GB2122841A

GB2122841A - Loudspeakers

Info

Publication number: GB2122841A
Application number: GB08217646A
Authority: GB
Inventors: Shin-Ming Hwang
Original assignee: SUPERIOR ELECTRONICS CORP
Current assignee: SUPERIOR ELECTRONICS CORP
Priority date: 1982-06-18
Filing date: 1982-06-18
Publication date: 1984-01-18

Abstract

A loudspeaker for heavy-duty use has an external trumpet portion 1 having a sealed rear section 12 for accommodating the loudspeaker parts and a front section 11 and is made of a metallic alloy having a good conductivity. The heat produced by the coil 61 is dissipated via the permanent magnet 2 to the external trumpet portion 1 which effectively conducts the heat away to the surroundings. <IMAGE>

Description

SPECIFICATION Loudspeakers This invention relates to loudspeakers particularly in the form of trumpet horns for heavy-duty use, for example as alarms.

Conventional loudspeakers work on the principle that an alternating electrical current is passed through a coil in the field of a permanent magnet. Resultant variation of the magnetic field strength causes a diaphragm to vibrate and transmit an audible signal by vibration of the air.

Trumpet loudspeakers are designed on the same principle for heavy-duty use. Conventional trumpet loudspeakers have a plastics enclosure inside for surrounding the magnet and the interior is sealed with soft rubber, each for providing moisture resistance. This has several drawbacks, however, as follows: 1. Short circuiting A high output power of the speaker will result in excessive heat which, because of the difficulty in dissipation via the plastics and rubber, in turn causes the magnetic components of the speaker to expand. After a while, further expansion of the parts involved will cause the voice coil of the speaker to come in contact with the magnetic components as it vibrates. This will cause the surface insulation of the intricate winding of the voice coil to break, thereby resulting in short circuits.

2. Open circuiting As stated above circumstances in which there is a high output power, a long period of operation will lead to short circuiting of the voice coil. If the output power remains high after the short circuit occurs, the voice coil will become damaged due to overloading, thus resulting in open circuits.

3. Operations disorders As the temperature of the voice coil rises due to a high output power, the magnetic elements will be disturbed and their operation disrupted.

This will weaken the magnetic flux, which in turn results in a poor quality of sound reproduction quality, low level response, noise, and lowering of the output power. Further, the gap between the voice coil and the magnet tends to reduce, causing friction of the voice coil and resulting in noise and deformation of the voice coil.

4. Distortion The above problems will produce serious distortion of the output audio signal.

A main object of this invention is to overcome the above drawbacks.

According to this invention we propose a loudspeaker for heavy-duty use, comprising an external trumpet portion made of a material having a good thermal conductivity, housing behind a sealing partition a permanent magnet, a voice coil and a diaphragm wherein the coil and the magnet are in heat dissipative communication with the external trumpet portion such that the loudspeaker can be used for long periods at high output power.

The invention involves an external trumpet made of a metallic alloy with good thermal conductivity through a special one-piece moulding process. The structure allows the bottom of the external trumpet to be in direct contact with a lower magnetic strip inside so that any heat produced by the voice coil may be quickly removed through the magnet, the magnetic strips, and the external trumpet and dissipated into air. This effectively prevents overheating of the voice coil and the permanent magnet, which would otherwise cause such problems as short or open circuits of the voice coil, magnetic breakdown of the magnetic flux of the round permanent magnet, weakening of the magnetic force which results in a low level response, deformation of the voice coil, distortion of sound, and so forth.

An embodiment of the invention is described by way of example, with reference to the drawings, in which: Figure 1 risen outer view of the present invention; and Figure 2 shows the interior structure of the present invention.

The external trumpet portion of the speaker 1 is a one-piece moulding, having a front section 11 and a rear section 12. The rear section 12 has a U-shape in cross-section with a hole for a screw at the centre to secure the components of the horn together. The front section 11 curves outwardly and has a flared mouth in the form of a trumpet horn. The front section 11 has a larger diameter than the rear section 1 2 by an external overhanging surface 1 4 or "step". The external trumpet portion is made of a metallic alloy having good thermal conductivity.

An annular permanent magnet 2 is employed.

Located beside its two ends are an upper magnetic strip 3 and a lower magnetic strip 4.

The upper magnetic strip 3 is flat and annular, while the lower magnetic strip 4 has a flat annular, outer portion with a central projecting cylindrical portion 41 located within the space defined within the magnet and the upper magnetic strip. A fixed gap 5 is thus defined between the cylindrical portion 41 and the annular permanent magnet 2 with the upper magnetic strip 3.

The gap 5 is for accommodating a voice coil 61 and to ensure that the gap 5 is fixed, the permanent magnet 2, the upper magnetic strip 3 and the lower magnetic strip 4 are adhered together. The gap 5 is where there is the greatest amount of magnetic flux and hence, where the magnetic field intensity is the highest.

A mounting frame 6 is integral with the voice coil and a diaphragm 62. Above the mounting frame 6 there is a plastics bracket 7 which has a step-wise recess complementary to the external step 14 so that the smaller lower end 71 fits into the rear section 12 of the external trumpet portion 1, while the larger upper end 72 is seated on the internal face of the step 14 of the external trumpet 1.

To prevent the structure from being loosened, a piece of soft rubber 8 is placed between the contacting surfaces of the bracket 7 and the external trumpet portion 1. A mounting hole 73 is provided on the bracket 7 for screws to secure the bracket 7 and the mounting frame 6 to the upper magnetic strip 3. Partitions 74 defining hollow, funnel-shaped tubes are provided over the middle of the bracket 7 and these act as a transmitting path for the sound produced by vibration of the diaphragm. A speaker jacket 9 is fixed to the bracket 7 by a screw through a hole 75 and this covers the partitions 74.

As an alternating electrical audio signal passes through the voice coil 61, an alternating magnetic field is produced by the voice coil 61 and the permanent magnet 2 which causes the diaphragm 62 to vibrate and produce an audible frequency signal. This signal follows the funnelshape tubes and causes resonance of the speaker jacket 9. It is then transmitted to the surroundings.

The formation of an alternating magnetic field causes a side-effect in that the voice coil produces excessive heat which then is conducted to the nearby permanent magnet and the upper and lower magnetic strips. Conventionally the upper and lower magnetic strips and the permanent magnet are surrounded by plastics material which makes it difficult for the heat to dissipate. Under a long period of operation involving high output power, the permanent magnet, the upper and lower magnetic strips and the voice coil will all expand as heat accumulates, thereby reducing the gap 5. Therefore, as the voice coil 61 vibrates it tends to touch the upper magnetic strip 3, the round permanent magnet 2 and the cylindrical portion 41 of the lower magnetic strip 4, resulting in such problems as noise, distortion, and short and open circuits of the voice coil, as earlier described. Furthermore, excessive heat will damage the magnetic force of the permanent magnet 2, resulting in a low level response.

With the present invention the external trumpet portion 1 confronts and contacts the lower magnetic strip 4 via thereby enabling heat to be dissipated from the coil 61 via the upper magnetic strip and the permanent magnet to space through the external trumpet and maintaining a constant heat level within the structure. It therefore allows the trumpet loudspeaker to be used for long periods of reliable operation with a high output powder.

The present invention may also be realized with the front section 11 and the rear section 12 of the external trumpet 1 being separate parts while the rear section 1 2 is integral with the back cover 1 0.

Claims

1. A loudspeaker for heavy-duty use, comprising an external trumpet portion made of a material having a good thermal conductivity, housing behind a sealing partition a permanent magnet, a voice coil and a diaphragm wherein the coil and the magnet are in heat dissipative communication with the external trumpet portion such that the loudspeaker can be used for long periods at high output power.

2. A loudspeaker according to claim 1, wherein the external trumpet portion has two separate portions.

3. A loudspeaker according to claim 1 or 2, having an upper magnetic strip and a lower magnetic strip with the permanent magnet disposed therebetween, in which the lower magnetic strip has a cylindrical projecting portion which is housed within and spaced apart from the permanent magnet, the space therebetween accommodating the voice coil.

4. A loudspeaker accprding to claim 1, 2 or 3 wherein the external trumpet portion is made of a metallic alloy.

5. A loudspeaker constructed and arranged substantially as herein described with reference to any of the drawings.