Hi all,
I want to know indepth how a ported subwoofer/speaker works. I think I have a relatively good understanding, but I want to know more. So I want to give my explanation and if there are any errors or misunderstandings then please clarify.
A ported subwoofer has a port which is tuned to a specific frequency depending on the diameter and length of the port. Above the tuning the frequency, the driver is still moving most. The port is not really working, it isn't coupled, I should say, to the driver above resonance or tuning.
The reason for this, I'm not sure. Is it because the driver is moving too quick for the mass of air to move out the port ? Perhaps someone will clarify. Now people I have spoken to on AVS forum have told me that ported systems behave very much if not identically to sealed above their tuning frequency where the port is not activated.
Now close to tuning, the port will start to couple to the driver and suck air in and this will counteract the drivers own high excursion. So cone excursion will be reduced because the energy will work against the drivers motion. Not too sure on that, sounds a bit shaky, so perhaps someone can clarify.
Below tuning the enclosure acts like a sealed enclosure with a big hole in it. So the port will start to work out of phase with the driver.
Many people that I know assume that the port is working all the time in a ported enclosure but that isn't the case. The driver is doing most of the work most of the time well above tuning but it's when frequency drops that the port starts to contribute.
So ported system is a 4th order system. 2 order for the driver and 2 order for the port. Summed it is 4th order. I don't know what it means, what 2 or 4th order means, perhaps someone can clarify, but I know that 2nd order drop off equals 12 dB/per octave and 4th order drop off or roll off is 24 dB per octave.
So 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. So less excursion. Less distortion.
One last thing. Minimum phase. What does that mean exactly ? I hear that term a lot on some forums but I have no clue what it means. I've heard it referenced in acoustics and in terms of subwoofers.
I hope the more experienced users here can clarify some of the issues I have with my understanding and help me to improve what I know on the subject. I hope all my questions will be answered.
Thank you.
--Sincerely,
I want to know indepth how a ported subwoofer/speaker works. I think I have a relatively good understanding, but I want to know more. So I want to give my explanation and if there are any errors or misunderstandings then please clarify.
A ported subwoofer has a port which is tuned to a specific frequency depending on the diameter and length of the port. Above the tuning the frequency, the driver is still moving most. The port is not really working, it isn't coupled, I should say, to the driver above resonance or tuning.
The reason for this, I'm not sure. Is it because the driver is moving too quick for the mass of air to move out the port ? Perhaps someone will clarify. Now people I have spoken to on AVS forum have told me that ported systems behave very much if not identically to sealed above their tuning frequency where the port is not activated.
Now close to tuning, the port will start to couple to the driver and suck air in and this will counteract the drivers own high excursion. So cone excursion will be reduced because the energy will work against the drivers motion. Not too sure on that, sounds a bit shaky, so perhaps someone can clarify.
Below tuning the enclosure acts like a sealed enclosure with a big hole in it. So the port will start to work out of phase with the driver.
Many people that I know assume that the port is working all the time in a ported enclosure but that isn't the case. The driver is doing most of the work most of the time well above tuning but it's when frequency drops that the port starts to contribute.
So ported system is a 4th order system. 2 order for the driver and 2 order for the port. Summed it is 4th order. I don't know what it means, what 2 or 4th order means, perhaps someone can clarify, but I know that 2nd order drop off equals 12 dB/per octave and 4th order drop off or roll off is 24 dB per octave.
So 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. So less excursion. Less distortion.
One last thing. Minimum phase. What does that mean exactly ? I hear that term a lot on some forums but I have no clue what it means. I've heard it referenced in acoustics and in terms of subwoofers.
I hope the more experienced users here can clarify some of the issues I have with my understanding and help me to improve what I know on the subject. I hope all my questions will be answered.
Thank you.
--Sincerely,
Hi Vaughan,
I think your best bet would be to find a good book on the subjects you're interested in.
The Loudspeaker design cookbook by Vance Dickason is considered to be pretty good I think.
Some good websites are
http://www.geocities.com/kreskovs/John1.html
and his new site
http://www.musicanddesign.com/
and of course
www.linkwitzlab.com
I think your best bet would be to find a good book on the subjects you're interested in.
The Loudspeaker design cookbook by Vance Dickason is considered to be pretty good I think.
Some good websites are
http://www.geocities.com/kreskovs/John1.html
and his new site
http://www.musicanddesign.com/
and of course
www.linkwitzlab.com
Thank you for posting the links. I have the Vance Dickason book and it is good, but it doesn't go into theory very well.
Just a few lines here and there describing port operation but nothing indepth. It doesn't explain the how or the why.
This is what I need to know. This is what I want to learn. I want to grasp the physics of ported design better. I will check those links you posted.
I still would appreciate feedback from those with more knowledge than myself. Thank you very much.
--Sincerely,
Just a few lines here and there describing port operation but nothing indepth. It doesn't explain the how or the why.
This is what I need to know. This is what I want to learn. I want to grasp the physics of ported design better. I will check those links you posted.
I still would appreciate feedback from those with more knowledge than myself. Thank you very much.
--Sincerely,
Depends on speaker & Cabinet volume
Even I was confused like U, when I stared new in designing ported enclosures. Nobody could answer in a thumb rule funda.
The thumb rule is:
Every speaker has its own free air resonance frequency, when u install it in a sealed enclosure the resonance frequency goes higher depending on the box volume but with a less pronounced resonance peak. In this case the box volume determines ur lowest bass frequency output and/or it is equalised by active bass boost circuit(12db octave) to achieve flat or boosted bass response. Hence the drivers used for sealed applications are of high Xmax long throw voice coils and a powerfull motor.
Now regarding ported box(as I would say) it is wrong to say that it is tuned at 30Hz with a particular port Dia and length (regardless of speaker). Basically to achieve a almost flat frequency response (only Bass, Mid & highs has got nothing to do with the port) we have to suppress the resonance peak(speaker mounted) and at the same time allow controlled excursion below resonance to achieve good Bass.
After u mount the speaker in a ported box u will get 2 resonance peaks but smaller in amplitude compared to free air Fr. Now the trick is to adjust the port Dia & Length to get equal amplitude of peaks, so that u get almost flat response. If the peak below fr is higher, than the cone excursion will be more at lower frequencies which is dangerous and if the peak above Fr is higher u loose everything in quality.
The Box volume and the ports Dia-Length plays a very critical role to tune a particular speaker to achieve good results. It's not a carpenters job.
I think the file attached will help u to some extent. (got it from somewhere on the web .... very uesfull).
Even I was confused like U, when I stared new in designing ported enclosures. Nobody could answer in a thumb rule funda.
The thumb rule is:
Every speaker has its own free air resonance frequency, when u install it in a sealed enclosure the resonance frequency goes higher depending on the box volume but with a less pronounced resonance peak. In this case the box volume determines ur lowest bass frequency output and/or it is equalised by active bass boost circuit(12db octave) to achieve flat or boosted bass response. Hence the drivers used for sealed applications are of high Xmax long throw voice coils and a powerfull motor.
Now regarding ported box(as I would say) it is wrong to say that it is tuned at 30Hz with a particular port Dia and length (regardless of speaker). Basically to achieve a almost flat frequency response (only Bass, Mid & highs has got nothing to do with the port) we have to suppress the resonance peak(speaker mounted) and at the same time allow controlled excursion below resonance to achieve good Bass.
After u mount the speaker in a ported box u will get 2 resonance peaks but smaller in amplitude compared to free air Fr. Now the trick is to adjust the port Dia & Length to get equal amplitude of peaks, so that u get almost flat response. If the peak below fr is higher, than the cone excursion will be more at lower frequencies which is dangerous and if the peak above Fr is higher u loose everything in quality.
The Box volume and the ports Dia-Length plays a very critical role to tune a particular speaker to achieve good results. It's not a carpenters job.
I think the file attached will help u to some extent. (got it from somewhere on the web .... very uesfull).
Attachments
Ajazz, thank you for posting. But I want to learn how ports operate. The actual physics behind the operation. What happens that allows the port to move air in and out. How the driver excursion is at a minimum at resonance.
The why and the how. This is what I need to know. It's not about building ported speakers. I'm sure there are many people out there who have built ported speakers but didn't have a clue how they operate in physical terms.
This is what I want to understand better. Thanks again.
--Sincerely,
The why and the how. This is what I need to know. It's not about building ported speakers. I'm sure there are many people out there who have built ported speakers but didn't have a clue how they operate in physical terms.
This is what I want to understand better. Thanks again.
--Sincerely,
Vaughan - discussion should fill you in on everything you need to know.this
Near the end, through measurements Ilkka has posted, you'll also see why the difference in group delay between a sealed sub and a ported sub around its tuning is of little to no consequence, especially the lower you tune. You'll also notice that a beefy class AB amp provides current much faster than a digital switching amp with built in EQ and a high pass filter.
Near the end, through measurements Ilkka has posted, you'll also see why the difference in group delay between a sealed sub and a ported sub around its tuning is of little to no consequence, especially the lower you tune. You'll also notice that a beefy class AB amp provides current much faster than a digital switching amp with built in EQ and a high pass filter.
Thank you for posting the link ! A big thread. I'll need some time to go through it. BTW, is my understanding of ported operation accurate (first post) ? Please correct any errors or misunderstandings that I've made. I really would like to get input from you.
Thanks.
--Sincerely,
Thanks.
--Sincerely,
A search on 'Helmholtz resonator' should turn up what you need - some fairly mathematical treatments, and others which explain by analogy.
Somewhat - the tune is achieved by the port diameter and length in relation to the internal volume. Above the range of the tuning frequency, yes, the port isn't really providing much if any output, and the driver behaves like it would if it were sealed. Near tuning, the air inside the port is coupled to the driver, meaning it will move back and forth in relation to the movement of the driver - blowing and sucking.BTW, is my understanding of ported operation accurate (first post) ?
Noah once used an example of a rubber ball with a string attached to a paddle. Move the paddle too fast or too slow and the ball won't bounce in unison with it. Move it at just the right speed (or frequency) and the ball will be coupled to the movement on the paddle - and very little paddle motion is needed to keep the ball bouncing.
As for phase, the port output is going to be 180 degrees out of phase with the driver output at tuning - I believe group delay is a measure of the timing associated with this phase change per frequency. The more excursion the driver is seeing at and around tuning, the larger the group delay will be. However, the audibility of group delay is typically a non issue as long as it stays below one cycle of a given frequency, and the length of each cycle increases as the frequency lowers. Yet another reason why a low tune is desired for a ported sub.
Thanks for the information Steve ! In the thread, Noah spoke about the port needing to "ramp up" to full amplitude. I don't know what he means by that.
And I hear all the time that ported systems are 4th order. What does that mean ? What is the "order" ? Sorry, I'm just trying to fully understand this as best I can.
Thank you again for putting up with me. 🙂
--Sincerely,
And I hear all the time that ported systems are 4th order. What does that mean ? What is the "order" ? Sorry, I'm just trying to fully understand this as best I can.
Thank you again for putting up with me. 🙂
--Sincerely,
You can think of the "order" of the rolloff simply as the slope, or how quickly it drops off. I believe the conclusion we reached from that thread was that the time the port needs to "ramp up" is simply the time associated with the phase change between driver and port output - in other words, group delay.
We also found the difference in group delay between a 20hz tuned sub and a sealed sub (both using the same driver/amp) when playing a sine wave at the tuning point (20hz) is only 0.035 seconds. I don't know about anyone else, but I'm not going to claim I can differentiate 0.035 seconds of 20hz output 😀 Thus, when people tend to describe sealed subs as being tighter or faster, they are actually referring to in room FR, in which case a typical sealed sub usually has a boosted high end and little to no low end (giving the impression of much punch), while a typical ported sub usually has too big a bump in the low end due to the designer incorrectly trying to keep anechoic FR flat (giving the impression of muddy sound).
We also found the difference in group delay between a 20hz tuned sub and a sealed sub (both using the same driver/amp) when playing a sine wave at the tuning point (20hz) is only 0.035 seconds. I don't know about anyone else, but I'm not going to claim I can differentiate 0.035 seconds of 20hz output 😀 Thus, when people tend to describe sealed subs as being tighter or faster, they are actually referring to in room FR, in which case a typical sealed sub usually has a boosted high end and little to no low end (giving the impression of much punch), while a typical ported sub usually has too big a bump in the low end due to the designer incorrectly trying to keep anechoic FR flat (giving the impression of muddy sound).
One question : 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 ?
And one more. 😀 I understand now that the order is the roll off characteristics of the design. I know that sealed design drops off by 12 dB per octave and ported design is 24 dB.
Why is that ? Why is it that a sealed design drops off by 12 dB per octave ? How does one calculate this ?
Just out of curiosity. Thanks a lot !
--Sincerely,
And one more. 😀 I understand now that the order is the roll off characteristics of the design. I know that sealed design drops off by 12 dB per octave and ported design is 24 dB.
Why is that ? Why is it that a sealed design drops off by 12 dB per octave ? How does one calculate this ?
Just out of curiosity. Thanks a lot !
--Sincerely,
I believe the 12db/octave rolloff of a sealed driver is due to air compliance. As the cone tries to use more and more excursion, it sees more and more resistance from the air inside the sealed enclosure - as the air has to expand during the driver's front wave and contract during the driver's back wave. I don't know the exact equations, but this compliance ends up creating a 12db/octave rolloff, just as perfect room gain (in a room that is air tight and built like a tank) would equate to 12db/octave. In reality, most rooms do not get that full 12db/octave gain.
Below tuning, the port just acts like a hole in the enclosure, and much of the energy of the driver is simply lost.
Below tuning, the port just acts like a hole in the enclosure, and much of the energy of the driver is simply lost.
The sealed system is a 2nd-order system - that's how many poles the mathematical system representation has. Once you add a vent you add another resonant system with two poles again, making a 4th-order system.
You might like to read what Mssrs Small and Thiele wrote, on my website in the 'articles' section www.readresearch.co.uk
You might like to read what Mssrs Small and Thiele wrote, on my website in the 'articles' section www.readresearch.co.uk
Thanks a lot ! A lot of good information to read. Again, thanks.
One more question concerning ported design that just popped into my head. 🙂 Why is it that the port only starts to couple at or near resonance ?
I've heard from some people that the driver is moving too quickly for the port to be *activated*. I don't understand that too well. Perhaps someone could elaborate.
Because as I understand it, many people are under the assumption that a port in a ported subwoofer or speaker is used all the time. And I get the idea that it's basically a big misconception.
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.
Even though this thread is to help me to learn more how this system works, I'm sure several people reading will benefit too. So thanks for all those who contributed so far.
I really appreciate the advice.
--Sincerely,
One more question concerning ported design that just popped into my head. 🙂 Why is it that the port only starts to couple at or near resonance ?
I've heard from some people that the driver is moving too quickly for the port to be *activated*. I don't understand that too well. Perhaps someone could elaborate.
Because as I understand it, many people are under the assumption that a port in a ported subwoofer or speaker is used all the time. And I get the idea that it's basically a big misconception.
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.
Even though this thread is to help me to learn more how this system works, I'm sure several people reading will benefit too. So thanks for all those who contributed so far.
I really appreciate the advice.
--Sincerely,
The vent is a resonant system of it's own. That means it only operates over a narrow band. The air in the vent has a mass which when excited will want to move at a specific frequency - it's resonant frequency.
Think of a weight on a spring and what happens when you move your hand holding the weight/spring up and down at different speeds.
Above resonance the weight stays almost still, so it's like the vent is blocked. Below resonance, the weight is being moved slower than it's moving itself, so it's like the vent is open.
Note that the vent does not actually suck or blow any air at all. All that happens is that the air already in the vent moves back and forth.
Think of a weight on a spring and what happens when you move your hand holding the weight/spring up and down at different speeds.
Above resonance the weight stays almost still, so it's like the vent is blocked. Below resonance, the weight is being moved slower than it's moving itself, so it's like the vent is open.
Note that the vent does not actually suck or blow any air at all. All that happens is that the air already in the vent moves back and forth.
Again, noah katz could explain this better than me, but the air in the port has it's own resonant frequency that is determined by the diameter and length in relation to the volume of the subwoofer enclosure. Thus, that air in the port is only excited when the driver is reproducing frequencies in the range of the port's own resonance. Also, again, the port does not only suck in air, it also blows air out - it should alternate with the motion of the cone.
Where is Noah Katz ? 🙂
Excellent point. Although richie disagrees that the port sucks in air or blows air. Don't know why though.
--Sincerely,
Also, again, the port does not only suck in air, it also blows air out - it should alternate with the motion of the cone.
Excellent point. Although richie disagrees that the port sucks in air or blows air. Don't know why though.
--Sincerely,
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