Quiet-List 1997

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Re: Need Technical Advice


>    In several tests the R.S. unit actually surpassed the B & K unit!
> The point that I was trying to make is that people who buy these R.S. 
>units should feel confident that readings taken are very close to those 
>taken with much more expensive "professional" units.

I've already seen that site, confirming my suppositions. The tests were
performed in a non acoustically conditioned room. The author even admits the
presence of traffic and computers noise, and the only thing he says is that
"the signal level is always above the noise level". He doesn't explain how
much above.

Second, he uses two Bruel & Kjaer instruments, one of which, at least, is
uncalibrated. Perhaps the Radio Shack unit, which allegedly exhibited better
calibration than the Bruel & Kjaer one, was a new unit, recently
factory-calibrated. As a matter of fact, Bruel provides no guarantee as to
the precision attained in measurements made with an uncalibrated meter, so
the comparison between them is at least unfair. As far as I know, Radio
Shack doesn't provide a calibrator for it's meters.   

Third, when you calibrate a sound level meter, you use either one of two
standard sound levels: 94 dB or 114 dB. When you measure levels far away
from these values, for example 70 dB, you must change ranges, and there is
no guarantee that the calibration valid for one range (that containing the
94 dB or the 114 dB values) keeps valid for other selected ranges.

Fourth, the author measures frequencies up to 20 KHz, whereas both
instruments specifications declare not being able to measure, in the "C"
frequency weighting (that one used in the test), further than 10 KHz. Thus,
he's actually measuring that part of the environmental noise (traffic +
computers noise) spectrum falling below 10 KHz.

Last, the purpose was rather to test the audio signal contained in the Audio
CD, and not to test the Radio Shack versus the Bruel & Kjaer sound level meter.

I'll post a copy of the page to Bruel & Kjaer because I'm curious to know
what they have to say.

I'd like to make it clear that I'm not advocating for Bruel & Kjaer nor
against Radio Shack--on the contrary, I possess a Radio Shack unit and not a
Bruel & Kjaer (because of congenital poverty!), and it has helped me in many
occasions. But if I could purchase a Bruel & Kjaer, no doubt I would. 


Federico Miyara

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