Is it bad to measure too close?

[icor1031]

I did a measurement further away, as I was told to do, to account for (? BSC? Something Baffle? ?)

Apart from that, is it a bad thing to measure with the mic very close - about 1ft - 2ft?

I get better measurements that way, all I did here was move the mic closer:

[greywarden]

Re: Is it bad to measure too close?

When you move the mic closer, there's less of the room reflections in the measurement. Ideally, the mic is positioned closer to the speaker than the closest thing (furniture, wall, etc.) in the room. When I finish building my new 3-way adjustable speaker stand, I'll be measuring the speaker from about 6ft away with the ground being somewhat further, and nothing else around.

[icor1031]

Re: Is it bad to measure too close?

Why 6' and not 1'?

[greywarden]

Re: Is it bad to measure too close?

The further away you measure, the more accurate the low end is, given the wavelength of the frequency. My old room was about 4ft, limited by the 8ft ceiling.

[icor1031]

Re: Is it bad to measure too close?

That makes this much easier, thanks. I just measured at 29".

Also, for some odd reason I appear to not need any BSC. I'll post more info later.

The further away you measure, the more accurate the low end is, given the wavelength of the frequency. My old room was about 4ft, limited by the 8ft ceiling.

[LouC]

Re: Is it bad to measure too close?

Answer: It depends. If you are doing a single point measurement with all drivers in the box to get the combined effect of BSC, edge diffraction, and acoustic centers, etc. then you need to be far enough away for the drivers to blend. Typically I find 3-4 feet for a two way, 5-6 feet for a tower. I also use ground plane, outdoors whenever practical (5-6 feet) . For multiple woofers (TMWW type design) I will supplement with a nearfield splice (2-3 inches) of the woofers.

If you are emulating test baffle/mfg type then you can probably go a bit closer, but you'll need to adjust sensitivity on all your measurements to keep them consistent.

IMHO measuring indoors in a small room is fine for final setup, but adds a lot of obfuscating information from standing waves and reflections. You can reduce gate times to minimize this, but you sacrifice any real data for the low end. For a typical 2 way, no big deal. For a 3 way, you might want that sub 300 hz info.

[cjd]

Re: Is it bad to measure too close?

For evaluating a driver, it's the only way to go.

If you're plan into use the data for a crossover, you're missing far too much important information. Lou covered the basics.

Measuring and getting usable data is harder than twiddling so on axis looks nice... though really getting a design to gel again takes understanding.

Fun stuff.

[donradick]

Re: Is it bad to measure too close?

I would never disagree with cjd and LouC, but:
I measure at 1 meter on the tweeter axis (in my smallish room), then import that into PCD for modeling.
I measure the woofer and ports close miked (1-2 inches) and splice that into the
1 meter to see the low end.
When I had questions about distortion measurements, I was advised to set an overall
level at 1 meter (85,90,95 dB), then move the mike close and get the distortion graphs.

Oh yeah, I should measure off axis also - just starting to understand the importance
of good off axis power response.

[ErinH]

Re: Is it bad to measure too close?

Basically, the closer you move the mic, the more high frequency resolution you lose. The rule for the near field cut off frequency on the high end is given in relation to the size of the driver. At/around this frequency, it is best to splice the far field measurement with the nearfield measurement to get a pseudo-anechoic response of the speaker.

D'Appolito's Testing Loudspeakers book goes in to detail on this. Or you can read here for a summarized method of testing and learning the limits of nearfield placement.

http://www.klippel.de/fileadmin/klippel/Files/Know_How/Application_Notes/AN_39_Merging_Near_and_Farfield_Measurements.pdf

[cjd]

Re: Is it bad to measure too close?

I would never disagree with cjd and LouC, but:
I measure at 1 meter on the tweeter axis (in my smallish room), then import that into PCD for modeling.
I measure the woofer and ports close miked (1-2 inches) and splice that into the
1 meter to see the low end.
When I had questions about distortion measurements, I was advised to set an overall
level at 1 meter (85,90,95 dB), then move the mike close and get the distortion graphs.

Oh yeah, I should measure off axis also - just starting to understand the importance
of good off axis power response.

You're not disagreeing with us at all. I don't think... Assuming you accurately derive acoustic offset so you can HBT your spliced data, you're right there. I think we both glossed over a lot of details (covered well elsewhere) but if I had to design using one, and just one measurement, it would be at 1-2M (speaker size depending) with zero movement of anything; Box modeling tells me enough about how the bottom end will respond, I don't need to see it in my graphs (crossing below gate, however... ok, complexities avoided here ok? ) ...

For evaluating a specific driver, I like to remove any external influences. I measure at voltage, and inspect (mostly) strictly for things-that-should-not-be. I also set them up and listen without a baffle, with a baffle, and all sorts of stuff... but... anyhow.

It seems so simple.

[jmb]

Re: Is it bad to measure too close?

Just a couple of comments and clarifications. Keele suggested that to get an accurate nearfield measurement, it is best to measure within .11*radius of the driver. This is the generally accepted standard. For a 6 inch driver, you are talking about 1/3 of an inch. It is true that the closer you measure to the driver compared to the distance to the nearest reflection, the more you can window out the room, but the amount of information steadily decreases as you get closer to the enclosure. This includes baffle step, diffraction, etc. In addition, your mid to high frequency data suffers as you get phase interference from baffle reflection. Farfield measurements should demonstrate a 6 dB drop with each doubling of distance. The measurements do not arrive at this in unison, so as you get closer to the true farfield, you get a more uniform consistency in measurement that does not change with changes in position. In general, with a non tower speaker, you are safe at about a meter. For towers, as CJD and Lou said above, a couple of meters or ground plane measurements are likely best.

I did a fair number of measurements and built a little program to calculate the farfield distance given your tolerable error. It is in the form of an Excel spreadsheet that I posted over at diyaudio.com under multiway speakers entitled the real farfield distance. Make certain that you download the last one that I posted as it was a work in progress until it matched my actual findings and the available mathematical theory.

Jay

[jmb]

Re: Is it bad to measure too close?

Just to make it easier, here is a link to the response with the spreadsheet:

http://www.diyaudio.com/forums/multi-way/212394-real-farfield-distance-6.html#post3110009

Jay