If you go to my site, and read the TL Anatomy article it describes how a TL enclosure and a driver couple to form a new resonant system. Exactly the same thing happens when a bass reflex enclosure and a driver are coupled together. These enclosures work because you change the dynamic properties of the system and create new resonant frequencies and mode shapes. The combination of the mode shapes, at different frequencies, determine the resulting motions of the driver and the air in the port. This I believe is the engineering solution to your question without all of the math derived.
I suppose you could use the terms suck or blow in a loose manner to provide a laymans understanding. As I hoped I had made clear, the air at the inside end of the vent for example, does not ever get blown right out into the outside environment - the whole 'block' of air in the vent just moves back and forth.
And what about at resonance ? Does the air then blow out or does the air never blow out in any condition ?
--Sincerely,
--Sincerely,
I'm on about resonance... The 'block' of air always hovers in and out unless you force the cone out by hand, wait a few seconds for the air to disperse then pull it back in again.
I guess I was a little confused when he said that the air does not move out the port because with my PB10, I feel the bass on really, really low passages on some material, like Attack of the Clones where I assume the port is giving it's all.
And ROTK, when Frodo is about to drop the ring into the lava, I can feel air movement and the scene is primarily centered at 17-20 hz, which is tuned to the port.
--Sincerely,
And ROTK, when Frodo is about to drop the ring into the lava, I can feel air movement and the scene is primarily centered at 17-20 hz, which is tuned to the port.
--Sincerely,
PB10? Ready to upgrade yet?Condensed version of my TL Anatomy article. Posted a few years ago on a different forum.
"I have not been following your thread until this afternoon. Resonances are alway of interest so I will chime in with my take on resonances and how they change the driver's impedance curve. If you see a peak in th driver's impedance magnitude curve, and an accompanying rapid phase fluctuation, then this is a sure sign of a resonance of some form. The way I see it is as follows.
1. Driver in free space or in an infinite baffle - a resonance condition will occur at or very near fs of the driver. There will be a single tall impedance peak along with a phase swing that approaches 180 degrees.
fs = (1/(2 x pi)) x (k_ms/m_ms)^1/2
k_ms = driver suspension stiffness (newton/m)
k_ms = 1/c_ms
c_ms = driver suspension compliance (m/newton)
m_ms = driver mechanical moving mass (kg)
2. Driver in a closed box - by adding a closed box to the back of the driver you are adding a second spring in parallel with the driver's suspension and raising the fs to a new frequency fc. This is predictable from the equation for the natural frequency of a spring and mass
fc = (1/(2 x pi)) x ((k_ms + k_mb)/m_ms)^1/2
k_mb = stiffness of the air in the box
k_mb = 1/ c_mb
3. Driver in a resonant enclosure - by adding a resonant enclosure, either a ported box or a TL tuned to fb, new resonant frequencies are generated. For a ported box the resonant frequenct is determined by
fb = (1/(2 x pi)) x (k_mb/m_mb)^1/2
k_mb = stiffness of the air in the box (newton/m)
k_mb = 1/c_mb
c_mb = compliance of the air in the box (m/newton)
m_mb = moving mass of the air in the port (kg)
For a straight classic TL the fundamental resonance is a function of the length
fb = 1/4 c/L
c = speed of sound (m/sec)
L = length of the line (m)
with harmonics at
fb = n/4 c/L
n = 3,5,7,9, ....
The interesting phenominon occurs when you combine two resonant systems, the driver and the enclosure, having approximately equal fundamental frequencies fs ~ fb. It does not matter if it is a ported box (bass reflex) ot some form of quarter wave enclosure, the behavior of the resulting resonances is the same. When two systems, with approximately equal fundamental resonances are combined, the resulting system will have two new resonances that bracket the original resonances as shown below.
f_low < fs~fb < f_high
The new resonances at f_low and f_high are the two impedance peaks you see for a bass reflex enclosure and an unstuffed TL.
The lower resonance, f_low, is the driver moving into the enclosure pushing air out of the open end or port and this produces the 24 dB/octave roll-off of a bass reflex or TL design. The mode shape (vibration theory term - the motion of vibrating systems can be completely described by their natural frequencies and mode shapes) has the driver mass moving into the enclosure and the open end air mass moving out of the enclosure.
The higher resonance, f_high, is the driver and the air at the enclosure opening moving out of phase combining to produce SPL. As you move up in frequency the driver's output dominates and you get the SPL curve of the driver. The mode shape has the driver mass moving out of the enclosure and the open end air mass moving out of the enclosure.
The common misconception is what happens at fs~fb which is the minimum between the two impedance peaks. This is not a resonance condition in the combined driver/enclosure system. This is the point between the two resonances where the mode shapes combine and result in the driver mass almost stopping (mode shapes cancelling the driver motion) while the motion of the open end air mass combines (mode shapes reinforcing the motion) to be a maximum. When the driver almost stops moving the only significant impedance is the resistance of the voice coil which is the minimum between the two resonant peaks.
Adding stuffing to the bass reflex or TL enclosure will tend to damp out the first resonant peak. Many people claim a TL has only one resonance peak which is incorrect. As you add more and more stuffing you tend to attenuate the lower impedance peak, at f_low, resulting in a single humped impedance curve. To determine the number of resonaces and mode shapes analyze the system without damping present, for a TL this means empty."
"I have not been following your thread until this afternoon. Resonances are alway of interest so I will chime in with my take on resonances and how they change the driver's impedance curve. If you see a peak in th driver's impedance magnitude curve, and an accompanying rapid phase fluctuation, then this is a sure sign of a resonance of some form. The way I see it is as follows.
1. Driver in free space or in an infinite baffle - a resonance condition will occur at or very near fs of the driver. There will be a single tall impedance peak along with a phase swing that approaches 180 degrees.
fs = (1/(2 x pi)) x (k_ms/m_ms)^1/2
k_ms = driver suspension stiffness (newton/m)
k_ms = 1/c_ms
c_ms = driver suspension compliance (m/newton)
m_ms = driver mechanical moving mass (kg)
2. Driver in a closed box - by adding a closed box to the back of the driver you are adding a second spring in parallel with the driver's suspension and raising the fs to a new frequency fc. This is predictable from the equation for the natural frequency of a spring and mass
fc = (1/(2 x pi)) x ((k_ms + k_mb)/m_ms)^1/2
k_mb = stiffness of the air in the box
k_mb = 1/ c_mb
3. Driver in a resonant enclosure - by adding a resonant enclosure, either a ported box or a TL tuned to fb, new resonant frequencies are generated. For a ported box the resonant frequenct is determined by
fb = (1/(2 x pi)) x (k_mb/m_mb)^1/2
k_mb = stiffness of the air in the box (newton/m)
k_mb = 1/c_mb
c_mb = compliance of the air in the box (m/newton)
m_mb = moving mass of the air in the port (kg)
For a straight classic TL the fundamental resonance is a function of the length
fb = 1/4 c/L
c = speed of sound (m/sec)
L = length of the line (m)
with harmonics at
fb = n/4 c/L
n = 3,5,7,9, ....
The interesting phenominon occurs when you combine two resonant systems, the driver and the enclosure, having approximately equal fundamental frequencies fs ~ fb. It does not matter if it is a ported box (bass reflex) ot some form of quarter wave enclosure, the behavior of the resulting resonances is the same. When two systems, with approximately equal fundamental resonances are combined, the resulting system will have two new resonances that bracket the original resonances as shown below.
f_low < fs~fb < f_high
The new resonances at f_low and f_high are the two impedance peaks you see for a bass reflex enclosure and an unstuffed TL.
The lower resonance, f_low, is the driver moving into the enclosure pushing air out of the open end or port and this produces the 24 dB/octave roll-off of a bass reflex or TL design. The mode shape (vibration theory term - the motion of vibrating systems can be completely described by their natural frequencies and mode shapes) has the driver mass moving into the enclosure and the open end air mass moving out of the enclosure.
The higher resonance, f_high, is the driver and the air at the enclosure opening moving out of phase combining to produce SPL. As you move up in frequency the driver's output dominates and you get the SPL curve of the driver. The mode shape has the driver mass moving out of the enclosure and the open end air mass moving out of the enclosure.
The common misconception is what happens at fs~fb which is the minimum between the two impedance peaks. This is not a resonance condition in the combined driver/enclosure system. This is the point between the two resonances where the mode shapes combine and result in the driver mass almost stopping (mode shapes cancelling the driver motion) while the motion of the open end air mass combines (mode shapes reinforcing the motion) to be a maximum. When the driver almost stops moving the only significant impedance is the resistance of the voice coil which is the minimum between the two resonant peaks.
Adding stuffing to the bass reflex or TL enclosure will tend to damp out the first resonant peak. Many people claim a TL has only one resonance peak which is incorrect. As you add more and more stuffing you tend to attenuate the lower impedance peak, at f_low, resulting in a single humped impedance curve. To determine the number of resonaces and mode shapes analyze the system without damping present, for a TL this means empty."
Steve, am I supposed to be underwhelmed with what I've got ?
The SVS PB10 is not a bad subwoofer at all. Now it's not going to compete with yours but still. For the money spent, there really is no competition.
And it does have twice the output of the big M&K MX-350 down to 20 hz. Not bad at all, let me tell you. 🙂
But yes. An upgrade I need. 😀 I'm thinking of going IB next.
--Sincerely,
The SVS PB10 is not a bad subwoofer at all. Now it's not going to compete with yours but still. For the money spent, there really is no competition.
And it does have twice the output of the big M&K MX-350 down to 20 hz. Not bad at all, let me tell you. 🙂
But yes. An upgrade I need. 😀 I'm thinking of going IB next.
--Sincerely,
I guess if you haven't heard better, then no. And I mean no offense by that.
I gotta disagree. A large and low tuned ported enclosure using a Dayton DVC 15" and a 240 watt plate amp would easily best it for less cost. I'd use 350 liters and an 8" diameter port that is 30" long for a 17.5hz tuning. It's the low budget PB10 killer.
Steve, no offense taken. The PB10 is the first subwoofer that I've ever owned. I have heard better. I have heard the Velodyne DD-18. Fantastic sub.
Far too expensive. I could purchase 8 PB10's for the price of one DD -18 in S.A. and still have spare change.
I have never heard a DIY subwoofer before though. I know that your subwoofer is apparently tuned to below 14hz and is massive. Probably sounds fantastic too. Pity I can't hear one.
Concerning the performance of my PB10, perhaps I should have been more specific, but it is practically unbeatable in a price/performance ratio compared to commercial designs.
I don't think you'll disagree with me on that. 🙂 The thing has less distortion and more output (25 hz and below), in fact it has twice the clean output compared to the big M&K MX-350 which is a sealed design. This was in a German subwoofer shootout.
That's like $2500 worth of sub there. In terms of DIY, sure, you could outperform it for less. No disagreement from me there.
--Sincerely,
Far too expensive. I could purchase 8 PB10's for the price of one DD -18 in S.A. and still have spare change.
I have never heard a DIY subwoofer before though. I know that your subwoofer is apparently tuned to below 14hz and is massive. Probably sounds fantastic too. Pity I can't hear one.
Concerning the performance of my PB10, perhaps I should have been more specific, but it is practically unbeatable in a price/performance ratio compared to commercial designs.
I don't think you'll disagree with me on that. 🙂 The thing has less distortion and more output (25 hz and below), in fact it has twice the clean output compared to the big M&K MX-350 which is a sealed design. This was in a German subwoofer shootout.
That's like $2500 worth of sub there. In terms of DIY, sure, you could outperform it for less. No disagreement from me there.
--Sincerely,
When considering commercial subs, yes, SVS is definitely at the top of the heap in regards to performance/dollar, and I don't know of anything that can match the PB10 at anywhere near its cost aside from the next model up SVS.
Vaughan said:
That's simply the sound waves you can feel. Imagine how much air would have to gush out of the vent to make a difference to the air in the room...
Think of the vent as an invisible speaker driver - does that move totally in and out, or does it just move back and forth...
I have a question which is not related to the thread but something I wanted to know about sealed design.
The compliance of air in the enclosure. Is the air springy because the air molecules in the small enclosure are densely packed ? I'm talking about acoustic suspension sealed subwoofers here.
The bigger the enclosure, the more spaced the molecules will be, hence more driver movement, hence less power required to reach any given SPL, hence less power compression. Hence. 😀
Am I right in this ?
Concerning ported subwoofers acting like a sealed sub above Fb, can someone please explain again why this is so ? Is there not a "leak" in the response because of the port ?
I don't understand fully why, as some suggest, the sub acts like a sealed above it's tuning frequency. I think some have mentioned that above Fb, the port is "blocked off", or something.
Please explain in more detail on that. Thank you very much.
--Sincerely,
The compliance of air in the enclosure. Is the air springy because the air molecules in the small enclosure are densely packed ? I'm talking about acoustic suspension sealed subwoofers here.
The bigger the enclosure, the more spaced the molecules will be, hence more driver movement, hence less power required to reach any given SPL, hence less power compression. Hence. 😀
Am I right in this ?
Concerning ported subwoofers acting like a sealed sub above Fb, can someone please explain again why this is so ? Is there not a "leak" in the response because of the port ?
I don't understand fully why, as some suggest, the sub acts like a sealed above it's tuning frequency. I think some have mentioned that above Fb, the port is "blocked off", or something.
Please explain in more detail on that. Thank you very much.
--Sincerely,
As you alluded to, in a large enough sealed design, above a certain frequency, the cone won't be moving far enough to compress the air in the enclosure.
In a ported design, if the tune is low enough and the FR is flat, the sub will behave almost identically to a sealed sub in the musical range (which I consider above 30hz) because the frequencies aren't low enough to be "activating" the port, at least not significantly. Earlier we discussed that the diameter and length of the port in relation to the size of the enclosure determines the resonant frequency of the port - if we stay high enough above that resonant frequency, it won't be producing any output, only the driver will.
Depending on the tune, you may get a few extra dbs with the ported design as you get closer to 30hz (this would mean the port is just beginning to be "activated"), but by and large, the output is the same, as the driver is providing nearly 100% of the output in both cases. If the enclosures are the same size, FR, group delay, transient response, and distortion levels should be almost identical in the musical range.
In a ported design, if the tune is low enough and the FR is flat, the sub will behave almost identically to a sealed sub in the musical range (which I consider above 30hz) because the frequencies aren't low enough to be "activating" the port, at least not significantly. Earlier we discussed that the diameter and length of the port in relation to the size of the enclosure determines the resonant frequency of the port - if we stay high enough above that resonant frequency, it won't be producing any output, only the driver will.
Depending on the tune, you may get a few extra dbs with the ported design as you get closer to 30hz (this would mean the port is just beginning to be "activated"), but by and large, the output is the same, as the driver is providing nearly 100% of the output in both cases. If the enclosures are the same size, FR, group delay, transient response, and distortion levels should be almost identical in the musical range.
I saw this thread but have been avoiding it; I’ve been spending hours a day online and not making any progress on my projects.
Vaughan, it sounds like you haven’t read the AVSForum thread yet; it has some good explanations.
I’m just going to touch upon some things quickly.
It’s kind of misleading to say the port sucks and blows; it does, but everything it does is driven by the woofer.
As has been said, the air in the port is a mass on the spring formed by the air in the box, and is coupled to the driver through that spring.
“ in a nutshell, the port sucks air forcefully at frequencies close to tuning which to a large degree cancels out the force of the drivers motion as it is moving forward because the port is coupling to the driver. “
The driver’s force is not being canceled; in fact at Fb the loading on the driver is maximum, due to the resonating port air moving into the enclosure and pushing in just when the driver is trying to push out.
“:the tune is achieved by the port diameter and length in relation to the internal volume.”
Making the port longer increases the mass of air in it and lowers Fb.
Making the port bigger raises the freq for the same reason a big driver’s Fsc is raised more when put in a given box than a small driver’s.
Which is because for a given excursion of the port air or driver, more air is being displaced, and displacing a bigger % of the box air requires more force aka a stiffer spring.
Or to think of it in hydraulic terms, to displace a larger diameter piston against the same pressure requires more force.
“Noah spoke about the port needing to "ramp up" to full amplitude. I don't know what he means by that.”
Resonance occurs because of the accumulation of energy over several cycles. Think of pushing someone on a swing, which is a pendulum resonator with gravity as the restoring force (spring).
Pushing with same effort for several swings (cycles) is required to reach max height (amplitude), which occurs when the energy dissipated by damping force, which increase with amplitude, equals the driving force per cycle.
”And I hear all the time that ported systems are 4th order. What does that mean ? What is the "order" ?”
I don’t have a solid intuitive grasp on this one; but there’s one order for each reactive element, which in mechanical systems is a mass or a spring.
The driver has a mass and spring, and the box stiffness and port air mass are two more.
As has been said (I think), each order contributes 6 dB/oct to the slope of the rolloff.
“Why does the port start to decouple or go out of phase with the driver below tuning ? Is it because the port was not tuned to those frequencies below fB ?”:
Think of the extreme case when the driver is moving very slowly, like if you just push in on it with your hand. It will just push the box air it displaced out of the port.
Again I suggest `the AVS thread for the basics of what’s happening below, at, and above resonance.
“many people are under the assumption that a port in a ported subwoofer or speaker is used all the time.”
I’m not sure why you think that, but in any case, download Unibox and look at the outputs for the system, driver, and port; you’ll see that the port output is a relatively narrow range.
“So perhaps someone could explain why the port only starts to suck in air from the internal air pressure near resonance or near the tuning frequency rather than at higher frequencies.”
At higher frequency the inertia of the port air makes it too sluggish to move back and forth as fast as the driver.
“describes how a TL enclosure and a driver couple to form a new resonant system. Exactly the same thing happens when a bass reflex enclosure and a driver are coupled together.”
I believe they are quite different. BR is a Helmholtz resonator (mass/spring system), whereas a TL is an acoustic resonator that depends on the speed of sound and the length of the resonant pipe.
“the air at the inside end of the vent for example, does not ever get blown right out into the outside environment”
My first sub was a Cerwin Vega ported design that blew pieces of fiberglass stuffing out the port and 6 ft into the room on the 1812 Overture cannon shots.
Shoot, there went another 15 min. I don't know how Mark Seaton does it.
Vaughan, it sounds like you haven’t read the AVSForum thread yet; it has some good explanations.
I’m just going to touch upon some things quickly.
It’s kind of misleading to say the port sucks and blows; it does, but everything it does is driven by the woofer.
As has been said, the air in the port is a mass on the spring formed by the air in the box, and is coupled to the driver through that spring.
“ in a nutshell, the port sucks air forcefully at frequencies close to tuning which to a large degree cancels out the force of the drivers motion as it is moving forward because the port is coupling to the driver. “
The driver’s force is not being canceled; in fact at Fb the loading on the driver is maximum, due to the resonating port air moving into the enclosure and pushing in just when the driver is trying to push out.
“:the tune is achieved by the port diameter and length in relation to the internal volume.”
Making the port longer increases the mass of air in it and lowers Fb.
Making the port bigger raises the freq for the same reason a big driver’s Fsc is raised more when put in a given box than a small driver’s.
Which is because for a given excursion of the port air or driver, more air is being displaced, and displacing a bigger % of the box air requires more force aka a stiffer spring.
Or to think of it in hydraulic terms, to displace a larger diameter piston against the same pressure requires more force.
“Noah spoke about the port needing to "ramp up" to full amplitude. I don't know what he means by that.”
Resonance occurs because of the accumulation of energy over several cycles. Think of pushing someone on a swing, which is a pendulum resonator with gravity as the restoring force (spring).
Pushing with same effort for several swings (cycles) is required to reach max height (amplitude), which occurs when the energy dissipated by damping force, which increase with amplitude, equals the driving force per cycle.
”And I hear all the time that ported systems are 4th order. What does that mean ? What is the "order" ?”
I don’t have a solid intuitive grasp on this one; but there’s one order for each reactive element, which in mechanical systems is a mass or a spring.
The driver has a mass and spring, and the box stiffness and port air mass are two more.
As has been said (I think), each order contributes 6 dB/oct to the slope of the rolloff.
“Why does the port start to decouple or go out of phase with the driver below tuning ? Is it because the port was not tuned to those frequencies below fB ?”:
Think of the extreme case when the driver is moving very slowly, like if you just push in on it with your hand. It will just push the box air it displaced out of the port.
Again I suggest `the AVS thread for the basics of what’s happening below, at, and above resonance.
“many people are under the assumption that a port in a ported subwoofer or speaker is used all the time.”
I’m not sure why you think that, but in any case, download Unibox and look at the outputs for the system, driver, and port; you’ll see that the port output is a relatively narrow range.
“So perhaps someone could explain why the port only starts to suck in air from the internal air pressure near resonance or near the tuning frequency rather than at higher frequencies.”
At higher frequency the inertia of the port air makes it too sluggish to move back and forth as fast as the driver.
“describes how a TL enclosure and a driver couple to form a new resonant system. Exactly the same thing happens when a bass reflex enclosure and a driver are coupled together.”
I believe they are quite different. BR is a Helmholtz resonator (mass/spring system), whereas a TL is an acoustic resonator that depends on the speed of sound and the length of the resonant pipe.
“the air at the inside end of the vent for example, does not ever get blown right out into the outside environment”
My first sub was a Cerwin Vega ported design that blew pieces of fiberglass stuffing out the port and 6 ft into the room on the 1812 Overture cannon shots.
Shoot, there went another 15 min. I don't know how Mark Seaton does it.
Oh, one more thing.
If you're wondering why below resonance is stiffness limited, stiffness driven behavior, and above resonance mass-limited, or mass driven behavior:
Above resonance, there's less time per cycle before the driver reverses direction, so displacements decrease with increasing frequency.
The spring force then is also decreasing; the spring constant (stiffness) is defined as force per displacement, so less displacement means less force.
OTOH, as freq increases the mass is being yanked back and forth faster and faster, which is resisted by inertia - thus mass-limited.
As freq decreases below resonance, the inertial forces are decreasing. With longer time available per cycle before reversing direction, the spring force resisting displacement are increasing - thus stiffness-limited.
If you're wondering why below resonance is stiffness limited, stiffness driven behavior, and above resonance mass-limited, or mass driven behavior:
Above resonance, there's less time per cycle before the driver reverses direction, so displacements decrease with increasing frequency.
The spring force then is also decreasing; the spring constant (stiffness) is defined as force per displacement, so less displacement means less force.
OTOH, as freq increases the mass is being yanked back and forth faster and faster, which is resisted by inertia - thus mass-limited.
As freq decreases below resonance, the inertial forces are decreasing. With longer time available per cycle before reversing direction, the spring force resisting displacement are increasing - thus stiffness-limited.
Noah, thank you for posting. There is one thing that you brought up which I don't fully grasp.
According to the essay on porting (home theater hifi) , the driver will be moving the least at resonance. The port will be doing all the work.
So I thought that since the port is sucking air so forcefully through the internal air pressure at and around Fb, that it would counterbalance the excursion from the driver and "cancel" the movement to a degree.
You say that that isn't the case. But then where does the low distortion come into play in a ported system ? I had always thought that the driver wouldn't be working as hard at resonance.
I am really confused at this point.
--Sincerely,
According to the essay on porting (home theater hifi) , the driver will be moving the least at resonance. The port will be doing all the work.
So I thought that since the port is sucking air so forcefully through the internal air pressure at and around Fb, that it would counterbalance the excursion from the driver and "cancel" the movement to a degree.
You say that that isn't the case. But then where does the low distortion come into play in a ported system ? I had always thought that the driver wouldn't be working as hard at resonance.
I am really confused at this point.
--Sincerely,
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