Nearfield measurements

[gedlee]

Some time back someone posted an excelent writup of the blending of a nearfield measurement to a far field one. But now I can't find it. Anyone know the link or remember the thread?

Thanks

[dantheman]

How far back are we talking?

there was just this one:
Measurement approach, 2-way closed box

[john k...]

Originally Posted by dantheman

How far back are we talking?

there was just this one:
Measurement approach, 2-way closed box

The problem with that discussion is that it assumes the far field response extends all the way to the frequencies where the response is in full 4Pi radiation. In general it is going to be difficult to gate the far field response with long enough window to extend that low in frequency. The method most commonly used is to obtain a far field measurement that extends to the frequency where the baffle step is still developing, typically between 200 and 300 Hz. Then a near field measurement is made. Next the 2Pi to 4Pi transition is modeled using a baffle diffraction simulation. This is then superimposed on the near field response to correct it for far field effects through the baffle step region. Then the far field and correct near field are overlaid which will show a region of overlap, again typically between 200 and 300 Hz. The near and corrected far field data can then be merged at a frequency where the overlap is best.

[PLB]

Hi Earl,

I'm not sure if this pdf has appeared in any DIYAUDIO threads, but it may be of interest.

http://www.fesb.hr/~mateljan/arta/Ap...d-Rev03eng.pdf

Regards.

Peter

[gedlee]

Originally Posted by PLB

Hi Earl,

I'm not sure if this pdf has appeared in any DIYAUDIO threads, but it may be of interest.

http://www.fesb.hr/~mateljan/arta/Ap...d-Rev03eng.pdf

Regards.

Peter

Peter

That was it thanks. Good writup. When I went to save it I already had it. I must be lossing something.

[gedlee]

Originally Posted by john k...

The problem with that discussion is ...

John - Was this for my benefit?

[john k...]

Originally Posted by gedlee

John - Was this for my benefit?

It was for the benefit of anyone reading the thread.

The Arta writeup and approach is better, provided that the wave length is large compared to the box dimensions.

Of course there are other "tricks" than can be played to get the low frequency response..

Cepstral editing

and Macthed Filters

[dantheman]

Originally Posted by gedlee

Peter

That was it thanks. Good writup. When I went to save it I already had it. I must be lossing something.

Get a mac.

Dan

[gedlee]

Originally Posted by john k...

Of course there are other "tricks" than can be played to get the low frequency response..

I did an AES paper on using Spectral Estimation techniques to extend the impulse response almost twenty years ago. This is the technique used in Praxis.

[john k...]

Originally Posted by gedlee

I did an AES paper on using Spectral Estimation techniques to extend the impulse response almost twenty years ago. This is the technique used in Praxis.

The only thing I see in the Praxis manual about extending low frequency response is the method suggested by Fincham, J. Audio Eng. Soc.,Vol.33,No.3, 1985March1985. The matched filter approach suggested by Benjamin, AES
Convention paper 6218, 2004, is an improvement on that technique.