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Old 22nd July 2006, 04:46 PM   #1
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Default Understanding ported systems

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,
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Old 22nd July 2006, 08:24 PM   #2
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Anyone ?

--Sincerely,
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Old 22nd July 2006, 09:05 PM   #3
Dave is offline Dave  New Zealand
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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
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Old 22nd July 2006, 09:21 PM   #4
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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,
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Old 22nd July 2006, 09:32 PM   #5
ajazz is offline ajazz  India
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Default 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:[B]
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).
Attached Files
File Type: zip tsparam.zip (9.6 KB, 52 views)
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Old 22nd July 2006, 09:47 PM   #6
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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,
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Old 22nd July 2006, 10:06 PM   #7
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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.
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Old 22nd July 2006, 10:14 PM   #8
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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,
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Old 22nd July 2006, 10:33 PM   #9
dnsey is offline dnsey  United Kingdom
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A search on 'Helmholtz resonator' should turn up what you need - some fairly mathematical treatments, and others which explain by analogy.
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Old 23rd July 2006, 01:14 AM   #10
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Quote:
BTW, is my understanding of ported operation accurate (first post) ?
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.

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.
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