Replacing a HT subwoofer with a car sub.
Jul 3, 2004 at 7:14 PM Post #33 of 89
Wow, it is nice that they tell you how to do this, but it is also odd that they did not just add vaiable pots to do the job. Let me think about this one a bit. I can add the filter to my simulation.

How goes progress? You should be making noise by now.


gerG
 
Jul 3, 2004 at 7:17 PM Post #34 of 89
I have finished the major construction and now its basically just getting the final looks right. However, I have a ton of port noise when its moving alot of air. I have tried to sand the plactic smooth, but that hasnt really helped. Any tips on getting rid of the noise? Other than the port noise, its sounding great! really beats you in the chest!
smily_headphones1.gif


Pics: (Thanks for the new pic hosting, D-EJ915)
one
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The vinyl in the corners was a bit tuff to do, but the cover I made covers most of it up, so it isnt too bad. I still have to do the back with vinyl.
 
Jul 3, 2004 at 8:05 PM Post #35 of 89
Wow! Very nice!

It looks like you have a lot of baffle space left. Though it's a hassle, I would consider calculating what it takes to add another port. That should reduce the velocity of the air moving through them. Flaring the end of the port is supposed to help too but it doesn't look doable with your premade pvc port.
 
Jul 3, 2004 at 8:09 PM Post #36 of 89
Hm....I might be able to add another port. Is there any recommendation on the size and length of it? The one I have in there now is roughly 4 and 3/4 inchs and has an adjustable length.

Also, what are the chances of the speaker blowing out? In some movie scenes the cone is realllllly moving up and down and its kinda of making me nervous. Maybe Im just overreacting....


Thanks again!
 
Jul 3, 2004 at 8:44 PM Post #37 of 89
To keep the same tuning, you will need to increase the port length of each port by a factor of almost 2. That may be tough given your box size...well, 1/2 meter long ports are pretty rough on any box size
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Jul 3, 2004 at 8:52 PM Post #38 of 89
The port it suppose to be 14 inches long. However the one I ordered could only go 11 inchs. If I were to get this port and replace it with the one I have would this get rid of the port noise?

To make that work I think I would have to get a thin piece of wood cut the 4 inch hole and then glue it over the current hole.
 
Jul 3, 2004 at 9:01 PM Post #39 of 89
That would be something I personally would be willing to experiment with.
 
Jul 3, 2004 at 10:29 PM Post #41 of 89
It is odd that you are getting so much cone movement and port noise. Part of the problem is the shorter tube length. By tuning to a higher frequency you allow more power to be dumped into the range below the cabinet tuning, which is not a good thing. That energy is nothing but wasted power, wasted excursion, and port noise. I would try adding a 90 degree fitting and some more duct length. Don't worry, the 90 won't hurt anything. For it's contribution to the length, just use the centerline length of the fitting. You don't have to glue it, but make sure that it is braced.

I have a very easy method of cabinet tuning, but you will have to lay hands on a signal generator and a frequency meter. Let me know if you can get hold of that gear. Otherwise we just go by the theoretical frequency.

Dual ports are not advised. Flared ports help a bit, but I don't think that exit effects are the problem. Bringing the low frequency cutoff higher is a solution as well. If you have a subsonic or rumble filter on your receiver, try turning it on.


gerG
 
Jul 4, 2004 at 2:55 AM Post #43 of 89
I think that it will help. Be sure to leave some room to tune lower (longer port) should you need lower extension. I want to run some simulations with different filter Q values for that low cut filter.

I thought of another thing to try (fun). Go to trueaudio.com and download Truerta. It is a spectrum analyzer program, but don't worry about that just yet. It also has a sine wave generator. You can put in signals at various frequencies and see how the sub behaves. Cone motion should be a minimum at the tuning frequency. It will go gonzo below that frequency, unless you have a subsonic filter. I want you to try tones at various frequencies to try to find the resonance frequency, and see what happens below that. I am guessing that the f is high, and the filter is not intervening fast enough. Tuning lower will get closer to that cutoff. Mean time I need to look at the numbers in the sub manual.


gerG
 
Jul 4, 2004 at 5:56 PM Post #44 of 89
Downloaded the program and gave it a try. The volume was at my moderatly loud listening level(laptop output a full; onkyo reciever at 66). Frequencies below 67 had noticable port noise and around 45 is when the cone started to noticably shake.
 
Jul 4, 2004 at 9:37 PM Post #45 of 89
Something is fishy here. You results would indicate that your cabinet is tuned around 60 hz, which is way too high. I want to double check some basics. Are these correct:

Woofer cabinet: 17.5" x 17.5" x 16.5" with a wall thickness of .75" (this excludes the part of the cabinet for the amp). Assuming some volumes for the driver and braces I get an internal volume a bit under 2 cubic feet.

The port internal diameter is 4", and the length is 11".

No other holes or leaks in the cabinet.

Make sure that you have a good seal around the woofer (light weatherstripping or some gasket material).

Make sure that you have a good seal around the edge of the port. RTV or caulking putty around the inside will do the trick.


gerG
 
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