There’s one more frequently overlooked loudspeaker problem : resonance. Why is this another fahgeddaboudit? It’s not for lack of consequence to the sound quality. No, together these factors separate the good from the outstanding. Rather, it’s for convenience. These issues get complicated. And that’s not the only reason they’re not talked about. The number one reason is that manufacturers think you’re too stupid to understand, and what’s more, if you did understand you’d cross their speakers off your short list in a heartbeat.
Once again, we’re talking about something that’s complex, and once again, I’m going to over simplify it in order to make the point understandable to others who, like me, don’t have an advanced engineering degree.
Resonance comes from three primary sources, the drivers, the air inside the cabinet, and the cabinet walls. These three sources interact with each other, but for clarity I’m going to take each one on its own. Every driver has a resonance frequency, i.e., a frequency at which it naturally vibrates, noted as the Fs. At the Fs, a driver tends to continue vibrating (or ringing like a bell) after the signal fed to it from the amplifier stops—not good. For good resolution (definition, detail, clarity) the speaker needs to reproduce only the signal fed into it, no more, no less. Any continued resonance will cloud the sound and mask finer details in the recording. To avoid this, it’s best not to feed a driver frequencies at or near its Fs, preferably only frequencies an octave or more above the Fs. But there’s more to the story. Drivers have other lesser resonances that are revealed with a measurement of their cumulative spectral decay (CDS). This measures how long a driver continues vibrating after the signal stops over its entire bandwidth. Good drivers are well damped, and amps with high damping factors also help to control the driver’s resonance. However, when a passive crossover is placed between amp and driver, all bets are off. The amp no longer has full, direct control over the driver; damping factor is rendered irrelevant.
This brings us to the air volume inside the cabinet. Smaller volumes raise both the resonance frequency of the driver and the Q. Q is a measure of the ‘quality’ of the resonance. The higher the Q, the narrower the frequency range and stronger the resonance. High Q is also slower to respond, both starting and stopping takes longer causing the trailing end of one note to blur into the attack of the next, transients spread and lose their punch. Conversely, the lower the Q, the weaker and more widespread the frequency range of resonance, and the sharper the transients. Larger air volumes are better for keeping the Q and the Fc (resonance frequency in the enclosure) low. An enclosure can only be made so small before there are serious consequences. This is only half the story. Most speakers use the air inside the cabinet to boost bass response either through a port, or by channeling the driver’s back wave through a series of internal guides, often called a transmission line (TL). Some TLs are closed to completely absorb the back wave, but most are open ended, acting similarly to a port. Open enclosures use the internal air volume to acoustically amplify the low bass notes, much like the body of a musical instrument, e.g., guitars, violins, pianos. They behave as a resonator with a high Q and slow response time (group delay). This is the antithesis of good sound reproduction. Only open-baffle, sealed acoustic suspension, and infinite baffle designs avoid the added resonances that ports, passive radiators, and open-ended transmission lines add to what you hear. The point is to hear the music, not the speakers.
And finally, there’s the resonance of the cabinet material itself. Avoiding this problem can be done in a number of ways. The most common means is to use internal cross bracing to control the panel resonances of a box. For woofers, copious amounts of bracing are needed to control the forces inside the cabinet. Unfortunately, bracing takes up significant space, reducing the internal air volume and raising the Q. Another means is to use dense, massive materials, and thick cabinet walls. This is more effective, but it also gets costly and extremely heavy. A simpler solution is to let the laws of physics do the work. Instead of flat walled boxes, spheres or cylinders have great natural strength to resist vibration. Problem solved the easy way.
The Parallel Audio Project uses cylindrical, low Q, sealed enclosures with carefully selected drivers directly controlled by the amps to reduce resonances to a minimum and keep transients intact. Rather than listening to the sound of speakers, you hear the clear sound of music.
Read about bass reflex systems (ported), how they differ from acoustic suspension (sealed), and be sure to note the animation illustrating how bass reflex uses resonance to boost bass output (about 3/4s down the page). The author’s five bullet points on the misconceptions of ported speakers need qualifying. Points 1,2, and 4 are, as he notes, incomplete and misleading. Point 3, I’ve never heard before and can’t imagine how anyone could construe such a thing. However, point 5 is true, and the author provides the evidence for it throughout the essay. See [How a Hole in a Box Works]