US3566310A

US3566310A - Waveguide for producing artificial reverberation

Info

Publication number: US3566310A
Application number: US741418A
Authority: US
Inventors: Werner Fidi; Bernhard Weingartner
Original assignee: AKG Akustische und Kino Geraete GmbH
Current assignee: AKG Acoustics GmbH
Priority date: 1967-07-05
Filing date: 1968-07-01
Publication date: 1971-02-23
Anticipated expiration: 1988-02-23
Also published as: BE717613A; NL6809390A; CH477737A; DE1762545B2; FR1572809A; NL139403B; GB1206868A; DE1762545A1

Abstract

THE SURFACE OF A WAVEGUIDE IS FORMED WITH IRREGULARITIES HAVING GEOMETRICAL DIMENSIONS WHICH ARE SMALLER THAN THE WAVELENGTH AT THE UPPER LIMIT OF A PREDETERMINED FREQUENCY RANGE. THE IRREGULARITIES ARE ARRANGED TO IMPART, TO THE PROPAGATION CONSTANT OF WAVES TRAVELING ALONG SAID WAVEGUIDE, A STATISTICAL VARIATION THROUGHOUT SAID FREQUENCY RANGE AND ABOUT A MEAN VALUE WHICH WOULD BE OBTAINED IN A WAVEGUIDE WHICH IS FREE OF SUCH IRREGULARITIES AND OTHERWISE IDENTICAL.

Description

" -Feb. 23,1911 wm. Em 3,566,310

Y wAvEGuIDE Fon PRoDUcING ARTIFcAL REVERBERATION y Filed July 1'. 19615- mvfnrons WERNER FIDI Mamma wemsnmzn ATTORNEY!) United States Patent Office 3,566,310. Patented Feb. 23, 1971 U.S. Cl. 333--30 8 Claims ABSTRACT OF THE DISCLOSURE The surface of a waveguide is formed with irregularities having geometrical dimensions which are smaller than the wavelength at the upper limit of a predetermined frequency range. The irregularities are arranged to impart, to the propagation constant of waves traveling along said waveguide, a statistical variation throughout said frequency range and about a mean value which would be obtained in a waveguide which is free of such irregularities and otherwise identical.

This invention relates to a oneor multi-dimensional mechanical waveguide, particularly a coil spring, for use in devices for producing artificial reverberation. In these devices, the audiofrequency signal is applied to the waveguide 4by an electromechanical transducer and is similarly picked up from the waveguide. The coil springs are preferably excited to perform torsional vibration.

Some basic requirements must be fulfilled for an artiiicial production of a good reverberation. Any reverberator has a comb-like frequency response curve consisting of a succession of polar portions and zero portions, and these polar and zero portions must have a high density of distribution. This requires sufficiently long delay times. Periodicities in the time and frequency spectra must be avoided. The signal which is applied should be subjected to a diffuse disintegration by a random dispersion. If the density of the distribution of the polar and zero portions of the frequency response curve is not sufiicient, any periodicity will result in a highly metallic, hollow and unnatural sound. With a higher density of distribution, such periodicity will result in -utter echoes or true echoes. The use of one-dimensional homogenous delay lines results in strictly periodic frequency response curves. The frequency response of the reverberation time is another significant factor and should drop toward the basses and toward the trebles.

In artificial systems, the reverberation is produced as in the reverberatory chamber, in that the applied signal is delayed and is reflected many times. The internal friction of the system will determine the reverberation time and the frequency response thereof. In View of the abovementioned requirement, measures are called for which ensure mainly the required dispersion as well as an adequate density of distribution of the frequency response curve. This is an object of the present invention.

There have been disclosed coil spring systems in which the dispersion is continuously varied, in dependence on frequency by, a uniform change of the turn diameter. As a result, the basses are highly delayed whereas the delay time decreases toward the trebles. In this way, the flutter echoes are reduced (U.S. Pat. No. 3,199,053).

In natural reverberation, disturbing regularities are avoided as a result of the design of the reverberatory chamber, so that an adequate dispersion and disintegration can be relatively easily achieved. A significant factor is the presence of three dimensions. With a dense distribution of polar and zero portions, it becomes increasingly Cil more difiicult to obtain the necessary disintegration. The subjective evaluation of the quality of reverberation by the human sense of hearing Orients itself in the chamber so that some of the above requirements are satisfied in a natural way.

In order to impart these characteristics also to devices for producing artificial reverberation, which devices comprise at least one transit time element consisting of a oneor multi-dimensional waveguide, e.g., at least one coil spring which is excitable to produce torsional vibration, the invention provides a waveguide. which is characterized in that it has a surface to which `an irregular shape has been imparted by a technological process so that the geometric dimensions of irregularities of said surface are smaller than the wavelength of the highest frequency to -be transmitted.

As a result of this measure, the propagation constant of waves travelling along the waveguide is so changed that a statistical variation is imparted to the propagation constant throughout the frequency range to be transmitted, and about a mean 'value which would be obtained in a waveguide which is free of such irregularities and other- Wise identical, so that the waveguide according to the invention results in a statistical dispersion of the sound transmitted by it and the final result is highly analogous to the reverberation conditions in the natural sound field.

Various processes, preferably etching processes, may be used to produce the irregular surface according to the invention. For instance, an irregularly distributed, acidresisting protecting layer (eg. of a varnish, a grease, or another acid-resisting substance) may be applied to the ybright surface of a waveguide. The acid-resisting substance may be sprayed onto the surface to form small and very small, partly coherent droplets thereon. When the waveguide is subsequently etched, the non-protected areas of the surface will be attacked immediately. The etchant will partly act under or through the protective layer after some time, so that a surface having any desired roughness or irregular texture can be obtained in dependence on the duration of the etching treatment.

If the waveguide material, the etchant, the temperature and the etching time are properly selected, and defects on the surface of the waveguides are properly utilized, it may not be necessary to provide a protective layer before the etching treatment is performed. Such defects may consist of minute cracks and scores which are produced when the wire is being drawn and is wound to form a coil spring. These defects result in an irregular attack by the etchant so that the desired irregular surface is produced.

Alternatively, the irregular surface may be produced by Sandblasting the waveguide. In this case, the particle size and the nature of the treatment may be selected to produce a surface which embodies the inventive concept.

The required irregularities may be produced not only by a removal of material but also by changes effected in the structure of the material and preferably on the surface itself. Whereas such structural changes affect only the spring component, so that the result is less pronounced, it can well be utilized for the purposes of the invention. A suitable material may be treated to have local zones differing in hardness and entirely irregularly distributed over the surface. These effects can be controlled by suitable tempering treatments.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. l is a partial longitudinal sectional View of a portion of a waveguide, embodying the invention, and in the form of a coil spring; and

FIG. 2 is a transverse sectional view of the coil spring.

3 DESCRIPTION OF THE PREFERRED EMBODIMENT It is apparent from the two figures that the surface 2 of the spring wire 1 is entirely irregularly shaped. EX- tremely bizarre configurations 3 may be present, so that the propagation constant of waves traveling along the waveguide is so changed, in depedence on frequency, that it statistically varies through the frequency range to be transmitted and about a mean value which would -be obtained in a waveguide which is free of such irregularities and otherwise identical.

The variations in wire diameter due t the irregularities amount to about -30%. This variation has a particularly large effect on the mass components and spring components of a waveguide consisting of a coil spring, because the mass components are proportional to r3a2 and the spring components are proportional to r/4, where r represents the mean radius of the spring and a represents the Wire radius.

In contrast to a waveguide having a smooth surface, the waveguide according to the invention affords a close approach to the conditions which control the production of artificial reverberation. As a result, a reverberation device provided with a waveguide according to the invention has a high fidelity.

What is claimed is:

1. A waveguide for producing artificial reverberation within a predetermined frequency range,

said waveguide having a surface formed entirely with irregularities having geometric dimensions which are `smaller than the wavelength at the upper limit of said range, said irregularities being arranged to impart, to the propagation constant of waves traveling along said waveguide throughout said frequency range, a statistical variation about a mean value which would be obtained in an otherwise identical waveguide which is free of said irregularities. 2. A waveguide as set forth in claim 1, which is onedimensional.

3. A waveguide as set forth in claim 1, which is multidimensional.

4. A waveguide as set forth in claim 1, which comprises a torsionally vibratile coil spring.

5. A process of manufacturing a waveguide for producing artificial reverberation within a predetermined frequency range,

said waveguide having a surface formed with irregularities having geometrical dimensions which are smaller than the wavelength at the upper limit of said range,

said irregularities being arranged to impart, to the propagation constant of waves traveling along said waveguide throughout said frequency range, a statistical variation about a mean value which would be obtained in a waveguide which is free of said irregularities and otherwise identical,

said process comprising:

irregularly applying an acid-resisting protective layer to a smooth surface of a waveguide blank, and

subsequently etching said blank to produce said irregularities.

6. A process Of manufacturing a waveguide for pro- 4 ducing artificial reverberation within a predetermined frequency range,

said irregularities being arranged to impart, to the propagation constant of waves traveling along said waveguide throughout said frequency range, a statistical variation about a mean value which would be 0btained in a waveguide which is free of said irregularities and otherwise identical,

said process comprising Sandblasting a surface of a waveguide blank to produce said irregularities.

7. A process of manufacturing a waveguide for producing artificial reverberation within a predetermined frequency range,

said irregularities being arranged to impart, to the propagation constant of waves traveling along said waveguide throughout said frequency range, a statistical variation about a mean value which could be obtained in a waveguide which is free of said irregularities and otherwise identical,

said process comprising:

providing a. waveguide blank having local zones of different hardness, and subjecting said blank to a technological treatment to produce said irregularities with utilization of said zones of different hardness.

8. A process of manufacturing waveguide for producing artificial reverberation within a predetermined frequency range,

said waveguide having a surface formed with irregularities having geometrical dimensions which are smaller than the wavelength at the upper limit of said range, said irregularities being arranged to impart, to the propagation constant of waves traveling along said waveguide throughout said frequency range, a statistical variation about a mean value which would be obtained in a waveguide which is free of said irregularities and otherwise identical, said processing comprising:

providing a waveguide blank having an irregular material structure, and etching a surface of said blank with utilization of said irregular structure, to produce said irregularities.

References Cited UNITED STATES PATENTS 2,923,369 2/1960 Kuhl 181-31 3,092,792 6/1963 Daniel 333-30 3,136,853 6/1964 Bissonette et al. 84-1.25X 3,264,583 8/1966 'Fitch 333-30 3,271,704 9/1966 Fitch 333--71 3,288,931 1l/l966 Bunger 333-30X HERMAN K. SAA-LBACH, Primary Examiner M. NUSSBAUM, Assistant Examiner U.S Cl. X.R. 3 33-71