Electrical audio signal processing systems and devices – Electro-acoustic audio transducer – Having acoustic wave modifying structure
Reexamination Certificate
2000-03-16
2004-08-24
Ni, Suhan (Department: 2743)
Electrical audio signal processing systems and devices
Electro-acoustic audio transducer
Having acoustic wave modifying structure
C381S345000, C381S186000, C381S349000, C381S353000, C181S145000, C181S163000
Reexamination Certificate
active
06782112
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates to acoustical transducer enclosure methodologies and ways to improve their efficiency.
2. Description of Prior Art
Transducer enclosure design for audio loudspeaker reproduction is a highly evolved science. The art of these designs goes back nearly one hundred years and yet there have been numerous recent advances in this art. The basic design methodologies are well described in the classical works of Novak “Performance of Enclosures for Low Resonance High Compliance Loudspeakers”, Thiele “Loudspeakers in Vented Boxes” Parts I and II, Small “Vented-Box Loudspeaker Systems” Parts I, II, III, and IV, and Geddes “An Introduction to Bandpass Loudspeaker Enclosures”. All of these articles can be found in the Loudspeaker Anthology series available from the Audio Engineering Society, New York, N.Y. A combined reference to these works would encompasses most of the current state-of-the-art in commercial loudspeaker enclosure design.
The recent trend towards the bandpass type of enclosure stems from the desire to produce more output with less energy i.e.—improved efficiency. By tuning single or multiple resonant acoustical systems the loading presented to the loudspeaker can be increased with a corresponding increase in efficiency. A rule of thumb for these designs is that the narrower the bandwidth of the bandpass system the higher the acoustic gain and the greater the efficiency improvement. This limitation of high efficiency—low bandwidth or low efficiency—high bandwidth is sometimes stated as—the efficiency bandwidth product for a bandpass loudspeaker system must remain constant. Mathematically this is not exactly true, but practically speaking it is.
It would be highly desirable to be able to increase the acoustic load presented to the loudspeaker without a corresponding decrease in the bandwidth of the resulting system.
Other novel attempts at increasing the radiating efficiency have been attempted by such inventors as Dusanek in his 1981 patent “Woofer Loudspeaker” U.S. Pat. No. 4,301,332. In this patent the rear of a loudspeaker is attached to “an inner and an outer passive speaker cone” such that the outer radiating cone is larger than the inner cone. An example from this patent is shown in
FIG. 1. A
mechanical amplification of the loudspeaker cone motion is thus created. The inventor claims that this results in “a combination of lower frequency response and higher efficiency from a . . . small enclosure . . . ”. The frequency response of the system may be lower, but the efficiency in the passband cannot be increased with this design. This is because the larger cone motion of the passive radiator will increase the sound radiation only in a very narrow range of frequencies around box resonance. Below this resonance the front and rear radiation will cancel one another (as in any ported enclosure) defeating any gains in radiation efficiency that might otherwise have been produced by a mechanical amplifier. Above resonance a passive radiator becomes decoupled from the loudspeaker and the pass band efficiency of the system must remain that of the direct radiator since there is no output from the passive radiator. At resonance an increase in efficiency will be evident and this improved efficiency can be utilized only by tuning the box lower than otherwise would be the case. Dusanek failed to realize that to be truly effective all of the radiating sound must be directed through the dual cone mechanical amplifier. This design has never seen commercial success.
Other forms of mechanical advantage have been tried. Niewendijk, et. al (1985) disclosed in U.S. Pat. No. 4,547,631, the use of a mechanical arm acting as a lever between a traditional voice coil and a bellows. The idea is to create a large volume displacement of the radiating surface from a much smaller motion of the voice coil or other actuating motor. The disadvantages of this design are extreme complexity in design and manufacturing and highly questionable reliability. This design has also never seen commercial implementation.
A similar (identical?) design to that of Dusanek was disclosed by Clarke in U.S. Pat. No. 4,076,097, “Augmented Passive-Radiator Loudspeaker Systems” (1979). In this invention a dual cone unit is again coupled to the back of a freely radiating driver except that the sound energy is directed to the junction between two passive radiator cones. An example from this patent is shown as FIG.
2
. The inventor claims that this new design offers an improved response over a standard passive radiator design due to the possibility of controlling the net compliance of the passive radiator by the addition of the second box. This “improvement” is of little consequence since any effect of a passive radiator's compliance would take place below resonance where the acoustical output is negligible. In practice the compliance of a passive radiator is not very important—thus controlling it is of little interest. This design never saw commercialization.
Referring now to
FIG. 5
, transducer
70
is energized by connecting its motor to an amplifier (not shown) via wires
95
. In this manner acoustic energy from the transducer is introduced into chamber
30
. The sound pressure that results from this acoustic energy acts on the driven area of the lever
85
, which faces the transducer. Lever
80
will be displaced by this action in an amount that is substantially equal to the ratio of the transducer radiating area to the driven area of the acoustic lever multiplied by the transducers cone displacement. That is, if x
d
is the displacement of a transducer having area A
d
and x
1
is the displacement of the driven surface of an acoustic lever having area A
ld
then:
x
1
=
A
d
S
d
·
x
d
An acoustic lever moves as a unit and thus the displacement of the radiating area of the acoustic lever is also x
1
. The radiating area, A
lr
, faces the exterior fluid medium. The volume of air displaced (displacement times area) by the radiating area of the acoustic lever, A
lr
, will be:
A
lr
=
A
lr
·
x
1
=
A
lr
·
A
d
A
ld
·
x
d
=
A
lr
A
ld
·
A
d
·
x
d
=
A
lr
A
ld
·
V
d
where V
d
is the volume of air displaced by the transducer. If the radiating area is greater than the driven area then the volume velocity of air radiated by the acoustic lever will be greater than that of the transducer by the ratio of the acoustic levers radiating area to its driven area. This transformation of radiating volume velocity is very similar to the function of a transformer in electrical terms or a lever in mechanical terms. Hence the name acoustic lever.
As an example of the relationship given above, consider an acoustic lever made from two cones, the driven side is constructed with a projected area of 200 cm.
2
and the radiating side is constructed with a projected area of 400 cm.
2
. From the above equation it is shown that the radiated volume displacement will be twice that of the electro-acoustic transducers cone displacement. This means that the radiated volume velocity will also be twice that of the electro-acoustic transducer and that the Sound Pressure Level (SPL) resulting from this sound radiation will be increased by approximately 6 dB as a result of the presence of the acoustic lever.
The above derived relationship will not hold at all frequencies. At frequencies above the frequency of resonance defined by the volume of chamber
30
and the acoustic mass of lever
80
the amplification effect will disappear and the radiated sound will diminish. This roll-off can be made steeper, if desired, by making the connection between the radiating surface and driven surface of lever
80
flexible instead of rigid. The volume of chamber
30
and or the acoustic mass of lever
80
can be determined from this relationship and the desired upper frequency of operation.
Chamber
20
will act so as to decrease the compliance of the acoustic lever. The acoustic compliance of chamber
20
will add (in parallel) to the
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