Rear Firing Vents…

So, why do I place the vents on the front of the cabinet when a number of my competitors locate it on the rear?

Answer: You will perceive a stronger and more defined bass output with the vent on the front of the cabinet – period. You will realize a better impulse response and proper sense of timing when the vent is located on the front of the baffle – period. If you’re reading something or if someone is telling you differently, run! Vent placement is very rudimentary acoustical physics. In full-range design (with its inherent bass-producing limitations; for example, small driver surface area, very low moving mass, etc.), squeezing the maximum amount of extended bass out of the system is a specific design goal.

Having the vent radiate from the same plane as the transducer is classic Helmholtz resonator acoustics. Have you ever seen opening slits on the rear side of a guitar or violin? No one in their right mind would ever put them there. The resonating slits are right there, directly below or directly adjacent the strings – which is exactly where they should be. Loudspeakers are a bit more forgiving (typically tuned and operating at much lower frequencies than guitars and violins, so rear vents are actually quite common).

Rear vents came onto the scene when some guy realized that he didn’t have room for the vent on the front baffle of his loudspeaker. Sorry if this rains on your parade, but it’s the plain and simple truth!

I place the vent on the rear side of the cabinet when necessity dictates that I do so. And I insure that I’m using a powerful driver that is capable of overcoming the subtle effects of a rear cabinet location.

Vent noise: Regardless of where the vent is located in the system (front or back, top or bottom), if the vent(s) is too small in relation to the driver’s surface area (effective piston size), the air has the potential to vibrate at the vent faster than Mach 1.0. Since we are working with sound, note that forcing the air (sound) out of an orifice at speeds approaching Mach 1.0 (or higher) will not convert to accurate sound reproduction! When a loudspeaker has an undersized vent(s), and when the driver is operating at high levels of excursion, the frequency of vibration is forced into a resistive type compression, and the pitch must go up in frequency and produce massive amounts of distortion! In other words, it creates the dreaded vent noise anomaly! You will not have to worry about vent noise with my lineup of loudspeakers.

For example: Calmly blow into a flute, and you get a wonderful tone. In contrast, blow with twice as much pressure into the flute, and it magically jumps up a complete octave in pitch. If you have enough lung capacity and muscle strength, double it again, and it will jump up another full octave! I know… you don’t have a flute on hand to try this. Okay, simply calmly whistle a pure note through your mouth. Now, while holding that note steady slowly increase your lung pressure and hold the the pitch constant at the same time… Pretty cool, eh!?

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