Narrow, shallow rectangular port

cabbagerat · 4th February 2008, 12:51 PM

First, some background - feel free not to read this if it bores you

I am busy designing and building a 2.1 system for music and a little bit of theater. The left and right channels are full rangers, using the Fostex FE167E, mounted in a 15l ported box. The front channels cross over to a single 12" sub at 95Hz with a fourth order active Linkwitz-Riley crossover.

Phase adjustment and sub level adjustment controls are included in the crossover I have designed.

The front channels will be powered with a LM3886 based amp for now, and the sub with a 200W amp of my own design. So far I have designed and built the sub, the amps and the crossover and am busy with the front channel boxes.

Now, my question.

I already own a pair of well braced 15l internal volume boxes made of 19mm MDF, which I would like to use. The FE167E data sheet has an example of this driver in a 15l box, with a port tuned to 67.25Hz. I would like to keep this frequency, as it works well with my sub. In this example, the port is a 5cm diameter by 5cm long circular port. Port velocity peaks at about 6m/s according to WinISD pro.

WinISD tells me that I can replace the circular port with a rectangular port 16mm x 82mm and 19mm deep - and get the same port tuning. Here, the velocity is still ok - peaking at about 12m/s. The advantage of doing this is that the box material is 19mm thick, so I can just cut a slot in the front baffle and get the port I need.

I mainly want to do this for looks. Are there any disadvantages to this narrow rectangular port approach that WinISD can't tell me about?

Scottmoose · 4th February 2008, 03:21 PM

Not AFAIK in this particular instance. You'd technically get a touch more turbulance, but as the vent air velocity is within reasonable limits, that shouldn't be audible.

cabbagerat · 5th February 2008, 04:45 AM

Thanks Scottmoose

Does the port need to be approximately equal distance from the speaker all the way along? For example, could I build something like the picture I have attached, with a vertical slot port?

planet10 · 5th February 2008, 05:32 AM

The only problam i see with that port is that it is going to make the baflle eveb flopier than it is... you do need to ensure it is well braced.

dave

cabbagerat · 5th February 2008, 08:04 AM

Thanks plantet10. The front baffle is very well braced, but I will feel how stiff it is after I have cut the slot.

I am going to try it out, and post measurements and photos when I'm done. If it doesn't work, I can always just make a new front baffle.

planet10 · 5th February 2008, 08:26 AM

Quote:

Originally posted by cabbagerat
Thanks plantet10. The front baffle is very well braced, but I will feel how stiff it is after I have cut the slot.

I am going to try it out, and post measurements and photos when I'm done. If it doesn't work, I can always just make a new front baffle.

Worth a try. If it doesn't hang together it is easy enuff to rebuild that box -- i'd suggest many-ply void free plywood.

dave

cabbagerat · 14th February 2008, 07:41 AM

I slotted the boxes, and they sound really good now. There is no audible port noise at normal listening levels (as predicted), and it looks fairly nice, too.

I'll so some measurements and take some photos soon, and post results.

The boxes are very stiff, but currently have no fill or damping. What is recommended for BR boxes of this size?

Scottmoose · 14th February 2008, 10:03 AM

If you can't hear any box problems, & you like the way they sound, then I simply wouldn't bother. If it ain't broke...

If you simply can't help yourself (DIY audio does tend to get addictive), try lining the top, back & one side-wall with wool carpet underlay.

4th February 2008, 12:51 PM	#1
cabbagerat diyAudio Member Join Date: Jan 2004 Location: Cape Town	Narrow, shallow rectangular port First, some background - feel free not to read this if it bores you I am busy designing and building a 2.1 system for music and a little bit of theater. The left and right channels are full rangers, using the Fostex FE167E, mounted in a 15l ported box. The front channels cross over to a single 12" sub at 95Hz with a fourth order active Linkwitz-Riley crossover. Phase adjustment and sub level adjustment controls are included in the crossover I have designed. The front channels will be powered with a LM3886 based amp for now, and the sub with a 200W amp of my own design. So far I have designed and built the sub, the amps and the crossover and am busy with the front channel boxes. Now, my question. I already own a pair of well braced 15l internal volume boxes made of 19mm MDF, which I would like to use. The FE167E data sheet has an example of this driver in a 15l box, with a port tuned to 67.25Hz. I would like to keep this frequency, as it works well with my sub. In this example, the port is a 5cm diameter by 5cm long circular port. Port velocity peaks at about 6m/s according to WinISD pro. WinISD tells me that I can replace the circular port with a rectangular port 16mm x 82mm and 19mm deep - and get the same port tuning. Here, the velocity is still ok - peaking at about 12m/s. The advantage of doing this is that the box material is 19mm thick, so I can just cut a slot in the front baffle and get the port I need. I mainly want to do this for looks. Are there any disadvantages to this narrow rectangular port approach that WinISD can't tell me about?

5th February 2008, 05:32 AM	#4
planet10 frugal-phile(tm) diyAudio Moderator Join Date: Oct 2001 Location: Victoria, BC, NA, Sol III Blog Entries: 5	The only problam i see with that port is that it is going to make the baflle eveb flopier than it is... you do need to ensure it is well braced. dave __________________ community sites t-linespeakers.org, frugal-horn.com, frugal-phile.com ........ commercial site planet10-HiFi p10-hifi forum here at diyA

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Port question, round vs rectangular	kvk	Full Range	6	11th February 2008 08:16 AM
End correction formula for rectangular port ?	philippe_95	Multi-Way	13	3rd March 2007 06:48 PM
MJK worksheets – rectangular port help	sunil	Multi-Way	3	5th February 2005 08:58 PM
narrow vs wide baffle and port location....?	AudioGeek	Multi-Way	3	9th May 2004 09:19 PM
rectangular port questions	Jimmy154	Multi-Way	8	30th June 2002 08:37 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

4th February 2008, 03:21 PM	#2
Scottmoose diyAudio Member Join Date: Mar 2005 Location: UK	Not AFAIK in this particular instance. You'd technically get a touch more turbulance, but as the vent air velocity is within reasonable limits, that shouldn't be audible.

5th February 2008, 08:04 AM	#5
cabbagerat diyAudio Member Join Date: Jan 2004 Location: Cape Town	Thanks plantet10. The front baffle is very well braced, but I will feel how stiff it is after I have cut the slot. I am going to try it out, and post measurements and photos when I'm done. If it doesn't work, I can always just make a new front baffle.

14th February 2008, 07:41 AM	#7
cabbagerat diyAudio Member Join Date: Jan 2004 Location: Cape Town	I slotted the boxes, and they sound really good now. There is no audible port noise at normal listening levels (as predicted), and it looks fairly nice, too. I'll so some measurements and take some photos soon, and post results. The boxes are very stiff, but currently have no fill or damping. What is recommended for BR boxes of this size?

14th February 2008, 10:03 AM	#8
Scottmoose diyAudio Member Join Date: Mar 2005 Location: UK	If you can't hear any box problems, & you like the way they sound, then I simply wouldn't bother. If it ain't broke... If you simply can't help yourself (DIY audio does tend to get addictive), try lining the top, back & one side-wall with wool carpet underlay.