Electrical audio signal processing systems and devices – Loudspeaker feedback
Reexamination Certificate
1997-08-27
2002-03-05
Mei, Xu (Department: 2644)
Electrical audio signal processing systems and devices
Loudspeaker feedback
C381S059000
Reexamination Certificate
active
06353670
ABSTRACT:
BACKGROUND
Sound transducers convert electrical signals into sound waves. A loudspeaker has one or more output sound transducers supported in a housing. The housing has a front opening for each output transducer. The volume of space behind the output sound transducer is the speaker cavity. Each sound transducer has a diaphragm that vibrates in response to the amplitude and frequency of applied electrical signals. With all passive designs, the shape and the size of the cavity influence the output of transducer. Under normal atmospheric conditions at frequencies above a few hundred hertz even a small cavity can be used to trap and prevent out-of-phase sound waves produced by a speaker, or, in general, any sound transducer, from interfering with the desired waves. But at frequencies below a few hundred hertz, the enclosure volume and resonance effects become significant. At low frequencies the size of the cavity creates pressure effects that alter the transducer's output compared to the ideal enclosure. The pressure effects create sound output amplitude decreases with lower frequency (roll off), distortion with decreasing frequency, and unwanted resonances. Present speaker designs generally rely upon passive acoustic methods to compensate for enclosure effects on the transducer output. Some examples of passive compensation include acoustic suspension, bass reflex, or the use of materials such as fiberglass to increase the effective cavity size. Another design uses the phase effects of several radiating speakers sharing the same enclosure to alter normal passive effects produced by the enclosure. Since accurate and loud base reproduction is hard or impossible to achieve with simple passive designs, the designers use the resonance effects to create a “booming sound” or false base that is not an accurate reproduction but has the sound of loud low sounds.
In a passive design, the inherent qualities of the structure of the design are used to counteract roll off and resonance without expending energy to artificially control cavity characteristics. Examples of such passive designs are:
1. Cavity size can be increased so that roll off is experienced at lower frequencies.
2. The cavity is filled with material, such as fiberglass, to increase the effective acoustical size of the cavity.
3. Special dampening materials can be used to reduce structural vibrations.
Despite efforts of others to passively control the cavity pressure, there remains an unfulfilled need for small speakers that can accurately reproduce sound, especially low frequency sound. There is a need for a speaker that will minimize low frequency distortion. There is also a need for a speaker that will reinforce certain frequencies to provide resonance at one or more desired frequencies. There is a further need for a speaker whose output is adjustable to both null certain frequencies and to reinforce others. There is a need for a speaker that is adjustable so the ranges of the nulled and reinforced frequencies are not fixed, as in passive designs, but are controllable and variable as selected by the user. There is a need to control these properties independent of constraints on the cavity's shape and volume. These and other needs are met by the invention described and claimed below.
Some prior art attempts have sensed the pressure in the cavity behind the output transducer and have used negative feedback to null the pressure in the output cavity. See, for example U.S. Pat. Nos. 5,461,676 and 5,327,304. However, it is difficult to accurately provide a null closed loop feedback system that has acceptable distortion. often the distortion of such systems are unacceptable to listeners. Moreover, such closed loop null systems can require complex control electronics. Even when such systems are used, they are unstable at certain frequencies that cause unwanted oscillations. It has been noted that even when pressure is nulled, the cavity will continue to vibrate. This indicates the need for positive pressure to reduce the spurious vibrations. Such systems cannot be adjusted to provide positive pressures or artificial resonances at arbitrary frequencies and rely only on nulling the pressure in the output cavity.
U.S. Pat. Nos. 5,461,676 and 5,5,327,304 apply to the restricted case of closed loop cases only. The above patents make no reference or implications to any of the other more general cases of active resonances and minimums, open loop designs, static pressures other than equal to outside, other gases, or the infinite cavity. The latter is not in a nulled condition but consist of traveling waves of similar amplitude but 180 degrees out of phase with the output. They also do not infer simulating passive effects such as resonances and minimums, or other active effects not possible with passive designs in the general sense of total arbitrary control of the cavity pressures and their effects on the speaker output. Closed loops, while helpful, are nevertheless restrictive in the amount and types of controls that can be applied to the speaker cavity. The cited patents are limited to nulling cases. However, full control of the cavity would allow the user to selectively increase or decrease the pressure in the speaker cavity as a function of variables other than cavity pressure, e.g. the frequency of the driver signal. Applicant's open loop design provides such flexibility.
The effects of
FIGS. 3.2
,
3
.
3
and the example design of
FIG. 5.1
are not suggested in the two previous patents. The example design of
FIG. 5.1
is totally open loop in operation. The sensors previously shown in the parent application were used only in characterizing the design. None of the data displayed in
FIGS. 6 and 7
for the normal operating mode of the speakers are made with a closed loop or nulled cavity. The example design relies on the principles of artificial resonances and minimums only. These effects are in no way mentioned of implied in the previous patents. The circuit of
FIG. 7.4
which produces the necessary phase shifts, is part of an open loop system with no sensor input. Setup of the adjustments shown is done once initially using only the sound level at the listener in the specific environment in order to produce the flat or other desired final response. The phase settings are different for different amplifiers and acoustic effects of different listening environments. The data of
FIG. 6
was made at single frequencies with the open loop circuit C
1
, the same circuit used to produce the flat responses of
FIGS. 7.1
and
7
.
3
.
The invention reduces or eliminates the “booming sound” due to resonances. That result is in not implied, mentioned, or obvious in the art of record. In contrast, it was determined experimentally. It is not obvious or true that all other active designs other than the example will eliminate the booming sound in all cases and it is definitely not true if the resonance frequency is not specifically driven. The open loop design was initially chosen because a closed loop design must use complex digital electronics to prevent oscillations and instabilities if a good null or other active effects was to be obtained, due to the complex phase and amplitude changes occurring in the active cavity. The closed loop oscillations that occur using an analog circuit could not be filtered out since they are contained within the desired band. In contrast, the open loop designs can be very simple and practical to produce cheaply, as the design shown, C
1
which uses a single integrated circuit. In practice C
1
is just connected in series with one of the existing subwoofer amplifiers, is adjusted once on setup, and no other wires such as those for sensors being required. The normal dual subwoofers are then replaced with the single active design. The original dual channel subwoofer amplified is used to drive the single active subwoofer.
The U.S. Pat. No. 5,327,504 of Jul. 5, 1994 and U.S. Pat. No. 5,461,676 of Oct. 24, 1995 apply only to speaker with a closed loop null with cavity pressure equal to the outside, i.e. clos
Jaeckle Fleischmann & Mugel LLP
Mei Xu
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