How to measure to determine correct port length in bass-part?

[jakja83]

I am about to finish my bass-compartments. They have 8" drivers in them, but I am having difficulties deciding on port length.

What measurements can I make with e.g. REW to check what works? Is there any way to do e.g. a 10 - 200 hz sweep and then look at curves?

 

[freddi]

if you don't have means to measure impedance, then you can place the mic very close to the cone. Be sure to turn the drive level way down so nothing is clipped. You will see a "notch" at tuning with direct radiator vented cabinets

An externally hosted image should be here but it was not working when we last tested it.

 

[jakja83]

I have a PC and a miniDSP UMIK-1. The cabinet is a isobaric one with dual 8" woofers, but when I run the numbers in Winisd I get a 1.6 meter long port tube suggested. Not easy to fit in a 15 litre enclosure.

 

[bigbottom]

What tuning are you aiming for with the port, that sounds about 20Hz...? 50-75mm port diameter is plenty for a single 8" which is what you get isobarically.

A Tangband ABR is about 35Hz in 15 litres:

TangBand WT-1427H Passiv Membran | DIY

 

[BTwelve]

Slot ports can be built into the bottom/back of boxes for extra length, but sounds like it's too late for that.

BTW, used this for a sub-woofer test the other day:
Free Online Frequency Sweep Generator | OnlineToneGenerator.com

 

[AndrewT]

isobaric results in a half volume box.
When that is vented, it becomes a bit difficult finding room for the port volume AND keeping the port end away from adjacent internal surfaces AND away from the back of the driver cone.


You may have to build two bigger boxes and use two bass speakers to solve this vent port problem.


BTW, a smaller box requires a LONGER port, than a bigger box tuned to the same frequency.

 

[jakja83]

Well, if it helps, the box is the bottom of a Kudos Titan T88. I bought cabinets and I am making my own variant.

The drivers are Seas CD22RN4X x 2 in each box.

 

[Ron E]

So it seems this is not really about port design, but about reverse engineering something you like the look of? Sometimes designers make compromises to make things look good, Your difficulties with WinISD are only beginning to make that clear to you. Are you using 2 of those woofers in parallel or series? Not that it matters significantly for box design (except for inductance)...

What are your SPL expectations? If you don't expect to drive them to limits, a 40-50mm diameter port might be sufficient for a 10" woofer, if you flare the entrance and exit generously. If you want lots of SPL, I would suggest you use a passive radiator instead of the port.

Here's a (peak) port velocity chart for two woofers in parallel with 2.83Vrms input, 15 liters tuned to 32Hz with a 40mm dia x ~220mm long vent. So if you take the 17m/s rule of thumb (~34m/s flared), this design is good to 6-12Vrms before port noise becomes objectionable.

 

[jakja83]

So it seems this is not really about port design, but about reverse engineering something you like the look of? Sometimes designers make compromises to make things look good, Your difficulties with WinISD are only beginning to make that clear to you. Are you using 2 of those woofers in parallel or series? Not that it matters significantly for box design (except for inductance)...

What are your SPL expectations? If you don't expect to drive them to limits, a 40-50mm diameter port might be sufficient for a 10" woofer, if you flare the entrance and exit generously. If you want lots of SPL, I would suggest you use a passive radiator instead of the port.

Here's a (peak) port velocity chart for two woofers in parallel with 2.83Vrms input, 15 liters tuned to 32Hz with a 40mm dia x ~220mm long vent. So if you take the 17m/s rule of thumb (~34m/s flared), this design is good to 6-12Vrms before port noise becomes objectionable.

Two woofers in parallell.

I have not any specific SPL expectations, but I never listen at very high levels (90+dB)

I can also fit a larger vent, but then would have to use a 90-degree angled one.