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Old 17th December 2011, 05:39 PM   #19351
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Quote:
Originally Posted by simon7000 View Post
Reciprocity refers to a reciprocal relationship. The microphone standard uses three as a minimum, as it is intended to give converging results. I don't think that is required for a simple is it flat question. It is if you want to know how flat!
I recently bought 32 cheap Karaoke microphones, going to order a pair of enclosures in Tap Plastics for 16 capsules each, arranged as line arrays. Let's check how Karaokish will they sound driven by transformerless tube outputs.
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Old 17th December 2011, 06:16 PM   #19352
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Originally Posted by Wavebourn View Post
I recently bought 32 cheap Karaoke microphones, going to order a pair of enclosures in Tap Plastics for 16 capsules each, arranged as line arrays. Let's check how Karaokish will they sound driven by transformerless tube outputs.
Thank You Thank You Thank You That means there will now be 32 less chances for Karaoke now!!!
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Old 17th December 2011, 06:55 PM   #19353
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Quote:
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Thank You Thank You Thank You That means there will now be 32 less chances for Karaoke now!!!
It depends on reciprocity. Don't you expect to hear a choir of 32 Karaoke singers?
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Old 17th December 2011, 07:46 PM   #19354
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My last post to the idea of current drive of a dynamic speaker. 1st plot shows impedance plot vs. frequency calculated from the equivalent diagram shown. The 2nd plot shows transfer function of this equivalent speaker circuit with increasing voice coil resistance as a parameter (higher resistance = closer to current drive). We can see that when increasing voice coil resistance, i.e. approaching to current drive condition, the only thing we get is a resonance peak at bass frequencies and decreasing efficiency. Exactly as expected.
Those curves look like they could be used for a "loudness control" feature
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Old 17th December 2011, 07:52 PM   #19355
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Originally Posted by simon7000 View Post
Scott,

As to NIST traceability accuracy that is what a pistonphone and barometer are for. My acoustic standards are better than many local testing labs! That is needed as sound level meter calibrators need to be checked each year. SLM's are checked before each use. The cheapies can drift by several db between uses and operating temperature. The local noise ordinance allows for 5 db calibration error!

ES
Pistonphones are only absolute SPL at one frequency. I repeat I'm trying give the average DIY'er something they can build for a few bucks and spare parts that gets close enough for DIY. BTW the medical pressure transducers are extremely stable as barometers -3dB equals
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Old 17th December 2011, 08:37 PM   #19356
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I just spoke to Jack Bybee today. He was complaining that one of the front tires on his Mercedes started to disintegrate while he was driving, after only 10,000 mi. I asked him if there was something wrong with the car, but he said no, it has to do with the variable suspension, he was told. I'm glad that I don't drive that car, I could not afford the tires. '-) For the record, he won't be at CES this year, he just don't want to exert the effort, and I don't blame him. As we get older, it is harder and harder to do CES. My office partner, Brian Cheney of VMPS will be there, however, showing with Jim Bongorno (sp) which should be interesting.
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Old 17th December 2011, 08:53 PM   #19357
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Quote:
Originally Posted by myhrrhleine View Post
Those curves look like they could be used for a "loudness control" feature
The effect of amplifiers with high output impedance - of many tube amps.

Click the image to open in full size.

http://www.stereophile.com/content/c...r-measurements
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Last edited by PMA; 17th December 2011 at 09:13 PM.
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Old 17th December 2011, 09:04 PM   #19358
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PMA, you exaggerate the effects of tube amps on being a tone control. Yes, the frequency response will change somewhat, but it might even get better, rather than worse. I am now perhaps driving my WATT's with 0.5 ohms, at least, and it sounds just fine. This is a damping factor of about 10. My Dyna tube amps have a damping factor of 15, so why exaggerate the problem? You did the same with current drive. Who is to say that current drive, IN SOME CIRCUMSTANCES, isn't better? Your experience curve is fairly limited, as I found this out almost 50 years ago.
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Old 17th December 2011, 10:01 PM   #19359
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Originally Posted by scott wurcer View Post
Pistonphones are only absolute SPL at one frequency.
Actually, no. They are accurate across the range they work across, usually not much above 350 Hz, or you get the equivalent of valve float. Since a pistonphone is a very constant known displacement transducer the pressure is constant (or so the manual says, with instructions for doing just this).

A few other comments-
Quad used the cancellation trick in the factory floor for QA of the ESL63 from the beginning. It works really well and its easy, but not face to face. Two side by side out of phase will get a really good sharp null if they are working right.

Reciprocity calibration is nice for national labs but not too practical for anything else. The measurement mikes are normally done with an electrostatic process with a special screen that fits on the mike to drive it. Works well but won't work with the Panasonic. Also opens up the can of worms between pressure and free field microphones. Fortunately, B&K supply correction curves that permit moving from one mode to the other in effect.

The Earthworks guys were pretty forthcoming on their spark testing and indicated that the 1/4" Microtech Gefell was the most accurate transducer they have tested. They got quiet when I started asking about the mechanical arrangements.

I'm very interested in the spark calibration method. My concern is that the energy at low frequencies is so low that there will be little useful information but I would be happy to explore it. I have a large collection of measurement microphones as well but the Girardin (Panasonic) mikes that come with Praxis make everything too easy so they don't see much use.

I have been using ground plane measurements time windowed in a basketball court in the warehouse very effectively. No reflections for more than 50 mS gives very good low frequency extension to the measurements. It even works when the crusher is running at the other end of the warehouse.
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Old 17th December 2011, 10:04 PM   #19360
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It simply depends on how the loudspeaker is designed and filtered, how the current drive affects the frequency response....in some cases i may simply null the effects of filtering, in others is will not matter to any significant degree...
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