WO1998054924A2

WO1998054924A2 - Ultra structure subwoofer

Info

Publication number: WO1998054924A2
Application number: PCT/US1998/011086
Authority: WO
Inventors: Clifford L. Blodget
Original assignee: Ultra Research, Inc.
Priority date: 1997-05-31
Filing date: 1998-05-29
Publication date: 1998-12-03
Also published as: WO1998054924A3; AU7710798A

Abstract

The invention provides a loud speaker (1) or a magnet structure (4) for a loudspeaker having an annular magnetic gap for receiving a voice coil (8) and means for centering the voice coil within the annular gap while allowing for pistonic movement, having a voice coil with a length of at least 75 % of the thickness of the top plate (2) and a top plate (2) of at least 50 % greater thickness than it would have if it were operated at the point of magnetic saturation and a pole piece extending beyond the top plate (2) by an amount sufficient to remain in proximity to the voice coil (8) at all points in the reciprocation of the voice coil (8) and preventing the voice coil (8) from becoming misaligned with the gap even at high power levels and large excursions, having a plurality of ceramic ring magnets (4, 5) in a stacked configuration and wherein the increased thickness of the top plate (2) increases the distance from the center of the voice coil (8) in the resting position to the back plate (6). The design permits greater power handling capacity and larger excursions in bass sound reproduction.

Description

ULTRA STRUCTURE SUB WOOFER

FIELD OF THE INVENTION

The present invention relates to a new loudspeaker design for producing low frequency audio sound at high sound pressure levels. Most conventional loudspeaker systems used a cone type diaphragm driven by a movable voice coil suspended between the pole piece and top plate of a permanent magnet assembly. The interaction between the current passing through the voice coil and the magnetic field produced by the permanent magnet causes the voice coil to reciprocate in a linear path in accordance with the applied electrical signal, and drive the diaphragm to produce sound. The diaphragm and voice coil are centered by a spider and surround that allow for this linear pistonic travel given the forward and backward direction on the central axis to the loudspeaker. This type of speaker is commonly known as the permanent magnet dynamic type loudspeaker. BACKGROUND OF THE INVENTION It is widely believed that it is necessary to saturate the top or bottom plates to precisely locate the lines of magnetic flux during operation of a loudspeaker and to maintain consistency from unit to unit through the manufacturing process. Any movement or change of intensity of the flux during operation is considered bad by the standards of commonly accepted wisdom in the industry. There is even a name for this flux movement, it is referred to as "flux modulation". The thickness of the top and bottom plates of the magnetic is precisely calculated in prior art to achieve some level of saturation or come close to saturation so that the flux levels in the magnetic circuit will remain stable during operation. The theory is that if the circuit is already saturated then the flux level cannot increase by an appreciable amount. On the other hand the electromagnetic interaction of the voice coil will not be strong enough to cause the flux level to decrease by any meaningful amount.

Low frequency loudspeakers or sub woofers operate primarily in the frequency range of 10 Hz to 125 Hz. In conventional sub woofers or low frequency loudspeakers the diaphragm is small relative to the wave length being reproduced. This causes the output level to decrease substantially as the frequency gets lower. To compensate for this decrease in the low frequency output level, the loudspeaker is designed to operate in a broad band resonant mode at low frequencies. This is accomplished by increasing the moving mass and decreasing the magnetic strength or electromechanical coupling to the acoustic load.

In prior art devices the low frequency output is limited by the inescapable compromise between raising the magnetic field strength to get more drive power and losing output at low frequencies due to over damping and the accompanying effect of decreasing the broad band resonance that accounts for a large part of the output at these low frequencies. SUMMARY OF THE INVENTION In a low frequency loudspeaker, optionally of essentially conventional design, comprising: optionally a basket, optionally a diaphragm movably suspended from the basket capable of pistonic movement, a voice coil, optionally connected to the diaphragm responsive to an applied electric current, a magnetic structure optionally connected to the basket and having an annular magnetic gap for receiving the voice coil and means for centering the voice coil within the annular gap while allowing for pistonic movement the invention provides an improvement comprising a magnetic structure designed to maintain levels of magnetic flux density substantially below the magnetic saturation level of the flux carrying components in the proximity of the magnetic gap to allow the lines of magnetic flux to modulate in position during normal operation and utilizing a voice coil with a length of at least 75% of the thickness of the top plate. In a preferred embodiment the voice coil has a length equal to 85% of the thickness of the top plate. More preferably of equal length.

In a preferred embodiment, the means for allowing the lines of magnetic flux to modulate comprises a magnet structure having a top plate about 50% thicker than it would be if it were at the point of magnetic saturation, preferably 200% thicker. In a preferred embodiment the loudspeaker as a pole piece extending beyond the top plate an amount sufficient to remain in proximity to the voice coil at all points in the reciprocation of the voice coil and preventing the voice coil from becoming misaligned with the gap even at high power levels and large excursions. In a preferred embodiment a plurality of ceramic ring magnets is used in a stacked configuration to allow clearance for voice coil to reciprocate over longer distances. In a preferred embodiment and extra thick top plate increases the distance from the center of the voice coil in the resting position to the back plate, thereby increasing the excursion capability of the loudspeaker. In a preferred embodiment the top plate and the bottom plate are substantially the same outside diameter as the ceramic ring magnets and the top plate has an L shaped profile. In another embodiment the invention provides a magnet structure for a loudspeaker having an annular magnetic gap for receiving a voice coil and means for centering the voice coil within the annular gap while allowing for pistonic movement, a top plate of at least 50 % greater thickness than it would have if it were operated at the point of magnetic saturation and a pole piece extending beyond the top plate by an amount sufficient to remain in proximity to a voice coil at all points in the reciprocation of the voice coil and preventing a voice coil from becoming misaligned with the gap even at high power levels and large excursions, having a plurality of ceramic ring magnets in a stacked configuration and wherein the increased thickness of the top plate increases the^~ distance from the center of a voice coil in the resting position to the back plate.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig 1 is cross-sectional view of the magnet structure of the invention. Fig 2 is cross-sectional view of the magnet structure of the prior art.

Fig 3 is cross-sectional view of the magnet structure of a preferred embodiment and best mode of the invention.

DETAILED DESCRIPTION OF THE INVENTION

By providing a low saturation magnetic circuit that allows for the movement of the magnetic field in a low frequency loudspeaker both efficiency and extended response can be achieved by increasing drive strength while allowing for broad band resonance.

The result is more accurate and defined lower range response.

Turning to Fig. 1, a cross-sectional view of a speaker according to the invention is shown. An extended pole piece 1 is positioned within top plate 2, preferably having an extended diameter portion 3 to give the plate an L shaped profile. Extended diameter

3 is the same outer diameter as magnets 4 and 5 which are preferably at least two ceramic ring magnets. The magnetic circuit is completed by bottom plate 6. The voice coil former 7 is positioned around pole piece 1 and carries voice coil 8. In contrast to prior art designs the illustrated loudspeaker has a much thicker than normal top plate which is at least 50% thicker than the thickness necessary to achieve saturation, preferably 75% more preferably 100% still more preferably 200%.

In Fig. 2 a typical prior art design is shown in cross-section. In contrast to the invention, the pole piece is only slightly above the top plate 2 which is a smaller diameter than single magnet 4. Bottom plate 6 and top plate 2 are substantially the same thickness in contrast to the much thicker top plate of Fig. 1. Former 7 caπying coil 8 travels axially with regard to pole piece 1 an a saturated magnetic field provided by the concentrated flux in top plate 2. Fig. 3 Illustrates a preferred embodiment featuring rounded corners and a severe — L shaped profile.

The result of the design shown in Figs. 1 and 3 is that less force pushes the coil out of the gap at the very end of travel and more force is available at the beginning of the travel to pull the coil back into the gap due to the traveling flux field in the gap of the ultra structure of top plate 2. The thick back plate 6 conducts heat away better form the center pole because of increased metal cross sectional area. The thick top plate conducts heat away better through direct proximity to metal of the voice coil. The low flux density design looses less leakage flux. Stacked magnets 4 and 5 put more distance between the top and bottom plates 2 and 6 so that the plates can extend out to the edge of the magnets with minimal leakage for greater efficiency of the magnetic circuit and utilization of a greater percentage of the magnetic material. The stacked magnets give greater clearance for long excursions, and the stacked magnets or a single magnet combined with the thick top plate gives even greater clearance by moving the center of the gap higher (further away for the bottom plate 6). The voice coil 8 is substantially the same length as the top plate for greater efficiency, against commonly accepted practice and wisdom. In conjunction with the sliding flux field, greater heat transfer and cooling, the invention provides a significant improvement over the prior art. In addition the L shaped profile saves metal and weight. Using the same outer diameter for the top plate and magnets increases efficiency.

Reduced back emf makes amps run cooler and coil run cooler. The amplifier damping factor is not as critical because the damping is done by eddy currents in the pole and top plate not the amp and back emf. This backfeeds less power into the amp letting the amp and the coil run cooler. A loudspeaker design that can produce bass and sub bass low frequency audio and have greater power handling capabilities than the currently available technology would be highly desirable. An embodiment of the present invention produces bass and sub bass low frequency audio in the range of about 10 to 300 Hz. The current invention may have- available power handling of greater than 5,000 watts.

A commercially available state of the art 18 inch (46 cm) low frequency transducer has a diaphragm surface area of approximately 200 square inches (1290 sq. cm) and amaximum excursion of approximately one inch (2.54 cm) yielding a maximum acoustic output of 1.3 watts at 25 Hz or 5.2 watts at 50 Hz. One embodiment of the present invention has a diaphragm surface area of approximately 200 square inches (1290 sq. cm) and an excursion of 6 inches (15.24 cm) yielding an acoustic output of 7.9 watts at 25 Hz and 31.6 watts at 50 Hz. The present invention provides acoustic output unattainable by conventional systems currently available. Preferred embodiments have the capacity to handle in excess of 5,000 watts output from the amplifier and to provide excursions of at least 3 inches preferably 6 inches.

Many variations of this design will be apparent to those skilled in the art of loudspeaker design, the ratios of the dimensions given above may be varied in a broad range, however the size specified is the currently preferred mode of the inventor and represents the best mode known to the inventor for practicing the invention. The embodiments described above are submitted for illustration of the invention and not as limitation. The invention is defined and limited by the claims set out below.

Claims

CLAIMS:I claim:

1. In a low frequency loudspeaker of essentially conventional design comprising a basket, a diaphragm movably suspended from the basket capable of pistonic movement, a voice coil connected to the diaphragm responsive to an applied electrical current, a magnetic structure connected to the basket having an annular magnetic gap for receiving the voice coil and means for centering the voice coil within the annular gap while allowing for pistonic movement, the improvement comprising a magnetic structure means to maintain levels of magnetic flux density substantially below the magnetic saturation level of the flux ca╧Çying components in the proximity of the magnetic gap and means to allow the lines of magnetic flux to modulate in position during normal operation and utilizing a voice coil with a length of at least 75% of the thickness of the top plate.

2. .A loudspeaker as claimed in claim 1, having a voice coil with a length of at least 85% of the thickness of the top plate.

3. A loudspeaker as claimed in claim 1 having a voice coil with a length at least equal to the thickness of the top plate.

4. A loudspeaker as claimed in claim 1, wherein the means for allowing the lines of magnetic flux to modulate comprises a magnetic circuit having a top plate at least 50% thicker that it would be if it were at the point of magnetic saturation. .

5. A loudspeaker as claimed in claim 4, further having a pole piece extending beyond the top plate an amount sufficient to remain in proximity to the voice coil at all points in the reciprocation of the voice coil and preventing the voice coil form becoming- misaligned with the gap even at high power levels and large excursions.

6. A loudspeaker as claimed in claim 4 wherein a plurality of ceramic ring magnets are used in a stacked configuration to allow more clearance for the voice coil to reciprocate over longer distances.

7. A loud speaker as claimed in claim 4 wherein the thickness of the top plate increases the distance from the center of the voice coil in the resting position to the back plate.

8. A loudspeaker as claimed in claim 6 wherein a portion of the top plate is substantially the same outside diameter as the ceramic ring magnets.

9. A loudspeaker as claimed in claim 4, wherein the top plate has an L shaped profile.

10. A low frequency loudspeaker comprising a voice coil connected to a diaphragm responsive to an applied electrical cu╧Çent, a magnetic structure having an annular magnetic gap for receiving the voice coil and means for centering the voice coil within the annular gap while allowing for pistonic movement, having a voice coil with a length of at least 75% of the thickness of the top plate and a top plate at least 50 % thicker than it would be if it were at the point of magnetic saturation and a pole piece extending beyond the top plate by an amount sufficient to remain in proximity to the voice coil at all points in the reciprocation of the voice coil and preventing the voice coil from becoming misaligned within the gap.

11. A loudspeaker as claimed in claim 10 wherein a plurality of ceramic ring magnets^" are used in stacked configuration to allow more clearance for the voice coil to reciprocate over longer distances.

12. A loud speaker as claimed in claim 10 wherein the thickness of the top plate increases the distance from the center of the voice coil in the resting position to the back plate

13. A loudspeaker as claimed in claim 11 wherein a portion of the top plate is substantially the same outside diameter as the ceramic ring magnets

14. A loudspeaker as claimed in claim 10, wherein the top plate has an L shaped profile

15. A low frequency loudspeaker comprising a voice coil responsive to an applied electrical cu╧Çent, a magnetic structure having an annular magnetic gap for receiving the voice coil and means for centering the voice coil within the annular gap while allowing for pistonic movement, having a voice coil with a length of at least 75% of the thickness of the top plate and a top plate of at least 50 % greater thickness than it would have if it were operated at the point of magnetic saturation and a pole piece extending beyond the top plate by an amount sufficient to remain in proximity to the voice coil at all points in the reciprocation of the voice coil and preventing the voice coil from becoming misaligned with the gap even at high power levels and large excursions, having a plurality of ceramic ring magnets in a stacked configuration and wherein the increased thickness of the top plate increases the distance from the center of the voice coil in the resting position to the back plate.

16. A loudspeaker as claimed in claim 15 wherein a portion of the top plate is^" substantially the same outside diameter as the ceramic ring magnets

17. A loudspeaker as claimed in claim 15, wherein the top plate has an L shaped profile

18. A loudspeaker according to claim 15 having a voice coil reciprocation of at least 3 inches (7.62 cm).

19. A loudspeaker according to claim 15 having a power handling capacity of at least 5,000 watts.