Newbie needs help with D.I.Y. sub - Page 5 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 09-18-10, 06:58 PM
Senior Shackster
7

Join Date: Jan 2008
Posts: 790
Re: Newbie needs help with D.I.Y. sub

Quote:
booberj wrote: View Post
this is a complete noob queston... why is it the bigger you go the longer the port and what's the advantage of one over the other.
A ported speaker enclosure is a Helmholtz resonator. The length and the diameter of the port as well as the volume of the enclosure relate to the tuning frequency of the system.

Port Length
The port length required to tune a volume of air to a specific frequency can be calculated by using the following equation:

Lv = (23562.5*Dv²*Np/(Fb²*Vb))-(k*Dv), where:
Dv = port diameter (cm)
Fb = tuning frequency (Hz)
Vb = net volume (litres)
Lv = length of each port (cm)
Np = number of ports
k = end correction (normally 0.732)
Borrowed from here.

So if you double the port diameter everything else staying the same, the length increases by (almost) 4x to get the same tuning frequency in the box.

A ported speaker behaves like a sealed speaker well above resonance of the port. At resonance the driver is contributing very little to the sound output and the port is doing all the work. As ports are invariably smaller in diameter than the driver itself, the slug of air in the port that is moving back and forth to produce sound needs to move several times as far as the driver would, so it's potentially moving quite fast. In the SDX example, the cone area is 300cm² and the port 47cm².

Fast airflow in a port becomes turbulent at some speed and especially so if the port has no flare to smooth the transition from the port to the baffle. Above some speed at some frequencies this flow becomes audibly disturbed and can draw attention to itself; this is often called chuffing. To get lower air speed, you need a larger port, but a larger port is longer for the same box volume and tuning frequency.

Quote:
booberj wrote: View Post
I also here people talking about port air speed... how much is too much and what does that look like in WinISD Pro?
The best study available easily is Collo's and is well written, so no point in me repeating it here. Whilst you're there, you might find his excellent Boxnotes software of some use.

My own personal preference is to keep it to 5% Mach or less at any expected input power if possible, with 10% a worst case.

Looking at the CSS app note example for the box, this is how it models with the same conditions as previously.

For some reason this gets scrambled when I upload to the gallery.

A 45m/s peak is too high for my tasted, and is the reason I suggested the Trio12 PR box would be better; PR's effectively replace ports and they have no noise.

Quote:
booberj wrote: View Post
told everyone I'd be full of questions
Questions are good.
Lucky7! is offline

Old 09-18-10, 09:09 PM Thread Starter
Shackster

Join Date: Sep 2010
Location: Jefferson, GA
Posts: 42
Re: Newbie needs help with D.I.Y. sub

holy information overload... good stuff... I'm starting to understand how ports work a little better
Thanks A9x
booberj is offline
Old 09-19-10, 02:16 AM
Shackster

Join Date: Sep 2010
Location: Tacoma,wa.
Posts: 8
Re: Newbie needs help with D.I.Y. sub

winISD will work with 7 but you have togo properties and set it to run as winxp
Old 09-19-10, 11:41 AM
Elite Shackster

Mike

Join Date: Apr 2007
Posts: 19,397
Re: Newbie needs help with D.I.Y. sub

What sub have you decided on?
Mike P. is offline
Old 09-19-10, 11:51 AM
Senior Shackster
tim

Join Date: Mar 2007
Location: vermont
Posts: 298
Re: Newbie needs help with D.I.Y. sub

I think he went with the TC sounds..

http://www.amazon.com/TC-Sounds-LMS-.../dp/B0039OTTHO
tundraSQ is offline
Old 09-19-10, 11:54 AM Thread Starter
Shackster

Join Date: Sep 2010
Location: Jefferson, GA
Posts: 42
Re: Newbie needs help with D.I.Y. sub

Quote:
Mike P. wrote: View Post
What sub have you decided on?
I'm going with the one I originally looked at the TC Sound LMS-R... 12 a 1.8 cubic ft box and I'm not sure what to get to drive it either a plate amp or a pro amp (I'm leaning towards a pro amp Behringer EP4000). It's someplace to start, make mistakes and learn. More important than that it's a decision so I can head in a direction.

-Gary
booberj is offline
Old 09-19-10, 12:07 PM
Elite Shackster

Mike

Join Date: Apr 2007
Posts: 19,397
Re: Newbie needs help with D.I.Y. sub

If you want low end extension for movies you're better off with a bigger box tuned lower as the comparison shows. Whatever box size you choose porting will be a challenge, PR's would be simpler and take up less box volume compared to a port.

Mike P. is offline
Old 09-19-10, 12:27 PM Thread Starter
Shackster

Join Date: Sep 2010
Location: Jefferson, GA
Posts: 42
Re: Newbie needs help with D.I.Y. sub

Quote:
Mike P. wrote: View Post
If you want low end extension for movies you're better off with a bigger box tuned lower as the comparison shows. Whatever box size you choose porting will be a challenge, PR's would be simpler and take up less box volume compared to a port.
The guy over at TC Sound recommended a PR enclosure as well 19x18.5x17.5 with 2 12" VMPs. It's something that I'm considering at a later point in my D.I.Y. "career". I have absolutely no experience with a PR design so that makes me a little hesitant. The canned solution that you offered is an easy way out but I like to make mistakes and learn.

I like the idea of the lower tuned box, I'll see if can design one that will fall under my space limitations and have a net 3.0 cubic ft enclosure.

-Gary
booberj is offline
Old 09-19-10, 12:53 PM
Elite Shackster

Mike

Join Date: Apr 2007
Posts: 19,397
Re: Newbie needs help with D.I.Y. sub

You'll need a slot port 2" H x 12" W x 49" L.
Mike P. is offline
Old 09-20-10, 11:08 AM Thread Starter
Shackster

Join Date: Sep 2010
Location: Jefferson, GA
Posts: 42
Re: Newbie needs help with D.I.Y. sub

Quote:
Mike P. wrote: View Post
You'll need a slot port 2" H x 12" W x 49" L.
Can I make the port a little narrower so it wouldn't have to be so long?
booberj is offline

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