Patent Application
20080031474
By combining acoustic sources to form arrays and processing acoustic signals that are delivered to the sources and to the arrays, the radiation patterns of a loudspeaker system that includes the arrays can be controlled to achieve a variety of goals for the acoustic energy that is radiated by the loudspeaker system to a listener, including generating various types of radiation patterns which can be more complex than the radiation patterns of the individual sources. The acoustic signal processing can include delaying, inverting, filtering, phase-shifting, or level-shifting the signals applied to each transducer relative to the signals applied to other transducers. At given points in space in the vicinity of the system, the acoustic output from the transducers may, for example, interfere constructively (increasing sound pressure) or destructively (decreasing sound pressure). Nulls can be created to take desired shapes and steered to a desired angles. For simplicity of understanding, we will view directivity in a descriptively useful plane, such as a horizontal plane. In the horizontal plane, we may discuss steering a “null axis” to a desired angle. However it should be understood that in three-dimensional space the null may have a three dimensional shape, such as a conical shell, where the angle of the shell walls are varied. For the case of a dipole-type source, the cone angle is 180 degrees, and the shape of the null deteriorates to a simple plane. For a cardioid shape, the cone angle is zero degrees, and the null shape deteriorates to a simple line.
Some aspects of driving acoustic transducers are discussed in co-pending application titled “Reducing Resonant Motion in Undriven Loudspeaker Drivers,” filed Aug. 4, 2006, and incorporated here by reference.
Because the effects of the signal processing on the radiated acoustic energy are dependent on the frequencies of the signals (and therefore of the acoustic waves) and on the relative positions of the transducers, various combinations of signal processing and groupings of transducers may be used to create desired acoustic effects in various ranges of frequencies.
The signal processing may be performed using either analog or digital signal processing techniques. Analog signal processing systems typically use analog filters formed using op amps and various passive components arranged to accomplish desired filtering functions. Digital signal processing can be accomplished in various types of digital systems, such as a general-purpose computer, controlled by software of firmware, or a dedicated device such as a digital signal processing (DSP) processor. Discrete components and analog and digital systems may be used in combination. These signal processing components and systems may be centrally located or distributed (or a combination of the two) among the speaker arrays, individual transducers, or other system components, such as receivers, amplifiers, and equalizers.
Trade-offs among efficiency, frequency range, and control of directivity are required when using a destructive interference. In some examples, a predetermined radiation pattern with a null along a null axis oriented at a desired angle can be achieved up to a frequency for which the spacing between two transducers is one-half the wavelength of the acoustic output. Above such a frequency, multiple lobes and nulls begin to appear, which may conflict with an intended effect. The efficiency of a system (the amount of acoustic energy, or power, that can be delivered to the listening environment, for a fixed amount of power input) directly depends on the spacing between the speakers. Larger spacing gives higher efficiency but (as explained) reduces the maximum frequency at which directivity can be controlled. In some examples, an array may have small spacing between its own transducers to maintain control at high frequencies, and large spacing between transducers from different arrays, to provide sufficient output power at low frequencies.
In some examples, as shown in
Each array 100L, 100R includes two transducers, which we refer to as left outer transducer 104, left inner transducer 106, right inner transducer 108, and right outer transducer 110. The transducers may or may not be identical. In one frequency range, for example, a higher frequency range (frequencies with a wavelength less than twice the separation between individual transducers within each array), each array works independently and only one transducer is used in each array, so no nulls are produced. At moderate frequencies (for example, frequencies with a wavelength less than twice the separation between the separate arrays), each array again works independently to reproduce its corresponding left and right signals and to steer those signals using the combination of that array's transducers to produce nulls. At lower frequencies, the arrays work together using one or both transducers in each.
For a left channel signal, the left array 100L steers a null in a desired direction, shown by null axis 112, by using its two transducers 104, 106 with appropriate signal processing to achieve a predetermined radiation pattern. An example of appropriate signal processing feeds a left channel signal to the outside transducer 104 and an identical but out-of-phase left channel signal to the inside transducer 106. (This assumes the two transducers 104 and 106 are identical. If they are not, the two signals may not be identical.) The desired null axis direction can be controlled by introducing delay between the two identical but out-of-phase left channel signals, or by filtering the signal fed to one transducer differently than the signal fed to the other transducer. If desired, the efficiency of array 100L can be increased by attenuating the signal applied to the transducer 106 relative to that applied to the transducer 104 (or attenuating the signal applied to transducer 104 relative to that applied to transducer 106). Similar behavior occurs for a right channel signal, with a null along the null axis 116 arising from the right array 100R.
The two transducers of each of the two arrays have a relatively small spacing 107, 109, for example, in the range of 5 cm to 7 cm on center, while the spacing 111 between the two arrays is wider, for example, in the range of 50 cm to 70 cm. This allows the arrays to be conveniently placed on either side of a typical computer or television monitor. In some examples, the transducers within each array are 6.5 cm apart on center.
At lower frequencies, the two more widely spaced arrays can be used together as if they were a single speaker array. In one lower frequency range, e.g., 550 Hz-1 kHz, one transducer from each array, e.g., outer transducers 104 and 110, are used together as two elements of an array driven so that their acoustic outputs interfere destructively to create a desired radiation pattern, characterized by a null along the null axis 114 between them. The wider element spacing in this frequency range results in increased efficiency of sound radiation by the combined arrays. In another low frequency range, e.g., below 550 Hz, the transducers 104 and 106 from the left array 100L are fed identical signals and are used to form a first acoustic source; the transducers 108 and 110 from the right array 100R are also fed identical signals and are used to form a second source, where the two sources combine to form a single array. The signals sent to the opposite side from which they were intended (i.e., left-side signals fed to the right array 100R) are sometimes referred to in this description as cross-feed signals. The signals sent to the first source and second source are processed as described earlier to create a null along the same null axis 114 described above for higher frequencies. That is, the signal fed to the transducers 104 and 106, in this low frequency range, is identical but of opposite polarity relative to the signal fed to the transducers 108 and 110. One signal may also be delayed with respect to the other, may be filtered with respect to the other, and/or may be attenuated with respect to the other. For example, the signal fed to the transducers 108 and 110 may be delayed relative to the signal fed to the transducers 104 and 106, it may be attenuated by some amount (e.g. 2 dB), and/or it may be filtered (for example, with a low pass filter). A benefit of this arrangement is that the system has more radiating area in this frequency range, (i.e., from all four transducers) which increases the system's maximum output capability. This serves to both achieve the desired radiation pattern and increase the overall output power capability of the system. In general, for arrays with multiple transducers, selectively altering the numbers of transducers that are operating in various frequency ranges can be used to improve system efficiency and maximum output capability, while achieving a desired radiation pattern over a wider range of frequencies.
Another effect of the arrays is that sound images can be placed well to the left of the left array or well to the right of the right array. This can be accomplished by orienting the null axis in a desired direction. The locations of these sound images (the location from which a listener interprets sound as originating) are referred to as the left and right perceptual axes 118 and 120. The orientation of perceptual axes can be controlled by controlling the orientation of null axes. An example of the signal processing used to crate nulls along the null axes is described below, in increasing detail starting from the most basic array building block and adding each functional feature of the signal processing in turn. For the sake of simplicity, this description focuses on the left input signal. As will be seen, the same processing is applied to deliver the right input signal to the appropriate transducers.
The null along the left null axis 112 is created by splitting the left input signal 204 into two paths and applying a low-pass filter 202 to the signal sent to the left inner transducer 106, as shown in
This signal filter 202 used in conjunction with the signal splitting and transducer geometry shown in
To improve the low frequency efficiency of the array, the right outer transducer can be used as the canceling transducer for low frequencies. In effect, the right array 100R is used as if it were a part of the left array 100L, rather than as a separate loudspeaker intended for right-channel signals. In the example of
With the canceling signal below 1 kHz now cross-fed to array 100R, it is useful to eliminate output from transducers 106 and 108 over this frequency range in a way that does not disrupt the phase relationship already established between the left inner and outer transducers. This can be achieved, for example, by using a pair of high-pass filters 310 and 312 and matching all-pass filters 302 and 314 (dashed arrows 322 and 324 indicate phase matching). The all-pass filters 302 and 314 also phase-matched to each other, as shown by the dashed arrow 325.
Applying the 1 kHz high pass filter 310 to the left inner transducer 106 without the matching all-pass filter would introduce a new phase shift that would disrupt the established null along the null axis 112. To avoid disturbing the null along the null axis 112, the phase of the all-pass filter should match that of the highpass filter over the band of interest (<1 kHz, in this example) within a tolerance of approximately +/−30 degrees. Performance can be improved if the phase match occurs over a larger frequency range, and phase is matched to a tighter degree, such as to approx. +/−15 degrees. Another all-pass filter 304 is applied to the left array input and phase-matched (again within +/−30 degrees) to the right low-pass filter 306 to keep the cross-feed signal in phase with the primary signal. The null formed by the combined outputs of the left transducers 104 and 106 is restricted to the frequency range of 1 kHz to 3 kHz due to the operation of the filters 202 and 310. In other words, for a left input signal 204 within the frequency range of 1 kHz˜3 kHz, the left array 100L independently achieves a null along the null axis 112. For a left input signal 204 in the frequency range below 1 kHz, the left outer transducer 104 and the right outer transducer 110 together combine to form a null along the null axis 114. A right signal can be processed in a similar fashion.
The low frequency performance of this system can be enhanced by using the inner transducers in combination with their corresponding outer transducers in a selected frequency range, for example, a frequency range lower than the frequency range described earlier where only the outer transducers were operating (for example, below 550 Hz). As shown in
As shown in
In
For left channel signal below 550 Hz, as shown by row 602 and
In the range of 550 Hz to 1 kHz of the left channel signal, shown by row 612 and
The null along the null axis 112 in the range of 1 to 3 kHz for the left channel signal is produced from the left transducers only, as shown in row 622 and
Above 3 kHz, as shown in row 632 and
In general, by using the respective elements of each individual array to independently control that array's radiation pattern at higher frequencies, and using both arrays jointly in some manner to control the radiation pattern of the combined array output at lower frequencies, efficiency can be maintained or improved at low frequencies and directivity controlled over a wider frequency range. Since the widely-spaced arrays improve total system efficiency, the system can deliver more power at low frequencies, compared to a system that only used each array to control its own side's signal.
As noted above, similar techniques can be used to deploy arrays having any number of transducers. The details of frequencies to filter, which signal to invert, shift, or delay, and where to position the transducers will depend on such factors as the number of transducers, characteristics of the transducers, the output desired, the environment where the arrays are to be used, and the power output capability of each transducer.
Other embodiments are within the scope of the following claims.
