My port doesn't work!

Tom Ato

Member
2018-04-13 8:09 pm
I am currently building speakers with the woofer being the Dayton Audio PA310-8, and thanks to all the handy simulation programs and internet calculators, I made my enclosure roughly 40 liters, and chose a 4" diameter port (I measured the ID at 11cm). Based on my calculations, the port should be about 21cm long, tuned to about 50Hz. I did some basic measurements with the port sealed (not perfectly, but blocked), the port 21cm, and a few other lengths and it doesn't seem to be...working.


attachment.php


The purple line slightly separated from the other lines is when I blocked the port, but the other lines are 16, 21, and 27cm, and they are almost all exactly the same, and none of them shows what I would consider "improvements" compared to the one with the blocked port. Based on a time I build a set of small bookshelf speakers, I would expect a difference between the port lengths, and a much more pronounced difference compared to when the port was blocked. As it stands, it almost seems like a nicer response without the port!

Is there something obvious I am doing wrong? Or am I just expecting too much out of this port?
 

Attachments

  • Screen Shot 2019-10-13 at 21.40.23.png
    Screen Shot 2019-10-13 at 21.40.23.png
    275.1 KB · Views: 519
Hi,

a port should work, if the cabinet doesn't leak. What is your measurement setup? Did you measure mic near the driver cone? At and near the tuning frequency output is from the port and not from the driver. Try measuring further a way (outside/in large space), or measure the port separately and then try to combine the measurements.
 

Tom Ato

Member
2018-04-13 8:09 pm
Hi,

a port should work, if the cabinet doesn't leak. What is your measurement setup? Did you measure mic near the driver cone? At and near the tuning frequency output is from the port and not from the driver. Try measuring further a way (outside/in large space), or measure the port separately and then try to combine the measurements.


Hmm...maybe I should look for leaks. Since I haven't done the crossover yet, I didn't seal all the seams. That could be it.


I was measuring from maybe 1.5-2ft away from it, and I tried to center it between the driver and port. Maybe I should move the mic even farther back.


My bet is you've stuffed the box with polyester fill or similar.

Can you measure impedance?

Chris



I actually haven't stuffed the box, so that shouldn't be an issue.
 

bullittstang

Member
Paid Member
2013-05-09 2:14 pm
Ft. Worth, TX
... I was measuring from maybe 1.5-2ft away from it, and I tried to center it between the driver and port. Maybe I should move the mic even farther back.

Nope - you should be measuring very close - like within 1-2 inches of the woofer cone. Your measurement should show you a severe suck-out at the tuning frequency - that severe suck-out is what will be filled in by your port.

See the attached as reference - Red (Port) and Blue (Woofer). Shows a port tuning of ~45Hz.
 

Attachments

  • 711DASSfig3.jpg
    711DASSfig3.jpg
    63.3 KB · Views: 175
Looks about right actually

Tom Not surprised what you are seeing.... 40 liters is WAY too small for the 310. Should be more like 65L if you are using an optimized vented set up. I have found Thiele's alignment based on QT to work well with this driver. 66 Liters with 4" diameter Vent, 7" long. Will tune from 38-41 Hz. The small variance is due to typical differences in actual driver specs and box leakage. This set up really works. In my set up I add 0.5 ohm of series Resistance Gain to cover various resistances usually found in the real world. This set up yields a near text book symmetrical double impedance hump and excellent group delay. You also need to measure the output at the port opening. You should see a small (3 DB or so) peak somewhere around the 50 Hz tuning frequency if that is where your current set up is providing a box - port resonance. (FB) Judging from your data your resonance is around 48 Hz give or take. Boxes tuned high like you now have tend to have very poor group delay and ring like crazy. Not all, but most. J
 

Tom Ato

Member
2018-04-13 8:09 pm
It seems I had multiple issues, and as you can see on the graph below, I have made some decent (8-10dB in places) improvements.


attachment.php




The green line is what I had from before I did any modifications. I went and sealed a bunch of things on the box that apparently were leaking a lot(!) of air, and the purple line resulted. Then, I increased my box size and retuned the port. That is the yellowish line at the top.


This is definitely much more like the frequency response I had been expecting originally!


Thanks for all the comments :cheers:
 

Attachments

  • Screen Shot 2019-10-16 at 19.20.45.png
    Screen Shot 2019-10-16 at 19.20.45.png
    222 KB · Views: 141