Port mystery...

[soulshine]

Hello my friends. I’ve just finished a pair of 15” monitros. I have used a jbl woofer and a p.audio driver. The Vb of my cab is 54 litre, this means that i can not tune it too low (but this is not the problem). Every cab has two ports that are 2cm (wide) x 16cm (high) x 3,5cm (deep). Acording to this my tiny 54litre cab is tuned at 65,87Hz. Till now everything looks ok... but what realy hapens with the rear port air velocity? This is something i thought after building them (dumb people have always unique ways to make difference). At 100Hz the rear port air velocity is 22,6m/s peak, at 65Hz (the tuning frequency) 33,3m/s peak and in 50Hz 40,4m/s peak. This is the problem! The air is coming noticeable fast from the ports (and so is and a knife in my heart when i hear it).

• Something i thought that would help, is to add two more ports, but since the empty left space that takes place down of the woofer isn’t enough, their shape must be round. Will this be a problem?
• Should this two new ports have the same tuning frequency with the two firstly described ports (2cm (wide) x 16cm (high) x 3,5cm (deep) or can they have enother?
• The final rear port air velocity will be the result of the abstraction of the two differents types of ports?

Hope to hear from you soon.

[Eton]

Without knowing the measured TS parameters of the 15” inch drivers (not published specs) You’re pretty much wasting time in which case 54 liter box and ports are all guessing game.

[noah katz]

If you double the port area you also have to double the effective length to maintain the same tuning.

I say effective length because end effects make the effective length longer than the physical length.

[soulshine]

What exactly is the effective length and how can i double it in order to maintain the same tuning?

[richie00boy]

Simply put you have to increase length of existing ports and add new ports of the same dimensions. With just 54 litres to play with and pretty high tuning frequency (hence lots of air movement) I suspect you are going to have real problems implementing this.

Ideally you should have modelled the response before building the cabinet.

[ocool_15]

Other options are to:
replace your current ports with larger ones if possible
Lower the tuning frequency.
have the ports external to the box
Possible active HP crossover
flaring port ends makes the effective length longer and reduces turbulence

[noah katz]

"What exactly is the effective length and how can i double it in order to maintain the same tuning?"

Look in Loudspeaker Design Cookbook or download WinISD and play with it.

In any case, I wouldn't trust a calculation; best to make them too long and cut them down as required when Fb can be measured.

[ocool_15]

Originally posted by noah katz
"What exactly is the effective length and how can i double it in order to maintain the same tuning?"

Look in Loudspeaker Design Cookbook or download WinISD and play with it.

In any case, I wouldn't trust a calculation; best to make them too long and cut them down as required when Fb can be measured.

To clarify how to find the tuning frequency a simple way is to put a resistor in series with the speaker. You measure the voltage drop across the speaker and for a properly tuned enclosure you will have 2 peaks with a low area. The lowest Vdrop between the 2 humps is Fb. Assuming your design was right the lowest point will be around 56Hz. I often find actual tuning is higher then calculated tuning. If you were to temporarily seal the box you would end up with one large peak.

[Andy G]

Try putting in a couple of pieces of aerated foam, cut to fit the holes. Not your standard foam that they use in mattresses etc, hunt for something with bigger air pockets.