Patent Grant
8406445
The present invention generally relates to loudspeaker systems and more particularly to full range directional loudspeaker systems. The invention has particular application in cinema sound reinforcement systems.
A common goal in loudspeaker system design is to achieve a constant beamwidth, or directivity, in both the horizontal and vertical planes over the loudspeaker's operating frequency range. Often the desired beamwidth in the horizontal plane is kept relatively wide (70 to 100 degrees) in order to direct sound uniformly over the width of the room or audience from a single acoustic point. Within this range, the desired horizontal beamwidth will depend on the width of the room, reflective properties of walls in the room, and the location of the loudspeaker. For example, a horizontal beamwidth at the low end of the range will avoid destructive acoustic reflections from nearby walls in the room, whereas a horizontal beamwidth on the high end of the range can be used where the walls of the room are acoustically absorbent or not in destructive proximity to the sound field. In either case, it is desirable to create a beam of sound having a horizontal beamwidth that meets the room conditions but yet is sufficiently large to cover the entire audience to its outer edges over the operating frequency range of the loudspeaker.
To accommodate different room conditions two different horizontal beamwidths are sometimes offered in a loudspeaker system. This is accomplished by providing two different waveguide horns, either of which can be installed into the loudspeaker enclosure. In that case the waveguide horns are kept similar in all respects except for the shape of the horizontal expansion of the horn. The horns with different horizontal expansions produce polar patterns with different horizontal beamwidths, and can be designed to produce horizontal beamwidths in a range between 70 to 100 degrees.
The desired beamwidth in the vertical plane is usually a much smaller angle than the horizontal. This is because the depth of the seating for an audience as seen by the loudspeaker is normally relatively shallow and only requires a relatively small vertical beamwidth angle to achieve the desired coverage. A vertical beamwidth of 40 to 50 degrees is often desired to concentrate acoustic energy to the audience and prevent acoustic energy from spreading elsewhere in the room.
Beamwidth is typically defined as the angle at which the magnitude of the acoustic pressure wave is 6 dB lower than the measured pressure on-axis to the loudspeaker. It is understood by those of ordinary skill in the art that, if the pressure has not attenuated more than 6 dB over a range of seats, the sound will generally be observed as similar to the on-axis response, provided a large portion of the frequency band attenuates equally. This is considered uniform coverage in the field of acoustics.
To obtain this goal, the beamwidth must be kept constant over the widest possible range of frequencies. In a two way loudspeaker system a crossover is used to split the full range audio signal into a high frequency channel and a low frequency channel. The high frequency channel uses a waveguide horn to control the vertical and horizontal beamwidth. To obtain a wide range of frequencies where the beamwidth is kept constant, the waveguide horn is made large to work well at long wavelengths and the crossover frequency is set at a relatively low frequency. A dimension of 24 inches by 48 inches at the face of the horn is not uncommon to obtain directional control down to 1000 Hz. When the angle of beamwidth control is small, such as 40 degrees as required in the vertical plane, the horn must be made physically deeper and larger to obtain control at the lower end of its operating frequency range.
The above-mentioned approaches to achieving beamwidth control in a waveguide horn have significant drawbacks certain applications, and particularly in cinema applications. In cinemas applications the loudspeakers are placed behind a cinema screen, and using loudspeakers having large or deeper waveguide horns can detrimentally affect the beamwidth and frequency response of the loudspeaker system due to waves reflecting back and forth between the screen and the surface of the horn. Reflections from the back of the cinema screen propagate back onto the walls of the horn causing the sound waves to reflect in undesirable directions. Sometimes referred to as beam spreading, this phenomenon degrades beamwidth control. The present invention overcomes the drawbacks of these previous approaches by permitting the use of a smaller horn behind a cinema screen while maintaining the desired beamwidth control in the vertical and horizontal directions. Because of this, the affect of the screen on the system's frequency response and beamwidth control over a range of frequencies is substantially reduced or eliminated.
The present invention is directed to an at least two-way loudspeaker system comprised of a high channel transducer attached to a waveguide horn, a low channel cone transducer mounted in close proximity above or below the high channel waveguide, both of which are mounted in an enclosure, which can be either a sealed or vented enclosure for operating the low channel transducer over a suitable frequency range. A high channel signal processing circuit and a low channel signal processing circuit are provided to control the relative phase and magnitude of the acoustic waves propagating from each transducer. The high channel processing circuit and low channel processing circuit are designed to allow the horn to operate below its normal cut-off frequency, that is, below the point where, due to its physical size, the horn would cease being directional (for example, around 1.5 kHz). The signal processing circuits for the hi and low channels also control the beamwidth of the combined acoustic outputs of the horn and cone driver in a frequency range below the horns normal inherent cut-off frequency, for example, down from 1.5 kHz down to about 500 Hz. The result is a full range loudspeaker system having a controlled beamwidth over substantially its entire frequency range.
The invention can be practiced in different audio frequency bands by selecting and appropriately scaling all the physical geometries of the horn and transducers and signal processing. For example, the horn of the loudspeaker system can be physically designed to provide constant horizontal and vertical beamwidth control from the highest audible frequencies (approximately 15 kHz) down to the mid audio frequencies (approximately 1.5 kHz), with a wide horizontal beamwidth (for example, approximately 100 degrees), and narrow vertical beamwidth (for example, approximately 50 degrees). The waveguide horn can then combined with signal processing, a low channel transducer, and an enclosure to produce an extended low frequency narrow beamwidth (down to about 500 Hz), which is otherwise not achievable by the waveguide horn alone.
Referring now to the drawings,
The horn 13 and low driver 17 are mounted to the front baffle wall of the enclosure in close proximity to each other. Selection of the spacing between the horn axis and the center of the cone driver is important to the ability to extend control over the beamwidth of the horn down into lower frequency ranges. Generally, where the spacing is too great, the ability to control beamwidth at lower frequencies with signal processing as described below will be lost. If the spacing is too close, distortion will be introduced into the acoustic output of the loudspeaker. To permit an optimal axis-to-center spacing that might not otherwise be possible due to mechanical interference between parts, the lower edge 33 of the mouth of horn 13, which includes outer horn's flange 23, can be provided with a cut-out 35 into which the perimeter rim 16 of the cone driver can be fitted. Preferably, this perimeter rim has a radius that conforms to the radius of the cone driver's perimeter rim.
It can be seen that the ported loudspeaker enclosure shown in
The shape (curvature) and size of the vertical and horizontal expansion walls 39, 41 of horn 13 will control the horn's inherent vertical and horizontal beamwidth over portions of the operating frequency range above the horn's inherent vertical and horizontal beamwidth cut-off frequencies. As used herein, the inherent (or normal) horizontal beamwidth cut-off frequency is the lowest frequency the horn can operate by itself before there is a loss of horizontal beamwidth control. The inherent vertical beamwidth cut-off frequency is the lowest frequency at which the horn can operate by itself before there is a loss of vertical beamwidth control. While cut-off frequencies cannot be precisely determined, they can established qualitatively by looking at beamwidth versus frequency graphs generated by acoustic measurements of the horn by itself outside of an enclosure and without signal processing.
The performance of the horn can further be described in reference to the to-scale exemplary horn shown in
The expansion shape and size of the horizontal expansion walls 39 can similarly be chosen to allow for a relatively constant vertical beamwidth above the inherent cut-off frequency for the horn's vertical beamwidth. Here, the depth of the horn, denoted “dh” in
With respect to the exemplary horn mentioned above,
Further referring to the beamwidth versus frequency plot in
To obtain extended vertical beamwidth control down to lower frequencies, the low frequency transducer 17 of the loudspeaker 11 shown in
The above horn axis to driver center spacing (s) was determined empirically and was chosen because it provided a spacing between the horn 13 and cone driver 15 that allowed the greatest control over the vertical beamwidth of the horn below the horns natural cut-off frequency using signal processing. Because this spacing causes the rim of the cone driver to overlap the horn's lower edge 33, the lower edge is provided with cut-out 35 as above described. It is contemplated that in most cases of a two-way loudspeaker system in accordance with the invention, the spacing (s) between the low transducer and waveguide horn will be less than 15 inches.
In the present invention the low driver is made to operate together with the high horn loaded driver with signal processing below the normal cut-off frequency of 1.5 kHz to obtain extended vertical beamwidth control.
The relationship between the phase and magnitude for the high channel and low channel of the exemplary loudspeaker system can be seen by
Generally, the amplitude and phase response of both the low and high channels can be created by various combinations of filters and signal processing. A loudspeaker in accordance with the invention will function as intended as long as a magnitude and phase relationship between the low and high frequency channels is achieved such as shown in
It is noted that the overall amplitude and phase shape of the low and high channels shown in
The exemplary signal processing system used to produce the vertical beamwidth control shown in
Referring to
Other circuit portions (blocks) of the illustrated signal processing circuit perform equalization functions over the operating frequency range of the loudspeaker, including equalization within the cross-over region. In the high frequency channel, block 109 is a second order band reduction circuit portion for the cross-over region, which affects the bandwidth of the cross-over region. Blocks 111, 129 are second order band boost circuit portions for providing equalization at high frequencies. Blocks 113 and 131 are first order low pass circuit portions having high cut-off frequencies and are added to contribute cumulative phase shift. The all-pass circuit blocks 119, 121, 123, 125, 127 are added to restore transient response and don't affect bandwidth. The first order low-pass filter 107 at the front end of the high channel is added to roll-off high frequencies in the ultrasonic region above 20 kHz. Additional circuit portions in the high channel include inverting op amp with gain adjustment 117 before the all-pass filter 119, and the power amplifier 135 at the end of high channel 101.
The low frequency channel of the illustrated signal processing circuit additionally includes band boost and band reduction circuit portions 137, 139 to provide equalization at low frequencies. The band boost circuit section 147 is added to the low channel to produce equalization in the cross-over region and to balance the contribution between the high and low transducers to the overall acoustic response. The second order high-pass circuit section 149 simply rolls off the response at the low end of the operating frequency range. Other circuit portions in the low channel include the op amp 143 for providing gain added before low-pass filter 145 and the audio power amplifier 151 at the end of low channel 103.
It is seen the additional equalization can be provide at the input before the high and low frequency channels 101, 103, such as second order band reduction circuit portion 153.
While a specific embodiment of the invention has been described in considerable detail above specification and accompanying drawings, it is not intended that the invention be limited to such detail except as necessitated by the following claims.
This application claims the benefit of U.S. provisional application No. 61/247,845 filed Oct. 1, 2009, which is incorporated herein by reference.
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| 6513622 | Gelow et al. | Feb 2003 | B1 |
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| 20090226019 | Koren et al. | Sep 2009 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 61247845 | Oct 2009 | US |
