Hi Jim,
Of course I agree with you. This is true of any speaker system -- whether
they are built with many identical drivers and an active equalizer (al la
Bose 901s) or any other system using two or more different drivers and
passive equalizers -- called cross-over networks. The theory was that many
small resonances in the system (due to the coupling between the identical
drivers) would be easier to "flatten" than a few resonances from widely
disparate drivers.
Note that the original acoustic research Bose and group did was in Boston's
Symphony Hall using binaural manikins placed around the hall. The research
was to simulate the sound of a full orchestra in a setting like Symphony
Hall. Many early critics claimed they sounded fine for orchestral music and
bad for small groups.
What does this have to do with antennas? In many ways it is similar to a
triband beam vs. a log periodic. The triband has three peaks (al la
tweeter, midrange, bass) and the log periodic has a "smooth but lumpy"
response across a wider bandwidth. (yea -- a bit of a reach...)
73 -- Larry -- W1DYJ
----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: <towertalk@contesting.com>
Sent: Saturday, August 31, 2013 11:06 PM
Subject: Re: [TowerTalk] Bose 901 antenna theory
On 8/31/2013 7:23 PM, Larry Banks wrote:
Then, adding an inverse response in series with the total speaker
response -- the equalizer -- created what is known as a quasi-linear
system.
There's a HUGE problem with all of this analysis -- the loudspeaker
response is the sum of all of the drivers, with the directivity of each
driver superimposed, as well as the acoustic time of flight from each to
each observer. AND the response at every point is the COMPLEX sum
(magnitude and phase) of all of those drivers, and because the TIME is
different to each observer, which means phase shift that increases
linearly with frequency, AND is different to each observer. This means
that every observer hears DIFFERENT peaks and dips in the response. In the
pro audio world, we call these peaks and dips caused by cancellation of
multiple arrivals "comb filtering," because the amplitude response looks
like the teeth of a comb held vertically.
SO -- in the real world, with a gargantuan equalizer, you might
theoretically obtain a flat response for one point in space, but you would
be creating a worse mess everywhere else. :)
Bose speaker systems do include equalizers, but they are compensating for
the overall response of the system, not for the comb filtering produced by
the multiple drivers.
73, Jim Brown K9YC (Retired from pro audio)
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