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Arny Krueger
 
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Default My equipment review of the Bose 901

"Peter Sammon" wrote in message


The small drivers themselves boast heavy magnets and high quality
voice coils as they must move an enormous amount of air.


Myth number one: A reasonable number of small drivers can move as much air
as a single large one, all other things being equal.

The hidden agenda is linear stroke.

Let's compare a 4" driver and an 8" driver. How many 4" drivers does it take
to move as much air as an 8" driver?

The naive answer is 4, presuming that cone area is proportional to diameter,
squared. True for theoretical geometry, not true for speakers.

Speakers have flexible surrounds and a fraction of the area of the surround
must be deducted from the area of the cone. However, the width of the
surround is not proportional to the diameter of the speaker, but rather is
dependent on the designer's goals for maximum linear stroke. IOW an 8"
speaker with a certain linear stroke will have a surround that is a certain
width. If the 4" speaker has the same linear stroke, then its surround will
have to be the same width. However, since the diameter of the smaller
speaker is quite a bit smaller, the surround takes up a greater proportion
of the diameter of the speaker. IOW, the area of the actual moving part of
the smaller cone is even less than proportionately smaller.

For small speakers, it may take 5 4" speakers to have the same moving cone
area as an 8" speaker due to the loss of effective moving diaphragm area
because of the width of the surround.

Secondly, smaller speakers, all other things being equal, don't have the
same linear stroke as larger speakers. Speakers tend to scale in all
dimensions. Not only is a 4" speaker half the diameter of an 8" speaker, but
it will have half the linear stroke, all other things being equal.

In short, it might take as many as 10 4" speakers to have the air-moving
capacity of a single 8" speaker, all other things being equal.

The multi
chambered ACOUSTIC MATRIX enclosure vents the speaker in a most
unusual way with air
speeds exceeding 60 mph! Three ports or jets protrude through the
rear of the 901 enclosure.


Knowledgeable speaker designers know that high air velocity in ports is
anathema. High air velocities lead to higher turbulence, and turbulent flow
tends to be noisy flow. One sign of a quality ported speaker is a large,
low-velocity port.

Because of the use of small full range drivers, an active equalizer
which is absolutely essential to the system is used in place of the
normal CROSSOVERS that introduce distortion no matter how well
engineered. The 901 EQ simply smooths out any bumps or irregularities in

the system's
power response.


Plan B: build a system that is essentially acoustically flat. Not mission
impossible in this day and age.

Now for the seriously damning facts about the Bose 901. When the 901 was
built, not that much was generally known about optimizing the performance of
arrays of small drivers. It turns out that arrays of small drivers can be a
real can of worms, if smooth, well-balanced frequency response is desired.
I'm quite sure that if Bose were to design the 901 today with a clean piece
of paper, they'd do it quite a bit differently.

Probably the most serious problem with designs based on ad-hoc collections
of identical small drivers is that they can have very rough frequency
response (lobing and comb-filter effects) when you move slightly off-axis,
and they can also have frequency response that is a strong function of
distance from the speaker to the listener. There are ways to manage these
problems with generally involve putting the drivers on curved baffles,
and/or adjusting the level and/or frequency response of the drive to the
various drivers. However, the classic Bose 901 design really does neither of
these things well.

Here's an AES paper that points out these problems and potential solutions
in some detail:

Author(s): Keele, Jr., D. B.
Publication: JAES Volume 38 Number 10 pp. 723·748; October 1990
Abstract: The Bessel array is a configuration of five, seven, or nine
identical loudspeakers in an equal-spaced line array that provides the same
overall polar pattern as a single loudspeaker of the array

One important point is that the no way is the Bose 901 a Bessel array.
A second important point is that of all the Bessel arrays analyzed, 5
drivers in a row was found to be the only one that was recommended.