Cab. design: I had a thought, you'll have a laugh

**cradeldorf** · October 16th, 2010, 05:47 PM

I don't consider myself an expert on cabinet design nor do I consider myself an all unknowing fool. I had a brainstorm and I just thought I'd share it here, flame me, chastise me or praise me freedom of speech is everybody's right.
Ok so here's what ran through my head, a speaker cabinet that has the ability to breathe when it needs to but not without some effort. Correct me if I'm wrong but it appears as though the higher the frequency the less a speaker diaphragm has to move so a horn is like a sealed unit because it moves very little air and the woofer is open because it moves a lot of air. therefore a large port would give the best lows but nothing at mid range and the horns take over above that so it is removed from the equation. So my brain fart is a cabinet with a set of valves similar to a reed valve on a 2 stroke motor one to let air in and one to let air out. This would then allow the cabinet to operate like a closed cabinet when mid ranges are playing and ported when the lows kick in providing it takes some effort to open and close them. Tuning would be achieved by the amount of pressure it takes to open and close the valves. Like I said you'll get a laugh.

**Old Guy** · October 16th, 2010, 08:50 PM

Well, sorta kinda.

The bass port only affects the woofer, not the horn. Since the woofer doesn't have appreciable highs, except maybe peaks, there would be no effect.

The reason the horn is sealed is that it acts like a transformer. It converts a small high pressure movement into a larger low pressure movement. Kinda sorta like when you step 120 volts down to 12 the current will go up.

A bass horn does the same thing. The trick is to get more air movement for a given cone movement.

The reason for the bass port is usually the horn is too small. Think A7. Or even 210. The bass port turns the rear wave around to reinforce the front wave, at a given frequency. Varying the box tuning would simply move the peak in the LF around, as I said without affecting the highs.

Even with a single full range speaker, high frequencies are too short to be "turned around"...they won't make it round the bend.

GM · October 17th, 2010, 01:16 AM

Originally Posted by cradeldorf

Ok so here's what ran through my head, a speaker cabinet that has the ability to breathe when it needs to but not without some effort.

.......therefore a large port would give the best lows but nothing at mid range and the horns take over above that so it is removed from the equation. So my brain fart is a cabinet with a set of valves similar to a reed valve on a 2 stroke motor one to let air in and one to let air out. This would then allow the cabinet to operate like a closed cabinet when mid ranges are playing and ported when the lows kick in providing it takes some effort to open and close them. Tuning would be achieved by the amount of pressure it takes to open and close the valves.

You're describing a Helmholtz Resonator (AKA bass reflex vented alignment, acoustic low pass filter), so no valves needed and while a larger vent is more efficient it has its own pipe harmonics that can make the vent 'sing' along. At this point then, a passive radiator (PR) in lieu of a proper vent is a viable option/desirable, so would be your 'valve' so-to-speak in that it would only allow very low frequencies (VLF) to be radiated by the cab.

GM