Port FR scaling - check my math

[DeZZar]

Apologies to anyone who finds this a stupidly basic question...just want to check!


So when measuring bass reflex the process to combine the port response and woofer response is to scale the port response before merging.

The degree to which the port response is scaled is expressed as:

20 LOG(Port Diam / Woofer Diam)

So as a practical example:
20 x Log(50mm/150mm) = -9.542 (db)

However, principally its just ratio of port to woofer....and I want to scale ports that are not circular. In these cases I had just been using the eyeball method of aligning the tails of the woofer and port response before merging.

If we convert to using area instead of diameter I found that:

20 x LOG(... != does not work...

However:
10 x Log(1963.4mm2 / 17671.46mm2) = -9.542 (db)....same result

So am I correct in assuming that I can re-state the above as:

10 LOG(Port Area / Woofer SD)

[fpitas]

Apologies to anyone who finds this a stupidly basic question...just want to check!


So when measuring bass reflex the process to combine the port response and woofer response is to scale the port response before merging.

The degree to which the port response is scaled is expressed as:

20 LOG(Port Diam / Woofer Diam)

So as a practical example:
20 x Log(50mm/150mm) = -9.542 (db)

However, principally its just ratio of port to woofer....and I want to scale ports that are not circular. In these cases I had just been using the eyeball method of aligning the tails of the woofer and port response before merging.

If we convert to using area instead of diameter I found that:

20 x LOG(... != does not work...

However:
10 x Log(1963.4mm2 / 17671.46mm2) = -9.542 (db)....same result

So am I correct in assuming that I can re-state the above as:

10 LOG(Port Area / Woofer SD)

Yes, makes sense since area is proportional to diameter squared.

[billfitzmaurice]

Use speaker modeling software and it will do all the calculations for you.

[DeZZar]

Use speaker modeling software and it will do all the calculations for you.

Yeah? And which modelling software can you recommend that deals with the above measurement specifically?

[billfitzmaurice]

WinISD allows you to specify port shape and size.

[DeZZar]

WinISD allows you to specify port shape and size.

Yes it does. How does that help apply scaling to the measured result for combining port and woofer near field measurements?

[billfitzmaurice]

My mistake, I didn't realize that's what you were trying to do. I never bother with that myself. I do all measurements outdoors at a minimum of two meters. If I wanted to model port and cone output separately I'd use HornResp.

[DeZZar]

No worries. Outdoor measurements aren't an option for me so I have to follow the quasi anechoic process. This is ultimately the measurement to validate model, its just the only white paper that details this formula explicitly assumed round ports.

[a4eaudio]

So am I correct in assuming that I can re-state the above as:

10 LOG(Port Area / Woofer SD)

Yes

10 LOG (Area1 / Area2)
If both are circles, then
= 10 LOG ( pi * R1^2 / pi * R2^2)
= 10 LOG (R1/R2)^2
= 20 LOG (R1/R2)
= 20 LOG (0.5*D1 / 0.5*D2)
= 20 LOG (D1 / D2)

[DeZZar]

Thanks mate! 👍

[wogg]

Yes it does. How does that help apply scaling to the measured result for combining port and woofer near field measurements?

VirtuixCAD does have a tool for that. Imports FRD files and will calculate and adjust the close mic port and cone measurements. It'll also merge that to a far field measurement. I've only stumbled through it once though, so by no means an expert. Still a bit of a Virtuix noob.

[DeZZar]

VirtuixCAD does have a tool for that...

Is that under the Merger tool?

There's so much in Vituix I'm still finding things after using it for a couple of years!

[wogg]

Yup, the merger tool!