves back and forth as a rigid whole.
To achieve this the moving voice coil of the loudspeaker is placed in the air gap of a strong permanent magnet. The moving coil is attached to a conical diaphragm which is supported by flexible suspension to keep the motion axial. The basket attached to the magnet supports the rim of the diaphragm. As electrical current flows through the coil an axial force is generated according to the law of inductance. This force brings the diaphragm into motion thus producing sound waves.[2]
Even though later other methods of transduction were invented - using electromagnetic, electrostatic or piezoelectric forces to set the diaphragm into motion - the basic principle of the operation, pistonic motion, has remained the same.
As we are now more or less familiar with the operation of the conventional loudspeaker let us try to identify the physical limitations of such a system.
Variation in directivity with frequency is one of the great bugbears of loudspeaker design. If we listened to reproduced sound in anechoic environments it would not be a problem: we would hear the diaphragm's on-axis output and nothing else. But the usual listening environment is far from anechoic, so a loudspeaker's output off the listening axis has a significant effect on what we hear. Because of frequency dependent directivity the direct, reflected and reverberant sounds in a room all have different tonal balances. Even if a conventional loudspeaker had an absolutely flat on-axis response and was entirely free of resonance - a high expectation - it would still sound colored and introduce imaging aberrations.
The reason for this phenomenon is the following. At low frequencies, where the wavelength in air is large compared with the diaphragm dimensions the acoustic power output is constant. As frequency continues to rise, though, and the wavelength in air reduces to the point where it becomes comparable with the diaphragm dimensions, a major change occurs. (Figure 2)
Because of various reasons the diaphragm's acoustic power output now begins to fall at a rate of 12dB per octave. This does not mean that the on-axis (in front of the speaker) pressure response falls away: what generally happens is that the diaphragm's acoustic output becomes restricted to progressively narrower solid angles. In other words, it becomes directional; it begins to beam. This is the main reasons why we need to use more than one speaker and a crossover network in better quality speakers. This will reduce the problems related to directivity but at the same time the crossover networks will introduce new disturbances in the signal chain. The crossover networks and their effect will be described later in a little bit more detail.
The second group of problems is resonances. Resonances stem from the fact that we live in a real world and we do not have ideal, completely rigid materials. Resonances appear at certain frequencies, depending on the inherent qualities of the materials. If the source signal contains these frequencies, the resonances of the different parts of a loudspeaker system will distort the sound at those given frequencies.
For starters we have the resonances of the diaphragm itself. An ideal diaphragm would move as a rigid whole. In reality we can never achieve such a pistonic motion. A real diaphragm is never completely rigid, therefore it resonates which results in further colorations in the reproduced sound since the speaker emits such sound components which were not in the original source. In the following table some of the fundamental bending modes of the diaphragm are illustrated.
Furthermore there is the resonance of the enclosure. We need an enclosure because the front and the rear of the diaphragm must be separated in order to avoid acoustical shortcircuit. (The distance between the rear and the front side must be larger than the largest wavelength we would like to reproduce) This is achieved by building the speaker either into the wall or an enclosure. Here we have another source of distortion in sound: the resonance of the enclosure. Similarly to the diaphragm, the sides of the enclosure will also have their own bending modes at different frequencies.
Now let us go back a bit. As it was mentioned earlier when the problem of directivity was discussed, we need to apply multiple loudspeaker systems and crossover networks.
The point of having multiple loudspeakers is that each of them covers just a part of the entire audible range, the part where it can operate without becoming too directional. For this we need an electric network which splits up the incoming signal according to the necessary frequency ranges. This is what a crossover network is. (Usually the signal is split into three parts: low, mid and high range)
A crossover consists of resistive, capacitive and inductive elements. Anyone who has learned a bit of electronics knows that the last two elements are reactive, that is they shift the phase of the signal which passes through them. The result of this will be for example, that the sound coming from the mid range speaker will have a different phase than the sound coming from the low range speaker. This would not be a problem if it were not for a special quality of our hearing. Namely, that we are able to hear phase differences. Therefore a crossover network will introduce phase distortions in the reproduced sound.
In monophonic listening situations this would not be such a significant problem, but in stereophonic situations these phase differences will greatly reduce the stereo image of the sound.
So far we have seen that there are physical limitations on the sound quality that can be achieved by conventional speakers.
Thus we can conclude that because of its physical limitations not even an ideal conventional speaker will reproduce the original signal perfectly, therefore the answer for our first question is that new loudspeaker paradigms are necessary to improve the overall quality of sound reproduction.
Now, we can move on to discuss our second question: what are the current developments in the field of loudspeaker design?
Presently, there are two new designs currently being developed which are worth looking at. The first one is the so called NXT, or Flat speaker, the second is the HSS, HyperSonicSpeaker.
The NXT is based on what we term distributed-mode (DM) operation. Essentially this involves encouraging the diaphragm, in this case a special panel, to produce the maximum number of bending resonances, evenly distributed in frequency. The resulting vibration is so complex that it approximates random motion. This means that each small area of the panel vibrates, in effect, independently of its neighbors, rather than in the fixed, coordinated fashion of a pistonic diaphragm. Think of it as an array of very small drive units, each radiating a different, uncorrelated signal but summing to produce the desired output[3]. (see Fig. 3)
Fig.3
Snapshot of panel motion
Because of this quasi random vibration of the panel there is no need for an enclosure and since we are actually using the resonances of the panel to reproduce sound we have no resonance problems as with the conventional loudspeaker.
Also, the problems related to directivity disappear since, because of the nearly random vibration of the panel, power is transferred into sound through the mechanical resistance of the panel, which is constant with frequency. The radiation resistance is now insignificant because the air close to the panel also moves in a random fashion, reducing the effective air load. This means that diaphragm dimensions no longer control directivity: you can make the radiating area as large as you want without high frequency output becoming confined to a narrow solid angle about the forward axis.
The HSS technology uses ultrasonic (frequency range above 200 kHz) emitters. The process of sound reproduction is based on the non-linear characteristics of air that give way to Tartini Tones, or the frequency differences between two original sounds. Firstly the source signal (e.g. music) is added to an ultrasonic signal, then this is amplified and sent to the emitters. One emitter will radiate this signal, the other one only the ultrasonic signal. Here come the characteristics of air in the game because these two signals will interfere in a special way, such that their sum and their difference (the Tartini Tones) will be present. Because of our limited hearing range we will only hear their difference, which is the original source signal.
We do not have problems with resonances since there is no diaphragm and even if the emitter has resonances, they are way above our hearing range.
As for the problem of directivity, we noted in the earlier discussion of the conventional loudspeaker that if we listened to the reproduced sound in an anechoic environment - this means that we are not disturbed by sound reflected back from the environment - the variation with directivity would not be a problem: we would hear the diaphragm's on-axis output and nothing else. In case of the HSS system its high directivity solves the problem. Even if we are not in a anechoic enviroment we only hear the on axis output.
In summary, it is true to say that the design goals for a conventional loudspeaker have to be a compromise. You are trying to deliver acoustical output across a wide bandwidth, yet when the radiated wavelength becomes smaller than the diaphragm circumference the loudspeaker's power output begins to fall. Because of this, and the need to provide sufficient diaphragm displacement to reproduce frequencies at the lower extreme of its passband, a conventional drive unit's power bandwidth is typically limited to four to five octaves. This is a physical fact that remains a limitation with pistonic speakers even if we could design and make a perfect pistonic radiator. Consequently conventional drive unit design always embodies trade-offs between bandwidth, directivity and smoothness of frequency response. In the finest conventional loudspeakers these engineering compromises are skilfully struck, but they remain compromises. Looking at the new designs the future appears promising. sincethe NXT or the HSS speakers oprerate on a completely different basis, they lack all the physical limitations of the conventional dynamic loudspeakers. Although these systems are still in the development stage, we hope that we will see their implementations and the improvements that they will bring to sound reproduction, in the next couple years.
Bibliography/Web-o-graphy:
- http://www.flatspeaker.com NXT speaker official website
- http://www.atcsd.com/ HSS official website
- Matthias Carstens: Zenei Elektronika, Cser kiado, Bp. 1996.
- Klinger: Hangdoboz építés, Franzis Kiado, München 1989., Marktech kft. Bp. 1991.
- Chronik der Technik, ,Chronic-Verlag, Dortmund 1988.
- Géher Károly: Híradástechnika, Műszaki Könyvkiadó Bp. 2000.
NXT™ is a trademark of New Transducers Ltd 2000
HSS™ is a trademark of the American Technology Corporation
________________________________________
[1]Chronik der Technik, p.364
[2] Géher Károly: Híradástechnika, p. 53
[3] http://www.flatspeaker.com/techology/technology.htm
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