Nearfield measurements of large woofers

Web Wolf · 30th April 2006, 01:15 PM

Hi, does anyone knows how to measure the response of a woofer bigger than 20 cm until 500Hz ?

In fact, as we know, the near field response is valid until the frequency Fmax = 10950/D where D is the baffle dimension in cm

So, the measurement of a 20 cm driver is valid (in near field) until the frequency Fmax=10950/20=548Hz but what about a driver larger than 20 cm ?

For example, a driver of 30cm is valid until 10950/30=365Hz !!!

I saw that farfield measurement is not valid under 500Hz in a non-anechoic chamber, so how about measuring the gap between (es.) 365 Hz and 500 Hz ?

RobWells · 30th April 2006, 05:13 PM

Hi,

I've found measuring ground plane style works - Place the speaker on the ground away from any walls, boundaries etc. Place the mic 2m away from the baffle (on the floor), and measure.

Here's a measurement of my Beyma LX60 15" driver in a sealed box, designed to be F3 at 80Hz. Below is Beyma's response chart for it.

quite similar.

Rob.

Svante · 30th April 2006, 11:26 PM

A woofer in a closed box or a vented box can also be measure inside the box, up to the first standing wave inside the box. I don't know if this helps as the upper limiting frequency is not necessarily higher than with the other method. Anyway, I think the inside-the-box method is kind of cool. You will have to tilt the response by 12 dB/octave, though.

Web Wolf · 30th April 2006, 11:50 PM

First of all, thanks for your reply

Is this measurement a sum of NF+FF ? Or it's only a FF ?

Thanks
Bye

RobWells · 1st May 2006, 12:15 AM

The idea behind the measurement is that if you measure a speaker in half space (on the ground with no other boundaries) at 2m , it will be the same as measuring the speaker at 1m in full space (4pi)

Do a search on 'ground plane'

Rob.

The top plot is exactly as I stated, measured at 2m from the baffle, mic on the floor, speaker on the floor, no other boundaries.

Web Wolf · 1st May 2006, 09:08 AM

Hum ...

OK

I understand now, but I know that the measurement in half space should be reduced of 6dB to compensate the half space effect.

However I'll do a research about ground plane

Bye

Web Wolf · 3rd May 2006, 10:56 AM

I saw the ground plane measurements and they are quite interesting, but now I've a question for you:

in these measurements the microphone is placed attached on the floor and I think it's good for lowest frequencies, but what about measuring a tweeter or a midrange (for upper frequencies) ? I mean, tweeters are quite directionals aren't they ? Should the microphone be placed a bit detached from the floor and in axis with the tweeter's center ?

RobWells · 3rd May 2006, 08:19 PM

'Testing Loudspeakers' by Joe d'apollito will answer all your questions and more, better than what I can.

http://www.audioxpress.com/bksprods/bkindexltheory.htm

I have the book, it's full of good info.

Cheers,

Rob.

elviukai · 3rd May 2006, 08:34 PM

hi Rob just send a PM to you

MethMan · 4th May 2006, 01:32 PM

Sorry for unpolite OT.
Rob, will you be so kind and share a measuremets of Beyma 15LX60 with me, perhaps via e-mail. I'm interested in raw driver impedance plot, but any other (T-S, distortion etc.) wouldn't hurt.
Thanks in advance. I'm in "consideration phase" and closer info than producer's DS will help.

30th April 2006, 01:15 PM	#1
Web Wolf diyAudio Member Join Date: Apr 2006 Location: Italy	Nearfield measurements of large woofers Hi, does anyone knows how to measure the response of a woofer bigger than 20 cm until 500Hz ? In fact, as we know, the near field response is valid until the frequency Fmax = 10950/D where D is the baffle dimension in cm So, the measurement of a 20 cm driver is valid (in near field) until the frequency Fmax=10950/20=548Hz but what about a driver larger than 20 cm ? For example, a driver of 30cm is valid until 10950/30=365Hz !!! I saw that farfield measurement is not valid under 500Hz in a non-anechoic chamber, so how about measuring the gap between (es.) 365 Hz and 500 Hz ?

30th April 2006, 11:26 PM	#3
Svante diyAudio Member Join Date: Feb 2004 Location: Stockholm	A woofer in a closed box or a vented box can also be measure inside the box, up to the first standing wave inside the box. I don't know if this helps as the upper limiting frequency is not necessarily higher than with the other method. Anyway, I think the inside-the-box method is kind of cool. You will have to tilt the response by 12 dB/octave, though. __________________ Simulate loudspeakers: Basta! Simulate the baffle step: The Edge

3rd May 2006, 08:34 PM	#9
elviukai diyAudio Member Join Date: Apr 2003 Location: EU- Electronic Union country	hi Rob just send a PM to you __________________ natural sound reproduction from traditional dynamic drivers?!! come on.. no no, it can be.. it must be!..

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30th April 2006, 11:50 PM	#4
Web Wolf diyAudio Member Join Date: Apr 2006 Location: Italy	First of all, thanks for your reply Is this measurement a sum of NF+FF ? Or it's only a FF ? Thanks Bye

1st May 2006, 12:15 AM	#5
RobWells diyAudio Member Join Date: Aug 2002 Location: UK	The idea behind the measurement is that if you measure a speaker in half space (on the ground with no other boundaries) at 2m , it will be the same as measuring the speaker at 1m in full space (4pi) Do a search on 'ground plane' Rob. The top plot is exactly as I stated, measured at 2m from the baffle, mic on the floor, speaker on the floor, no other boundaries.

1st May 2006, 09:08 AM	#6
Web Wolf diyAudio Member Join Date: Apr 2006 Location: Italy	Hum ... OK I understand now, but I know that the measurement in half space should be reduced of 6dB to compensate the half space effect. However I'll do a research about ground plane Bye

3rd May 2006, 10:56 AM	#7
Web Wolf diyAudio Member Join Date: Apr 2006 Location: Italy	I saw the ground plane measurements and they are quite interesting, but now I've a question for you: in these measurements the microphone is placed attached on the floor and I think it's good for lowest frequencies, but what about measuring a tweeter or a midrange (for upper frequencies) ? I mean, tweeters are quite directionals aren't they ? Should the microphone be placed a bit detached from the floor and in axis with the tweeter's center ?

3rd May 2006, 08:19 PM	#8
RobWells diyAudio Member Join Date: Aug 2002 Location: UK	'Testing Loudspeakers' by Joe d'apollito will answer all your questions and more, better than what I can. http://www.audioxpress.com/bksprods/bkindexltheory.htm I have the book, it's full of good info. Cheers, Rob.

4th May 2006, 01:32 PM	#10
MethMan diyAudio Member Join Date: Jun 2004 Location: Praha	Sorry for unpolite OT. Rob, will you be so kind and share a measuremets of Beyma 15LX60 with me, perhaps via e-mail. I'm interested in raw driver impedance plot, but any other (T-S, distortion etc.) wouldn't hurt. Thanks in advance. I'm in "consideration phase" and closer info than producer's DS will help.

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