You were correct in your initial interpretation. I am wondering if a 1 us pulse can be established and not smeared over a longer period of time. The source antenna emits a coherent wave with this pulse modulation envelope, but many different rays taking different paths converge at the receive antenna at different arrival times. As long as those delays are much shorter than 1 us (path difference much less than 300 meters), the original modulation is received. But if delays are too long, then the pulse smears. What I was looking for was how to relate the "smearing" to room size. As Mr. DeWitt pointed out, the dependence is not on room size alone, but also on the "Q" of the room. Since I don't know how to analytically predict any of this, and I don't know the "Q" of the room, I was asking if the previously submitted check seemed reasonable.
> From: John Woodgate <j...@jmwa.demon.co.uk> > Reply-To: John Woodgate <j...@jmwa.demon.co.uk> > Date: Sun, 27 Jul 2003 18:52:33 +0100 > To: emc-p...@majordomo.ieee.org > Subject: Re: pulse modulation in reverb chambers > > > I read in !emc-pstc that Ken Javor <ken.ja...@emccompliance.com> wrote > (in <bb49768d.3717%ken.ja...@emccompliance.com>) about 'pulse modulation > in reverb chambers' on Sun, 27 Jul 2003: >> So if I am worried whether the 1 us pulse width can be sustained, and I >> don't know how to determine it analytically, my plan is as follows: >> >> I put a wire probe in the room, run it to a spectrum analyzer tuned to >> the transmit frequency, put the analyzer in zero span mode with a 1 or 3 >> MHz bandwidth and look at the modulation waveform. If it is >> significantly longer than 1 us, I know my constructive interference path >> delays are smearing the modulation away. > > I originally thought you wanted to establish a reverberant field with a > 1 us pulse. Now I'm not so sure. > > By 'modulation', you mean the envelope of the 1 us pulse? Isn't the idea > of a reverberation chamber that once it is excited the energy takes a > long time to die away? That means that the original pulse will be > s t r e t c h e d, dying away exponentially if the room is a good one. A > too-short pule would almost certainly produce a delay that was not > exponential. > > You can only 'sustain' the 1 us pulse if your receiver is in the 'direct > field' of the antenna, i.e. the field strength due to the direct > propagation path from sending antenna to receiving antenna is at least 3 > dB above the combined field strength due to all indirect paths. You can > easily see this if you consider just a few simple specular reflections, > which appear at the receiver as a series of overlapping (since their > path lengths are less than 300 m) 1 us pulses of varying level. > -- > Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk > Interested in professional sound reinforcement and distribution? Then go to > http://www.isce.org.uk > PLEASE do NOT copy news posts to me by E-MAIL! > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > > To cancel your subscription, send mail to: > majord...@ieee.org > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Ron Pickard: emc-p...@hypercom.com > Dave Heald: emc_p...@symbol.com > > For policy questions, send mail to: > Richard Nute: ri...@ieee.org > Jim Bacher: j.bac...@ieee.org > > Archive is being moved, we will announce when it is back on-line. > All emc-pstc postings are archived and searchable on the web at: > http://www.ieeecommunities.org/emc-pstc > This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
