Source: Tom's Guide US | Keywords: decibels, noise, canceling | Themes: Audio/Video Players
14. A Healthy Jiggle
It turns out that the primary culprit of that spike you see near the middle of the chart was none other than the NEC DVD-ROM drive. We replaced the drive with a Lite-On DVD, and the offending emission level dropped noticeably. I know—Lite-On over NEC. Go figure.
Mike Haines took a look at the chart and said that he felt we might be able to drop some of the remaining spikes by changing some of the internal cable positions. This turned out to be little more than grabbing the wad of cables coming off the power supply and twisting them this way and that. It reminded me of trying to tune a TV with rabbit ears. We watched the live readout of the still-running antenna sensors overhead change as a half-inch this way or that made several percent difference in the emissions. (Again, the slow exposure is making the graph you see look misty. At the time, the lines did appear sharp and jittery.) With more time and some decent cable management, we might have been able to remedy many of the signal overages.
Mike Haines was kind enough to send me this synopsis of our testing activities: “As you saw with the DVD-ROM drive, we are at the mercy of all the peripheral vendors to pass this test. The all-acrylic case presents us with a problem of grounding all these internal devices. The FCC, for open chassis testing, only requires that the top is off and two sides exposed. That still leaves a lot of metal to ground everything to. With this case we only have the I/O shield in the back and then it is all grounded through the power supply. I suspect in a typical chassis these peripherals pass to the legal limit just fine.
“We had three frequencies that ended up over the legal limit. The first is 128.88 MHz and failed by 9.3 dB, then 100.2 MHz that failed by 2.8 dB and finally 750 MHz that failed by 0.1 dB. None of these frequencies were on the test done in the metal chassis with the cover off.
“I debugged the 128.88 MHz and 750 MHz frequencies to the DVD-ROM again. I unplugged it and did one more scan. The 100.2 MHz was still over the limit and we had a new one at 160 MHz pop probably due to cable routing that was close to the limit. I disconnected the internal hard drive and the 100.2 MHz went away. The bottom line is this: Depending on what peripherals are used in the system you may or may not be passing to the legal limit.”
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One word: Google.
Gosh, we're not the general public... we're nerds
correction, physical pain doesn't set in at 130db, feeling sets in at about 130 db, so that bass that you feel thumping is above 130db, but since volume is a function of decibels and frequency it is very quiet. but children wailing is very high pitched so them at 80 db is significantly louder than the bass in your car. if you want to learn more than just do some more googling around. try searching "volume"
A clarification on ANC theory. There is no "anti-noise" generated. Ambient sounds strike the speaker cone moving it. Essentially the ANC signal applied to the diaphragm resists this motion of the cone, holding it still. Without movement the sound energy is dissipated as heat on the back surface of the cone. It is not canceled out in the classic sense, the compression and rarefaction of sound waves are mechanically destroyed.
One more really simple thing to add here. Put some really high-quality foam earplugs in your ear canal AND use your ANC headphones. This is what I do when I fly (Bose QC2's and custom-fit plugs). You've got to have enough sound without distortion in your headphones to overcome the attenuation of the plugs but for the most part their response curve is flat (if not a bit high on the low end). So your tinny-sounding earphones are actually going to have a bit more bass overall.
I've also been known to wear my Shure earbuds under my QC2's but they stick out far enough that I have to be careful not to touch them against the inside of the headphones. The nice bit is that even sitting in the back of an MD-80 I've only got to put my iPod at about 25% volume to hear every detail.
Shures with QC2 sounds like Heaven...assuming Heaven is a very quiet place. :-)
Regarding joeman42's comment, first, I'm going to say that I am definitely not a sound engineer nor any kind of other acoustic specialist, so the depth of my understanding may need improvement. However, the description I gave of ANC fits every description of the technology I've been exposed to over the years. For example, check this paper: http://doctord.dyndns.org:8000/Pubs/POTENT.htm, which describes the ANC process like so: "the noise is modeled to produce an anti-noise waveform at the output speaker." Given this guy's title as Vice President of R&D for Noise Cancellation Technologies, Inc. and the fact that this paper appeared in an IEEE publication, I'm pretty confident of the source.
Another very entertaining article from you.
“(Once again, you can tell something about the quality of a site’s content by the frequency of typos and errors in its text.)”
You might want to change that line before someone of bestofmedia knocks on your door and gives you a kick in the nuts when you open.
Another very entertaining article from you.“(Once again, you can tell something about the quality of a site’s content by the frequency of typos and errors in its text.)”You might want to change that line before someone of bestofmedia knocks on your door and gives you a kick in the nuts when you open.
Yeah, I can add that to the list of reasons I've given them. ;-) But really, you can tell the difference between a rushed schedule and borderline illiteracy or outright disregard for quality. If I've got to take one in the giggleberries for saying that Web sites in general need better quality control, so be it, but I think Bestof has a good crew that does good work. I'd rather have a few typos and solid reporting from people who care about the readers' best interests than a lot of the over-polished, under-thought dreck common in the field.
Thanks for the kind words.
Interesting article
I have hearing loss. I was born since birth, and therefore, I can't use headphones or go to rock concerts, lest I risk damage to my ears. I have never used an i Pod for more than a few minutes, and I'm glad.
I'm 15 years old, and I estimate that by the time I'm 40, my generation will have worse hearing than me. I'll be laughing "I HAD hearing loss BEFORE it was POPULAR!"
I should hope so.