Column loudspeakers have seen a resurgence of use, especially in the installation world. While the basic design dates back to venerable products like the Shure Vocal Master, advances in driver technology put modern column loudspeakers in a very different class from their historic cousins. Broadly, column loudspeakers consist of arrays of many small, closely spaced drivers. These may be full range drivers or multi-way systems. Column loudspeakers are, by nature, tall and narrow. They are usually installed in a vertically-oriented position, such as against the face of a proscenium, or next to a whiteboard.
Column loudspeakers, with their small visual footprint and minimal space requirements, are increasingly the correct solution for situations such as classrooms, houses of worship, museums, digital signage displays, theme parks, boardrooms and train stations. In many of these examples, aesthetics trump sonic considerations. The consultant, integrator, installer, or sound company is left to pick the best overall sonic compromise. That’s not to say they’re necessarily poorer performers than traditional loudspeakers — many modern columns offer extended frequency response and can solve many pattern control problems for speech and light music.
At the high end of the column loudspeaker market are advanced loudspeakers in which each driver has its own amplification and processing. These arrays often allow for tailoring of the in-room response, using beam steering, allowing custom response for the room or even coverage that is switchable between several presets. At the low end of the market are simple columns where all the drivers receive the same signal. Mid-level systems typically utilize multiple arrays of different sized drivers, sometimes with some slight array curvature and/or array tapering, to try and get the most consistent pattern throughout the response range of the column. All three types of systems have their place and usage cases where they can be effective.
In this article, we discuss the physical behavior and use of column loudspeakers that do not have individual driver amplification and processing, but rather where each array of close-spaced identical drivers is driven and processed as a whole. This represents the most affordable and commonly-installed type of column loudspeaker. From this article, you will learn how these loudspeakers have advanced, the way they direct sound and how to properly locate them in a venue for even coverage and frequency response.
Not Your Father’s Column
The physics principles of closely-spaced drivers in a column aren’t new. Indeed, they are covered extensively in Leo Beranek’s 1954 reference book, Acoustics. Professional loudspeaker drivers (i.e., transducers), however, have improved tremendously from the days of Acoustics or the Vocal Master, and the small drivers in modern column loudspeakers are beneficiaries of most of these transducer advances.
Driver design has progressed tremendously over the last four decades. Loudspeaker driver improvements have touched virtually every aspect of their performance. While these improvements could easily comprise multiple articles of their own, here are several specific examples that help the performance of small drivers:
• Computer modeling of cone profile – Computers have enabled driver designers to refine and improve the profile of loudspeaker cones for smoother response, fewer resonances and extended frequency response.
• Laser measurement – Laser interferometry allows driver manufacturers to visualize the specific behavior of each location on the cone surface. This allows them to spot resonances, rocking behavior, or problems with speaker surrounds.
• Improved voice coils – Voice coils consist of thin, insulated wire wrapped around a support structure. The support structures, also known as formers, now utilize (literally) space-age polymers and adhesive to be able to withstand much higher temperatures. Further, the way wire is wound on voice coils has improved magnetic strength, linearity and heat dissipation.
• NdFeB magnets – “Neo” permanent magnets, originally invented by General Motors and Sumitomo Chemical in the early 1980s, have revolutionized the size to strength ratio of driver magnetic structures. Small drivers today have stronger magnet systems than ever because of neodymium magnets.
• Electromagnetic modeling – Computers now allow accurate prediction of magnetic field behavior inside complicated shapes near a moving voice coil. This also improves driver “motor” strength and reduces distortion.
The list above is by no means comprehensive, but it provides a portrait of why today’s small loudspeakers can play louder, with less distortion and higher fidelity than their predecessors. The frequency extension and amount of output available out of a modern four-inch driver is very impressive.
The rising tide of improved driver design has raised the boat of even the venerable column loudspeaker. Columns are now lighter, handle more input, have smoother response and provide more output than their grandfathers. In particular, performance at extreme high frequencies, or extreme low frequencies, has seen the most improvement.
The updated high and low frequency performance of the drivers that make up column loudspeakers only serves to highlight the compromises in behavior that a physics imposes on a vertical array of drivers. To understand this more fully, we now unpack some of the aspects of individual driver performance followed by driver performance in a vertical column.
Individual Drivers and Horizontal Coverage
The performance limitations of individual drivers at low frequencies are somewhat intuitive. Larger driver cones can physically couple better to the air, and larger drivers generally have a longer voice coil that allows them more excursion (i.e., they can move in and out further). Larger cones plus longer movement yields more low frequency output. Large drivers generally aren’t required to produce high frequencies, and therefore the additional mass of a high excursion suspension and voice coil is not of much concern.
Full-range drivers, on the other hand, have the contradictory requirements of needing to be small and light to reproduce high frequency information while requiring extended excursion for low frequency output. The mass of the voice coil former and voice coil wire is quite appreciable relative to the mass of the cone and suspension for a small driver, so there are always trade offs in low-frequency output for high-frequency behavior. Columns that have a second array of even smaller high frequency drivers (i.e., tweeters) can have an advantage, as the tweeters bear responsibility for high frequency reproduction and the burden for full-range reproduction is reduced.
Other limitations of individual driver performance on column behavior are subtler. At low- and mid-frequencies, the wavelengths of sound are several feet long — much bigger than the driver diameter. At 1000 Hz, the wavelength is about a foot, or approximately three driver diameters for a typical column loudspeaker. 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 begins to narrow. As frequencies get higher, the driver becomes progressively more directional. Figure 1 shows this effect graphically.
The driver in Figure 1 becomes more directional because of phase, specifically the phase difference between sound that comes from different points on the speaker cone. Imagine that you are standing directly in front of the speaker driver, listening. When you are standing directly in front of the speaker, the arrival time of the sound from the driver cone to you is uniform, as you are the same relative distance from the cone.
Now imagine standing off to the left of the loudspeaker driver and listening. Sounds from the left side of the speaker cone will arrive slightly sooner than sounds from the right side of the speaker cone. This is because you are farther from the right side of the speaker cone than the left side. At low and midrange frequencies, this difference in distance has little effect. The wavelengths are very long, and therefore, the phase difference is minimal. However, when the wavelengths become shorter, comparable to the dimensions of the cone, the phase difference is quite high, and the directivity becomes narrow.
Practically speaking, this means that the horizontal coverage of a column loudspeaker will grow increasingly narrow above the frequency whose wavelength is equal to the diameter of the speaker cone. In the case of a four-inch loudspeaker, that will be 3375 Hz (i.e., 3.3kHz). Thus, a column loudspeaker composed exclusively of four-inch full-range drivers will have very broad horizontal coverage through the range of speech frequencies, but the higher frequency reproduction will become increasingly narrow. This means that a column composed exclusively of four-inch full range drivers could be well suited to project a professor’s voice to the corners of a lecture hall, but would not be ideal for full range music reproduction.
Now that we understand how the horizontal coverage of the column loudspeaker will change at high frequencies, it is time to turn our attention to the vertical coverage behavior of column loudspeakers.
Column Arrays and Vertical Performance
Largely because of the rise of line array systems, there has been much discussion of the behavior of sound emitted from vertically arranged sources. Unfortunately, a fair amount of that discussion has been tainted with marketing hype or over-simplifications. In the case of column loudspeakers, all of the good and bad aspects of vertically spaced sources are clearly on display.
Discussions on vertically-arrayed sources typically begin with describing their behavior up close, usually at a distance no greater than the height of the array. Then the discussion moves to ever-farther distances from the array. Conceptually, however, it is much easier to understand the behavior of a vertical array by starting at a point very far from the array, and then moving closer.
Just like sun, a very large object up close, appears as a small round ball in the sky, a tall vertical array looks like a small dot when we move far enough away. At this extreme distance, every driver in the vertical array is essentially the same distance away from us, and the array’s sound behavior is much like standing directly on axis of an individual driver like we discussed above. Technically stated, “from far enough away, every array is a point source.”
As we move closer and closer to the array, the relative distances between each individual driver and our ears increases, because of simple geometry. The drivers at the far ends of the array are farther from us, and therefore increasingly out of phase at the listening position at higher frequencies. The result is analogous to the effect of listening off axis to a single driver (as discussed above).
There are two important effects that result from these phase differences. First, at high frequencies, where the array is many wavelengths tall, there is a “beam” of in-phase energy directly on axis with the array. Above and below the array, the phase differences at higher frequencies cause the drivers to cancel almost completely, resulting in very narrow vertical coverage that causes high frequencies to essentially disappear once you move above or below the array’s height.
The second effect we experience when moving closer to the array is a high frequency roll-off, due to the phase of each high frequency arrival becoming increasingly spaced in time due to array geometry. The wider the spacing between drivers, the more pronounced this effect becomes.
Consider that doubling the number of sources that are perfectly in phase adds together as to provide a 6dB increase in output. That is essentially what happens with a column loudspeaker at midrange frequencies, because the total phase difference between the arrivals from each individual driver is comparatively small.
Contrast this effect with doubling the number of sources that are not completely in phase. It can be shown this only provides 3dB more output. This is exactly what happens at high frequencies as the spacing between drivers in the column insures that the drivers cannot sum completely in phase at the listener.
The net result is that the high frequency response, when measured on the axis of the column, is attenuated relative to the midrange frequencies. This apparent high frequency “droop” requires either passive equalization circuitry inside the column’s crossover network, active equalization provided by external processing, or a bank of HF drivers. All column loudspeakers, to some extent, will need compensation for this effect.
Low Frequency Performance
Just as column speakers need processing to balance their high frequency response, the very lowest frequencies are not immune from effects of the vertical array. At low frequencies, the array is not physically tall enough to avoid spilling bass frequencies above and below the array. It takes a very tall array to narrow the vertical coverage in the low frequencies.
A consequence of low frequencies spilling above and below the main axis of the array is that less bass energy is directed out into the audience on axis with the array. This manifests itself as a roll-off in the on axis response of column’s low frequencies.
An un-equalized column loudspeaker — that is, one that does not boost the low frequencies and high frequencies to balance the axial response — will typically have good reproduction in the range of speech, but will be thin and dull for music. Both low and high frequency boost will usually be needed for balanced full range music reproduction. This should not be viewed as a problem with a product, but rather as a consequence of the physics of the array.
Column Loudspeaker Placement
The first, most practical advice when designing column loudspeakers is to utilize the tallest column possible. Taller columns are capable of more output, have better control of low frequencies, generally require less low frequency equalization, and usually provide more even coverage. Most sound reinforcement columns allow for joining multiple modules together to create columns of arbitrary height. Always utilize the tallest column the customer will accept.
The column should be made tall enough that every audience member’s ears lie within the axial height of the column array. Ideally, it is better for the column’s height to extend somewhat below the ear level of the lowest listener and above the ear level of the highest listener. This is because the main on-axis coverage lobe is slightly shorter than the total array height. Extra height ensures the listener is directly in the main coverage lobe.
Since practical loudspeaker placement will frequently preclude the very tall columns advocated here, it is important to mention that column loudspeakers have some pretty strong “side lobes” above and below their on-axis coverage response. These lobes can be seen in Figure 1. The lobes are frequency and angle dependent, produce very uneven response, and cause havoc when designing for high gain-before-feedback (GBF). It is generally best to avoid placing audience members, or vocal mics, directly below the primary axis of a column array. Ideally, the lobes will be absorbed by architectural features above and below the array, leaving the comparatively even response of the on-axis column coverage zone. Always endeavor to make sure the lobes below the column are not aimed on the audience, or at open vocal mics.
Conclusion
Used with the guidelines discussed above, column loudspeakers can be very effective at providing even audio coverage with minimal visual impact. A tall column loudspeaker array also has the intangible psychoacoustic benefit of sounding very “big” and spacious to the audience. I have personally seen this used to great effect in a theme park installation, and it is a useful addition to the bag of tricks for other types of installations, such as museum exhibits. Column loudspeakers are back again, and will provide great audio performance in carefully-chosen situations for many years to come.
Figures are courtesy of Leo Beranek’s reference standard, Acoustics, 1954; www.leoberanek.com/pages/acoustics.html.
Phil Graham, a principal of PASSBAND, llc in Atlanta, GA, a professional audio consultancy, started building subwoofer arrays more than a decade ago. Email him at: [email protected].
