Typically, most professional loudspeakers have been comprised of a low frequency driver and a high frequency driver paired together in the manner common to speakers on sticks. Today though, manufacturers have branched out into many different driver orientations in the search for better performance, better coverage, more output and reduced footprint.
This month we’ll take a practical look at some of the tradeoffs made by manufacturers as they consider the driver configuration of their products. Specifically, we will look at the tradeoffs between using a single low frequency driver and two low frequency drivers. This article is divided into three sections. The first looks at how crossovers are influenced by the choice of one or two woofers. The second part looks at the low-frequency tradeoffs involved between a single driver and multiple drivers. Finally, we consider how power handling is influenced by the choice of one or more drivers.
Crossovers and Multiple Drivers
The core of a loudspeaker crossover is a collection of “elements” capable of routing electrical signals to different loudspeaker transducers designed to reproduce different ranges (i.e., frequencies) of the sound in the most optimal manner. Crossovers exhibit two basic methods of operation that collectively direct the music’s electrical signal to the correct transducer.
The first type of operation is known as a low pass filter. Low pass filters operate as their name indicates. They allow lower frequencies to pass through easily, while impeding higher frequencies. The complement to the low pass filter is termed a high pass filter. High pass filters provide an easy path to higher frequencies, while impeding lower frequencies.
Crossovers are typically configured with multiple high pass and low pass filters, or pairs of filters in sequence (e.g., a high pass followed by a low pass). One feeds each of these filter groups the same input signal, and the filter group provides multiple paths for specific ranges of the input signal. These ranges of frequencies are in turn sent to the correct loudspeaker driver. Regardless of the method of construction, the specific details of how the filters function (or the ultimate frequency response of each electrical part of the crossover), the end result is splitting the incoming signal in a way that is best for each driver.
Unfortunately, crossover design is not as simple as merely building any filter that sends low frequencies to the low transducer(s) and high frequencies to the horn and driver. There are a number of variables in play that determine proper crossover function. Almost all crossovers have a transition that occurs over a range of frequencies, and that means there will be a collection of frequencies that both transducers are producing simultaneously. The crossover must be designed with a mindset for performance through this transition region.
In the transition region, where high and low drivers are playing the same frequencies simultaneously, the interference between the transducers influences the loudspeaker’s angles of coverage, or polar pattern. The upper half of Fig. 1 is a schematic diagram of what happens to the polar pattern of a loudspeaker where the single woofer is not aligned in time and/or the phase response of the crossover has influenced the polar pattern. From this picture, it can be seen that the axis of polar pattern shifts through the crossover region.
By contrast, the lower half of Fig. 1 shows what happens when the loudspeaker has two woofers arranged in an axis-symmetric fashion. Even if there is a misalignment of the crossover, the polar pattern remains symmetric through the crossover frequency range because of the woofer configuration. In fact, one can influence the shape of the polar pattern through the crossover of an axis-symmetric design by careful “mis” alignment of the two woofers and high frequency driver.
Fig. 2 is a montage of polar patterns for a single woofer/single HF driver configuration with a range of crossover filter types and orders. Here, the drivers are simulated to be in perfect alignment, delay-wise, and the changes in polar pattern are due only to the crossover filter phase response. It should be clear from the images that only the Linkwitz-Riley (LR) filter type provides symmetric polar patterns at the crossover point for a single woofer configuration. By contrast, the axis-symmetric dual woofer configuration provides inherently symmetric polar behavior, and opens up the possibility of using filter types other than Linkwitz-Riley.
Of course, there is no free lunch. Cone loudspeakers exhibit a narrowing of directivity as frequency increases, as shown in Fig. 3. The speaker designer wants to choose the crossover point so that the directional response of the drivers matches through the crossover transition. This is to ensure the directional response stays as consistent as possible. For a loudspeaker with two woofers, the directivity behavior at the crossover must consider both the narrowing pattern of the drivers and the woofers’ interaction with each other. This usually increases the complexity of the crossover design and choice of crossover frequency.
Low Frequency Performance
Just with the give and take in directivity behavior with one or two woofers, there are tradeoffs in the low frequency performance capability when using one or two drivers. At first glance, it seems straightforward to say “more is better,” but this is not always a clear-cut choice when practical restrictions on speaker size and weight come into play. We will consider the low frequency compromises for the ubiquitous vented boxes used by the vast majority of speaker manufacturers.
The acoustic performance (e.g., frequency response) of vented loudspeaker enclosures is calculated using a group of loudspeaker driver parameters commonly known as the Thiele-Small (T-S) parameters. Researchers Neville Thiele and Richard Small developed mathematical models of enclosure behavior, and then defined parameters that could be measured for real loudspeaker drivers and inserted into the models. There are a number of T-S parameters, and we will not discuss all of them presently, but below we discuss the key parameters that influence box size.
In a loudspeaker, the voice coil is attached to the speaker cone which, in turn, is connected to the driver’s suspension system. The suspension system is typically composed of one or more “spiders” that keep the voice coil centered in the driver’s permanent magnetic field and a flexible “surround” around the edge of the speaker that lets the cone move in and out. The mechanical performance of the driver suspension influences the correct volume of the vented enclosure. There are two parameters that describe the behavior of the surround and spider(s): “Cms” and “Rms.”
• Cms is the compliance of the suspension. Compliance is the inverse of stiffness. A loudspeaker with high compliance moves easily when the cone is pressed, while a loudspeaker with low compliance moves very little when pressed.
• Rms is a measure of the damping of the driver suspension. After pressing on the cone, it does not move back and forth indefinitely, but instead quickly comes to a stop. The energy imparted to the cone by pressing on it is dampened, or dissipated, by the suspension, stopping the driver movement.
As the loudspeaker cone moves, the compliance (i.e., Cms) of the suspension centers the cone’s movement, but also opposes it. The compliance is working against the force from the voice coil moving in the magnetic field, always guiding the driver toward its rest position. As a rule of thumb, drivers with a stiffer suspension (i.e., lower Cms) tend to have higher Rms.
Cms directly influences the vented box size. In short, the stiffer the suspension, the smaller the enclosure volume can be. Larger woofers tend to have larger and/or more spiders, which allows for stiffer suspension design. Larger woofers also tend to have bigger, more powerful voice coils that are better equipped to deal with the higher Rms of stiff suspension designs. This does not mean that larger woofers need a smaller overall enclosure volume, but rather enclosure volume normalized by cone area can be less for a larger woofer.
The end result is that, for a given box size, a single large woofer may provide more low frequency output or extension than a collection of two (or more) smaller drivers, even if they have similar total cone area. Larger woofers also tend to have more linear excursion, further aiding overall output.
Power Handling Performance
Our third area in the one woofer versus two discussion involves power handling. Here is another circumstance where the performance of a single larger driver may eclipse the performance of multiple smaller drivers. The dominant parameter in loudspeaker power handling is voice coil surface area. Voice coil surface area is proportional to the square of the voice coil radius, so a larger diameter voice coil has a greatly increased surface area.
As a practical example, a single 4-inch voice coil has 2.5-times the surface area of a 2.5-inch voice coil of the same length. Now the relationship between power handling and voice coil diameter is far from linear, but more voice coil area is the first place to increase a loudspeaker’s long-term power handling. Here again, a single larger driver has the potential to outperform a pair (or more) of smaller drivers.
Conclusion
If a single larger driver can be better for low frequency performance and power handling, and a pair of drivers has the potential to be more flexible for even audience coverage, where does this leave our three tradeoffs? There is no clear configuration that is superior for all performance contexts, and here is where the art of engineering comes into play.
Often other forces, hidden from the public eye, have as much influence on product design as the underlying physical restrictions. Clearly, in the consideration of product design, the additional cost of using a second woofer will be a factor in deciding how to approach the design. The loudspeaker designer balances the tradeoffs in this article against market demand, product cost requirements and time to market. It is out of this stew that new and exciting products for the market are born
