For the last couple of months we have been answering some FAQ’s relating to sub drivers and enclosures (AKA, bass boxes) that are well suited to the requirements of concert sound production. We delved into the history of Hoffman, his Iron Law and Thiele-Small parameters of low frequency drovers (to predict LF performance). This month, we provide expert input from a leading woofer manufacturer on subwoofer venting tradeoffs.
While I haven’t been doing sound as long as our fearless editor (and my mentors), I am coming up on a half a century of doing live sound and reading countless audio design books and sound magazines. Having done so, I had a little understanding, early on, of the benefits of bass ports in loudspeakers. For decades, my focus was more on acoustic, array and sound system design; and frankly, I had access to loudspeaker experts to detail my custom loudspeaker design concepts.
So, it wasn’t until about a dozen years ago that I started using bass box modeling software to get a better understanding of how to design bass vents into my custom loudspeaker designs. About that time, my predecessor, Phil Graham, wrote a fascinating FRONT OF HOUSE article about how the most powerful P.A. woofers required larger vents to keep from overloading the vents. He wrote about limiting vent chuffing, noise and woofer distortion — and more tech talk about how laminar air flow through the sub vent is better than turbulent air flow. So, I actually hired Phil to help me optimize the vent for my 3 kW dual 18” subs, resulting in an aerodynamically optimized vent with a 200 square-inch mouth. Most of the leading concert loudspeaker manufacturers have been increasing their vent sizes over the years, to keep up with the ever-increasing higher output of the woofers they’re using.
After admiring and modeling the new Eminence NSW6021-6 21-inch (5,000W program power) sub driver from its Tour Grade series — one of the very best 21-inch subwoofers on the market — I discussed sub design concepts, with a few of the P.A. subwoofer market leaders. This month, we spoke to Eminence senior design engineer Matthew Marcum on the topic of vent size versus bass performance in subwoofer designs.
FOH: Could you explain why you wouldn’t say “bigger is better” in regards to bass box vent size and what are the disadvantages (besides requiring more cubic volume) of very large bass box vents?
Matthew Marcum: Vent resonances are primarily what came to mind first. The larger the vent, the lower in frequency these resonances occur. Large ports often have port resonances that lie in the woofer’s usable pass band and can add to distortion. Smaller ports can increase the port resonance frequency beyond the woofer’s usable frequency range, eliminating this added distortion.
The next tradeoff comes to mind is system cooling. The higher the vent air velocity, the greater cooling potential it has. If the volumetric flow is constant, large vents result in less velocity. Less velocity can spell lower power handling. As you pointed out, too high a velocity creates chuffing, noise, and distortion. It’s all a game of tradeoffs, no matter what. There may be a “better” or “best” for every application, but as the application changes, so do the tradeoffs and requirements.
A few years ago, we designed a 2-way system for testing our “new-at-the-time” compression driver, the N314T-8. It was the development of that test system that revealed to me how important vent design really is for reflex systems. We used our Impero 15C for the LF section, and designed a cabinet that would utilize that woofer along with our compression driver and H14EA horn. The initial design concept was about a 3.5-cubic foot total box volume tuned to about 60 Hz. We began with four big, oversized ports, with about 4.5” ID’s, and 11” long. Sure, the vent air velocity was kept low, as shown in Fig. 1.
But right off the bat, we noticed the impact of the vent resonances on frequency response as well as group delay, as shown in Figs. 2A/2B.
To highlight the performance of the N314T-8 compression driver, our intent was to cross the drivers over together somewhere around 500-800 Hz. However, the nasty vent resonances of that design had dramatic impacts in this bandwidth. We kept the box dimensions and tuning the same, but changed the vent design to 2 slots, 2” tall x 8” wide each, and nearly 4” deep. This not only reduced the amplitude of the vent resonance but also increased them to frequencies above our crossover point. (See Fig 3A/3B.)
The vent air velocity was increased some, as shown in Fig. 4.
However, it still remains below the “chuffing” threshold. As a result, this system is likely to have a lower operating temperature than the larger round vent system [due to turbulent air flow cooling the woofer] in addition to improved sound quality. A perfect example that bigger is not always better.
For help with live-sound design in large spaces see David Kennedy’s consulting sound design web site at D-K-A.com.
