posted
Could someone explain EXACTLY how a ported box works? I know how to design them, and how the different tuning affects the sound, but I don't know exactly how they work. Explain on an atomic level please. Does the port resistance cause the air particles to resonate, or what? Thanks.
------------------ rybaudio@freeze.com
The goal: Accurate musical reproduction in the mobile environment
SQ with a side of SPL
Posts: 3957 | From: State College, PA | Registered: Sep 2000
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posted
The air inside the vent acts like a piston and vibrates in response to the movement of the woofer’s diaphragm. The smaller the vent, the higher the air velocity and visa versa. Generally, the air velocity through the vent should be kept relatively low by using a vent of sufficient size. This avoids the problem of air turbulence noise in the vent. However the benefit of a large vent can be overshadowed by vent losses because they increase when the vent size increases—a major source of vent losses is the viscosity friction of air as it moves through the vent.
Some of the vent’s sound waves lag behind those of the woofer, creating a phase shift. At and above the resonance frequency of the woofer, the vent’s sound waves have the same phase as those of the woofer so they reinforce each other. At the box resonance frequency, the vent also damps the woofer so that its diaphragm moves very little while the air velocity in the vent reaches a maximum.
Below the box resonance frequency, the phase of the vent’s sound waves shift 180 degrees so that they are out of phase with the woofer’s sound waves. This is both good and bad. It is good because the sound waves of both the woofer and vent begin to interfere and cancel each other below the box resonance. This produces a rapid 24 dB/octave low-frequency cutoff rate which can protect the woofer from excessive excursion. It is bad because it “unloads” the woofer which can make it susceptible to excessive excursion at ultra-low frequencies in spite of the rapid cutoff rate mentioned previously. Fortunately, the “unloading” problem is easy to overcome. All it takes is a low-frequency (usually subsonic) high-pass filter to protect the woofer from sound waves below its cutoff point.
Without a subsonic high-pass filter to protect it from “unloading”, the power handling of a vented box is not as good as a comparable closed box. However, if this disadvantage is overcome with the filter mentioned above, it can be turned into an advantage. This is because the woofer in a vented box will require less excursion than a comparable woofer in a closed box so the vented box will generally have less nonlinear distortion from this source.
Another effect that results from the vent’s damping action on the woofer is a significant reduction in the electrical impedance in the vicinity of the box resonance frequency. This reduced impedance has a bad side-effect. It causes the voice coil temperature to increase because the voice coil will now draw more current from the amplifier and as its temperature increases so does its resistance (Re). This in turn affects many characteristics of the woofer such as its electrical Q (Qes), motor strength (BL) and efficiency (ho). The result: nonlinear distortion increases dynamically with changes in temperature.
A woofer that is designed for a vented box also tends to have a better midrange response than a woofer that is designed for a comparable closed box because the former can have a higher free air resonance (Fs). This means that the vented box woofer will have less moving mass (Mms) and be more efficient.
------------------ Ron Hawkins
Sometimes the majority only means that all the fools are on the same side.
Everyday I beat my own previous record for number of consecutive days I've stayed alive
posted
Yes, yes I know most of that, but why do certain ports tune to a particular frequency? And why does it dampen cone motion at that frequency? Its nice to know why it has such a steep rolloff below tuning, but why do the soundwaves change from in phase to out of phase? What properties cause such a thing go happen. I'm looking extremely in depth. I want to be able to examine it down to every atom. Thanks for the explainations, but I want to know more.
------------------ rybaudio@freeze.com
The goal: Accurate musical reproduction in the mobile environment
SQ with a side of SPL
[This message has been edited by Rybaudio (edited 06-07-2001).]
Posts: 3957 | From: State College, PA | Registered: Sep 2000
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quote:Originally posted by Rybaudio: Yes, yes I know most of that, but why do certain ports tune to a particular frequency? And why does it dampen cone motion at that frequency? Its nice to know why it has such a steep rolloff below tuning, but why do the soundwaves change from in phase to out of phase? What properties cause such a thing go happen. I'm looking extremely in depth. I want to be able to examine it down to every atom. Thanks for the explainations, but I want to know more.
Im getting really sick now and didnt get much sleep but ill try to help.. The woofer begins to unload below the ports tuned frequecny because the port mass is not large enough to resonate at the required note, so the woofer starts actually moving air out of the box trying to play the low note because the port isnt able to reinforce it properly. The lower the note beyond tuning, the more air the woofer tries to move and more excursion it will have until finally it has lost all damping from the enclosure and might as well be in no box at all. Honestly I am not sure about the phase relationships concering this and I think Im going back to sleep. now.. Gotta love muscle ralaxers, sorry if Im a little incoherent at the moment..
------------------ TEAM THUNDER TEAM NOISY TOYS From the creator of the "Wonder Box" and the "Hell Box" comes the newest creation- "The Coffin"
posted
Hey, thanks for some more info. So, I think I understand a little bit more now. I'm still looking for exactly why the port adds, and how does it add waves of the same phase when waves 180 degrees out of phase are coming from the rear of the speaker. Ok, and port mass, what exactly does that mean? I know there is a relation of crossectional area to length, what exactly is that relationship? I know, bigger area, longer for the same tuning, but why? Is the air pushed and pulled out of the box, or do you have to look at the air inside the port as a separate thing? Can someone go incredibly in depth, this is driving me nuts!!! By the way, thanks to everyone who has helped so far.
------------------ rybaudio@freeze.com
The goal: Accurate musical reproduction in the mobile environment
SQ with a side of SPL
Posts: 3957 | From: State College, PA | Registered: Sep 2000
| IP: Logged |
posted
The wave coming from the rear of the speaker is out of phase with the front but the port delays the rear wave enough so that it is in relative phase with the front by the time the wave exits the port. This is also related to the phasing between the waves. Also the port mass is the air inside the port. The air in the box modulates the air in the port. The small spring(port) hanging off of the large spring(box) is a good analogy. The air in the port doesnt actually exit the box but is made to resonate by the air in the enclosure. Think of the port as a second speaker that reinforces the sub's output. Air should actually never leave the port unless the woofer is unloading by playing well below the tuning frequency. Total port volume is not the correct way to figure a port, but port face area with respect to a given lentgh is what is needed. The larger the port area for a given box size, the longer it will need to be to tune to a given frequency. I believe this has to do with the total air mass in the port with respect to the resonace characteristics of the dimensions of the port. And the box volume is very important as well. Massive objects have a lower resonant frequency, and a larger box will give lower resonance than a smaller box with the same port dimensions. Hope this helps/makes sense, Im still sick and on my sleepy-medicine..
------------------ TEAM THUNDER TEAM NOISY TOYS From the creator of the "Wonder Box" and the "Hell Box" comes the newest creation- "The Coffin"
posted
the buzz just hit on what i think you are looking for. at box tuned frq and above the air in the box actually does not have enough excursion to physically leave the box. meaning the air from in the box doesn't actually penitrate the outer plane of the port. so the air in the box does not actually leave the enclosure. effectivly keeping the back wave in the box. the air you feal coming out of the port is resonating in and out in a push pull fashion. but the air you feal is the mass of the port. not the actual air in the box. when you go below res the box begins to unload. meaning the air in the box starts to leak out because the excursion on the port becomes larger then the actual ports mass. hense letting the back wave out and starts to cause cancelation. now when a port is too small the velocity gets too high. when velocity gets too high the port becomes a leak. because the air starts to move too fast in one direction and can't stop and turn around fast enough that the port looses it's mass. and in turn just becomes a hole in the box.
Common sense might tell you that the back wave of the sub is coming out of the port, and it should be out of phase with the sub.... so it shouldn't be as loud.... but in reality, common sense shouldn't be used.
Think of a slinky. You're holding a slinky up by one end, and the other end is dangling close to the ground. Now drop the slinky. Common sense might tell you that the bottom of the slinky will hit the ground right away, because of gravity, but in reality the bottom of the slinky actually travels UP, away from the ground.
Look at PRs(Passive radiators) The "mass" of a port is the AIR in the port. The "mass" of a passive radiator is the cone, surround, and anything else attached(usually a small weight).
If you've ever seen a PR, when you push the subwoofer inwards, the PR moves OUT, and is "out-of-phase" with the subwoofer. This is because you didn't push it in fast enough. If you could push it in faster, the PR will actually move INWARDS just like the subwoofer that you just pushed in. This is because when you push the subwoofer inwards, it's like you're replicating a 5 hertz tone(about), which is more than likely BELOW, I repeat, BELOW the tuned frequency of the enclosure. If you push the sub inwards real fast, you'll notice that the PR moves inwards just a little, then back outwards once the sub is pushed in. Well when there is music applied, as long as that music is ABOVE the tuned frequency, the PR will move WITH the subwoofer, defying "common sense". If you've ever owned a "Sunfire Signature Subwoofer", or an Earthquake MAGMA with PR, or others, you could experiment and see what i'm saying!
Also, below tuning frequency the port or PR does about all of the work.
------------------ *24/7 man! QoTSA Better Living Through Chemistry
[This message has been edited by I luv tHc (edited 06-15-2001).]
Posts: 141 | From: lala land | Registered: May 2001
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