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Discussion Starter · #1 ·
Hello all:


I am looking for an explanation as to how a ported sub (or any other bass reflex speaker) works to augment the low frequencies. I am confused as to the physics behind this.


For example: if the driver is moving outward (positive going signal), the air in front of the driver is being compressed. But the air behind it is being decompressed. Hence the outside air will travel into the port, while the air in front of the driver is moving away from the speaker. This would seem to create an "out of phase" situation with the sound waves eminating from the driver and the port, hence causing phase cancellation and a lowering of low frequency sound level, rather than an augmentation of the sound level.


In the case of subs where both the driver and the port are both downward firing (or are both forward firing) it would seem that this effect would be even more pronounced.


So what am I missing here in my understanding? How does the ported design work to actually increase low frequency output?


Thanks for any help in understanding this.


Frank
 

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Almost afraid to give my take on it. I don't really design subs, but I have desined a fair amount of speaker systems in my day.


But, never the less.


The port and cab effectively create a tuning (one tuning), so when it's resonating it really is only resonating at one frequency + harmonics of that frequency as a by product. That tuning is usually well below what the driver really can produce.


Generally there are no phase issues because it is not reproducing a frequency that is being reporduced by the driver.


Now, smart people... Well people who can work math, can usually use the driver characteristics, power considerations in conjunction with the cab/port tuning to create the nicest / flattest curve they can with a little room eq thrown in.



It seems your thinking of how a Passive Radiator works perhaps?
 

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The passive radiator works almost exactly like the air mass in a port. It's designed to resonate at at a particular tuning frequency to augment the output of the active driver as the active driver begins to roll off. It doesn't move out of phase with the active driver, but resonates in phase at and around the tuning frequency.
 

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I know that phase issues or at least potential phase issues are why many DIY'rs like myself through the years have stayed away from them.


I am way weak on the passive radiator subject though admittedly.


I'd be up for reading some links on the subject indeed.
 

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Quote:
Originally Posted by sound dropouts /forum/post/14189962


So how would one "tune" a radiator?

The passive radiator can be tuned by adjusting the mass of the driver. Increasing the mass lowers the tuning.

Quote:
Originally Posted by JOHNnDENVER /forum/post/14189981


I know that phase issues or at least potential phase issues are why many DIY'rs like myself through the years have stayed away from them.


I am way weak on the passive radiator subject though admittedly.


I'd be up for reading some links on the subject indeed.

Just like with a ported sub, the passive radiator begins to goe out of phase as you go to frequencies below tuning.
 

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Hi Frank,


Let's see if I can give a stab at this in terms of most basic function...


The confusion you are having comes from how and when the port operates. Trying to think of this in terms of what happens *when* will be confusing, as this function changes with frequency, so is not constant. If we focus on what happens at different frequencies, conceptual understanding is less complicated. Remember that especially in subwoofers, there is no separation of frequency response and what happens in time, as any change to one affects the other in a very predictable manner.


In simple terms, the port or PR are tuned to a resonant frequency based on size and length of the port or size and mass of the PR which will resonate at some frequency in given box size. This resonance isn't just at one frequency, but it is centered on what we call the tuning frequency. For frequencies above and below this tuning frequency, the strength of this resonance drops off. If we plot frequency in the horizontal and output in the vertical, we see that an ideal port or PR would have a bell shaped output centered on the tuning frequency. In simple terms the air in the port or the disk of the PR moves very easily near the tuning frequency, and much less so away from the tuning frequency.


It is important to note that the port/PR will have a tuning frequency that is independent of the woofer used or even being mounted in the closed box at all. When we do cut a hole and place a woofer in this enclosure, it is now acoustically coupled to the port. While not a precise analogy, to some degree you can think of the box as a mechanical lever connecting the motion of the cone to the motion of the air in the port or the movement of the PR. This "lever" effectively changes length with frequency. At the tuning frequency of the combined box and port/PR, the lever is long, and you get lots of port/PR motion with very little cone movement.


Just as with a real lever when small motion creates large motion on the other end, the woofer end is hardest to push when the lever produces the most port/PR motion from the cone motion. What we see in operation is that while the driver has to push very hard at the resonant frequency (where the lever makes the most motion in the port/PR), the cone movement is very small, even though the port/PR are moving a lot. "Stronger" drivers will push the port/PR more effectively here.


As I stated above, this virtual "lever effect" changes with frequency, and as we move further from the tuning frequency, the driver will feel less pushing back from the port/PR, and it will move much more. Well below the tuning frequency the driver will move close to what it would if there was no box at all, while the air in the port or PR cone will move opposite the driver. At the tuning frequency, the driver is not moving much, and the port/PR is moving greatly, allowing the port/PR to make sound while the cone of the woofer produces very little sound (it isn't moving much). The driver progressively moves more below the tuning frequency. Here the driver and port/PR are moving opposite eachother, and as the driver moves more, the output cancels more. This is why ported/PR boxes roll off faster than sealed boxes. The port and driver cancel eachother out more effectively as frequency goes lower.


Above the tuning frequency, the driver cannot efficiently move the port/PR, so the port/PR output is not enough to cancel the driver output. In ideal operation, the port or PR move very little air above the tuning frequency, and all, or at least the majority of the output, comes from the driver.


Of course you can get into all of the mechanics of resonators and the equivalent electro-mechanical circuits we use to model this behavior, but in the end the point is that at the tuning frequency the port makes the output, while the driver doesn't move much. Below the tuning frequency the driver moves more and the driver and port output are canceled more effectively the lower in frequency you go.


Hopefully some of that will make some sense.
 

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Quote:
Originally Posted by sound dropouts /forum/post/14189702


I have always wondered how passive radiators work....it seems the same as wiring two woofers out of phase....
Quote:
Originally Posted by sound dropouts /forum/post/14189962


So how would one "tune" a radiator?

A frequency of resonance (tuning frequency) will occur any time you have a spring (= volume of air in the box) coupled to a mass (slug of air in the port or mass of the PR). The size of a port or PR will affect the stiffness of the spring, where a smaller box or a larger port/PR area will make for a higher tuning frequency for a given length of port or weight (mass) of PR.


Once the size of the port or PR is chosen to allow enough output, the box size defines the "spring" in the mass-spring system. You can then adjust the mass of the system by either changing the mass of the air in the port, or by directly changing the mass of the PR. A longer port includes more air, which weighs more, which will resonate at a lower frequency. Shortening the port or removing mass from the PR will raise the tuning frequency.


A PR's mass can actually be calculated by modeling a port for the box having the same diameter as the PR. You can imagine that makes for a very long port when you have 12-18" nominal diameter ports! While there is some added complication of the suspension of the PR (surround + spider), if the PR is rather soft, the mass required for a given tuning frequency will be very close to the mass of the air in an equivalent diameter port tuned to the same frequency. Of course it's often easier to fit an 18" PR than it is to fit a 12' long port.
 
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Sweeet post....


You really made a complex subject, decently simple, and well, pretty clear on the reactions and counter reactions involved.


Nice, when you read links, and even books. The relationships explained in the above post are revealed, but typcially you have to draw out those conclusions combining information from several sections.


Thanks again Mark Seaton...
 

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Quote:
Originally Posted by JOHNnDENVER /forum/post/14190555


Sweeet post....


You really made a complex subject, decently simple, and well pretty clear on the reactions and counter reactions involved.


Nice, when you read links, and even books. The relationships explained in the obove post are revealed, but typcially you have to draw out those conclusions combining information from several sections.


Thanks again Mark Seaton...

+1. Thanks Mark!
 

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Agreed... I'm always learning something new from Mark...
 

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Discussion Starter · #14 ·

Quote:
Originally Posted by Mark Seaton /forum/post/14190200



Hi Frank,


.........Of course you can get into all of the mechanics of resonators and the equivalent electro-mechanical circuits we use to model this behavior, but in the end the point is that at the tuning frequency the port makes the output, while the driver doesn't move much. Below the tuning frequency the driver moves more and the driver and port output are canceled more effectively the lower in frequency you go.


Hopefully some of that will make some sense.

Hello Mark:


Thanks so much for your excellent explanation! This is just what I needed. It also somewhat explains why ports can be plugged to change the bass extension. I really appreciate the effort that you put into your response.


Given your explanation, I guess that it would be possible to design a ported enclosure so poorly that a given frequency center could indeed suffer reduced output due to overlap in the tunings for the port/PR and driver. In other words, the tunings for the port and driver completely overlap rather than complement each other. Then the output waveforms would truly be out of phase with respect to amplitude at that frequency center and would cancel each other.


Also, I assume that this same explanation holds true for any kind of bass port? I'm thinking specifically about subs or full range cabinets that use slotted ports rather than cylindrical.


Thanks again and Best wishes,
 

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Quote:
Originally Posted by Yardman 49 /forum/post/14191555


Hello Mark:


Thanks so much for your excellent explanation! This is just what I needed. It also somewhat explains why ports can be plugged to change the bass extension. I really appreciate the effort that you put into your response.

Hi Frank,


It is always nice to know when a bit of what was typed is read and understood, and even better if appreciated.



It sounds like you got most of it, but might have misunderstood some other parts.


Before getting too far ahead, we do have to remember to separate function of the system vs. how well it is implemented. Understanding the type of system and its defining variables make it clear what happens in ideal operation. Whether or not the resulting behavior is a response which provides good sound is separate from understanding the operation. Understanding the operation lets us know how to change the system to get a more desireable response. The defining variables and assumed conditions must also behave ideally for the system to respond as we expect. Non-linearities such as a driver or PR running out of excursion, or a port constricting airflow cause operation to deviate from the ideal operation. Sometimes this deviation is small, and sometimes it is very large.

Quote:
Given your explanation, I guess that it would be possible to design a ported enclosure so poorly that a given frequency center could indeed suffer reduced output due to overlap in the tunings for the port/PR and driver. In other words, the tunings for the port and driver completely overlap rather than complement each other. Then the output waveforms would truly be out of phase with respect to amplitude at that frequency center and would cancel each other.


Also, I assume that this same explanation holds true for any kind of bass port? I'm thinking specifically about subs or full range cabinets that use slotted ports rather than cylindrical.


Thanks again and Best wishes,

I'm not quite sure I understand your question above, but a "poor design" only can be determined once you define what the desired or target design is. Depending on the application, what is good for one application will be useless for another.


In simple terms, a typical reflex design is not generally used much below the tuning frequency. While there are exceptions, the output of a ported box will typically be falling quickly below ~85% of the tuning frequency, and getting significant SPL below there requires lots of excursion from the driver. At some range well above the tuning frequency, the sound produced is dominated by the driver (assuming the port isn't making any un-intended output). The port's dominant effect will be near the tuning frequency and below, but the driver will still be able to play much higher in frequency regardless of the port's operation.


There is no requirement for a ported/PR design to be tuned very low in frequency. There are plenty of cases where we will want higher frequency tunings, especially in full range speakers. You can even tune speakers above 200Hz with the right combination of parts and sizes. In every case, the driver will move less at the tuning frequency and the port produces the majority of the sound output (more than the driver).


Problems do arise when the port doesn't behave as a port, or has other issues such as a very short in length and very large diameter port, which would obviously let many other frequencies out. Even so, this is a secondary behavior which is not defined by the reflex enclosure, but rather by a large opening which allows un-intended frequencies to leak out. In a reflex box design, a port is intended to have a defined resonance. When shapes get too irregular, ports are not reasonably constant in opening area, or ports get very short (or long), operation will not meet the assumptions of an assumed port, or simple Helmholtz resonator. It will still have some operation in common, but there will also be other affects which may or may not be useful.
 

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I'm sorry for this very stupid question. I'm curious if you can put a port for the tweeter (assuming the tweeter has its own enclosure) to extend the lower end for the tweeter?
 

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Quote:
Originally Posted by chengbin /forum/post/14192262


I'm sorry for this very stupid question. I'm curious if you can put a port for the tweeter (assuming the tweeter has its own enclosure) to extend the lower end for the tweeter?

No and most tweeters are self contained and why would you want to extend it.
 

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Quote:
Originally Posted by chengbin /forum/post/14192262


I'm sorry for this very stupid question. I'm curious if you can put a port for the tweeter (assuming the tweeter has its own enclosure) to extend the lower end for the tweeter?

Possible yes. Practical, or easy to construct? No.



The sizes and frequency ranges involved are not very conducive to both locating the port in a useful location (remembering high frequencies are much more directional), nor getting the sizes and high tuning frequencies without having other non-port like effects. If you had wider range driver that extended well below 1kHz this would be a more attractive option, but still one with many obstacles to navigate.
 

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Wilson has ported midranges, a lot of people seem to think their speakers sound pretty good (please ignore price tags or "value"). Another application of higher tunings would be 6th order bandpass boxes where you want the top end tuned to around 60-80hz. I'd love to do a 6th order reflex system with a PR for the top end but I don't know of any PR's suitable for high tuning like this, they would be pretty easy to make though.
 
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