Is there any way to model or estimate the effect of using an aperiodic vent? Or do you just have to build and test?
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Re: aperiodic vent modelling?
I built and tested with my ear but you will have better results with proper measuring equipment.
Here is a good read on aperiodic vents
http://diyaudioprojects.com/Technica...udspeakers.htm
PWR knows his stuff about aperiodic vents, hopefully he will chime in.

Re: aperiodic vent modelling?
Originally posted by donprice View PostIs there any way to model or estimate the effect of using an aperiodic vent? Or do you just have to build and test?
When you understand its behaviour it's actually pretty easy to model. What it does is give you a very leaky closed box. It is pretty accurately modeled by entering a value of around 3 for Ql in a box modeling program that is complete enough to include Ql, Qa, Qp, etc. A value of Ql indicates a very low Q due to a high degree of leakage in the box.
It basically increases the compliance of the air spring of the air inside the box by letting some of the pressurization escape. The result isn't the same as if you had a larger box, like some people say, it's more like if you had a woofer with a lower Qts instead.
I hope this helps.
Jeff B.
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Re: aperiodic vent modelling?
Hi,
If you search "my" threads on the project QA25, you can follow my findings on aperiodic vents.
http://techtalk.partsexpress.com/album.php?albumid=445
Regards//lassePerry Mason talking to his dentist:
"Do you swear to take the tooth, the whole tooth and nothing but the tooth, so help you God?"
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Re: aperiodic vent modelling?
I like speakers because of the math. There is a certain joy to creating a series of equations that have not been explored and turning it into something you can touch. I never realized before you started this thread that the aperiodic is a third order transfer function. I have never worked with equations that assume the third order component is predetermined. It's an interesting idea.
If you ever see any equations or math calculations about an aperiodic that would be awesome! TIA.
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Re: aperiodic vent modelling?
Originally posted by moron#99 View PostI like speakers because of the math. There is a certain joy to creating a series of equations that have not been explored and turning it into something you can touch. I never realized before you started this thread that the aperiodic is a third order transfer function. I have never worked with equations that assume the third order component is predetermined. It's an interesting idea.
If you ever see any equations or math calculations about an aperiodic that would be awesome! TIA.
Even a vented box is really just two second order systems combined; both the cone's output and the vent output rolloff second order. What makes the summed rolloff fourth order is due to the output of the vent moving to a more and more outofphase relationship with the cone. This causes the two to begin to cancel and resultant is a fourth order rolloff.
Again, an aperiodic box is second order system.
Jeff B.
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Re: aperiodic vent modelling?
Originally posted by Jeff B. View PostThat is another misconception, the aperiodic enclosure is not third order, it is still second order. Even a simple thought experiment will reveal this, as there is no mechanism for an additional pole to introduce an additional 6dB/octave rolloff. It simply behaves as a damped second order system.
Even a vented box is really just two second order systems combined; both the cone's output and the vent output rolloff second order. What makes the summed rolloff fourth order is due to the output of the vent moving to a more and more outofphase relationship with the cone. This causes the two to begin to cancel and resultant is a fourth order rolloff.
Again, an aperiodic box is second order system.
Jeff B.
but I like Pete's idea better. It's simpler. he says it's really a bass reflex tuned to either zero or infinity or somewhere else with a really wide Q. I'm dying to plug this into small equations and see what pops out the other side.
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Re: aperiodic vent modelling?
Originally posted by moron#99 View PostI read somewhere that all aperiodics have an 18dB rolloff. I have been racking my brains trying to figure out how the vent could introduce a single pole. My best guess to date was that Smalls equations combine the driver compliance with the compliance of trapped air in box =>> Cat ~ Cas*Cab. When you combine these two compliances into one then the equivalent circuit becomes second order. However, if the vent placed a finite resistance across the Cab compliance then it would not longer be like two capacitors in series. One of the capacitors would have a resistor across it and bleed off. Therefore, our second order equation would no longer be valid. Our new circuit model would have two caps, one coil, and two resistors.
but I like Pete's idea better. It's simpler. he says it's really a bass reflex tuned to either zero or infinity or somewhere else with a really wide Q. I'm dying to plug this into small equations and see what pops out the other side.
The correct way of looking at an aperiodic system is by looking at the impedance plot. This will reveal that it really is aperiodic, with no periodic resonance frequency, and with no impedance peak associated with one. What you will find is that the impedance peak is flattened significantly  in same way as if Qms were lowered, which is essentially what is happening  the system's mechanical Q is being damped by the leakage. This is how Ql models it in the Benson model, and that's why using a Ql of around 3 gives you a very accurate model of an aperiodic system. It's really pretty simple.
Jeff B.
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Re: aperiodic vent modelling?
Originally posted by Jeff B. View PostNo offense, but if that is what Pete said, then that is incorrect. If you enter zero for Fb into Small's polynominal for a vented box terms cancel out and it reduces to the math for a sealed box. In fact, it will produce the indentical response curve as that of a second order system polynomial.
The correct way of looking at an aperiodic system is by looking at the impedance plot. This will reveal that it really is aperiodic, with no periodic resonance frequency, and with no impedance peak associated with one. What you will find is that the impedance peak is flattened significantly  in same way as if Qms were lowered, which is essentially what is happening  the system's mechanical Q is being damped by the leakage. This is how Ql models it in the Benson model, and that's why using a Ql of around 3 gives you a very accurate model of an aperiodic system. It's really pretty simple.
Jeff B.
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Re: aperiodic vent modelling?
The 18dB thing IS misinformation (and unfortunately exists in the first article on google) 
Dickason, however, notes that the effect is equitable to simply stuffing a box.I am trolling you.
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Re: aperiodic vent modelling?
Originally posted by moron#99 View PostI read in a couple places that aperiodics display 18db slopes. If true, then the A/S model isn't a good approximation. I don't think Pete meant absolute zero. I think the idea is that it was so far removed from the tested area that it did not show up.
Jeff B.
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Re: aperiodic vent modelling?
Originally posted by Jeff B. View PostNo offense, but if that is what Pete said, then that is incorrect. If you enter zero for Fb into Small's polynominal for a vented box terms cancel out and it reduces to the math for a sealed box. In fact, it will produce the indentical response curve as that of a second order system polynomial.
The correct way of looking at an aperiodic system is by looking at the impedance plot. This will reveal that it really is aperiodic, with no periodic resonance frequency, and with no impedance peak associated with one. What you will find is that the impedance peak is flattened significantly  in same way as if Qms were lowered, which is essentially what is happening  the system's mechanical Q is being damped by the leakage. This is how Ql models it in the Benson model, and that's why using a Ql of around 3 gives you a very accurate model of an aperiodic system. It's really pretty simple.
Jeff B.
The impedance plot tells us what the cone motion is doing because the driver is a reciprocal network and motion on the secondary is reflected back into the primary as a voltage. All the impedance plot tells us is what the cone it doing and if the cone is loaded by an acoustical network then that alters the motion. However, the total response of the system the the sum of the volume velocities out of the box as I'm sure you are familiar with per Thiele and Small. The system input impedance tells us nothing of the output from vents and leaks and the complex transfer function involved.
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Re: aperiodic vent modelling?
Originally posted by Pete Basel View PostConsider what an "aperiodic" system does at DC as compared to an ideal closed box. If you push the cone in with your hand, or simply apply DC to the voice coil, the vent acts as a leak completely out of phase and equal to the cone displacement  the combined output is zero. A vented system also acts this way, and a sealed system does not. It is a highly damped vented system there is no way to dispute this. Some argue that the resistive vent absorbs or dissipates the port output but given the depth of the port and damping I doubt that it is very effective at eliminating the port output even at 30 Hz. The aperiodic vent adds resistive damping and a significant acoustical leak.
Jeff
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