Since the beginnings of professional audio, we have needed more output, coverage, and frequency response than a single loudspeaker transducer could provide. Due to the limitations of loudspeaker drivers, much of the effort in professional loudspeaker design has been expended in combining multiple drivers in a single loudspeaker box, and then combining multiple boxes together into arrays. Even with dramatic increases in modern transducer performance, it would seem that combining multiple loudspeakers together — whether for response, coverage or output — will continue to be a perpetual fixture of the industry.
Unfortunately, the simple placement of boxes in an array means a potential series of compromises in terms of evenness of coverage and deleterious interactions between boxes. At the end of the day, loudspeakers are asked to produce a wide range of wavelengths, from much smaller than the box to much larger. In this article we will highlight some key aspects of acoustics and then speaker performance. We will then combine those insights to help you better understand setting up different loudspeaker coverage zones.
Acoustics Principles
Acoustics are influenced by the physical properties of air: Sound requires a medium to propagate in, a way to get from the sound system to the architecture. For us, that is a fluid known as air, which is composed mostly of oxygen and nitrogen. Temperature and humidity of air can have profound effects on how sound travels to the audience and about the room.
Sound covers a huge range of wavelengths: Wavelength is a measure of how far a periodic signal (like sound) travels in space before the wave repeats. In the range of human hearing, the shortest wavelengths are on the order of one inch, and the longest 60 feet! This huge range of wavelengths is the source of most issues in professional audio, because it is hard to provide the same degree of control over both very short and very long wavelengths.
Room surfaces interact with sound in a frequency-dependent manner: It would be convenient if room surfaces all absorbed sound in a consistent manner, but this isn’t the case. Whether brick or drywall, painted or unpainted, wood floors or carpet, all change how the sound arriving from the speakers is returned back to the room.
Segmenting the Audience
While the logistics of events often get in the way, segmenting the audience vertically within the venue generally makes the most sense. Audience members near to the loudspeakers need comparatively low sound levels, and experience only small amounts of high frequency absorption. Audience members far from the array require greater output, and more compensation for high frequency absorption to maintain even coverage.
Splitting an array into vertical zones, where the upper portion covers distant audience members and the bottom of the array the closer audience, allows for tailoring the sound arriving at each audience cross-section in a more even manner. Further, there is a psychoacoustic advantage to managing inter-box transitions in the vertical plane. Human hearing is much less sensitive to the artifacts of where the boxes overlap in the vertical plane than in the horizontal. Splitting audiences vertically also helps manage common architectural features like the dreaded mezzanine front wall, or spilling sound into an empty balcony.
Despite best attempts to divide the audience vertically, loudspeakers will never behave like a laser beam. Loudspeaker coverage patterns widen and narrow in a frequency-dependent manner, and the performance of each loudspeaker in an array depends on its neighbors. The variable directional performance of loudspeakers is a consequence of the huge range of wavelengths they are asked to reproduce.
Pesky Physics
The physics of acoustic waves dictates that they bend fairly easily around objects whose dimensions are comparable to (or smaller than) a given wavelength. No product or manufacturer has immunity from this effect. You can illustrate this for yourself by taking a loudspeaker and turning it so the drivers face away from you. The low frequencies will still be clearly audible behind the box, as they have bent around the loudspeaker enclosure.
As wavelengths get longer, and frequencies get lower, the ability of an individual loudspeaker to provide directional control decreases. Sound bends around the edges of the box, and spills on the room features around the loudspeaker. Sound also bleeds onto adjacent loudspeakers in the array. This means that the coverage behavior and frequency response of a loudspeaker in the middle of an array is different from a loudspeaker near the top or bottom of the array. It also means that processing applied to individual boxes in the array is not independent, and will influence the entire array’s response. Even further, it means that boxes near the edge of an array should receive different processing to maintain a response consistent with a speaker in the middle of the array!
Array Interactions
A general principle of arrays is that the drivers must be physically spaced in the plane they wish to control directional behavior. If you want to have control over the vertical coverage of an array for a broad range of frequencies, then the array should be as tall as feasible. An array’s physical size must be some appreciable fraction of the wavelength of the lowest frequency where the control is to take place. Once the wavelength becomes much larger than the array, the pattern control is lost. No amount of processing can overcome the limitations behind the relative physical spacing of the drivers with respect to wavelength.
At very short wavelengths, the loudspeaker’s physical size will be sufficient to provide strong control over how sound travels. A useful guideline? Virtually all individual professional loudspeakers retain excellent directional control for frequencies above approximately 4 kHz. This is akin to saying that one can treat each loudspeaker driver as independent from its neighbors above 4 kHz. This also means that the physical processing applied to drivers in an array at high frequencies has less effect on the surrounding drivers or boxes.
By contrast, at low and mid frequencies, the wavelengths of sound are several feet long. This is bigger than the driver diameter. As the frequencies get higher, the driver’s dimensions become comparable to the wavelength of sound being reproduced. In this frequency realm, an interesting effect occurs where the driver’s coverage angle starts to narrow. As frequencies get higher, the driver becomes progressively more directional. Ultimately this is due to differences in the phase of sound arriving from different points on the cone. This phase differential is an inescapable consequence of the driver’s design and the fact that it becomes physically similar in size to the wavelength.
Conceptually, it’s useful to think about the directivity of an array by starting at point very far from the array, and then moving closer. Even a very tall vertical array looks like a small dot when we move far enough away from it. Think of the Sun, which is a very large object indeed, and yet appears as a small round ball in our sky. At an extreme distance, the Sun — or every driver in an array — is essentially the same distance away from us, and the array’s behavior is much like standing directly on-axis of an individual driver. Technically stated, from far enough away, every array behaves like a point source.
As we move closer to the array, the relative distance between each individual driver to the listener increases because of geometry. The drivers at the far ends of the array are farther from us, and therefore increasingly out of phase at a listening position equidistant from the array ends. At high frequencies where the array is many wavelengths tall, there is a “beam” of in-phase energy that is on-axis with the array. Above and below the array, the collective phase differences cause the drivers to cancel almost completely, resulting in narrow vertical coverage that causes high frequencies to essentially disappear once you move above or below the array’s height. Of course there are side lobes above and below, but most of the energy is concentrated in the main beam. As the frequency gets lower, and the wavelength longer, this effect becomes progressively less pronounced as the size of the array gets smaller with respect to the wavelength.
Back to Reality
The datasheet response curve is not what the audience hears: Most manufacturers place a frequency response curve on their datasheet as a measure of the loudspeaker’s performance. In reality that curve provides precious little useful information for the majority of the audience. Datasheet curves are typically taken directly on the main axis of a single loudspeaker. The frequency response they show represents the speaker’s response in the absence of any room influences. Back in the real world, your audience is standing inside of a box of questionable acoustic pedigree, and only a small fraction of them are camped out near the speaker’s main response axis.
There is a range of frequencies where the speakers interact most strongly with the room: As loudspeakers can’t shape the directional response of all frequencies with the same degree of control, lower frequencies will interact more strongly with the room. This is the first half of the speaker and room interaction. The second half of this equation is the absorbing behavior of the room features and surfaces. It is the absorbing character in the midrange (e.g., 150 Hz to 2 kHz) that has the greatest effect on coloring the sound in the room. Sometimes, the most dominant effect on the amount and nature of room absorption is the audience.
Multiple speakers used together radiate sound differently than individual speakers: When the datasheet shows the coverage pattern of a single speaker, and one is using multiple speakers for coverage and/or output, the aggregate output of multiple speakers is not simply a wider version of the single speaker. At some frequencies, the coverage will be wider but at other frequencies it will be narrower. The datasheet performance gives no indication of what the aggregate response of an array of multiple speakers will produce at some point in the venue. At very short wavelengths, an individual loudspeaker’s physical size will be sufficient to provide strong control over where the sound travels, but for most of the spectrum, the result is an aggregate of the full array.
Array Basics
Aim the speakers at the audience: The single best way to improve problems with room acoustics is to insure as much of the sound hits the audience as possible, and is therefore absorbed by them. It is important to remember the enclosure coverage pattern will most likely be the narrowest at the top end of the frequency spectrum, so the overall coverage at 12 kHz is arguably more relevant to getting good coverage than the pattern at lower frequencies.
Improve coverage evenness by using geometry: For typical point-source loudspeaker setups, a straightforward way to insure the audience has even coverage is to minimize the differences in distance between the loudspeaker and each audience member. Usually it is not practical to make these values the same for all audience members, but smaller differences usually result in more even coverage. In most spaces this dictates a vertically high placement of the loudspeaker, so that trigonometry allows similar distances to the first row and the last row. At the small local gig, the speakers are not necessarily going to be flown, but we should still endeavor to place them as high as feasible.
Use the minimum amount of loudspeakers possible — Every additional loudspeaker is another source to dump extra acoustic energy into the room, and those additional loudspeakers interact with each other and the room in complicated ways. As even very large loudspeakers have limited directional control below 400 Hz, using too many acoustic sources is a recipe for muddy low mids.
A Final Word of Caution
Avoid lumping a large number of speakers together to obtain more volume or “throw.” This was all too common back in the days when every box was trapezoidal. The result was usually too much mid-bass build-up and excessive comb filtering in the high frequency coverage. I still see this more commonly than I’d hope at the regional level. Please pick your boxes carefully, and default to using a box with wider coverage before haphazardly combining two narrow coverage boxes.
Phil Graham is FRONT of HOUSE’s regular technical contributor and resident scientist.
At a Glance:
Six Essential Nuggets of Coverage Wisdom
Synthesized together what do all of the guiding principles in this article mean from a practical sense, and what do they look like when applied to a system? There are many possible nuggets here, but here are a few that I find especially relevant when designing and tuning systems:
• Early wall reflections matter: No budget for room-wide acoustic treatment? Try to carve out some dollars for treatment immediately behind and beside the main speaker system.
• Rely on the energy from your main system: Even a large horn-loaded P.A. will have spill below. If the main array covers the audience in the orchestra pit well up to 300Hz, then it’s just fine to high pass the down fills at 300Hz (or above).
• Anchor delay systems to the main system: Same principle as the down fill speaker, but applied other places. Does the main P.A. cover under the balcony below 500 Hz? If so, it’s fine to lean on that and derive those frequencies from the delays.
• Equalization cannot solve spatially dependent issues: If the loudspeaker being equalized over- or under-covers the audience, equalization cannot mitigate how the loudspeakers are aimed. No amount of boost will compensate for a speaker whose coverage pattern collapses above 10 kHz.
• Align coverage zones in the lows and midrange: It makes little sense to get phase alignment between two zones at high frequencies. The wavelengths are too short to maintain that alignment over a large audience area. By contrast, a system that has phase alignment from 200 Hz to 1 kHz does has some chance to maintain that coherency through the whole range where both systems cover the audience.
• Beware low mid energy: For frequencies below 300 Hz, the energy of any one speaker ends up pretty much everywhere in the venue. The more of that energy that arrives from the main system in the main zone, the more coherent the overall sound will be. —Phil Graham