Bigger is Not Always Better
That huge 15" woofer may look impressive but ever wonder why they’re not on Polk speakers? There are several reasons why bigger is not always better when it comes to your speaker’s drivers.
While huge drivers with imposing and wide cabinets may look impressive, using multiple smaller drivers has tremendous advantages. With the exception of the largest car audio subwoofers, Polk Audio does not use drivers larger in diameter then 12" for subwoofers. Our larger bookshelf and tower speakers use multiple smaller drivers, usually 7" or less. What this means for you is more detailed, higher quality sound, in a less intrusive cabinet.
First, as driver diameter increases the actual cone becomes harder for the suspension to control, particularly at higher volumes. The larger driver with its bigger surface area needs to be more rigid than a smaller driver, otherwise it will flex. More rigidity means more structural reinforcement. More reinforcement means added mass. Added mass means poorer transient response because that huge driver is slower to accelerate when a signal is applied and slower to stop when the signal disappears. The smaller drivers yield faster transient response, which results in more detail and a tighter sound.
Second, if the driver flex is not properly dealt with, the flex causes inaccuracies in the frequency response as well as distortion. Larger drivers have more pronounced peaks in their frequency response, while the smaller drivers are usually more accurate. A more accurate driver with less distortion results in a more natural sounding speaker that is less fatiguing to listen to.
Since each small driver has its own suspension and motor assembly instead of just having one, far more control is possible. For example, having two weaker people lift a heavy cabinet is far more stable than having one strong person doing so. A well controlled speaker is a more realistic sounding speaker.
Next, as driver diameter increases, the sound’s dispersion pattern becomes narrower and narrower, particularly at the upper range of its operating spectrum. In other words, the dispersion pattern gets beamy. Think of a speaker projecting sound much like a light fixture projecting light. A narrowly focused beam will do a poor job lighting up your room, but a wide dispersing beam will cover a wider area. The same holds true for audio. With smaller drivers, a wider dispersion pattern is maintained, which yields a larger sweet spot for your listening area.
And let’s not forget a loudspeaker’s visual appeal. Because smaller, individual drivers are not physically enormous, they will not need a wide and imposing cabinet. Slender cabinets are sleeker looking and take up less space. But besides visual appeal, a slender cabinet also has less diffraction. Diffraction results when some of the sound coming from the sides of the individual speaker drivers partially reflects off the cabinet’s surface. These reflections color the sound. A narrower speaker cabinet has much less diffraction then a wide cabinet, resulting in purer sound.
We’ve gone through the positives of employing multiple smaller drivers. Are there any negatives? Well, yes and no. First, a smaller driver handles less power then a larger driver; thus, the smaller driver cannot move as much air, nor can it play as loud as a larger driver. To address this, multiple smaller drivers work together to increase total power handling, usually more than what the single larger driver can handle.
Another advantage of multiple small drivers, besides power handling, is less power compression. At higher volume levels, some of the energy powering the speakers becomes heat, rather then sound. Since multiple smaller drivers are used, each with their own suspension and voice coil, less power compression (loss of sound through heat) occurs. Less of your amplifier’s wattage is lost to heat.
Smaller drivers have less surface area and do not play as low in bandwidth. However, since small drivers are usually mounted in close proximity to one another, their acoustic output will sum together. In other words, their multiple smaller outputs combine to create one large output. The technical term is “mutual coupling.” This results in an effective surface area equal to or greater than the single larger diameter driver. It also means multiple small drivers can play as low, if not lower, then the single larger driver.
In both situations, the “weakness” of the smaller driver is easily overcome by using more of them. Multiple small drivers yield better results over single large drivers, and that means better overall audio quality for your listening enjoyment.
This article was last modified Apr 20, 2014