Note: Descriptions are shown in the official language in which they were submitted.
L76
BOS P37 BACKGROUND OF THE INVENTION
~777 The present invention relates in general to improving
woofer efficiency and more particularly conce.rns novel apparatus
and techniques for providing an economical compact relatively
efficient loudspeaker system with relatively good performance.
From the standpoint of achieving optimum sound repro-
duction, the best presently known approach is that of the
BOSE 901 DIRECT/REFLECTING loudspeaker system generally of the
type described in U.S. Patent No. 3,582,553. That system use~ a
multiplicity of full-range drivers in combination with an electronic
active equalizer having more than one hundred components. The use
of multiple full-range speakers helps eliminate audible resonances
and avoids the disadvantages associated with woofers, tweeters and
crossovers. However, the use of so many Loudspeakers and components
in the equalizer is costly. The more economical approach is to
use a woofer, tweeter and crossover network that directs low
frequency signals to the woofer and high frequency signals to the
tweeter in a vented box. A typical approach is to design for
a Butterworth Response characteristic in the manner described in
a paper by A. N. Thiele entitled "Loudspeakers in Vented Boxes"
in the May and June 1971 issues of JOURNAL OF THE AUDIO ENGINEERING
SOCIETY, pp. 382-91 and 471-83. Specific loudspeaker systems
using these princip~es are described in papers by Raymond J. Newman
entitled "A Loudspeaker System Design Utilizing a Sixth-Order
Butterworth Response Characteristic" in the JOURNAL OF THE AUDIO
ENGINEERING SOCIETY, July/August 1973, pp. 450~56, and
D. B. Keele, Jr., entitled "A New Set of Sixth-Order Vented-Box
Loudspeaker System Alignments" in the JOURNAL OF THE AUDIO
ENGINEERING SOCIETY, June 1975, pp. 354-60. These papers teach
0 establishing the closed box fundamental resonance of the woofer
at a frequency corresponding to the half power point in a system
consisting of the loudspeaker in its enclosure together with a
i
complementary filter of the type described by Thiele as
his alignment 15~ Although this approach provides a
reasonably uniform frequency response above the lower
half power point, efficiency is less than would be
desired.
Accordingly, it is an important object of
this invention to provide a woofer-tweeter loudspeaker
system with improved efficiency.
It is a further object of the invention to
achieve the preceding object while providing a good low
frequency response,
It is still a further object of the invention
to achieve one or more of the prececling objects economi- ,,
cally.
According to the a'bove objects, from a broad
aspect, the present invention provides a loudspeaker
system having a woofer and at least one -tweeter energized -~
through a crossover network in a vented cabinet. The
improvement comprises means including the woofer mass
and the compliance of the cabinet for establishing'the
closed box fundamental resonance of the woofer at least ~,
as high as of the order of 80 Hz and at least of -the
order of 20% higher than the half power frequenc~ of
the vented loudspeaker system relative to its constant
output region. Port means is also provided having a
mass so that the port resonant frequency of the port ,'
means mass with the compliance of the cabinet is lower
than the half power frequency of the vented loudspeaker
system. Input,~terminal means are provided for receiving ~'
an input signaln Active equalizing means to compensate
3 _
:, ,~ , .
for the fall-off in frequency response from a frequency
just above the half-power ~requency of the vented
loudspeaker system to a predetermined frequency signifi-
cantly below the half~power frequency so that the frequency
response of the equalized loudspeaker system between the
lower frequency and the frequency just above the half-
power frequency of the vented loudspeaker system is
substantially uniform while the efficiency of the system
ls significantly increased relative to the system with
the closed box fundamental resonance of the woofer being
the same as the half-power frequency. Means is further
provided including the active equalizing means for
coupling the input terminal means to the woofer and to
the tweeter.
SUMM~RY OF THE INVENTION
According to the inventionl there is means for
establishing the closed box fundamental resonance of the
woofer-tweeter loudspeaker system at a frequency higher
than the lower half power point frequency of the system,
and means for actively equalizing the woofer frequency
response to establish a substantially uniform frequency
response, such as 3 db between the lower half power
point frequency of the system and at least a ~requency
above the fundamental closed box resonance of the system.
The latter means preferably comprises a two-pole single-
transistor circuit.
'
- 3a -
Numerous other features, objeets and
advantages of the invention will beeome apparent from
the following specification when read in conneetion
with the accompanying drawing în which:
BRIEF DESCRIPTION OF THE DRAWI~G
FIG. 1 is a block diagram illustrating the
logieal arrangement of a system aceording to the
invention;
FIG. 2 is a perspective view of a preferred
embodiment of the loudspeaker eabinet aeeording to the
invention' :
FIG. 3 :illustrates a preferred form of cross-
over network,
FIG. 4 is a sehematic eireuit diagram of a
preferred
- 3b -
L7il~
BOS P37form of ac~ive equalizer; and
7 FIG. 5 is a graphical representation as a function of
frequency showing the woofer freq~aency response as a ~unction of
frequency for a cr~ssover ne~work of FIG. 4 with no equalization,
6 db equalization at 45 Hz and 10 db equalization at 45 Hz.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
With reference now to the drawing and more particularly
FIG. 1 thereof, there is shown a block diagram illustrating the
logical arrangement of a system according to the invention. A
signal on input terminal 11 is reproduced by woofer 12 and tweeter
13 in cabinet 14 energized through crossover network means 15
by power amplifying means 16. Active equalizing means 17 provides
an equalized signal to power amplifying means 16 to compensate for
the fall-of~ in response oE wooEer 12 frorn a frequency above the
closed box woofer resonance. In a preferred exemplary embodiment
the closed box fundamental resonance of the woofer is 81.5 Hz
and the 3 db downpoint of the unequalized system is 65 Hz.
Referring to FIG. 2, there is shown a perspective
view of a preEerred form of cabinet according to the invention
corresponding substantially in structure to the commercially
available BOSE Model 301 loudspeaker with the exception that the
fundamental closed box resonance o~ the woofer has been raised from
79 Hz to 81.5 Hz by reducing the woofer mass, such as by using fewer
voice coil turns. This reduction in turns and mass increases the
Bl ratio and damping while facilitating equalization with a
relatively inexpensive two-pole network. A conventional system
with a 40 Hz half-power point would require about twice the cabinet
volume to achieve the same damping and would have 1/4 the midband
efficiency.
0 Cabinet 14 supports woofer 12 in the front panel cand
tweeter 13 in the angled panel 21 that also carries vent tube 22.
A sound deflecting vane 23 depends from shaft 24 suspended from
BOS P37knob 2S for controlling the effective directivi~y of the tweeter.
7 Referring to FIG. 3, there is shown a schematic
circuit diagram of a preferred form of crossover network means 15
with specific parà`meter values.
Referring to FIG. 4, there is sho~l a schematic circuit
diagram of active equalizing means 17 which receives an input
signal on terminal 11 and drives power amplifying means 16 between
terminal 32 and ground. Terminal 31 receives collector potential
for this two-pole emitter follower circuit. Typical parameter
.0 values are:
Cl, C2 .022 ~f
Rl, R2 3.3 Megohm
R3 18 KQ
R4 47 KQ
Ql 2SC1335
C3, C4 .022 ~f
R5, R6 56 KQ
Relative to the frequency response at 1 kHz, the circuit of FIG. 4
produces a response 10 db higher at 45 Hz so that the overall system
~20 response is within ~ 3 db from 40 Hz to a frequency above 100 Hz
and falls off at 36 db per octave with the half power frequency
being 42 Hz.
Referring to FIG. 5, there is shown a graphical repre-
sentation of ~he system response as a function of frequency
without equalization represented by curve 41 showing the 3 db
down point at 65 HZ, with 6 db equalization at 45 Hz represented
by curve 42 and in a preferred form of the invention with 10 db
equalization at 45 Hz represented by cur~e 43 resulting in the
frequency response between 40 Hz and above 100 Hz being within
+ 3 db.
The prior art approach exemplified by the aforesaid
papers teaches establishing the closed box tuning frequency fb the
-5-
. .
7~i
BOS P37same as the lower half power frequency f3 of the system
~7~7 relative to its constant output region. In contrast it is an
important feature of the present invention to select the closed
box tuning frequency fb above the half-power frequency f3 of the
system relative to its constant output region and thereby effect
a material increase in efficiency while using relatively inex-
pensive two-pole ac~ive equalization to reta:in an exceptionally
good low frequency response for a system of this low cost and
small size so that the invention may readily be used with a 10 watt
o power amplifier providing exceptionally high sound levels
accurately reproducing relatively low bass tones.
In an actual embodiment of the invention, tweeter 11
was a 3 inch tweeter having a nominal voice coil impedance of
8 ohms, woofer 12 was an 8 inch woofer having a nominal impedance
of 8 ohms, the volume of cabinet 14 was 0.7 feet3 and vent tube 22
was 6 inches long and 2.5 inches in diameter.
~ There has been described novel apparatus and techniques
for appreciably increasing efficiency of an economical compact
woofer-tweeter loudspeaker system while providing exceptionally
good bass response for such a compact inexpensive system. It is
evident that those skilled in the art may now make numerous
uses and modifications of and departures from the specific embodi-
ments described herein without departing from the inventive concepts.
Consequently, the invention is to be construed as embracing each
and evéry novel feature and novel combination of features present
in or possessed by the apparatus and techniques herein disclosed
and limited solely b~ the spirit and scope of the appended claims.
What is claimed is:
.
