Loren42, I really like your ideas. I don't fully understand what they are thought. I know that larger ports would be the best, but I don't want to increase my holes for ports yet. As i said before I have a feeling that lower tuning might not solve my problem completely (it most likely will, but I have a feeling) if lowering tuning works well, I will obviously put larger ports. The problem would be that they would be really long. That's where you can help
This might sound a little retarded but... When doing this test, how do I measure cone displacement? What exactly am I looking for? Also, if I ever do it, what will the results mean? how can I interpret them?
The shelf port is a rectangular port instead of round. To keep your box internal volume the same I would add the port externally to the box. This is a quick and dirty sketch of the concept. Obviously, we need to determine the port length and make the piggy back tunnel the right length.
Two of the renderings have all of the original box's walls made invisible except the back wall of the cabinet. To make this thing you add the solid rendered wood to your original box and cut a large rectangular slot in the back of the cabinet. The actual details will vary once we get the designed dialed in, but I just wanted to give you a concept of the idea.
Also, tuning lower extends the bass, but also lowers the bass SPL level more and more as you go lower in frequency. It is a little like looking at the plots I did for you and as you lower the port tuning the response gets more like the red line (sealed cabinet) response. It goes deeper, but you also get the effect that the bass is weak.
An externally hosted image should be here but it was not working when we last tested it.
The idea of putting tape on the speaker and observing the cone's movement is simply a visual check to show that the cone moves more as the bass gets lower. It is very subjective and it is also a bit misleading.
It is subjective because you really can't quantitatively measure the amount of displacement by eye.
It is misleading because cone excursion, even in a sealed enclosure, is not linear with frequency. Cone excursion rapidly gets larger as the frequency gets lower.
If you could quantitatively measure the cone excursion in both a ported and sealed cabinet and plot them you would be able to see how the cone excursion gets very large below the tuning point of the cabinet.
You can see the exact same thing quantitatively on the graphs I did for you for xmax (3rd graph down on the right). The plots show the amount of cone excursion and they also show when the cone exceeds xmax for both sealed and ported designs (traces get lighter in color).
Bear in mind that both the sealed and ported designs both require some sort of low cutoff filter to prevent the cone from over extending. You can see from the graph that sealed and ported designs exceed xmax at just about the exact same frequency.
I hope you can see this thread from another forum. I thought that you might want to see an alternate approach to what you are doing using a different driver. This driver has stunning performance and in the right box makes an outstanding sub. Something you might want to consider if you ever want to do this again. It also contains a shelf port that is internal to the box.
The driver has since been obsoleted and this newer version is available that actually is better than the old one at a real bargain of $140.
As a last thing, make sure you have the speaker voice coils wired correctly. That driver has dual voice coils and I modeled everything with both coils in parallel.
Last edited:
How to SEE your tuning of loudspeaker port
Like the other guy said it is a visual check but this is the method and explanation.
Cone motion doubles to achieve the same SPL each time the frequency is reduced by half. So if the amplitude at 100Hz is 2mm at 50Hz it would be 4mm and 25Hz it would be 8mm. This assumes the woofer in the box is tuned below 25Hz. In your case with the port get the light good and the piece of white tape on the cone and apply like 60Hz at low volume. Adjust the volume until the cone amplitude is about 8mm peak to peak. Now sweep the frequency up an down and simply watch the cone motion. You will see the motion decrease with increasing frequency and increase with decreasing frequency until the port loading comes into play. As you decrease frequency the cone motion will decrease to a minimum. This minimum cone motion is near or at the port tuned frequency (near only if the woofer is not a Q=.383 and tuning is not perfect.) so this should be the port frequency. A floppy piece of paper strip (newspaper?) (for you this would be about 1 inch wide and about 2 inches long over the port see image) hung in front of the port will show maximum amplitude at the port tuned frequency. With just 5 minutes practice you will quickly be able to see the cone motion increase and decrease and be able to find the cone motion minimum frequency and port motion maximum. It truly is easy once you get the hang of it.
I suppose I have done this to many times because I can see it so well that I just read the numbers right off the oscillator and never hook up the rest of the test gear unless someone is paying. This give a direct reading of the actual tuned system and not the reflected electrical result and is therefore actually more accurate than any electrical measurement done with the speaker leads. I have seen errors as high as 30% off when measuring the electrical values rather than measuring the cone motion and port motion directly. Of course 30% is an extreme case but 10% off is not particularly unusual. In this whole speaker thing I have learned that direct measurement of every parameter is always the best choice as opposed to infer measurement through the electrical system.
As a note- this works with a sealed box also but the cone motion reaches a maximum and then stops increasing. This is not the tuned frequency but is a very good indicator of the -3dB point on the sealed box system. Of course nothing can tell us about the fabled transmission line.
If you need more please ask as specific a question as possible.
Like the other guy said it is a visual check but this is the method and explanation.
Cone motion doubles to achieve the same SPL each time the frequency is reduced by half. So if the amplitude at 100Hz is 2mm at 50Hz it would be 4mm and 25Hz it would be 8mm. This assumes the woofer in the box is tuned below 25Hz. In your case with the port get the light good and the piece of white tape on the cone and apply like 60Hz at low volume. Adjust the volume until the cone amplitude is about 8mm peak to peak. Now sweep the frequency up an down and simply watch the cone motion. You will see the motion decrease with increasing frequency and increase with decreasing frequency until the port loading comes into play. As you decrease frequency the cone motion will decrease to a minimum. This minimum cone motion is near or at the port tuned frequency (near only if the woofer is not a Q=.383 and tuning is not perfect.) so this should be the port frequency. A floppy piece of paper strip (newspaper?) (for you this would be about 1 inch wide and about 2 inches long over the port see image) hung in front of the port will show maximum amplitude at the port tuned frequency. With just 5 minutes practice you will quickly be able to see the cone motion increase and decrease and be able to find the cone motion minimum frequency and port motion maximum. It truly is easy once you get the hang of it.
I suppose I have done this to many times because I can see it so well that I just read the numbers right off the oscillator and never hook up the rest of the test gear unless someone is paying. This give a direct reading of the actual tuned system and not the reflected electrical result and is therefore actually more accurate than any electrical measurement done with the speaker leads. I have seen errors as high as 30% off when measuring the electrical values rather than measuring the cone motion and port motion directly. Of course 30% is an extreme case but 10% off is not particularly unusual. In this whole speaker thing I have learned that direct measurement of every parameter is always the best choice as opposed to infer measurement through the electrical system.
As a note- this works with a sealed box also but the cone motion reaches a maximum and then stops increasing. This is not the tuned frequency but is a very good indicator of the -3dB point on the sealed box system. Of course nothing can tell us about the fabled transmission line.
If you need more please ask as specific a question as possible.
Attachments
Last edited:
Hi fouchagalaga and all,
I just found this thread and the issues discussed are so close to those I'm currently considering I thought it might be relevant to ask my question here.
I am looking for the best way to implement a 4 inch diameter x 3 foot long round port with flares on both ends into my 10"sub-woofer cabinet design. ( one per channel )
I'm combining cheap & sturdy 4" PVC tubing & bends with Dayton flares from Parts Express and my question is about the most ideal place for the port to begin inside the cabinet.
Is it best for the port to begin on the outside of the cabinet as in diagram 1 or in the middle of the cabinet as in diagram 2 ?
Any advice welcome
thanks
mike
I just found this thread and the issues discussed are so close to those I'm currently considering I thought it might be relevant to ask my question here.
I am looking for the best way to implement a 4 inch diameter x 3 foot long round port with flares on both ends into my 10"sub-woofer cabinet design. ( one per channel )
I'm combining cheap & sturdy 4" PVC tubing & bends with Dayton flares from Parts Express and my question is about the most ideal place for the port to begin inside the cabinet.
Is it best for the port to begin on the outside of the cabinet as in diagram 1 or in the middle of the cabinet as in diagram 2 ?
Any advice welcome
thanks
mike
Attachments
Last edited:
I would try to contain all the pipe inside if possible, but technically, either 1 or 2 should work about the same.
Ports do not need to exit out the front baffle, so rear firing ports are fine, too.
For the port signal to add up with the active driver signal the port should be next to the active driver. At port tuning frequency in an ideal cabinet the port signal is exactly 1 full cycle behind the drive signal. To keep everything in the correct phase these two "sources" should be kept near each other.
That depends on the port tuning frequency. If the port is tuned to 30 Hz, one wavelength is 344 meters/ 30 = 11.5 meters.
If the port is on the backside of the cabinet and is .5 meters away from the front baffle, the margin of error is about (.5 meters/11.5 meters) * 100 = 4.4%.
What that means is the actual port tuning frequency will be slightly misaligned from the theoretical predicted value. The amount is not considered to be significant in the larger picture of things.
You should also be able to fine tune the pipe length to compensate for this when the cabinet is built and an impedance sweep is carried out.
There are many successful commercial speaker systems out there that employ this strategy.
I am a bit of purist and my ports exit off the front baffle, but this makes testing the bass frequency response a little more difficult. It would be nice to be able to take a near-field reading of the woofer and one of the port without acoustic interaction or "contamination" of the measurement. It is very hard, even in a professional anechoic chamber, to empirically measure a vented speaker's low end response.
Last edited:
In the cabinet the active driver has two impedance peaks. The entire region from the higher frequency peak down is "overlap" area.
Tuning a port at 30Hz means the delay is about 33.3ms which is dangerously close to echo. Some say echo is 40ms and all agree it is clearly echo at 50ms delay. Tuned at 30 mean a bump becomes a ba-bump because of the delay. Experience has shown tuning in the 20-40Hz region is not that great an idea. Above 40 the delay is less than 25ms and tuning below 20 it is out of the audio band.
Also note the 4" port will go into power compression as discussed earlier in this thread. Exactly where the power compression sets in is not to easy to say. If you have the choice to use two 4 inch ports it would be better.
Tuning a port at 30Hz means the delay is about 33.3ms which is dangerously close to echo. Some say echo is 40ms and all agree it is clearly echo at 50ms delay. Tuned at 30 mean a bump becomes a ba-bump because of the delay. Experience has shown tuning in the 20-40Hz region is not that great an idea. Above 40 the delay is less than 25ms and tuning below 20 it is out of the audio band.
Also note the 4" port will go into power compression as discussed earlier in this thread. Exactly where the power compression sets in is not to easy to say. If you have the choice to use two 4 inch ports it would be better.
Well because I have two subs, one for each channel and because my neighbours will probably come and beat me up if I play it loud enough to get chuffing from stereo 4" ports with flares both ends . . . . I think 4" will be ok - but I appreciate your logic
And I'm also going to try infinite baffle to see how that compares - perhaps BR for movies & IB for music - we'll see
This may sound a little off content right now... but I just want to say that I have installed the longer ports already a week ago now and the difference is very clear. Althought I only have 3'' ports now, I will eventually upgrade to 4'' that follow the sides of the enclosure so that it won't look silly. I am still not completely convinced with the performance of my sub, but it still does sound pretty good when I stay realistic. I will post later my Ideas to integrate ports better. It seems as my computer won't let me upload photos, so I just won't upload them.. And also... Hello mikelm! welcome to the thread!
Thanks for your welcome - I'm wondering do you have speaker design software on ur PC ?
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.