DESCRIPTION FIRE ALARM TECHNICAL FIELD The present invention relates to a fire alarm that can be installed in a room in a building and issues a fire alarm when detecting smoke or heat or the like. BACKGROUND ART In a conventional fire alarm that is installed in a room, as shown in Japanese Patent Publication No. 11-16064, a detection head is fixed to a base that is fixed to the ceiling, and smoke that is generated within the room enters the detection head, and fire determination is carried out depending on the smoke density. The detection head is enclosed by an insect-proof net barrel, to prevent the ingress of insects and other foreign matters into the detection head, to maintain accurate fire determination. This type of fire alarm is constituted to transmit a fire alarm signal to an external device when it is determined that a fire has occurred. Recently there has been demand for residents to be accurately warned of the occurrence of a fire by voice, not by an ambiguous sound such as a buzzer or the like, when a fire occurs, so it is desirable that a speaker that issues the fire alarm in the form of voice be added, in particular it is desirable that the speaker be incorporated in the fire alarm itself. SUMMARY OF THE INVENTION In an aspect, the present invention provides a fire alarm comprising: a base adapted to be installed in a room of a premises; a detector head mounted on said base for detection of a fire condition; and an insect-proof net barrel covering said detection head, wherein said detection head has a sensor for sensing smoke or heat, said insect-proof net barrel surrounds said detection head, and comprises an upper end coupled to said detection head or said base, and a lower end having a speaker, said base comprises an alarm sound generation circuit for processing an output from said sensor to control said speaker to generate a fire alarm sound based on a predetermined fire determination criteria, said insect-proof net barrel comprises open upper and lower ends, and is divided by a partition wall into an upper barrel and a lower barrel, said upper barrel surrounding said detection head, said lower barrel accommodating said speaker, and said upper barrel comprising a peripheral wall with net holes for allowing air to be introduced into said detection head, 1 2540701_1 (GHMallers) P81039.AU said detection head comprises a barrel having a number of slits for allowing smoke to be introduced into the detection head, said sensor is configured to detect light from a light source disposed in said detection head so as to determine a smoke density based upon a variation of the intensity of the detected light, a lower opening of said detection head is covered by said partition wall, and said partition wall comprises a plurality of holes for allowing sound pressure to be outputted from said speaker to an interior of said detection head, said holes being aligned with said speaker in order to prevent the light from entering into the detection head through said holes. It is also preferred that the lower barrel of the insect-proof net barrel is formed in its peripheral wall with air vents. In this instance, the air vents can release the pressure developed behind the vibrator of the speaker. It is possible to use the insect-proof net barrel without the partition wall. In such instance, the speaker held at the lower end of the insect-proof net barrel is used to close the lower end of the detection head. The upper barrel of the insect-proof net barrel may be tapered to have its diameter smaller towards its lower end. In this instance, the insect-proof net barrel is preferably provided with a plurality of circumferential ribs extending circumferentially of the upper barrel, and a plurality of vertical ribs crossing with the circumferential ribs to form a lattice which defines therein net holes opened radially outwardly as well as axially downwardly of the insect-proof net barrel. With this arrangement, foreign matters such as insects entrapped to the lattice can be dropped downward by their weight or by the vibration of the speaker for preventing the entry into the detection head. The fire alarm of the present invention is preferred to include a head cover configured to surround the insect-proof net barrel and cover the speaker. The head cover is a top-opened barrel and is detachably at its top to the base. The head cover is formed in its periphery with windows through which the sensor is exposed to the room air, and is formed in its bottom with sound holes for passing therethrough the sound of the speaker. Thus, the insect-proof net barrel and the speaker can be concealed within the head cover to give an improved aesthetic appearance to the fire alarm. BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be more clearly ascertained, embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a perspective diagram of a fire alarm according to an embodiment of the present invention; 2 2540701_1 (GHMatters) P81039 AU Fig. 2 is a front section diagram of the above fire alarm; Fig. 3 is an exploded perspective diagram of the above fire alarm; Fig. 4 is a bottom diagram showing the detection head of the above fire alarm; Fig. 5 is a perspective diagram showing the insect-proof net barrel used in the above fire alarm; Fig. 6 is a cross-section diagram showing the above insect-proof net barrel; Fig. 7 is an exploded perspective diagram of the main parts of the above fire alarm; Fig. 8 is an exploded front diagram of the main parts of the above fire alarm; Fig. 9 is an exploded section diagram of the main parts of the above fire alarm; Fig. 10 is an exploded perspective diagram showing a modified form of the insect-proof net barrel used in the above fire alarm; and Fig. 11 is an enlarged perspective diagram showing a part of the above insect-proof net barrel. DETAILED DESCRIPTION OF THE INVENTION A fire alarm according to an embodiment of the present invention is explained based on Figs. 1 through 9. The fire alarm is designed to determine whether a fire has occurred based on the density of smoke that occurs within a room, and is provided with a detection head 30, that measures the smoke density, on a base 10 that is fixed to the surface of the ceiling within a room. The fire alarm includes an insect-proof net barrel 50 that covers the periphery of the detection head 30, a head cover 60 that covers the periphery of the insect-proof net barrel 50, and a base cover 20 that covers the base 10 on the outside of the head cover 60, and a speaker 80 that sounds a fire alarm voice signal is retained on the bottom end of the insect-proof net barrel 50. The base 10 includes a first base 11 that is fixed to the constructed surface of the building, and a second base 12 to which the detection head 30 is fixed, and the two are joined by screws to become integral. A circuit board 14 on which electronic parts are mounted to constitute a circuit for fire determination is fixed to the second base 12. The detection head 30 is a cylindrical body provided with a plurality of slits 32 in the circumferential wall, and the top end is fixed to the circuit board 14. As shown in Fig. 4, the circumferential wall of the detection head 30 has a plurality of curved fins 31 arranged in the circumferential direction, with the slits formed between the fins. The fins 31 are constituted. 3 2540701_1 (GHMatlers) P81039 AU -4 to guide air from outside to the inside through the slits 32, but to prevent the ingress of light. An LED 34 that is mounted on the circuit board 14 and a photodiode (PD) 35 that receives the light from the LED project into the interior space of the detection head 30, and light scattered by smoke that is guided into the detection head 30 is detected by the photodiode 35, and the density of the smoke is obtained from the strength of the light. The smoke density is detected by a detection circuit that is realized by an IC chip 15 mounted on the circuit board 14. This detection circuit is configured to output the fire alarm signal when the detection circuit detects the smoke density which exceeds a predetermined threshold value. A voice generation circuit is likewise realized by the IC chip. The voice generation circuit generates a fire alarm message on the basis of the fire alarm signal, and outputs the voice from the speaker. The LED 34, the photodiode 36, and a battery 18 that supplies electrical power to the IC chip 15 are retained on the second base 12. A circuit is also formed in the IC chip 15 that monitors the capacity of the battery 18, and when the capacity of the battery is less than a predetermined value, the circuit generates a battery alarm signal by voice, and the battery alarm signal is output from the speaker 80. As shown in Fig. 5 and Fig. 6, the insect-proof net barrel 50 is formed in a cylindrical shape from resin with both the top and bottom ends open, and is divided into an upper barrel 51 and a lower barrel 52 by a partition wall 55 formed in the interior. A projection 53 is formed in the top end of the upper barrel 51. This projection 53 is engaged with a catch 33 which is provided in the outer periphery of the detection head 30, so that the insect-proof net barrel 50 is removably attached to the detection head 30, and the upper barrel 51 covers the area around the lower end of the detection head 30. A plurality of net holes 54 that communicate with the slits 32 of the detection head 30 is formed in the circumferential wall of the upper barrel 51, so that air within the room proceeds into the interior space of the detection head 30 via the net holes 54. The lower barrel 52 is provided with a structure to house the speaker 80. A voice coil case that projects from the rear surface of the speaker 80 is fixed and engaged to an installation recess 74 formed in the center of the partition wall 55, and a circular frame 82 that surrounds the outer periphery of the speaker 80 is fixed to the internal wall of the lower barrel 52. A plurality of tiny holes 58 that communicates with the space within the detection head 30 is formed in the partition wall 55, so that by opening the back of the speaker 80 to the outside via the detection head 30 the speaker becomes an open-type and the efficiency is increased. Also, projections 56, 57 that project into the detection head 30 are formed on the partition wall 55, and the projections are combined with an LED base 36 and a PD base 37 of the detection head 30, to constitute a guide for the light emitted from the LED and a mask for the light received by the PD respectively. In other words, the projection 56, 57 4 -5 cooperate with members inside the detection head 30 to construct the optical structure for detection of smoke by the detection head 30.The lower barrel 52 is provided at its circumferential wall with air bents 59 which open to the rear side of the speaker 80. Sound pressure is developed behind the speaker 80. The air vents 59 cooperate with tiny holes 58 to release the sound pressure to the outside. A lead wire (not shown on the drawings) that extends from the speaker 80 passes through a part of the tiny holes 58 and is connected electrically to the circuit board 14, for outputting the fire alarm messages and low battery alarms generated by the voice generation circuit. The head cover 60 is a cylindrical body which covers the insect-proof net barrel 50. The head cover 60 has a top end and the bottom end. The top end of the head cover is removably fitted to the peripheral edge of a central aperture 22 of the base cover 20 which covers the bottom surface of the base 10. The bottom end of the head cover 60 is formed with the bottom wall which conceals the speaker 80. A plurality of sound holes 62 is formed in the bottom wall of the head cover 60, and the voice output of the speaker is emitted into the room via the sound holes 62. The circumferential wall of the head cover 60 is formed with windows 64. These windows 64 are communicated with the slits 32 of the detection head 30 via the net holes 54 of the upper barrel 51 through the net holes 54, and also are communicated with the air vents 59 of the lower barrel 52 through the net holes 54. In the embodiment shown in the drawings, an example is shown in which a gap is provided between the outer periphery of the speaker 80 and the inside wall of the insect-proof net barrel 50, and the speaker 80 is fixed to the insect-proof net barrel 50, but the circular frame that surrounds the outer periphery of the speaker 80 may be in close contact with the inside wall of the lower barrel 52 of the insect-proof net barrel 50. Fig. 10 shows a modified example of the insect-proof net barrel that can be applied to the above embodiment. Similar to the above embodiment, this insect-proof net barrel 50A is a cylinder partitioned internally into an upper barrel 51A and a lower barrel 52A by a partition wall 55A, and the upper barrel 51 A is tapered to have its diameter smaller towards its lower end. The upper barrel 51A is provided with a plurality of circumferential ribs 71 extending circumferentially, and a plurality of vertical ribs 72 crossing with the circumferential ribs to form a lattice, and net holes 54A are formed in the lattice. By giving the upper barrel 51A a tapered shape, each net hole 54A is opened in the radial direction and in the axial downward direction of the insect-proof net barrel 50A as shown in Fig. 11. Therefore insects and dust that adhere to the lattice can be made to fall down by applying vibrations to the insect-proof net barrel 50A so it is possible to prevent the net holes 54A from becoming blocked. These insects and dust can also be expected to fall down due to gravity, but they can be effectively removed from the insect-proof net barrel 50A by external vibrations. In particular, by using the speaker 80, preferably the insect-proof net barrel 50A is vibrated by 5 the sound generated by the speaker. In this case, in order to make the sound pressure developed behind the speaker 80 act on the partition wall 55A, as shown in Fig. 10, preferably holes that communicate with the detection head are not provided in the partition wall 55A. In order to periodically clean the insect-proof net barrel 50A, an electrical circuit for periodically outputting an appropriate sound from the speaker may be provided separately from the voice generation circuit, and in this case the sound may be outside the audible range, instead of a voice. As shown in the drawings, air vents 59A that open into the rear surface of the speaker 80 may be provided in the circumferential wall of the lower barrel 52A, to make the speaker an open-type, but in order to effectively apply vibrations to the insect-proof net barrel 50A, the air vents 59A may be eliminated to give an enclosed speaker structure. In this modified form, the speaker 80 is retained by the circular frame 82, and by closely engaging the circular frame 82 within the lower barrel, the speaker 80 is retained within the insect-proof net barrel 50A. The circular frame 82 has a cover 84 that covers the vibrator of the speaker 80, and sound holes 86 are formed in the cover 84. It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. 6 25407011 (GHMatters) PS1039 AU