Speaker box

This invention relates generally to vented loudspeakers for the reproduction of musical sounds, but particularly to the design and location of ports or vents that tune the enclosure. The loudspeaker system has an enclosure, having a front baffle and a pair of loudspeaker drivers mounted in the baffle. The enclosure has a hexagonal cross-section. A pair of vents are each located at a juncture of the vertical side edges of the front baffle and its adjoining panels. The vents lead into the enclosure via a conduit, which ends in an inlet. The inlet is positioned within the enclosure to face the rear of the loudspeaker driver. By this arrangement high to mid frequency sound waves radiated within the boundaries of the enclosure and entering the inlet are substantially attenuated in the conduit and low frequency sound waves radiated within the boundaries of the enclosure are reinforced with sound waves directly radiated from the front of the loudspeaker driver.

FIELD OF THE INVENTION 
This invention relates generally to a vented loudspeaker system for the 
reproduction of musical sounds, and particularly to a two-way loudspeaker 
configuration. 
BACKGROUND OF THE INVENTION 
Procedures for the alignment of vented loudspeakers, utilizing standard 
formulae for cabinet tuning, have been thoroughly expounded by A. N. 
Thiele and Richard H. Small. However, interest in the dynamic conditions 
inside the enclosure remains high, and has led to innovative vent designs 
and the development of techniques for modifying the environment within 
loudspeaker cabinets to alter their effective sizes. 
The Venturi Vent design comes readily to mind, but perhaps a better know 
example is the Isobaric System in which one driver, located deep inside 
the enclosure, creates the acoustic environment for a second, external 
driver that radiates the sound. Needless to add, this latter example seems 
wasteful of driving units. 
Since the pressure distribution inside a loudspeaker enclosure becomes 
increasingly nonuniform above 50 Hz (Small, 1971), in the dynamic state in 
which a wide band of audio frequencies is being reproduced, there already 
exists, within the enclosure, conditions that allow for optimizing the low 
frequency performance of the system through careful design and placement 
of the vent or vents. In this regard the relationship between vent 
terminations and the pressure distribution inside loudspeaker enclosures 
remains to be fully explored. For example, vent termination away from high 
pressure areas and towards areas of relatively more rarefied air should 
have the effect of tuning a relatively larger box, and vice versa. In such 
cases, standard formulae for tuning, based on the principles of the 
Helmholtz resonator, are likely to yield results that require modification 
by a correction factor to optimize performance. 
Furthermore, although the use of absorbent materials to change conditions 
in closed-box systems has been thoroughly discussed (Moir, 1962) and 
(Small, 1971), the application of such materials to modify the behaviour 
of the air mass in a vented system, and thereby vary the tuning, has 
received far less attention. This latter concept, however, is fully 
embraced in the present invention. 
It is well known that the pressure distribution inside a loudspeaker 
enclosure is uniform below approximately 50 Hz but becomes increasingly 
non-uniform above that frequency. Intuitively, one senses that this could 
influence the techniques employed in the tuning of vented systems. Yet, 
surprisingly little has been said concerning the possibility of exploiting 
this phenomenon of pressure distribution by using vent size and placement 
to optimize performance at low frequencies. An important design 
consideration would be to avoid the restrictiveness of high pressure areas 
in the placement of vents; or to state the converse, the benefits of 
rarefication for simulating a larger enclosure should be investigated. 
To pursue this line of reasoning further, it is to be noted that although 
pressure inside an enclosure is uniform below 50 Hz, or at frequencies 
where most vented enclosures would normally be tuned, in the strictest of 
senses this state can only exist in a bandwidth sweep. In reality, many 
different frequencies are present at once in the reproduction of musical 
sounds, and forces tending towards pressure uniformity and non-uniformity 
occur simultaneously. Small observed that pressures tend to be higher than 
average near the back panel(s) and lower than average near the driver(s). 
He implied that pressure changes near the geometrical center of the 
enclosure are less extreme. 
Other background an which can be regarded as useful includes U.S. Pat. 
document No. US-A-4 837 839. This patent describes a compact speaker 
assembly with an improved low frequency response. This patent merely 
discloses a speaker as opposed to a speaker system which is the subject of 
the present invention. In this patent, a speaker transducer assembly is 
described which is comprised of a pair of speaker diaphragms superimposed 
on each other and separated by a intermediate partition or baffle. The 
spaces between the speaker diaphragms and the partition are vented to the 
outside of the speaker frame by suitable vent openings. This transducer is 
designed for use generally in motor vehicles or for use in a non-enclosure 
type mounting. 
SUMMARY OF THE INVENTION 
A primary object of the present invention is to exploit more fully the 
dynamic conditions inside the enclosure that can help to improve the 
quality of reproduced sound at the lower frequencies which, for the 
purpose of the present disclosure, are defined as frequencies between 40 
Hz and 250 Hz. 
A second object is to contain distortion-causing back waves in the 
midfrequencies that normally emanate from conventional vents. 
A third object is to develop an enclosure shape that would avoid the 
degradation of the reproduced sound by standing waves inside, and 
diffraction outside the enclosure. 
A fourth object of this invention is to employ a particular baffle 
configuration that would enhance off-axis stereo imaging. 
Yet another object is to provide a loudspeaker system of superior sonic 
quality over the entire frequency range for which it is designed, that is 
to say, from approximately 40 Hz to 20 kHz. 
The present invention seeks to provide a vented loudspeaker for reproducing 
musical sounds and in particular to providing a vented two-way loudspeaker 
system. 
In accordance with this invention, there is provided a loudspeaker system 
comprising; 
an enclosure means having a front baffle for receiving and supporting a 
driver therein; and 
vent means in the enclosure, the vent means including an inlet leading into 
the enclosure and directed to the rear of the driver, an outlet leading 
outside the boundaries of the enclosure, and a conduit connecting the 
inlet to the outlet, the vent means selectively positioned and arranged in 
the enclosure wherein high to mid frequency sound waves radiated within 
the boundaries of the enclosure and entering the inlet are substantially 
attenuated in the conduit and low frequency sound waves radiated within 
the boundaries of the enclosure are reinforced.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring to FIG. 1(a), an improved two-way loudspeaker system for the 
reproduction of musical sounds, especially in the stereo format, is 
indicated generally by numeral 1. A left hand unit of a stereo pair is 
shown having an enclosure 2 whose cross section describes an irregular 
hexagon, and has a top and bottom panel 27 and 28 respectively. The 
enclosure 2 is further defined by a front baffle panel 11 which supports a 
low frequency driver or woofer 10, and a high frequency driver or tweeter 
21. The enclosure is further defined by a pair of major panels 15 and 17. 
A pair of vents or ducts 12 and 18 are each positioned at a juncture 
between the major panels 15 and 17 and the front baffle 11. The vents 12 
and 18 lead into the enclosure 2. The rear of the enclosure is formed by a 
panel 16 extending between the major panels 15 and 17. A stand 24 for the 
enclosure 2 is shown in FIG. 1(b). 
The top and bottom panels 27 and 28 are arranged parallel to each other and 
are made of dissimilar materials. 
Turning to FIG. 2, it may be seen that the baffle panel 11 consists of a 
main front panel, with two narrow extensions 11.1 and 11.2 receding at 
45.degree. on either side of the front panel. These extension panels 11.1 
and 11.2 are fastened to the main panel with a high quality adhesive, such 
as Lepage's "Sure Grip".TM. adhesive, to form a single, integrated unit. 
The entire baffle panel 11, whose edges are all rounded, projects about 7 
millimeters from the edges of the enclosure 2. Referring back to FIG. 1, 
it can be seen that the tweeter 21 and the woofer 10 are aligned in an 
inclined configuration on the main baffle panel 11. In a stereo pair, the 
inclination will be in an opposite direction for a fight hand unit (not 
shown). 
A unique feature of the invention is the pair of vents or ducts 12, 18 
whose outside outlet ends terminate near the juncture of the vertical side 
edges of the baffle extensions, 11.1 and 11.2, and the major panels 15 and 
17. The greater proportion of each vent is integrated with the respective 
major panel 15 or 17 of enclosure 11, thereby stiffening the panels, while 
the smaller proportion is angled inward at 90.degree. to form a 
free-standing section 12.1, 18.1, each aimed approximately towards a 
vertical central axis 26 of the enclosure. 
The vents are further defined by vent panels 13, 14 which are parallel to 
the major panels 15, 17, to which the entire vent assemblies are 
permanently attached to form a rigid vent-panel structure, linked by the 
rear panel 16. 
The woofer 10 and vent inlet openings 12.1, 18.1 form a triangular 
arrangement about the major central portion of the air mass 19 inside the 
enclosure. The vents therefore "fire" away from the interior of the major 
panels 15 and 17 and approximately towards the vertical axis 26 of the 
enclosure. Absorbent material 20 is installed in a columnar arrangement 
between the top and bottom panels 27 and 28 and in a region of the 
vertical axis 26. Smaller pieces of absorbent material 25 are disposed 
horizontally approximately midway along the vertical dimension of the 
enclosure, without obstructing the openings of the vents 12.1, 18.1. The 
structure of the absorbent material 20 may be more clearly seen by 
referring to FIG. 4(b). 
Referring to FIG. 4(a) and particularly FIG. 5, it can be seen that back 
waves entering the vent inlet openings 12.1 and 18.1 will be deflected 
several times in the narrow, angled conduit, thereby sustaining 
considerable loss of energy and failing to escape the boundaries of the 
enclosure. Moreover, back waves that would normally escape through the 
woofer cone must travel twice through the centrally located absorbent 
material 20, again losing much energy in the process. (See arrow paths in 
FIG. 5). 
The state of the air mass 19 in the enclosure is further altered by the 
thermal effects of the major portion of absorbent material 20 which is 
centrally located within the enclosure. Thus the interrelationship among 
driver, vent, absorbent material and air mass, as described, determines to 
a large extent the controlled behaviour of the air mass, and the improved 
low frequency performance of the system. The placement of the drivers on 
the baffle may be explained with reference to FIG. 1. 
The woofer 10 is located near the bottom edge 11.4 of the panel, with its 
axis approximately 8 millimeters from a vertical center line of the baffle 
11, while the tweeter 21 is positioned near the top edge 11.6 of the 
panel, with its axis about 80 millimeters from the vertical center line on 
the opposite side of the center line, as the woofer 10. 
The woofer 10 is flush mounted on the slightly protruding baffle 11. This 
baffle protrusion, as well as the rounding of all baffle edges, 
effectively eliminates the problem of diffraction. The angular 
relationship between tweeter 21 and woofer 10 contributes to the final 
shape of the frequency response curve shown in FIG. 7. In a stereo 
arrangement, this helps to improve off-axis stereo imaging by introducing 
a small amount of attenuation in the tweeter nearer the listener, and 
conversely giving a slight advantage to the far tweeter. In addition, this 
baffle geometry also provides partial compensation for horizontal driver 
displacement in the on-axis listening situation. This occurs when each 
cabinet in a stereo pair is angled inward between 20 and 30 degrees, 
relative to the central listening position, and the acoustic center of 
each tweeter (which is normally forward of that of the woofer) is shifted 
backwards, and hence further away from the listener. 
Referring back to FIG. 3, an illustration of the method employed in 
establishing the proportions of the long section 12 and the short section 
12.1 (18, 18.1 ) is shown. Once the required length of the vent was 
established, the ratio between the long and short sections was varied 
until the best low frequency performance was obtained (see broken lines in 
the diagram). In the preferred embodiment, the ratio between short and 
long vent sections is approximately 1:6. 
Referring back to FIG. 4(a), the horizontal center line of each vent is 
slightly above the central horizontal plane 23 of the enclosure 2, while 
the central axis of the woofer is substantially below the horizontal plane 
23. The location of the covered-back tweeter 21 is indicated, and corner 
blocks 22 provide added structural rigidity to the enclosure. 
The vents 12, 18, which are integrated with two of the larger cabinet 
panels 15, 17, serve three important functions. The first is to tune the 
enclosure for optimum low frequency response. The second function is to 
stiffen the panels to reduce panel resonance. The third, resulting from 
their narrow, angled design is to render the escape through them of 
antiphase back waves virtually impossible. 
The procedures employed in tuning the enclosure are as follows: 
Given the required enclosure volume (v.sub.B), resonant frequency 
(f.sub.B), and the desired cross-sectional area of the vent (S.sub.v), 
vent length is established by applying the formula 
EQU L.sub.v /S.sub.v =1.84.times.10.sup.8 /.omega..sub.b.sup.2 V.sub.B (1) 
where L is the effective length of the vent in inches, S,, is given in 
square inches, and V.sup.B is in cubic inches. The variable .omega..sub.b 
=2.pi.f.sub.B. 
To calculate the necessary end correction for a vent with both ends 
flanged, the formula applied is 
EQU (L.sub.v /S.sub.v).sub.end =0.958/.sqroot.S.sub.v (2) 
or, for a vent flanged at one end only, 
EQU (L.sub.v /S.sub.v).sub.end =0.823/.sqroot.S.sub.v (3) 
While the vent length is calculated by applying the standard formulae 
above, the unique design of the vents, as well as the relationship among 
vent, driver, acoustic damping and air mass, makes it necessary to 
multiply the result by an empirically determined factor for more precise 
tuning. For example, significant improvement in low frequency performance 
was achieved when the result from applying equations (1) and (2) was 
corrected by a factor of 0.930. In the embodiment shown, the combined 
cross-sectional area of the twin vents is 8.5 square inches (54.84 
cm.sup.2), with the shortest dimension of one vent, that is, the distance 
from enclosure panel to vent panel being 9/16 inch (1.43 cm). 
In a preferred embodiment utilizing a 1.06 ft.sup.3 (30 L) enclosure, the 
equivalent diameter of the twin vents is 3.29 inches (8.35 cm). And as 
already mentioned, their internal terminations, together with the low 
frequency driver, are in a triangular arrangement (to which the hexagonal 
cabinet readily adapts itself). Vents 12, 18 and woofer 10 "fire" towards 
a region about the vertical axis 26 in the vicinity of the geometrical 
center of the enclosure. 
It should be noted that the vents cannot be described as being entirely 
free-standing, since the greater proportion of each rum parallel to, and 
is integrated with, one of the wide back panels. These vents may best be 
regarded as combining features of both the double flanged and 
free-standing types. 
The necessity for shortening the calculated length of the vents is 
consistent with the requirements for tuning an effectively larger 
enclosure to the same frequency. However, the increased resistance 
introduced by the vent angle and cross-sectional proportions may also be 
contributing factors. To what extent this may be the case could not be 
determined by the techniques I employed. What was established is that in 
the preferred embodiment, increasing the density of the column of 
absorbent material installed about the center of the enclosure lowered 
f.sub.B by as much as 6%, or to state it differently, required a reduction 
in vent length to hold f.sub.B constant. Fiberglass was the absorbent 
material chosen, and the quantity applied was in the order of 80 to 100 
grams. 
Tweeter position was established empirically by mounting the tweeter 
eccentrically on a circular, adjustable sub-baffle on a prototype 
enclosure. Rotation of the sub-baffle permitted various anechoic frequency 
response measurements to be taken with the tweeter in different positions, 
relative to the woofer. The most desirable response was obtained in this 
way. 
The system derives further advantages from the irregular shape of the 
enclosure 2, which renders the propagation of standing waves between any 
two vertical panels virtually impossible. Such waves will also lose energy 
when passing through the absorbent material 20. 
To deal with the special case of standing waves between the top panel 7 and 
bottom panel 28, which are parallel to each other, small additional pieces 
of absorbent material 25 are disposed horizontally, approximately midway 
around the central absorbent column 20 as shown in FIG. 4(b). A further 
refinement is that density and thickness differences in the top and bottom 
panels distribute their natural vibration periods and reduce the chance of 
their being excited at the same frequency. 
The top panel 27 and large back panels 15, 17 are made of 17.5 millimeter 
veneered particle board, while the bottom panel 28, baffle 11 and small 
back panel 16 are of 19 millimeter high density particle board. The vents 
are made of 9.5 millimeter plywood and solid wood. Bracing is applied to 
all 17.5 millimeter material, and inside surfaces of the enclosure are 
treated with bituminous damping material. 
The invention makes possible the use of vents of relatively large cross 
sectional area in small enclosures. This is often difficult to realize in 
conventional designs, since vents large enough to avoid turbulence and the 
generation of spurious sounds tend to be long and, in the case of those 
originating from the front baffle, "fire" internally towards the very 
regions where pressures are highest. In the present design, vent 
orientation away from regions of highest pressure overcomes this problem. 
In fact, internal box conditions are effectively exploited to enhance 
performance at low frequencies. 
The system as a whole provides several other advantages. One is that the 
non-rectangular shape of the enclosure is inherently anti-resonant, to the 
extent that standing waves cannot develop between opposite side walls 
whose varied sizes in addition, distribute their natural vibration 
periods. A second advantage is that antiphase back waves which emanate 
from conventional vents located on the front baffle will have difficulty 
escaping the narrow rectangular vents of the present design, since they 
would have to negotiate the angle of the vents and would in any case lose 
energy in bouncing between vent panels. In addition, the triangular 
woofer-vent configuration about the air mass inside the enclosure largely 
accounts for the controlled behaviour of the air mass. In a narrow sense, 
this is analogous to the stable behaviour of an inflated balloon of 
reasonable size held between the fingers of both hands and squeezed at 
intervals, compared to the behaviour of the same balloon held with one 
hand and squeezed in a similar way. 
Other refinements of the system preserve the advantages gained from the 
internal features already described. For example, the baffle's projection 
and rounded edges eliminate virtually all traces of diffraction. And as 
mentioned above, the angled position of the tweeter, relative to the 
woofer, enhances the stereo effect, permitting full stereo enjoyment even 
when the listener is sitting off axis and quite close to one of the 
enclosures in a stereo pair. 
It is noted that the tweeter is of a closed-back type that will not 
normally be affected by the pressure changes or reflected waves inside the 
enclosure. 
It is understood that the drivers are to be properly connected to a 
suitably designed crossover network that serves the crossover function, 
adjusts the system impedance dynamically, and establishes desirable phase 
relationships over the system frequency range. The crossover network in 
turn links the loudspeaker system to the amplifier output. FIG. 6 is a 
diagram of a Butterworth crossover network used in the subject design. A 
filter network of this type is well known in the art. 
While the unique vent-driver configuration is a principal feature of this 
invention, the cumulative benefits of other features of the system need to 
be appreciated as well. The overall sonic advantages are improved low 
frequency performance, remarkable spatial imaging, high sensitivity (89 
decibels anechoic), and exceptional clarity over the system's frequency 
range. This remains true even at high sound pressure levels, relative to 
system size. The on axis frequency response (anechoic) for an input of 1 
watt (2.83 V RMS) @1 meter is shown in FIG. 7. 
But although the angles and other dimensions stated in this disclosure 
concern the preferred embodiment, it is conceivable that persons 
knowledgeable in the field can, within the framework of the overall 
concept, modify certain dimensions and relationships to some extent, 
without significantly degrading the reproduced sound. What is particularly 
significant, though, is the comprehensive integration of important 
features that contribute to the superior sonic quality of the system. Box 
shape and the deployment of materials, vent design and location, placement 
of absorbent material, driver-vent-air mass relationship and baffle 
configuration are all advantageously integrated in this system. 
While the invention has been described in connection with a specific 
embodiment thereof and in a specific use, various modifications thereof 
will occur to those skilled in the art without departing from the spirit 
and scope of the invention as set forth in the appended claims. 
The terms and expressions which have been employed in the specification are 
used as terms of description and not of limitations, and there is no 
intention in the use of such terms and expressions to exclude any 
equivalents of the features shown and described or portions thereof, but 
it is recognized that various modifications are possible within the scope 
of the claims to the invention.