[TowerTalk] Bose 901 antenna theory

[Larry Banks]

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 at audiosystemsgroup.com>
To: <towertalk at 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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