Electro-acoustic transducers

Abstract

 An acoustic transducer such as a dome loudspeaker has a membrane of polyamide 12 mixed with plasticizer, offering an improved consistency of frequency characteristic above 5 kHz and a lower natural resonance frequency than previously used materials such as Supronyl.

Description

[0001] This invention relates to acoustic transducers, particularly, but not exclusively, loudspeakers.

[0002] A known type of loudspeaker has a dome shaped membrane to which a voice coil is attached. Such domes have previously been pressed from synthetic plastics foil or from fabric material which is subsequently impregnated with plastics material.

[0003] The present invention seeks to provide an improved acoustic transducer with a dome membrane having increased efficiency compared with transducers having dome membranes of the type previously mentioned.

[0004] According to the present invention there is provided an acoustic transducer having a dome membrane αΓpolyamide plastics material, preferably polyamide 12 mixed with a plasticizer.

[0005] The thickness of the dome membrane is preferably such as to provide an equalized frequency response in a desired frequency range above a resonant frequency. By the use of a plastics membrane of different thicknesses dome membranes for transducers of different frequency range can be produced.

[0006] For example, for a high frequency dome loudspeaker an equalised frequency response in the range 6 kHz - 15 kHz, can be achieved, with a resonance frequency some 200 Hz lower than the lower end of this range, using polyamide 12 as the dome membrane material.

[0007] It has been found that the manufacture of a dome membrane using polyamide 12 plastics material mixed with a plasticizer results in a membrane of lower natural frequency compared with similar domes made of previously used synthetic plastics materials or woven material. The lower end of the equalized frequency response, or take-up frequency, can therefore be chosen as a lower frequency than the take-up frequency of comparable domes of the previously used materials referred to above.

[0008] The dome membrane according to the invention finds particular application in acoustic transducers such as loudspeakers for high and medium frequency range operation.

[0009] The accompanying drawings illustrate by way of example comparative graphical frequency response curves obtained by measurements upon loudspeakers having dome membranes. Figure 1 shows the frequency response characteristic of a dome membrane of conventional woven material, for use in a high frequency loudspeaker, while Figure 2 shows the frequency response of a comparable dome membrane of the same shape and quality made of polyamide 12. Figure 3 shows the frequency response of a dome membrane of Supronyl for use in a medium frequency loudspeaker, and Figure 4 shows the frequency response of a comparable dome membrane of polyamide 12.

[0010] Compared with dome membranes of textile material, dome membranes of polyamide material have higher overall efficiency. When used in a high frequency loudspeaker a polyamide 12 dome exhibits a crest in the region from 9-20 kHz, as will be evident from a comparison of Figures 1 and 2. This is advantageous in a domestic loudspeaker installation, since the high frequency reproduction falls off in proportion to the distance of the loudspeaker from a listener.

[0011] Comparing Figures 3 and 4, it is evident that a dome membrane of polyamide 12 for a medium frequency loudspeaker has a more equalized frequency response and a higher efficiency than a comparable dome membrane of Supronyl, particularly in the important medium frequency loudspeaker range of 2-5 kHz. The resonance frequency of the polyamide 12 dome is also about 200 Hz lower than that of the comparable Supronyl zone, so that the medium frequency loudspeaker can be brought into effect earlier.

[0012] By using a dome membrane of polyamide 12 it is possible to provide a loudspeaker of considerably lower cost than currently available loudspeakers of comparable efficiency and resonance frequency.

[0013] Further advantages arise from the use of a dome membrane of polyamide 12, particularly as contrasted with a textile fabric dome, in a loudspeaker installed and operated in a housing having no frontal covering, that is, where the loudspeaker is visible and accessible to the touch:

(i) impressions caused by touching the dome membrane of polyamide 12 have no permanent effect and do not cause any acoustic variations; .

(ii) dust deposits can be brushed away without causing any permanent damage to the dome membrane of polyamide 12;

(iii) the tolerances in the material thickness of the dome membrane can be kept appreciably smaller.

Claims

1. An acoustic transducer having a vibratory membrane . supported peripherally, characterised in that the membrane is fabricated in polyamide plastics material.

2. An acoustic transducer according to Claim 1, in which the plastics material comprises polyamide 12 mixed with a plasticizer.

3. An acoustic transducer according to Claim 1 or Claim 2, in which the thickness of the dome membrane is such as to provide an equalized frequency response in a frequency range above a resonant frequency.

Drawing