Electrical audio signal processing systems and devices – Electro-acoustic audio transducer – Having acoustic wave modifying structure
Reexamination Certificate
2000-02-17
2002-05-14
Tran, Sinh (Department: 2743)
Electrical audio signal processing systems and devices
Electro-acoustic audio transducer
Having acoustic wave modifying structure
C381S351000, C381S349000, C381S350000
Reexamination Certificate
active
06389146
ABSTRACT:
BACKGROUND OF THE INVENTION AND RELATED ART
This invention relates to improved, low frequency bandpass loudspeaker systems. In the art of loudspeaker enclosures there are two basic types of systems that are most common. The sealed or acoustic suspension system, which consists of an electroacoustical transducer mounted in an enclosed volume that has the characterization of acoustic compliance. The second type is what is commonly called a bass-reflex system which includes an electroacoustic transducer mounted in an enclosure that utilizes a passive acoustic radiator which includes the characteristic of acoustic mass which interacts with the characteristic acoustic compliance of the enclosure volume to form a Helmholtz resonance. A reflex system, enclosure/vent—compliance/mass, that exhibits a Helmholtz resonance shall be referred to hereinafter as a Helmholtz-reflex.
One of the prior art configurations relevant to the invention is the multi-chamber bandpass woofer system. Historically it has been shown that for a given restricted band of frequencies an acoustical bandpass enclosure system can produce greater performance both in terms of the efficiency/bass extension/enclosure size factor and large signal output compared to non-bandpass systems such as the basic sealed or bass reflex enclosures. The basic forms of these bandpass systems are discussed in the literature. See for example A bandpass loudspeaker enclosure by L. R. Fincham, Audio Engineering Society convention preprint #1512, May.
The earliest patent reference to a single Helmholtz-reflex tuned bandpass woofer system is Lang, ‘Sound Reproducing System’ U.S. Pat. No. 2,689,016. This patent reference anticipates the most common version of bandpass woofer system that is used in many systems today. This type of system includes an enclosure with two separate chambers with an active transducer mounted in the dividing panel and communicating to both chambers. One chamber is sealed, acting as an acoustic suspension and the other is ported, operating as a vented system with a passive acoustic mass communicating to the environment outside the enclosure.
The single tuned prior art bandpass woofer systems suffer from a number of shortcomings. First, they tend to have a series of resonant amplitude peaks that appear above the pass band of the bandpass system. These are due to standing waves in the enclosure and are well documented in the article by Fincham listed above. Prior art solutions to this problem suggest the use of damping materials which unfortunately damp out useful system output at the same time they damp out the undesired resonances. Secondly, they have a cone excursion minimum at their Helmholtz-reflex frequency, but there is only one tuning and it is placed at a frequency near the highest frequency of interest where cone excursion is insignificant compared to the lower frequency range of the system. If the vent tuning is placed at a lower, more useful frequency then the system suffers from reduced high frequency bandwidth.
The next evolutionary step in complexity of a prior art bandpass woofer is expressed in the earliest patent reference to a dual Helmholtz-reflex bandpass woofer system in
FIG. 2
in D'Alton, “Acoustic Device” U.S. Pat. No. 1,969,704. This reference discloses an enclosure containing a two chamber bandpass woofer system with an active transducer mounted in the dividing panel and communicating to both chambers. Each chamber has a passive acoustic radiator communicating to the environment outside the enclosure. European patent 0125625 “Loudspeaker Enclosure with Integrated Acoustic Bandpass Filter” by Bernhard Puls and U.S. Pat. No. 4,549,631 “Multiple Porting Loudspeaker Systems” granted to Amar G. Bose are derived from the same basic structure as shown in the D'Alton reference.
An alternative arrangement of a dual Helmholtz-reflex bandpass system is disclosed in the U.S. Pat. No. 4,875,546 “Loudspeaker with Acoustic Band-pass Filter” granted to Palo Kmnan. This system includes an enclosure with two separate chambers with an active transducer mounted in the dividing panel and communicating to both chambers. One chamber is ported with a passive acoustic radiator communicating to the environment outside the enclosure. There is a second passive acoustic radiator communicating internally between the two chambers.
These dual tuned bandpass subwoofers suffer from the same out of band, high frequency resonances that are endemic to the single tuned bandpass system. Further, by venting the lowest frequency chamber the lower frequency, out of band performance suffers below vent Helmholtz-reflex tuning, resulting both in a reduction of amplitude of output and an increase in diaphragm amplitude with a corresponding increase in distortion. This causes a steeper rolloff slope and increased distortion at frequencies below system cutoff. Because of this the system of this type does not lend itself to equalization below the lowest vent tuning frequency and therefore does not have useable output below this vent tuning frequency.
U.S. Pat. No. 5,092,424 “Electroacoustical Transducing with at Least Three Cascaded Subchambers” granted to Schreiber, et al, is an extension of the above listed bandpass art. It utilizes an enclosure with at least three chambers such that it is substantially equivalent to the Bose ′631 patent listed above, but with an additional enclosure volume added to the outside of the main enclosure. This additional enclosure receives the two ports from the internal main chambers and an additional passive acoustic radiator communicates to the environment outside the system. This system suffers from the same low frequency problems as the dual tuned bandpass systems.
Each of the above patents have shortcomings that have limited the full potential of the bandpass approach for low frequency reproduction. In general, the above systems either suffer from both a steep, highpass cutoff in the bass range where the most output is desired and/or a slow, lowpass cutoff in the higher frequencies where the greatest extension with the sharpest cutoff is most desirable and unattenuated resonances that can cause audible distortion.
It would be desirable to have a woofer system that combines a mild 2nd order high pass rolloff characteristic at the low frequencies with an extended frequency, steep slope lowpass characteristic at the high frequencies.
SUMMARY AND OBJECTS OF THE INVENTION
It is an object of this invention to utilize a multiple low pass, acoustic filter characteristic to filter out internal resonances and minimize their acoustical output.
It is the further object of the invention to utilize at least a double, acoustical, low pass filter characteristic to filter out audible distortion components that are generated when producing high output levels.
It is the further object of the invention to provide smaller internal chambers in which any remaining standing wave resonances are moved up to a higher, out of band frequency, preferably removed from the operating range of the invention.
It is a further object of the invention to form a hybrid bandpass/high pass woofer system that can achieve extended frequency response and minimized cone excursion.
It is a further object of the invention to create an acoustic bandpass having a steep slope low pass characteristic to allow a higher crossover point and/or achieve acoustical filtering of transducer distortion while, also exhibiting a more gradual high pass characteristic, extending the lowest frequencies.
It is the still further object of the invention to utilize its extended response and steep slope to allow higher crossover frequency and reduced out of band distortion and therefore significantly reduce the size and cost requirements of the upper range satellite speakers being used with the invented woofer system.
These and other objects are realized by the present invention which in a preferred embodiment provides a novel loudspeaker system incorporating an enclosure with a total of three subchambers and two Helmholtz-reflex tunings. The first of
American Technology Corporation
Dabney P.
Thorpe North & Western LLP
Tran Sinh
LandOfFree
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