The Line on Arrays
By Will Craig
Multimedia Systems Consultant
Elert & Associates
Ask an eager sound system designer how to solve a difficult sound reinforcement problem in a classroom, and chances are, one of the first ideas that will be proposed will be “line array”. Is line array technology a panacea for all acoustical ills? Or is it a case of everything looking like a nail to a construction worker who loves using a hammer?
Audio system design for small classrooms is a straightforward affair. Need to hear the professor? Ask her/him to speak up. Need to hear audio accompanying a visual presentation? Put one or more small loudspeakers on or near the monitor or screen.
Sound system design for larger classrooms can be more challenging. Besides having a greater number of seats, large classrooms (75-500 seats) often have additional considerations beyond those of the smaller classrooms. On the “plus” side for the audio system designer, larger classrooms often are designed with some thought to acoustics. Sound-reflective materials are used to utilize the reverberation in the front of the room to help boost the level of the spoken word of the presenter in the rest of the room, while sound-absorbent materials absorb reflections off the rear and side walls, helping to increase intelligibility. In many well-designed large classrooms, no sound reinforcement is necessary for a presenter in the front of the room.
Even in situations where no microphone support is normally required, there are some reasons to consider sound system design:
- A professor may have a medical condition that
prevents them from projecting their voice effectively.
- Audio from sources other than the professor, such as
from their computer, iPod, or VCR/DVD source. The sound from these devices
will need to be intelligible and loud enough for all the users in the room.
- Capturing, streaming, and assistive listening applications. These do not pertain directly to sound reinforcement, but do affect decisions regarding microphone provision, placement, and priorities.
Where room acoustics are not good, nobody is happy. Causes of bad room acoustics can include:
- Excess noise within the room due to internal sources
(HVAC rumble and hiss, buzz from lighting ballasts, hum from cooling fans of
projectors, fiber optic network switches, and (ironically enough) audio
amplifiers).
- Excess noise within the room due to external sources
within the building (adjacent elevator equipment room, bathrooms, electrical
transformer, busy/noisy hallways).
- Excess noise from outside the building (trucks,
airplanes, trains, generators, lawn mowers).
- Undesirable/excessive reflection and reverberation,
causing reduced intelligibility.
- Excessive sound-absorption, creating a “dead” room where audio amplification is a necessity.
Enter the line array loudspeaker. The basic concept of line array speakers is that by arranging speaker elements in a vertical stacked arrangement, that the pattern of sound propagation can be controlled more tightly than in a conventional 2-way or 3-way speaker box.
Several manufacturers, including Renkus-Heinz, EAW and Duran Audio, have taken the line array concept further by integrating digital signal processing (DSP) and internal power amplification for each speaker element in the array to create digitally steerable arrays. Using a computer connected to the array, the user can “steer” the vertical beam of voice-range sound up and down. This has the practical effect of being able to precisely direct sound to the areas of a room where the listener’s ears are. Line arrays have wide horizontal dispersion pattern, making them suitable for use in wide rooms.
An additional capability available when configuring digitally steerable arrays is creating multiple lobes on the vertical plane. One lobe could be directed downward, toward the main floor audience, while a second lobe could be directed toward the seats in a balcony. The direction and angle of each lobe can be tweaked to avoid reflective surfaces, such as rear walls or balcony faces.
When evaluating the claims of someone proposing a digitally steerable line-array loudspeaker “solution” to an acoustical problem, be sure to ask questions of your vendor, consultant, or designer to make sure you aren’t being sold the currently fashionable solution irrespective of your needs:
- Do the frequencies that are steerable correspond with
the frequencies that are needed in the application? (Voice-range frequencies
are typically steerable, while higher and lower frequencies are not steerable
to the same extent, making them suitable for lecture halls but perhaps not
ideal for performance venues)
- D'es the width of the room pose a problem for a line
array’s wide dispersion pattern?
- Who will be doing the software-based setup of the
product – the technician who mounted the speaker, the factory rep, or the
salesperson? Will the DSP configurations be turned over to you, the customer,
or will the vendor keep them as proprietary information?
- Has an acoustic model of the room been created and
tested with the selected line array product specs, and how d'es this model
compare with other models using other loudspeaker technologies for coverage
and intelligibility?
- Who will ensure that the final configuration of the
DSP matches the performance and design intent of the acoustical model?
- Can the vendor/designer/consultant explain how line arrays work and why they are the best fit for your application?
Asking some probing questions when it comes to line arrays will help ensure that you are receiving the best solution for your room, not just an expensive flavor-of-the-month.
Will Craig CTS-D, is a multimedia systems consultant with Elert & Associates, a nationwide technology consulting firm specializing in working with higher education and K-12 clients.
