Electrical audio signal processing systems and devices – Electro-acoustic audio transducer – Having acoustic wave modifying structure
Reexamination Certificate
2002-01-14
2003-09-30
Le, Huyen (Department: 2643)
Electrical audio signal processing systems and devices
Electro-acoustic audio transducer
Having acoustic wave modifying structure
C381S340000, C381S343000, C181S152000
Reexamination Certificate
active
06628796
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is generally directed to loudspeaker systems and more particularly to loudspeaker systems which use sound chambers which progressively propagate entering annular mid frequency sound waves concentrically about high frequency sound waves to an output wherein the mid frequency sound waves are substantially parallel on opposite sides of the high frequency sound waves.
2. Brief Description of the Related Art
Most loudspeaker systems for commercial or professional applications require more than one transducer. There are two common reasons for this that stem from the limits of transducer technology: limited bandwidth; and/or limited sound power output of individual transducers.
The limited bandwidth of transducers, when compared with the wide bandwidth of the human ear dictates the need for multi-way loudspeaker systems. The wavelengths of sound audible to us range from nearly sixty feet to less than three quarters of an inch in length. No single transducer can reproduce this range of frequencies with acceptable levels of both distortion and efficiency.
The limited sound power capacity of a single multi-way loudspeaker unit when compared to the sound power and distribution required for large venues, dictates the need for multi-unit loudspeaker groups or arrays. This is the case in nearly all commercial use or professional loudspeaker systems. For the purposes of this discussion, multiple units of multi-way loudspeakers will be considered.
Clarity, referred to also as intelligibility and speech intelligibility, is affected by the degree to which the loudspeaker reconstructs the temporal and spectral response of the reproduced wavefront. Interference in the perception of that wavefront can be caused by environmental reflections of sound waves bearing the same spectral information which arrive near in time to the beginning of the wavefront.
Coherence of a wavefront refers to the degree to which the loudspeaker reconstructs the temporal response of the reproduced wavefront.
Uniformity of distribution refers to the similarity in the temporal and spectral nature of the reproduced sound when considered spatially.
Correction of the sound spectrum through equalization is easily achieved with signal processing equipment. Correction of the temporal aspects of sound referred to as impulse response equalization is considerably more complex. Correction of the spatial distribution of sound energy, after the sound has exited the loudspeaker system is not possible.
To fully understand all aspects concerning clarity in large loudspeaker systems, it is necessary to consider issues beyond those limited to the temporal and spectral performance of individual transducers and their related enclosures or waveguides. Wavefront coherence and uniformity must be considered concerning several aspects of the multi-way structure and the multi-unit array. In the multi-way loudspeaker the additional issues are twofold; the reconstruction of complex waveforms from two or more transducers not physically occupying the same location that reproduce different parts of the spectrum; and the temporal interference that occurs in the region of spectral overlap between transducers. In the multi-unit array a further consideration is added: the temporal interference between multiple transducers working together to reproduce the same part of the spectrum.
Complete and uniform energy summation occurs when two or more simple cone loudspeakers produce sound waves of the same frequency which propagate into the same space, where the wavelength propagated is approximately equal to or greater than the spacing of the loudspeakers. In cases such as this the devices are said to be mutually coupled; multiple devices work nearly as a single device.
Complex patterns of summation result in reduced spatial uniformity and lost efficiency when two or more transducers produce sound waves of the same frequency which propagate into the same space, where the wavelength propagated is smaller than the spacing of the transducers. These patterns are not easily integrated in systems and most often, the result is reduced coherence of the wavefront and therefore reduced sound quality.
It is evident that a useful approach to the problem of summation is to physically limit or eliminate the negative interaction between adjacent transducers through the design of wavefront modifying or directivity controlling mechanical geometry through which the sound waves are propagated. The mechanical control of such interactions are therefore of great interest in the development of better loudspeaker arrays.
From the ideal loudspeaker system, sound would appear to the listener as though it came from a point source floating in space. This goal is approachable in a single multi-way loudspeaker, but impossible in a large sound system. Nevertheless, audio engineers have sought over the years to come as close to the goal as possible through a number of interesting innovations.
In small systems, it can be said generally that for best coherency, the physical spacing between transducers of differing frequency ranges should be kept as small as possible. Whereas in large systems, more attention should be paid to the physical relationship between transducers operating in the same frequency range due to the overall size of the array.
The evolution of the co-axial loudspeaker has resulted in improved coherency in two-way systems. A typical variation is a two-way device consisting of a high frequency compression driver mounted on the back plate of a woofer magnet, so configured to allow the sound from the high frequency driver to pass through the woofer and emerge at the center of the cone of the woofer. The passageway through the low frequency magnet combined with the woofer cone, or other small horn device, serve to guide the high frequency energy. The addition of time compensation in the signal path to correct for the physical displacement of the two sound sources produces something very close to the ideal. In this described configuration a direct radiator is combined with a horn loaded driver.
However, the directivity cannot be controlled to the extent that might be desired at all frequencies in such a loudspeaker. Furthermore, a substantial part of the benefit of point source approximation is lost when multiple co-axial speakers are configured in an array spaced on the centers of the woofer. The larger size of the woofer may result in the space between high frequency drivers increasing beyond the dimension allowed by the smaller high frequency drivers, thus aggravating the interference problem between the high frequency components. It is evident that the co-axial driver can improve coherence in a small system, but where large multiples are deployed, no significant gain is likely to occur.
The recently introduced co-entrant horn disclosed in U.S. Pat. No. 5,526,456 to Heinz is a two way, mid frequency and high frequency horn loaded variation on the co-axial loudspeaker. In this variation, the high frequency compression driver is mounted on the back plate of a mid frequency compression driver magnet, so configured to allow the sound from the high frequency driver to pass through the mid frequency device and emerge through the center of the diaphragm of the mid frequency driver. The energy from the mid frequency diaphragm enters the throat of the horn through an annular slot adjacent to the high frequency opening. With suitable time compensation to align the acoustic output of the two devices in the time domain, the result is similar to the co-axial loudspeaker, but with the added advantages of increased mid frequency efficiency and control of mid frequency directivity through the horn loading of that band of energy. However, the discontinuity in the high frequency throat caused by the mid frequency entrance to the throat of the waveguide is quite close to the high frequency driver diaphragm. If the discontinuity is within one quarter wavelength of a given frequency, energy reflected back to the diaphragm will
Dabney P.
Dowell & Dowell , P.C.
Le Huyen
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