Mark,
Thank you for this post!!
I greatly appreciate your experience on this topic!
Duane :)
On Fri, 9 Feb 2007 09:02:46 +0100, "Mark van Wijk"
<mark.van.wijk@philips.com> said:
> The Jan/feb issue of the National Contest Journal has a great article on
> the subject of fighting local audible noise and QRM.
>
> The article describes:
> - experiences with some isolation headphones
> - how to make your own (extreme isolation headphone with small drivers
> build in D.I.Y. kit)
> - commercial noise canceling headphones
> - David Clark professional passive and active isolation headphones
>
> There is a graph showing the isolation of various tested products vs
> frequency.
> It also shows the human body limitation because at some level your bone
> structure is acting as a transmission channel and beyond that value there
> is no further attenuation possible.
>
> My current QRL does offers some 40-100 new models for the consumer
> electronics market each year. I do a lot of consumer testing with some
> models, especially on the wireless versions and those having noise
> cancelling. I have not found a decent one yet which truly allows me to
> dig
> deeper into the noise and copy that cw signal in noisy environments. The
> noise cancellation types do offer 'some' relief which becomes avid after
> many hours of use during contests and dx-pedition etc....
>
> Earlier this week I received my ordered "chopper-headphone" from
> http://www.davidclark.com/StereoHeadset.html.
> This device has great passive noise isolation with specs from -15dB at
> 125
> Hz to quickly into -25db in the midranges to -38dB at 4 - 8Khz.
> I wanted this special model, since it incorporates full range drivers
> (10Hz--20000Hz). From my former QRL (Dali Hi-Fi loudspeakers, Rogers
> Hi-Fi loudspeakers) I learned that some drivers with restricted ranges
> can
> be too stiff. They simply needed a lot of energy before they start moving
> following/replicating the swing of the input signal. Typical "burn-in"
> time solved this to some (minor) extent, but the especially designed high
> quality drivers were much better capable of setting up a detailed
> soundstage at an already much lower input volume. That is why I
> initially
> skipped the D/C 200Hz -5500 Hz versions which even has some 4dB better
> spec. i terms of passive noise cancellation. I cannot tell if my former
> experience with stiff cone material is also valid for current mylar
> headphone drivers as well and might affect copying weak signals in
> received HF audio, I simply did not want to take that risk.
>
> Anyway how does that green chopper headphone perform?
> It does great !
> Both at audio quality and especially at how it almost totally kills your
> noisy environment, almost scary to say the least.
> I compared it to a professional passive isolation headphone as is
> required
> on construction sites etc., the green chopper does soo much better.
> It does however allow you to focus on what you do want to hear.
> Price? exactly the same as a Heil pro set, but without the mic-boom which
> an be ordered as a seperate kit.
> I have no application for any mics at HF at my home :)
> Will use this green chopper during ARRL dx at PI4TUE , which shack is
> stacked with zillions of pc servers and other loud noise producing stuff
> (loud students, noisy YL-operators etc..)
>
>
> 73 Mark, PA5MW
>
>
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--
dw
dw@sover.net
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