General port info- As the speaker moves at a given frequency, the port air oscillates sympathetically. This moving air allows the compliance in the box to go up as the speaker is moving (that is, allows the speaker to move more freely). Thus, it allows the box to act less like a spring (therefore allowing the woofers back wave to couple with the front wave as well as allowing the speaker to unload more (than a sealed box of similar size).

At resonance - The tube and the air inside will have a resonance. At this resonance, the air in the port moves most easily. Because of this, the air acts very much like a little energy vacuum, sucking energy from the speaker to couple it to large amounts of port velocity. This causes the box to become less compliant, pushing against the sub, allowing it to excurt less and the port air velocity goes way up.

Under resonance - However, under the tuning frequency, the mass of air can no longer oscillate in the port. This makes the port become much like an open window, causing the box compliance to go WAY up. This makes the subwoofer unload until only the suspension of the driver (and a TINY bit of air resistance in the box) controls it.

Above resonance - Above the tuning frequency, the air in the port can oscillate quite well and vent velocity drops off quite fast (in one example I'm working with, vent velocity is 1/6th of the resonance velocity at 1 octave above tuning), as does Xmax (about 1/2 at 1 octave above resonance)

SPL effects - The primary factors in how loud a ported box plays are:

1) Phase - Between port wave and driver wave (how well they couple)

2)Xmax - How much acoustical energy the speaker is imparting to the total system of air

3)Vent Velocity - How much acoustical energy the port is generating

(NOTE- As the whole mass of air moves, and the shape of the mass is semi-constant (fluid) you could also apply a displacement (xmax) to the port air just as you could apply a velocity to the cone movement, the two are related.. Thus the mass of air in the port does act like a radiator of sound when moving, and it also acts like an energy sink (by dissipating acoustical energy through momentum, like a speaker)

Note, some-counter intuitivity. The vent velocity doesn't really make much sense if you think about it as air shooting out of the port. Since a sound wave is longitudinal (NOT transverse) when forced into a given shape like a cylinder, air moves back and forth in the port matching roughly the shape of the port. Note- This is a place vent losses can occur, as air near the sides of the port create friction and lose energy as heat)

Now, if you think about it, you can see why a ported box is generally loudest at about 1/2 octave above its tuning frequency. At this point, the waves of the port and woofer have a decently constructive phase relation, allowing them to couple, adding to the radiated acoustic energy.

Also, the box compliance is such that the speaker can move more freely than it can at any higher frequency.

And, the port air is close enough to resonance that it can move freely also.

Phase coupling + more xmax + more vent velocity = most overall radiated energy.

And dats wat its all about, right? Louda is mo' betta.

ShadowStar

(And obviously, if I screwed up anywhere, point it out.)

NICELY

ShadowStar

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Still looking for that CHEAP Thunderdome :D

Got Ears? Get Oz!

You can't build a reputation for what you're GOING to do.. But you can build one for TALKING about it!

-Eclipse-Rockford Fosgate-Oz Audio-Precision Power Inc.-AudioControl-Stinger-Tsunami-Clifford

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TEAM THUNDER
TEAM NOISY TOYS

From the creator of the "Wonder Box" and the "Hell Box" comes the newest creation- "The Coffin"


"Back seats are for SQ cars"

Couple of quick Qs..
What are your thoughts on (at the peak enclosure/speaker combo output frequency) which radiates more sound energy into the listening space- speaker or port?

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Hmm.. An interesting question.. I would wonder what things would affect it? I would guess that it would vary case by case depending on the woofer parameters and how loaded it is at that frequency.. Would be interesting to take some measurements, though!

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And what are your opinions on port/box size ratios and their effects and ultimate limits on efficiency and output?
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Hmm.. This would come into my earlier post of mass vs. velocity for wave energy a little bit, I think.. I might be treading water a bit deep for me here, but I would guess that so long as the box loads the speaker correctly, as big a port as possible would prove to couple the speaker to outside the box.

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Quote ow.di :

Is there any way to predict the acustical coupling between the port and driver? I seem to remember that the port wave is 180 degrees out of phase with the driver wave. Could you use this to your advantage by placing the port on the side of the box, then loading the port in some manner to encourge better acustical coupling?

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Umm.. I think this would also be pretty case by case..

But.. The rear wave energy is 180 degrees out of phase with the front wave energy, and then any time delay induced puts it more in phase with the next positive woofer cycle (before the port radiates said energy).. The ideal case would be 1 frequency time delay (for a 60hz note, 1/120th of a second, for example) for the energy to radiate from the back of the woofer to out the port.. That would provide full coupling of the rear energy to the front energy. However.. I would say that since this is frequency dependant, thats why you get a peak response.. Hmm.. maximizing this effect for a given frequency might net even a few more db from 1-2 woofer cars.. hint hint

ShadowStar

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Still looking for that CHEAP Thunderdome :D

Got Ears? Get Oz!

You can't build a reputation for what you're GOING to do.. But you can build one for TALKING about it!

-Eclipse-Rockford Fosgate-Oz Audio-Precision Power Inc.-AudioControl-Stinger-Tsunami-Clifford

Large Q, more spl out of port.
Lower Q, more output out of the sub(s).

Basically, the Q defines the alignment whether it be butterworth, chebychev, ect... A higher Q (chebychev) is when your box/port/speaker combo gives you a huge peak in the freq response curve. An example would be a DD in a huge ported box tuned high. In this case when you’re at that peak, more spl will come out of the port.
Another note, with parameters of qes roughly less than .30(the lower the better), as the box gets bigger than the Vas, the peak moves towards the tuning freq of the box (so does the minimum impedance).
I’m pretty sure that most equations and software don’t take into account the impedance mismatch at the boundary of the ports with respect to spl of the total system. That means don’t use small ports! If you can make them horn shaped, make them bigger than the subs.

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2-DD 9512's
2-2000X's (for now)
8.0ft^3 box in a civic trunk
One big ass port
?????dB's

Colin

quote:

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Originally posted by RWAudio:
now here's a guy who might be able to tell me why this works and how to make it better...
modeled as a standard ported box, sub (dd9515) fires into a 64sq" x 15.5" deep chamber, radiating surface of sub is roughly double the chamber opening. port is 80sq" and tuning is approx 40hz at the moment (port was modeled/tuned assuming box was simply ported) the driver "slot" is on the passenger side of the car, and port "slot" on the drivers. see pic.

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so are you sayinmg it's like a regular ported box, only the sub isn't firing into the cabin of the car, it's firing into a "chamber"? Well, I know that if you had a bandpass box that had a design like one you could pick up at Circuit City, and you ported the front chamber to exactly match the reaer chamber, than you would have zero output. That's if both sides had the same air space and same port dimensions. Basically, the sides will both produce bass but will be 180 degrees out-of-phase. I'm designing a 7th order superpressure multifractionaly-limited bandpass driver enclosure right now, and it should be hellasweet and loud, but we'll see

On the flip side, excursion limited power handling is is normally increased slightly and low end extension along with it. I'd be willing to bet that he has a plateau in frequency response if you do low level testing that makes it hard for him or at least did in the beginning make it hard to find the right burping frequency for that enclosure.