[Arctos]

Greetings,
Quick newbie question:  I have been doing alot of experimenting with Unibox and have been paying particular attention to port tube recommended length.  Understanding that I want to avoid the "chuffing" port noise from my DIY speakers, but also want to use the optimum port diameter, am I better off going with a shorter port tube with smaller diameter, or a longer port tube with larger diameter?

(I must say that I found it strange that as the port tube diameter dimensions increase the length also increases).

Thanks,

RDS

[ginger]

Do a search on Thiele Small and you will come up with heaps of info. In amougst that you will find recommended minimum port diameter for your amount of cone movement and diameter i.e. the volume of air you're shifting. Use the next standard size up that you can buy commercially.

The larger the port diameter the broader (generally) the bass peaking effect. Remember unless you are a serious "Dush Dush" music fan (under 25 years old) the you want a Qts not greater than 1.1 and preferrably arround 0.9 (0.707 is the theoretical ideal for max flat response)

Cheers,
Ginger

[ginger]

I did a quick scan of my admittedly crappy file system last night to see if I could find more info for you.

What I found suggests that a larger port diameter has some advantages. What we are talking about is air velocity in the port. You mentioned the "chuffing" effect if too small a diameter port is used. Actually you can run into trouble before the "chuffing" is audible because as air velocity goes up the air FLOW becomes less linear and more turbulent. Turbulent flow through the port will introduce distortion in the reinforcing bass frequecies coming out the port.

I did find the following recommendation
Bass Driver Diameter / Port Size
6 to 8 " use 3" port
8 to 10" use 4" port
10 to 12 use 5" port
12 to 15 use 6" port

If you find fitting a port of that diameter on the baffle is a problem simply use two smaller diameter ports to give the same cross sectional area.

Example: I have an old set of speakers I built 20 years ago. Has Electrovoice century 100B 8" Bass Driver and I have 2 x 2" Ports. This is the same as if I'd used one 2.8" port.

Hope this is of some help to you.

Cheers,
Ginger

[Arctos]

Ginger,
Thanks for the info!!  As I will be using the Audax HM210Z0 8" driver, I think I will use a 4" diameter port.  Unibox shows that with this port size, driver and with my 62L cabinet volume the port will be approximately 9" in length.  I found a cool looking port tube system at Partsexpress.com that has flares at both ends that I will try.

BTW, should I be concerned that Unibox calculates my Qts at .305?

Thanks again,

RDS

[ginger]

RDS,

Glad you found my response some use.
 
Now to confess my sins
- when I typed Qts in the above I actually meant Qtc. That is, the Q of the complete Cabinet plus driver not the Q of the speaker driver alone (which is what Qts is) - so NO you should not be concerned about The Qts value you quoted - it is fairly typical. I should know better than to rip of a reply without reviewing to see that I actually typed what I had intended.

Think "s" = speaker (driver), "c" = cabinet (loaded with the driver) when looking at Thiele Small Parameters.


Theile Small Parameters can be confusing enough without someone giving you crap information - SORRY.

For guidance on Qtc
"Boom Boxes" for "Dush Dush" music Qtc > 1.4
Small Bookshelf Speakers with lots of Bass Peaking - Typically Qtc approx. 1.4
Maximum Power Handling - Qtc = 1.1
Up Market Speaker Boxes - Qtc between 0.9 and 1.1
Theoretical maximimum flat frequency response and critically damped - Qtc = 0.707

Cheers
Ginger

[Arctos]

Ginger,
Thanks for the clarification!!  Don't beat yourself up, it was an honest mistake.  As I look at the Unibox software it really only calculates a Qtc for a sealed cabinet, not for a vented one.  Though when I look at the specs on a sealed cabinet with this driver it shows the Qtc at .714, which looks pretty good.  Again, I don't know if this really applies to my vented cabinet.

According to the Unibox program these drivers are cabable of a very flat response down to 40hz or so with my desired dimesions.  An internet friend that I met on HD (who is also from Australia) has built a number of pairs of the speakers I am building and he thinks they are great, so I am excited to finish them.  They are loosely based on the Audax KLS-9.

Thanks again for the help,

RDS