Influence of electromagnetic wave guide dispersion on the human perception of sound quality in audio systems
patented & patents pending

Abstract: This paper describes a method for optimizing percepted sound quality of an audio system, achieving enhanced time coherence of electroacoustic signals. The method uses a technique that will be normally applied in microwave electronics. Use of dispersive delay line structures with its influence on electromagnetic wave guide dispersion can explain the reason for differences in percepted quality of sound in different audio systems, and fine adjustment of sound quality of any audio equipment is possible.
© 2000 R.Heinrich mail: rh@finalinvention.de

Introduction
Sound quality and differences in sound quality between different electroacoustic audio systems are not yet fully explainable. In practice, every system has its own specific sound characteristic. Optimizing sound quality of new constructed audio equipment can be done by modifying active or passive electronic components of the circuit, with a listening test that follows any modification. With this procedure, the sound characteristic of an electroacoustic circuit can be optimized to a desired direction, but the work is time intensive and changes of sound quality can only made in coarse steps.
It is necessary to find the decisive parameter in the electrical part of an electroacoustic transmission system, which must be considered as the physical reason for the percepted quality of sound; this parameter should be influenced independent of other parameters of signal transmission. Finding this parameter will give control over the sound quality of an audio system, fine adjustment of sound quality will be possible. This has been the content of the past work. Parts of this work are disclosed in patent DE19740601.

The decisive parameter of electroacoustic signal transmission
The most important parameter that has influences on sound quality in an electroacoustic transmission system is the electromagnetic wave guide dispersion of the complete system, viewed as a non-separable unit from the signal source to the electroacoustic transducer. Every part of this system, a single electronic component, conductor, dielectrics or semiconductor has its own dispersive property depending from material and construction. The quality of sound will be decreased if the electromagnetic wave guide dispersion of the complete audio system can't be reduced or eliminated over the full audible frequency range. Reducing the electromagnetic wave guide dispersion of an audio system in the electrical part will result in an increased time coherence of the transmitted audio signal. This can be done by changing components or other parts of the system. In addition, connection wires between components of the system also have audible influence on the electromagnetic wave guide dispersion of the complete audio system. It is impossible to make fine changes of electromagnetic wave guide dispersion in a system because of the dispersive material property of any single component that has a fixed frequency dependent characteristic. The solution to have control over the electromagnetic wave guide dispersion in an audio system can be found having a look to high frequency microwave electronics. The dispersive delay line, normally used in compression receivers in radar technology solves the problem of decreased sound quality in audio systems.

Dispersive delay lines
The frequency dependent characteristic of electromagnetic wave guide dispersion of a complete audio system can be minimized with the aid of a dispersive delay line that has an inverse delay characteristic over the audible frequency range compared with the original system. The frequency components of the propagating electromagnetic wave front, passing through a system that was corrected with a dispersive delay line, are slightly changed in signal timing. The results are time coherent spectral components of the electromagnetic wave. This is audible as an enhanced quality of sound.
The dispersive delay line must be constructed on breaking up the limitation of constant diameter of a conventional electric wire. The material should have a lower electric conductivity than conventional conductors; the constructed prototypes are made of brass. This type of material was taken to increase the skin deep of the physical skin effect. The diameter of this conductor varies aperiodically over the complete length. The variations in diameter of the conductor must be rotation symmetric on its long axis. The mathematical function of the rotation symmetric contour of this conductor specifies the frequency characteristic of the wave guide dispersion of the delay line. This behavior can be explained with the displacement of current caused from skin effect. The higher frequency components of the electromagnetic wave are displaced to the outer regions of the conductor. The geometrical anomaly of the conductor will cause a delay of these frequency components; they have to pass a slightly longer transmission path. The amount of delay time may be estimated in the range of 10 ps or lower. Figure 1 shows an example of a dispersive delay line.

This example line is constructed with four regions, where the amplitude of the mathematical sinusoidal function of the diameter changes from high to low. The left-hand region has high amplitude of diameter, high and mid frequencies of the audio spectrum will be delayed, high frequencies more than mid frequencies. The skin effect will displace these components to the outer region of the conductor; they have to pass a longer signal path in a symmetrical bended course resulting from the geometric structure. Highest frequency components will take the longest path on the surface of the conductor. The right region of the conductor will only affect the higher frequency components; only these will get an additional delay. The low and mid frequency components are influenced less because the majority of these are not displaced to the outer region of the conductor. The mid regions of the conductor will have influences on high frequencies and low influence on mid frequencies. Low frequency components of the audio signal are nearly undelayed where mid- and high frequency components will be delayed more with increasing frequency. The result is a nonlinear frequency dependent delay of the spectral components of the electromagnetic wave depending on the geometric construction of the conductor.
In most audio systems the high frequency components are propagating slightly faster through the system, they should be delayed to optimize time coherence. In this case, the sound quality of the electroacoustic system can be increased by inserting a dispersive delay line with a correct geometrical construction in the signal path. The electromagnetic wave guide dispersion of the complete audio system and the dispersion of the delay line with its inverse characteristic will decrease timing errors of the electroacoustic signal. This enhances the percepted quality of sound over the audible frequency spectrum.

Audible influences
The effectiveness of the described delay line was reviewed in listening sessions. The construction of the line was made from experience because of a non-existing measurement technology. The sound of a high quality transistor amplifier can be modified to a great naturalness. The reproduction of musical signals and human voices is very clear. The audibility over long hearing sessions is extremely stress less. The technical reproduction character of sound is removed and the percepted sound signal is very natural with reduced time dependent distortion. The reproduction of stereophonic music material enhances the spatialness of these recordings. Time smearing effects are significantly reduced.
The characteristic of sound reproduction is sensitive to the geometrical construction of the dispersive delay line; small changes of geometry are still audible. Bass frequencies are reproduced very accurate. Formerly degraded sound quality is increased depending on construction of the dispersive delay line. Cascading two or more dispersive delay lines sometimes enhances sound quality if the line isn't constructed correctly, but in most cases the percepted sound quality will be decreased, depending on the fact that the optimum of sound quality over the complete audible frequency range is only reached with the correct specified geometrical construction of one or more cascaded lines.

Conclusion
The use of a dispersive delay line in audio systems with its audible influence on the percepted quality of sound indicates that sound quality is dependent from the overall electromagnetic wave guide dispersion of the system. Decreased quality of sound must be considered as a loss of time coherence caused from the dispersive properties of any single component in the signal path of a system. In the case that higher frequency components propagate slightly faster through an audio system, the loss of time coherence can be theoretically eliminated and practically reduced with the aid of a dispersive delay line. The frequency dependent delay characteristic of such a line is adjustable over the geometrical construction of it. The dispersive delay line only affects signal timing. Other parameters of signal transmission like frequency response won't be influenced. This makes a dispersive delay line suitable to adjust the quality of sound of any audio equipment.