New Port Idea

[davepellegrene]

Since I already have the boxes built for the project I am working on I didn't want to take up anymore room for the vent internally. I decided to go to a slot vent and to get it tuned to 72Hz I will need a slot .5" x 3" x 13". My cabinet it 4.5" wide x 9.25" tall. The port is going to be part of the back of the cabinet and the whole think removable. I think I am going to attach the crossover to the back, at the top, also. Looks good on paper. Does anyone see a problem with this?
Thanks Dave


http://lh6.ggpht.com/_lY-Ky2SsZ6U/S3SniIVR-EI/AAAAAAAAEUQ/D1X2TNMpHkg/s800/Stainless%20Cabinet%20Drawing.JPG

[davepellegrene]

Added cross braces and attachment screws.
http://lh6.ggpht.com/_lY-Ky2SsZ6U/S3TG8lZjYjI/AAAAAAAAEUY/a_ZQhZkbu4Y/s800/Stainless%20Cabinet%20Drawing.JPG

[philiparcario]

port noise as a whistle sound with the brace?

[davepellegrene]

port noise as a whistle sound with the brace?

I think Wolf put the braces between the 1/4" panels on a slot port on the side of one of his speakers. I think the vent was on the side of the cabinet vertically. I guess the nice thing is if it doesn't work I can take it off and redo it.:)
Dave

[dcibel]

I am wondering why you want this back port assembly to be removable? Just to access the inside of the cabinet, or to switch between ported and sealed?

If just to access the inside, I think a removable baffle is a simpler task. Why over-complicate it. [Edit, didn't read that you already built the cabinet]

If to change cabinet type "on-the-fly", I think this may work, provided you use some good gasketing to seal it up. You'll still have the slot at the bottom of the cabinet to deal with though.

I think the bolts would be better placed near the seal, at the top and bottom of the cabinet, and more than one bolt. Maybe a four-corners approach instead.

[bolland83]

Will you be using these with stands of some sort? If so, why not bottom port it and incorporate the port into the stand? Just another idea.

[CokewithLime]

Dave

I think this might be simpler and also stronger cabinet wise - the additional MDF pieces could just be biscuit / glue joined on and then you could use your trenching method to make it all seamless.

Could make the front more interesting with a nice flared squarish ovid port opening. Just a thought.

http://img214.imageshack.us/img214/4228/davep.jpg

[Wolf]

You mean these?

http://i18.photobucket.com/albums/b117/wolf_teeth_speaker/Deuterium_Deftech-Silkie/100_3669.jpg

I used a 1/4" high nylon spacer and some screws, but I think your method looks a lot easier/better than mine, and I wouldn't worry about the screws creating whistles. Mine didn't.

Have fun!
Wolf

[davepellegrene]

If to change cabinet type "on-the-fly", I think this may work, provided you use some good gasketing to seal it up. You'll still have the slot at the bottom of the cabinet to deal with though.

I think the bolts would be better placed near the seal, at the top and bottom of the cabinet, and more than one bolt. Maybe a four-corners approach instead.

I plan on using foam tape. I have some good quality tape I got from a glass company that was left over from a window install.
I did plan on two bolts. I got this idea off of one of Zaph's speakers.


Will you be using these with stands of some sort? If so, why not bottom port it and incorporate the port into the stand? Just another idea.

You mean like cokewithlime posted?


I think this might be simpler and also stronger cabinet wise - the additional MDF pieces could just be biscuit / glue joined on and then you could use your trenching method to make it all seamless.

Could make the front more interesting with a nice flared squarish ovid port opening. Just a thought.



That is a good idea. I have the cabinets ready for paint, so that is why I have the sides tapered back. I was going to paint the assembly black. I could still use your method with tapered sides I think.


I used a 1/4" high nylon spacer and some screws, but I think your method looks a lot easier/better than mine, and I wouldn't worry about the screws creating whistles. Mine didn't.

Have fun!
Wolf
Today 01:36 AM

Yep! That's the one. Good to hear it didn't whistle.

Thanks everyone for your input.
Dave

[billfitzmaurice]

What you have there is a shelf duct, and it should work well. An advantage to the way you have it is that you can easily alter the length of the last section, making it a simple affair to fine tune it. I would go with the same material for the duct as the rest of the box, which would probably make the additional bracing unnecessary.

[davepellegrene]

What you have there is a shelf duct, and it should work well. An advantage to the way you have it is that you can easily alter the length of the last section, making it a simple affair to fine tune it. I would go with the same material for the duct as the rest of the box, which would probably make the additional bracing unnecessary.

I am still not real clear on tuning. Here is a pic that shows how the vent will work. If I drop the length to 10 inches the hump goes up almost a decibel. If I make it 19" long it drops almost a decibel. That doesn't seem like much of a change for such a big difference in the length. The rest of the graph doesn't change much. The longer the vent the more the knee at 50Hz is pronounced. I wouldn't think you could hear that being 9 dBs down.
Dave

http://lh6.ggpht.com/_lY-Ky2SsZ6U/S3VrNiMW6MI/AAAAAAAAEVU/MpPHRuuyEYU/s800/WinISD%20graph.JPG

[billfitzmaurice]

I am still not real clear on tuning. Here is a pic that shows how the vent will work. If I drop the length to 10 inches the hump goes up almost a decibel. If I make it 19" long it drops almost a decibel. That doesn't seem like much of a change for such a big difference in the length. The rest of the graph doesn't change much. The longer the vent the more the knee at 50Hz is pronounced. I wouldn't think you could hear that being 9 dBs down.
Dave

http://lh6.ggpht.com/_lY-Ky2SsZ6U/S3VrNiMW6MI/AAAAAAAAEVU/MpPHRuuyEYU/s800/WinISD%20graph.JPGThat hump is characteristic of a box that's too small for the driver, not a tuning issue.

[JustinG]

Well since you are using the RS100S-8, it doesn't really model well in a vented enclosure... regardless of size unless you want to EQ that lower bump out. From what I'm guessing you have it in ~2 liters tuned to 72Hz... that really helps with the cone displacement but you get a really bad bump in response, which will be audible.

The issue arises with the Qts value I believe... it is above .707 so you will never get something that doesn't have a bump before roll off

[JustinG]

I am still not real clear on tuning. Here is a pic that shows how the vent will work. If I drop the length to 10 inches the hump goes up almost a decibel. If I make it 19" long it drops almost a decibel. That doesn't seem like much of a change for such a big difference in the length. The rest of the graph doesn't change much. The longer the vent the more the knee at 50Hz is pronounced. I wouldn't think you could hear that being 9 dBs down.
Dave

http://lh6.ggpht.com/_lY-Ky2SsZ6U/S3VrNiMW6MI/AAAAAAAAEVU/MpPHRuuyEYU/s800/WinISD%20graph.JPG

I said it below but the "bump" we are seeing is from the driver not the vent, so the vent is adding to the "bump" the closer you tune it to the original frequency the bump is (~120Hz) and tuning lower allows the port to not contribute to the bump. If you reduce the area of the port you should be able to tune it that low with out having that much length.

[davepellegrene]

I said it below but the "bump" we are seeing is from the driver not the vent, so the vent is adding to the "bump" the closer you tune it to the original frequency the bump is (~120Hz) and tuning lower allows the port to not contribute to the bump. If you reduce the area of the port you should be able to tune it that low with out having that much length.

The box is 1.8L. I have been allowing 12in^3 for driver/xover. That makes it 1.6. I thought a hump at 120Hz was a good thing for a small speaker?
I down sized the port and ended up with the same length and made it .5 x 1.5 in diameter. That tuned it to 51Hz. That lowered the hump. It put the air velocity at 28 @ 35Hz. Would this be a problem since it is way lower then the speaker will play. I also modeled it sealed and am wondering if this would be simpler. I haven't tried it in PCD yet though. I thought a high Qtc. ment vented? I must be thinking backwards.

The bottom one is sealed.
Dave

http://lh6.ggpht.com/_lY-Ky2SsZ6U/S3W2Vx7dMFI/AAAAAAAAEWY/fDQCdh4-Uq4/s800/WinISD%20sealed%20vs%20ported.JPG

[billfitzmaurice]

I thought a hump at 120Hz was a good thing for a small speaker?It is if you're trying to compensate for Allison Effect with the cab front 2.3 feet from the rear wall, otherwise it may sound boomy.

[JustinG]

The box is 1.8L. I have been allowing 12in^3 for driver/xover. That makes it 1.6. I thought a hump at 120Hz was a good thing for a small speaker?
I down sized the port and ended up with the same length and made it .5 x 1.5 in diameter. That tuned it to 51Hz. That lowered the hump. It put the air velocity at 28 @ 35Hz. Would this be a problem since it is way lower then the speaker will play. I also modeled it sealed and am wondering if this would be simpler. I haven't tried it in PCD yet though. I thought a high Qtc. ment vented? I must be thinking backwards.

The bottom one is sealed.
Dave


It depends on what your goal is, that hump/tuning will help protect the small woofer from excursion issues. If you can EQ the hump out I'd go with the 70+ Hz tuning as it will control cone movement better anything above 60Hz.

Sealed that woofer has excursion issues at ~90 dB from 100Hz on down so unless you are crossing them to the sub at a higher frequency you will have problems.

If it were me I'd stick to the first option tuning it to 70-75Hz, that way you can hit 95 dB with out too much of an issue, you'll just have to use some eq or deal with a 5-6 dB bump.

[rogoll]

The box is 1.8L. I have been allowing 12in^3 for driver/xover. That makes it 1.6. I thought a hump at 120Hz was a good thing for a small speaker?
I down sized the port and ended up with the same length and made it .5 x 1.5 in diameter. That tuned it to 51Hz. That lowered the hump. It put the air velocity at 28 @ 35Hz. Would this be a problem since it is way lower then the speaker will play. I also modeled it sealed and am wondering if this would be simpler. I haven't tried it in PCD yet though. I thought a high Qtc. ment vented? I must be thinking backwards.

The bottom one is sealed.
Dave

http://lh6.ggpht.com/_lY-Ky2SsZ6U/S3W2Vx7dMFI/AAAAAAAAEWY/fDQCdh4-Uq4/s800/WinISD%20sealed%20vs%20ported.JPG

I'm not one to be giving you advice, as you and others on here have far more experience than I. However, I do think you have it reveresed. The lower Qtc valued drivers are better for vented and vice-versa. The hump can be advantagous, though, to help with BSC.

[billfitzmaurice]

The hump can be advantagous, though, to help with BSC.Allison Effect yes, BSC, not so much. With the 4.5" width of his cab it will start rolling off around 3kHz.

[davepellegrene]

I got the duct port built over the weekend. The one on the right is the inside. The one on the left is the outside. It is upside down. I left the outside cover off so I can clamp it on and move it up to shorten the port. Then attach it when I get the tuning right.

http://lh4.ggpht.com/_lY-Ky2SsZ6U/S3nqc74LWSI/AAAAAAAAEXU/X8oxhnW2eDY/s640/duct%20port%20build%20005.jpg

This is how it will fit the cabinet.
http://lh5.ggpht.com/_lY-Ky2SsZ6U/S3nqdLGTA8I/AAAAAAAAEXY/gOIycPN66kk/s640/duct%20port%20build%20007.jpg

For the 180 degree bend I used a piece of PVC pip split in half. I figured it would make a smoother turn.
http://lh3.ggpht.com/_lY-Ky2SsZ6U/S3nprmDjU0I/AAAAAAAAEXE/CvPeHNJsWZg/s640/duct%20vent%20build%20002.jpg

Not sure how I am going to finish the top piece. I may angle the sides to match the rest so it looks molded in. I am also not sure what to do with the opening. I may cut it off on an angle. Have to put some thought into it. Any ideas?
I think I am going to paint the whole vent satin or flat black.
http://lh6.ggpht.com/_lY-Ky2SsZ6U/S3nprYE3yrI/AAAAAAAAEXA/yfoQY9zkoYc/s640/duct%20vent%20build%20001.jpg

Dave

[billfitzmaurice]

For the 180 degree bend I used a piece of PVC pip split in half. I figured it would make a smoother turn.
Moot with the long wavelengths involved. Rounded bends have no effect much below about 400 Hz.

[davepellegrene]

Moot with the long wavelengths involved. Rounded bends have no effect much below about 400 Hz.

Well burst my bubble! Seemed like a cool idea.

[Wolf]

Well burst my bubble! Seemed like a cool idea.

I don't see what it would hurt, Dave. Keep going!

Seems to me it's more of a feature of the design, so maybe a light stain? I would hate to just erase the idea visually.
Later,
Wolf

[davepellegrene]

I don't see what it would hurt, Dave. Keep going!

Seems to me it's more of a feature of the design, so maybe a light stain? I would hate to just erase the idea visually.
Later,
Wolf

There's no turning back now!. I always jump in with both feet then look for a life jacket. :eek: I had thought about stain. light stain would be a better choice then the dark I was picturing. I have been working on a brushed stainless steel paint for the cabinets. There is a fine line between can't see the brushed lines, done with 120 grit sand paper, to heavy fake lines, done with a sponge brush. Hopefully I will come up with something before I run out of paint. :rolleyes:
Dave