Jeff Bagby has written a white paper on an improved method of doing this which should be more accurate than the current approach. It showed up on a discussion forum on parts express:

New Paper available on How to Get Accurate Measurements Indoors Down to 10 Hz

The actual paper, worth a read, is in PDF form here:

https://app.box.com/s/fefis558wna1d6pd07r3

The thinking has been that a dustcap measurement will give a representative response for low frequencies that is free from the standing waves of your room (or even of an imperfect anechoic chamber). At upper frequencies we can use a gating approach to measure the "anechoic" response. All we have to do is splice the two curves together at somewhere around 200 Hz and we have a wideband and accurate reflection free response curve.

The problem is that the dustcap measurement doesn't properly represent what the woofer looks like in free space because it doesn't include the effects of baffle size and diffraction. Bagby uses a diffraction modeling program to create a 2pi to 4pi

*correction curve*and adds that to the mix. When he adds the "baffle step" correction to the dustcap measurement, it and the farfield measurement show much more agreement over a broader frequency range. Splicing becomes less arbitrary and the proper level of bass vs. midrange is revealed.

Recommended!

Note that every nearfield approach suffers this same issue, as well as microphone in the box measurements, Thiel/Small approaches, P-Spice models, etc. They all assume the driver is non-directional and don't take finite baffle size into account. In that regard they look more like the 2pi response, as infinite baffle mounting will force constant directivity from very low frequencies up to beyond piston band operation.

If you want to play with diffraction modeling try here:

Home of the Edge

Tolvan Data

(Click on “The Edge”)

Don Keele’s classic paper on nearfield measurements is here:

http://xlrtechs.com/dbkeele.com/PDF/Keele (1974-04 AES Published) - Nearfield Paper.pdf

He gives draw away curves for a woofer both mounted in a cabinet and in a large baffle. Draw away curves for the infinite baffle case retain the dustcap response shape while the 4 pi case varies as you move away from the driver.

Regards,

David