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Old 16th June 2017, 03:23 PM   #11
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Quote:
Originally Posted by Piersma View Post
@Mario Pankov

Which of the measurements would you consider the most valid?
None, Lojzek and ICG have already explained why. You must measure under the same conditions with no alterations to your setup and using the same settings for all drivers. The in house measurement should be gated. You can achieve close to anechoic measurements outside if you can raise the speaker some 2-2.5m above ground and there nothing in close proximity although with today`s technology this is barely required.
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Old 16th June 2017, 04:27 PM   #12
ICG is offline ICG  Germany
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Just another thing about the impulse response: On all measurements the first amplitude is negative, that means you've switched the polarity somewhere in the chain. That though doesn't change the frequency response measurement. I'd suggest you re-check the complete measurement chain.

@Lojzek: That should be at least fs/2.
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Old 16th June 2017, 05:13 PM   #13
Piersma is offline Piersma  Netherlands
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The outside situation:
The speaker was placed on a stand, 75cm above the ground.
Distance from the tweeter to ground level : 1.65 meters.
There were no reflective surfaces in the vicinity within a distance of 2,5 meters and on the microphone was a foam dust cap in place.
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Old 16th June 2017, 08:13 PM   #14
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A couple of thoughts:

- For sweeps turn off the highpass altogether and set the start frequency of the sweep around 100 Hz. I do this all the time in HolmImpulse and never have had a problem.

- I don't see any gate markers in your first post.

- For your outside situation you shouldn't see any ground reflections for 8 msec if the mic is 1 meter from the speaker. In your impulse response you should be able to verify that no reflections occur until after 8 msec.
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Old 16th June 2017, 08:40 PM   #15
ICG is offline ICG  Germany
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Quote:
Originally Posted by Piersma View Post
The outside situation:
The speaker was placed on a stand, 75cm above the ground.
Distance from the tweeter to ground level : 1.65 meters.
Ah, thank you.

Quote:
Originally Posted by Piersma View Post
There were no reflective surfaces in the vicinity within a distance of 2,5 meters and on the microphone was a foam dust cap in place.
On stone, concrete etc. ground you'd have to calculate with the 1,65m, which would be fine too in that range. On grass or sand, you can use the 2,5m excapt for the bass.

You were right to use the foam cap, you have to use it to eliminate wind noise. The microphone foam cap does change the frequency response of the microphone though and that might be one of the main reasons for different response measurements, it filters the treble (how much depending on the foam material) and it increases the 'baffle' of the microphone. You could do all measurements with the cap on to have comparable measurements, but that means you don't have a flat mic response because it differs from the calibration file. That's not ideal but no that bad either. Or you could measure the same speaker twice, one time with and one time without the cap and edit the calibration file for the differences - but you have to select the correct calibration file for each measurement then.
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Old 17th June 2017, 08:14 AM   #16
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In my setup the foam cap did introduce shifts in the recorded frequency response and I threw it in the trash Another thing you may want to consider is proper positioning of the mic stand. Best is to put it as far away as possible and the mic to be mounted as on-axis to the holder (so to look like an extension to it) as possible. Cable should be twisted around the mic holder and stand. You can also add absorbing material around these, including the microphone (for indoors use) to reduce reflections from the pencil that holds the capsule.

You can do a somewhat accurate measurement outdoors by putting the speaker to lay on a large concrete/tarmac plane, and putting the mic some 2-2.5m away lying on axis with the speaker. It is an old method that was widely used in the past.
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Old 17th June 2017, 01:14 PM   #17
ICG is offline ICG  Germany
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Originally Posted by Mario Pankov View Post
Cable should be twisted around the mic holder and stand.
No, don't do that. It puts stress on the cable and twisting the cable itself happens automatically. Instead use gaffa or, even better, cable clips.

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You can also add absorbing material around these, including the microphone (for indoors use) to reduce reflections from the pencil that holds the capsule.
Don't do that! There's a very good reason the mic got that shape, it's to avoid building up pressure, the absorbing material does not help you in any way but the 'baffle' of the mic increases. It affects the linearity and the distortion of the mic.

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You can do a somewhat accurate measurement outdoors by putting the speaker to lay on a large concrete/tarmac plane, and putting the mic some 2-2.5m away lying on axis with the speaker. It is an old method that was widely used in the past.
That's called a groundplane measurement. But you cannot lay the mic on axis, it got to be placed flat on the ground and that is a MUST! You get a higher level (+6dB) and it's not ideal for tweeter measurement because of the distance of the tweeter to the ground (it becomes 'mirrored' from the ground). That is especially a problem with wider speakers. A GP measurement is great for subs or bass, where the reflection of the ground would otherwise be a big issue, for other uses 4pi (free field) measurement is a lot better.
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Old 18th June 2017, 03:25 AM   #18
system7 is offline system7  United Kingdom
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I think that ring-radiators like the Vifa XT25 in single (30) or dual magnet (60) versions are a great idea. I say this without ever having heard one.

What we know about them, is they are non-ferrofluid and low impedance, hence have some impedance issues around 600Hz, which is fixable with crossover design.

Dome tweeters, especially soft-domes, go to pieces above a certain frequency. The big 3" ATC mids show this in spades at 5kHz on the waterfall.

There are certain theoretical mathematical grounds for thinking spherical domes are dreadful once they go into breakup. Exemplified by Leonard Susskind in his string theory lectures. A ring-radiator is a toroidal solution. IMO, it works better. In essence it pins the centre of a spherical dome, thus eliminating the troublesome breakup.

All is geometry in audio.
Attached Images
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File Type: png Vifa_XT25BG60-04_Picture.PNG (118.4 KB, 87 views)
File Type: jpg Jeff Bagby ATC.JPG (113.2 KB, 66 views)
File Type: jpg Waves on a sphere Prof. L Susskind.JPG (33.0 KB, 67 views)
File Type: jpg Gaussian Curvature in Geometry.jpg (101.1 KB, 66 views)
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Old 18th June 2017, 07:15 AM   #19
Sonce is offline Sonce  Macedonia
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There are certain theoretical mathematical grounds for thinking spherical domes are dreadful once they go into breakup. Exemplified by Leonard Susskind in his string theory lectures. A ring-radiator is a toroidal solution.
Please stop with that nonsense which you are proclaiming over and over. I have to quote myself answering you from other, earlier thread:
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Not true. Please bear in mind that some of us in this forum actually are educated in physics and mathematical topology. In fact, string theory and Prof. Susskind lecture has nothing in common with this topic. You have to understand what torus compactification and Ricci-flat actually means, before claiming that toroidal solution is more optimal. Ring from XT-25, or more precisely annulus, topologically is equivalent to an open cylinder, not a torus! Or, if "ring tweeter diaphragm" is not punctured, than it is topologically equivalent to a dome or cone, not a torus!
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Old 18th June 2017, 07:15 AM   #20
ICG is offline ICG  Germany
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Quote:
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Dome tweeters, especially soft-domes, go to pieces above a certain frequency. The big 3" ATC mids show this in spades at 5kHz on the waterfall.
I don't see a problem there - that's already outside of its usable range, regarding f-response aswell as beaming or useful upper crossover frequency. They are mid drivers after all.

Quote:
Originally Posted by system7 View Post
There are certain theoretical mathematical grounds for thinking spherical domes are dreadful once they go into breakup. Exemplified by Leonard Susskind in his string theory lectures. A ring-radiator is a toroidal solution. IMO, it works better. In essence it pins the centre of a spherical dome, thus eliminating the troublesome breakup.
In essence, it's a coil moving two huge cloth surrounds. That description might not appear to be appealing but the result speaks for itself. IMO an excellent concept.

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All is geometry in audio.
I couldn't agree more! Thank you!
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