SVC on Twitter    SVC on Facebook    SVC on LinkedIn

Related Articles

 

Technology Showcase:
Focused Loudspeaker Systems

Nov 1, 2005 12:00 PM, By Bruce Borgerson

Directing sound to small groups.


   Follow us on Twitter    

Brown Innovations Localizer

Most commercial loudspeaker systems are directional, at least to some extent. Line array principles and horn-type waveguides are useful in aiming loudspeakers to cover portions of a seating area or to minimize interference in clustered applications. Yet they are significantly less effective when it comes to focusing sound to one isolated point. To illustrate, when compared to the tightly focused light emitted by a stage follow-spot or a laser beam, the majority of loudspeakers can claim to be no more than relatively directional at best.

The loudspeakers profiled here are a somewhat different breed. When used in the near field (distances of 7ft. or less) they typically cover an area suitable for an “audience” of only one or two listeners, or a few more if they are close friends. Some of these devices can project sound distances of 160ft. or more, but here the idea is to spotlight higher frequencies (usually to compensate for air loss) in relatively limited areas where placement of distributed or delay speakers is impractical.

THREE SOLUTIONS

Focused sound products currently available on the market fall into three broad categories: reflector-based, ultrasonic, and beam steering.

Reflector-based products are the most familiar and enjoy widespread popularity in museum and trade show exhibit applications. All take advantage of the acoustical properties of three-dimensional curved surfaces, although the specific properties of each product vary with the exact type of “dishware” involved. Waves generated at the focal point of a parabolic reflector, for example, defy the inverse square law by falling off at only 3dB per doubling of distance — at least for a while. In comparison, hemispherical and hybrid domes are less effective at greater distances, yet these shapes can be used to generate stereo effects within a defined listening space.

The reflector approach is not without its disadvantages, however. The beaming effect is much more pronounced at higher frequencies, and the frequency response within the working bandwidth tends to be irregular because dishes of different sizes and shapes reflect some frequencies more efficiently than others. On the positive side, reflector-based systems are the simplest to set up and maintain and usually fall into the lowest cost bracket.

Museum Tools Secret Sound

The second approach, modulation of ultrasonic sound, employs the heterodyne principle — a concept long familiar in the radio frequency domain. First identified by Helmholtz, the underlying physics describe how two audio frequencies emitted in the same space will generate additional frequencies that represent the sum and difference of the two originals. That means that two ultrasonic frequencies can be modulated in such a way that the difference between the frequencies falls into the audible spectrum. Because the inaudible, high-frequency carrier wavelengths are so short, the sound travels in a narrow and tightly defined beam. When that ultrasonic beam strikes a solid object — either the eardrums directly or some exterior reflective surface — the difference signal emerges as sounds in the audible frequency range. Although the concept is elementary, making an ultrasonic sound beam work in practice involves a number of engineering hurdles.

The primary benefit of the high-frequency beam lies in its ability to project a very narrow beam over significant distances, spotlighting a chosen area while remaining inaudible in free space. In short-throw applications, sound can be focused for a single listener — in front of a video screen, for example — and remain inaudible a few inches away, as long as other surfaces in the beam's path (such as floors) provide near-total absorption. If the surface is reflective, then the sound becomes audible and appears to originate at that surface and not at the beam emitter itself.

The downside is a single device cannot do stereo, and using two beams to create the effect could prove problematic, not to mention expensive. Also, some listeners may find the effect of a direct beam disconcerting, as the apparent sound source is inside the listener's head.

The third technology uses arrays of multiple loudspeakers, with the signal to loudspeakers delayed by varying amounts to achieve what is popularly known as “beam steering.” The most common application of the principle involves linear arrays, a technique that allows the sound to be focused to varying degrees in the plane perpendicular to the array. (It also allows the coverage to be shifted up or down relative to the axis of the array.) However, some manufacturers now are offering two-dimensional arrays, a technique that allows the sound to be steered in both planes, converging if desired toward a single, adjustable focal point.

Because devices of this type use direct radiating drivers working in their “native” bandwidth, audio performance is generally superior to the other two approaches. Low-frequency extension and flatness of frequency response remain close to that of the drivers employed. Longer-throw performance is quite good as well, particularly with the “steerable” line array products and one higher-power, two-dimensional array. However, a single array cannot accommodate stereo, and system costs are on the high side because of the number of drivers and amplification channels required.

Holosonic Audio Spotlight

HEMISPHERICAL REFLECTORS

Brown Innovations of Chicago manufactures four Localizer hemispherical dome systems. The units employ the company's patented Virtual Audio Imaging (VAI) technique to reflect stereo sound from two drivers down from the dome to a fixed focal point in the space below. When the system is properly positioned, the listener experiences the left and right channels as if they were floating just outside each ear.

Hemispherical domes possess an inherent advantage in sharply focusing sound in the near field, and also in projecting a stereo image. Efficiency and bandwidth are good within the focal range, with the company claiming 95dB to 100dB SPL and a frequency response of as low as 80Hz for some models, but typically closer to 150Hz. However, hemispherical designs do require placement within a relatively limited range of distances to achieve optimal performance and the desired stereo effects.

The Localizer line includes models using both 20in. and 32in. domes, with respective coverage areas suitable for one, two, or four or more persons. With the three 32in. models, the coverage area, amount of sound isolation, and stereo separation are determined by driver type and arrangement of drivers within the dome and by the dome mounting height. Typically, stereo separation is between 6dB and 10dB, with a dropoff of 9dB to 12dB at a distance of 2ft. from the focused field. Dome prices range from $499 for the 20in. Mini to $1,399 for the WD (Wide Dispersion) model. A complementary Virtuoso amplifier ($179) includes EQ curves tailored to produce optimum response.

HYBRID DOMES

SoundTube Entertainment of Park City, Utah, offers two similar systems, each based on the company's patented Dual Parabolic design: the 20.25in. FP6020 and the 30.75in. FP6030. Sound Tube's dual-curve approach is designed to leverage the inherent projection characteristics of the parabolic reflector while modifying lower curvature angles to generate a reasonably coherent stereo image with some latitude for height adjustment. Domes are molded from PETG clear polymer, and both versions employ the same dual 1in., high-excursion, convex aluminum drivers. Because of the nature of the beast, coverage patterns vary with frequency. For the FP6030, the company specifies a coverage angle (-6dB at 2kHz) of 41 degrees averaged, with a Q of 14.23 and directivity index of 26.48 at that same frequency. Corresponding specifications for the FP6020 are a coverage angle of 43 degrees, a directivity factor of 27.9, and a directivity index of 14.5. Suggested list prices are $420 for the FP6020 and $470 for the FP6030.

Meyer Sound SB-1

PARABOLICS: NEAR AND FAR FIELD

Museum Tools in San Anselmo, Calif., has been marketing the Secret Sound parabolic transducers for nearly two decades, and the system has become a familiar fixture in scores of museum exhibits. The simple, 30in. dome design employs a single 2in. cone driver set in a molded composite housing at the focal point of the dish. Efficiency is high, with the manufacturer specifying only 5W of input power. Frequency response is given as 500Hz to 18kHz, with a high-pass filter kit included to remove unwanted sounds below the stated bandwidth in music program material. Though stereo is not available, the pure parabolic design of the Secret Sound dome makes it very forgiving in regard to flexible mounting height.

The Secret Sound system is available in clear ($695 list), white, and black ($645 list each) versions.

The Meyer Sound entries in the parabolic field are the odd ducks among the dishes, if we may mix metaphors. Both are significantly larger, and significantly more powerful, and thus are intended for much longer-throw applications.

The SB-1 is a hybrid transducer design, working as both a parabolic reflector and a direct radiator. The high-frequency component, housed in a pod at the focal point and aimed back at the dish, employs a 4in. diaphragm coupled to an aspherical horn aimed at the reflector. A single, band-limited 12in. cone driver embedded at the center of the 54in. dish fires forward. The entire system is self-powered (2×620W), allowing use of complex phase manipulation processing to minimize side lobing. Frequency response is given as 500Hz to 15kHz, with an extraordinary 10-degree (-6dB) coverage pattern and maximum SPL of 110dB, at more than 300ft. The list price of $16,710 implies a limited market, but the unit has found its niche in applications such as targeting the far corners of football stadiums as part of a point-source cluster.

Meyer Sound's SB-2 is the odder of the two ducks, being a parabolic device that does not reflect sound. Instead, 28 4in. cone drivers are mounted on the face of a parabolic surface in order to focus — without recourse to digital delays — far more acoustic power at lower frequencies than can be accommodated by a reflector. However, because cone drivers make poor reflectors, the high frequencies generated by a single 4in.-diaphragm compression driver must be focused by a deep, 20-degree symmetrical horn. Operating frequency range is given as 130Hz to 18kHz, with -6dB points of 20 degrees from 1kHz to 16kHz and 40 degrees at 500Hz. The list price of $12,880 again suggests specialized high-end applications. For example, SB-2 units serve as concealed front-fill loudspeakers at Celine Dion's Colosseum in Las Vegas.

ULTRASONIC SOLUTIONS

Holosonic Research Labs (Watertown, Mass.) was the first to bring this technology to the installation market, with its Audio Spotlight systems. Standard sizes are given as 18in. and 24in., though custom sizes are offered as well. The transducers are thin (about 0.5in.) and relatively lightweight at about 4lbs. each. Normal coverage patterns hardly apply here, as a plot of the beam looks like water coming out of a high-pressure fire hose. Isolation is extreme, with levels dropping by 20dB 2ft. in either direction in most indoor applications. Even though power is not exceptional (about 100dB at 6.6ft.) the beam maintains a useable range up to 66ft. and is considered audible at 660ft. High frequencies demodulate effectively up to about 16kHz; the low frequencies involve a tradeoff with output power, with a practical usability limit of around 200Hz to 400Hz, according to the company. All units include the accompanying amplifier processor. The AS-18 is priced at $1,999, with quantity discounts and dealer pricing available.

A second player in this emerging niche, American Technology (ATC) of San Diego, has scheduled shipment of its debut product, the HSS H450, for the fourth quarter of 2005. Smaller than the Audio Spotlight at 12"×6"×4", the HSS unit also exhibits a lower output power of 85dB SPL at 7ft., though it drops to only 60dB at a distance of 50ft. The coverage area is sharply defined within a diameter of about 1ft. at a distance of 7ft., expanding to a 5ft. circle at 26ft. Projected list price for the H450 is $699.

DSP BEAM SHAPING

Dakota Audio in Bismarck, N.D., has entered the near-field exhibit fray with a customizable range of circular, multi-speaker arrays that use differential delay times to focus sound. This two-dimensional beam-steering technique is accomplished by arranging multiple drivers in a specified pattern and then delaying the drivers in groups to achieve focus at the desired distance.

A typical company product is the FA501, a 36in. disc that incorporates 120 long-excursion, 2in. cone drivers. The drivers are arranged in eight concentric groups, with the signal to each group delayed (by the included digital processor and 8-channel amplifier) to achieve the desired focus distance.

Dakota Audio claims a fairly flat response of 80Hz to 15kHz, with low and low-mid frequencies undoubtedly better served by this approach. Peak SPL at about 3ft. is given as 95dB, with sound falling off by 28dB at about 5ft. away from the center axis. The price for the FA-501, which offers user-selectable preset focal lengths, is $1,890. Smaller units, and units with fixed focal lengths, are available at lower price levels.

Duran Audio (Zaltbommel, the Netherlands) takes the idea up a step or two in power and performance with its Intelli-Disc, a high-power, long-throw variant based on the same concept. The 37in. circular array contains 32 4in. wide-range drivers and nine dedicated high-frequency drivers, with power supplied by four 360W amplifiers. Performance parameters are given as 95dB peak SPL at around 100ft., with a 20-degree coverage angle. The custom-order Intelli-Disc is priced around $15,000.

STEERED LINE ARRAYS

The primary focus, pardon the pun, of this technology showcase is on products that use reflectors or other means to “spotlight” sound in two dimensions. However, mention should be made of other products — and certainly more are to come — that use sophisticated beam steering in higher-power arrays to focus and direct a sonic “wedge” of variable height that can be aimed up or down.

All of these systems work according to the same principle: Each driver in the line array is powered by its own internal amplifier, with software-controlled DSP selectively delaying individual drivers in the array to achieve the steering and focusing effects. In most cases, the software control program and onboard DSP also offer comprehensive signal compression and equalization capabilities.

Duran Audio was the first to offer this technology in commercial systems, introducing the Axys Intellivox arrays in 1996. The current line encompasses seven systems, ranging from the Intellivox 1b (six 4in. drivers and two dome tweeters) up to the 16ft.-long 6c, (32 4in. drivers).

The DSA series digitally steerable array loudspeakers from EAW offer similar functions using two complementary modules, the two-way DSA250 (8x 4in. and 8x 1in. drivers) and DSA230 (8 4in. drivers). As with the Duran Audio units, the control software easily accommodates stacking of multiple units of the same model or combinations of the two to achieve more power and greater beam-steering functionality.

The Renkus-Heinz entry in the field is the IC8 module. As with the EAW two-way system, the IC8 incorporates eight each of 4in. and 1in. transducers, although in this case the two drivers are combined in coaxial units. Software and DSP features are generally comparable, though as the fresh newcomer into the market, some IC8 specifications and details still are preliminary.

Manufacturer list prices for the steerable line arrays start in the neighborhood of $4,300, not only because everything is included — processing, amplifiers, and control software — but also because they can provide a degree of pattern control flexibility far beyond that available from conventional line arrays.


For More Information

American Technology
www.atcsd.com

Brown Innovations
www.browninnovations.com

Dakota Audio
www.dakotaaudio.com

Duran Audio
www.duran-audio.com

EAW
www.eaw.com

Holosonic Research Labs
www.holosonics.com

Meyer Sound
www.meyersound.com

Museum Tools
www.museumtools.com

Renkus-Heinz
www.renkus-heinz.com

Sound Tube Entertainment
www.soundtube.com


Bruce Borgerson is principal of Wavelength Communications (Ashland, Ore.), a writing and consulting firm for the AV industry.



Acceptable Use Policy
blog comments powered by Disqus

Browse Back Issues
BROWSE ISSUES
  September 2014 Sound & Video Contractor Cover August 2014 Sound & Video Contractor Cover July 2014 Sound & Video Contractor Cover June 2014 Sound & Video Contractor Cover May 2014 Sound & Video Contractor Cover April 2014 Sound & Video Contractor Cover  
September 2014 August 2014 July 2014 June 2014 May 2014 April 2014