------------------
"Hope is when we feel the pain that makes us try again." -CS

quote:

---

Originally posted by [meph]:
who dug this post up?
sheesh.. its a good one though =)

---

Uhh.... That would be me.

I came looking for this post and had forgotten I had dug it up once before.

------------------

Eric
SS 1-2 NW

quote:

---

Originally posted by Eli47:
Yeah that was sort of good reading, lot's of good information and theory.
Sort of reminds me of a cartoon on my page.
I tried to copy it here but was unsuccessful, sorry.

---

the tuning frequency of a ported enclosure is not going to be the loudest frequency in a ported SPL enclosure. The loudest frequency(not taking in account vehicle resonances... in other words, free air) will be where the port and subwoofer are closest to being in phase.

And... don't think of a port working because the back wave of the sub reflects off the inside of the box, and then goes out the port... If this "reflection" is what makes the sound from the port, then wouldn't the depth of the enclosure be an important factor when tuning enclosures? Nope. Also, the subwoofer's and port's phase relationship will vary depending on frequency. The closer they are to being in phase, the more output you will get out of the enclosure. If you try to picture how the bass is produced from the port you'll probably just give yourself a headaque. Physics is complicated. I like Bob Carver's anology..... get a slinky. Hold the slinky in air, dangling. Think of your hand as the woofer cone. Move your hand down. This represents, let's say, the subwoofer moving "inwards", into the box. BUT, when you move your hand(and slinky) down, you would think the bottom of the slinky would go down too... but it goes up. Now picture the subwoofer moving inside the box. You would think that when the woofer moves inwards, it pushes the air in the port outside of the box. This is not the case. This doesn't mean that the port and woofer are "in phase", because they're not... they're just closer to being in phase(0 degrees diff) than out of phase(180 degrees different), and how close they are to being in phase will depend on the frequency.... and I think this is because of the actual time it takes the woofer to move. Actually the woofer and port might be 180 degrees out of phase at a certain frequency... check out Richard Clarks forum at Carsound.com. Maybe the port is delayed by exactly how long it takes the woofer to complete it's inwards movement. I don't know..... (I never took physics or anything)

Build an enclosure that will have the port and subwoofer the most "in phase" at a vehicle's resonance, and watch the SPL meter get excited

Hammer One.... I'm not sure a parabolic design will yield higher SPL than a similar flat wall... radio frequencies are omni-directional, so they'll reflect right off a surface(which is a problem for engineers when designing a cellular network in a metropolitan area), bass frequencies aren't really. I'm not sure just how omnidirectional a 60 hertz signal would be... but if you have success in getting subsonic sound waves to reflect like you want them... let us know!... and goodluck

------------------

anotherwords, if one was to make a wall and set the port and drver on different plains, what should be protruded to make up for the phase difference?

how far would be logical? It would vary by the tone wouldn't it?

I wouldn't think the phase of the port could supercede the phase of the speaker...it could, I guess, but it would look like at that point out of phase but one whole period plus the fraction out of phase.

Putting ports on different planes would be like putting speakers on different planes, sometimes it works, sometimes it doesn't- but a general rule to have them on the same (or as close to the same plain) to avoid cancelations and to maximize acoustic coupling...correct?

It is late, my brain hurts...

I think what you are looking for is an explaination of group delay.

But what I was questioning when I started this, is whether or not ports are more effecient than another speaker? Ports MOVE air, and given proper phasing, could possibly help pressurize it, but creating another sound wave?

But all subs would be all pressure. with phase shifters, they can all create a wave together. Can ports be more efficient at doing that? I just don't get how that is possible.

I am not doubting 4 subs ported, is louder than 4 subs sealed. But wouldn't 6 subs be more powerful than 4 subs with 50% port area?

------------------
2001 BMW 330i
Harmon Kardon Sound (too broke to buy more)

ShadowStar

------------------
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

When you have a sub in a passive radiator design. You'd think the passive would be better than a port, considering it has much more surface area.
Personally, I'd take a port with the same area as the sub. I'm assuming I have a huge ass box to work with. The port couples directly with the environment, I'm sure the port would match the impedance of air more closely than a PR.

But yeah, ports always have losses. Longer is not always better, you could have pipe resonances.

Also.. Do passive radiators count as cone area?

quote:

---

Originally posted by ShadowStar:

Also.. Do passive radiators count as cone area?

ShadowStar

---

The mass of air in the port (1.18/area*Length + 1.462*sqrt(Area/pi))), coupled with the compliance of the air
(V/(345^2*1.18))
in your enclosure determines your fb.

At fb, the driver hardly moves. However, that little bit of movement creates a back wave. Now consider that a resonating circuit has a high impedance. when you force a small current(called volume velocity in acoustics) through that high impedance, you get a large voltage which is analagous to pressure in the acoustical context. At different frequencies, the air in the port doesn't act as a resonator and the volume velocity produced in the back wave decreases as well, thus the port stops making appreciable sound pressure. Driving your speaker to Xmax at fb would produce an incredible amount of output I would think. Maybe how some of the folks are putting up 170s with 1 or 2 drivers?

I am pretty sure that at Fb, the driver and port are 90 degrees out of phase. This is on the virge of destructive interference. thus, any output from the woofer cone would be inconsequential in the far field. Above and below Fb the general trend is towards 180 degrees of phase separation. Remember the half power bandwidth of the port output only includes about 1/2 octave above and below fb, so while the port output theoretically deconstructs the woofer output in the far field, it isn't large enough to make a difference.

In a perfect system, air would not flow in and out of the port. There is no perfect system and physically, air does move in and out, this is realized as a loss.

I hope this is a good explanation, and my 5 years of EE didn't fail me again, I think there are a few holes in it already Maybe somebody who does this for a living would care to comment?.

Also, to the comments about increasing the force of the air hitting the mic. I wonder if using a square wave input would DQ you? There would be frequency content present above the 80hz cutoff. A big strong motor on your speaker could help if you you weren't DQ'd in this case, other wise, with sine wave material, as long as you can reach xmax, it shouldn't matter how strong your motor is. The only way to get more force would be to add mass to the air, which would require more amplifier to reach xmax....

1st of all, the position of the port (eg, is it sticking out of the box, or is it on another side of the box etc) and the phase of the port air, in relation to the phase of the wave the woofer moves

*position of port*

2nd of all, Port air velocity! the air out of the port is travelling much slower than the air that the woofer is moving
Yes, the air coming out of the woofer at the tuning frequency is out of phase (also air moves easiest out of the box at the tuned freq, and less so the further away from the tuning freq you get)

*port velocity*

as it exits the box, the air causes cancellation, but after a while, the wave sort of builds up (after a short (very short) period of time the 2 waves would eventually become in phase) and produce constructive interference, hence group delay

3rd, the bigger the port, the longer you make it to restrict air movement (to tune the air movement to the freq)
and the bigger the port, the lower the velocity of the air coming out of the port
Think about that

(also, really long ports, and very narrow ports also have losses, indicated by chuffing, or the pipe organ effect of very long ports)

4th, as you go higher in freq above the tuning freq, (assuming the port is on the same plane as the woofer, its more complicated if the port is sticking out of the box, or on another side) the port wave becomes more in phase, until it is in phase with the woofer at a certain freq (how close the freq is to the tuning freq, depends on a few variables, but is most commonly 5-15hz away ) and then it goes out of phase again as the freq goes even higher
(I hope that makes sense..)

5th, think about how all these factors relate to group delay, and finally SPL

the pressure of air is P0 at point R0.