the present invention overcomes the problem of different headset plugs being incompatible with headset jacks . as discussed above , the problem is particularly prevalent in the commercial airline industry where passengers in different “ classes ” may be issued with different headsets having different capabilities . rather than having to change whole seat installations in order to change the jacks , the present invention allows a single multifunctional headphone jack to be used throughout an aircraft , bus or other passenger vehicle so that a number of different types of headset can be used . the invention achieves this task by providing plug pin sockets that are arranged to allow each common headset type to be plugged into the jack , and by providing the jack system with passive or active detection means to detect , from the plug pins that have been inserted into the socket , the type of plug and thus the type of headset so that the required audio and other signals can be provided to the necessary plug conductors to enable the connected headset to function correctly . as mentioned above , the invention is particularly applicable to noise reduction headphone systems . in order to fully understand the operation of the jack of the present invention , it is helpful it have a general understanding of headset types . airline entertainment headphones have been categorised by arinc into four different types , a , b , c and d . types a and b are older types . types c and d are intended for use with noise cancellation ( nc ) systems . type c headphones incorporate nc electronics , and type d are similar to type c except that the nc electronics is installed remotely in the seat area . a summary table of airline entertainment system headphones is provided below . it is also helpful to have some understanding of headset types for noise cancellation systems . existing and proposed headset types are set forth below . this headset has a three pin plug for which an associated three socket jack is required . this is usually installed into the armrest of the passenger seat . the plug is shown in fig1 having two 3 . 5 mm mono plug pins and one 2 . 5 mono plug pin . it is commonly known as an arinc “ c2 ” plug , as will be described further below . a mono pin is shown in fig2 in which it can be seen that the pin has two conductors , being a ring 21 and a tip 22 . a stereo plug pin is shown in fig3 from which it can be seen that the pin has three conductors , being a first ring 31 , and second ring 32 , and a tip 30 . returning again to fig1 the 2 . 5 mm pin supplies power to the noise cancelling electronics located in or adjunct to the headset . the noise cancellation ( nc ) electronics typically connect to the jack with preferably a six - conductor cable . two of these conductors are for power , one conductor for program audio left , one conductor for program audio right and two conductors for audio left ground and audio right ground . therefore , the jack for this type of headset is one that is designed to accept mono plug pins , not stereo plug pins . between the nc circuit medallion and the headset &# 39 ; s transducers are separate cables of two leads , each with four conductors . these provide program audio to the two speakers and noise signal from the two sensors . the sensor conductors are shielded . this is a variant of the type 1 headset . the plug of this type of noise cancellation headset connects to a jack comprising a single 3 . 5 mm stereo socket and a 2 . 5 mm stereo socket . the headset is fed programme audio from the passenger seat remote jack unit via the 3 . 5 mm stereo sockets . power is provided to the headset &# 39 ; s nc circuit via the 2 . 5 mm stereo socket . this headset does not comprise any noise cancellation circuitry and relies on an audio signal that has already been processed to provide noise cancellation . therefore , the nc circuitry is provided adjunct to the headphone , typically in the seat or within the remote jack unit itself . control features on the headset may be provided . the control features do not include a noise cancellation circuit , but do include a volume control and an nc gain control . these may both be provided in the form of variable resistors . these control features do not alter the number of conduction paths required or the configuration of the sockets in the jack . the cable from the jack to the headset or circuit is preferably seven - conductor assembly ( two conductors for audio left and right , two audio grounds and two shielded cables for the sensor left and right ). this is a variant of the type 2 headset and is proposed at the present time . it is planned as a personal headset to be sold to passengers who wish to buy an nc headset for personal audio use . this model will have battery power for use with 32 ohm low voltage sources . the batteries will be located in the headset or adjunct to the headset in a box or medallion . again , these requirements can be supplied using a plug according to the invention . as well as the noise cancellation headset types discussed above , it is desirable if the seat jack unit is provided with means to supply headsets , which are not noise cancellation headsets i . e . to supply simply mono or stereo audio without noise cancellation . known jack arrangements for commercial aircraft are specified by arinc ( aeronautical radio inc ). fig4 shows a standard airline single stereo plug , commonly referenced type a1 and type b1 . [ 0057 ] fig5 shows a standard airline dual mono and dual stereo plug , commonly referenced type a2 and type b2 . fig6 shows a standard dual stereo type airline plug , which is usually , referenced d2 . [ 0058 ] fig7 shows an arinc two pin plug , which is commonly called a type d1 plug . a table , referenced table 2 below , shows the use for each of the conductors provided on the pins of the plug discussed above . the jack of the present invention is shown in fig9 generally referenced 90 . as can be seen , it comprises two 3 . 5 mm plug sockets ( 92 , 93 ) and a 2 . 5 mm plug socket 94 connected to a seat circuitry connector 91 . the sockets are all capable of receiving pins , which have two or more conductors . this has the significant advantage that there is a maximum of nine possible conductors provided by the jack of the present invention . it will be seen that the plug sockets are arranged in a triangular form , which is compatible with that of the arinc three pin triangular c2 plug . furthermore , since a 2 . 5 mm socket is provided as well as a 3 . 5 mm socket , the jack of the present invention is able to receive an arinc two pin ( d1 ) plug . since two spaced 3 . 55 mm plug sockets are provided , the jack of the present invention enables existing dual mono and dual stereo plugs d2 , a2 and b2 to be used . also , a single stereo i . e . a1 or b1 plug can also be received . there is also a mechanical consideration . due to slight differences in the construction of plugs and pins it is highly desirable to allow some movement in the jack assembly . thus , in the preferred embodiment , one ( preferably the 2 . 5 mm ) or more of the jack sockets ( or jacks ) is allowed to “ float ” in the assembly . this is because the distance between the 2 . 5 mm pin and 3 . 5 mm pin in the c2 plug is slightly smaller ( 0 . 466 inches ) than the d1 , d2 plug ( 0 . 500 inches ). the degree of tolerance for this dimension needs to be in the order of 0 . 05 inches on the angle from the axis of the 2 . 5 mm jack to the 3 . 5 mm jack . the tolerance is indicated in fig9 . without this mechanical allowance the plug will be under strain and proper contact cannot be assured . referring again to table 1 , it will be seen that in order to operate the different noise cancelling headsets referred to above , and standard headsets , which use the various plugs of table 1 , appropriate connections need to be made from the “ smart jack ” of the present invention to the circuitry in the passenger seat or in the jack itself . referring to fig1 , a block diagram is shown illustrating an interface 1001 which is provided between the smart jack 1002 and the audio signal provided to a seat connector 1003 . the headset to which the jack is connected in use is referenced 1004 . the interface 1001 , in the preferred embodiment , comprises an active or passive network which is either enabled by or partially consists of , sockets in the jack of the present invention which provide an indication as to whether a pin has been inserted in the relevant socket . in fig1 , an electrical schematic diagram for a typical socket is shown . as can be seen , the socket has electrical contacts , which enable up to three conductors on a plug pin to be electrically connected to the socket . in particular , the conductor at the tip of the pin can open or close a connection between the adjacent conductors . this means that a signal is provided as to whether a pin is fully inserted into a relevant socket . therefore a logic table can be constructed to enable an identification to be made of the type of plug which has been inserted into the socket , and from that identification of the relevant headset can be made . therefore , the appropriate connections from the audio signal provided in the seat to the headset can be made . also , in the relevant instance , the appropriate connections can be made from noise cancellation circuitry provided in the seat to the relevant headset . by way of example , if only one of the 3 . 5 mm sockets and no other sockets register insertion of a pin , then it can be deduced that a single stereo plug has been inserted into the jack . if both the 3 . 5 mm plugs and not the 2 . 5 mm plug indicate that pins have been received in those sockets , then it can be deduced that the plug is of type a2 b2 or d2 . also from knowing whether there is a short between the conductors preceding the socket tip conductors , it will be know whether the pin that have been inserted are mono or stereo i . e . a distinction can be made between a2 and b2 and a d2 type plug . such deduction can also be carried out electronically by analysing the loading effect that a microphone would provide if connected in circuit to two of the conductors . [ 0067 ] fig1 illustrates such an approach with a voltage comparator where the window comparator detects whether the microphone dc voltage is within the range v h to v 1 . a resistor in parallel to the microphone defines a minimum microphone load which , in conjunction with the pull up resistor , ensures that v h & gt ; v in & gt ; v 1 giving a high output from the comparator to enable noise cancellation circuitry . if the microphone is disconnected then v in will equal v bias through the action of the pull up resistor and since v bias & gt ; v h the enable output will be low . if the microphone input is grounded then v in & lt ; v 1 and the enable output will also be low . for additional understanding reference may be made to fig1 , which illustrate how the plug arrangements for type a , b , c and d headsets can all be accommodated by the invention . the plug pin layouts are graphically represented in column c . column a describes the plug type , and column b diagrammatically shows the jack of the invention , with the plug type from column a overlaid in dashed outline . from the foregoing , it will be seen that a multifunctional headset jack is provided which allows a number of different headset types to be plugged into the jack , and still perform their expected function , whether the headset is a nc headset or otherwise . typically the jack and associated sensing circuitry is intended to allow a user to plug in a headphone of any type from a single plug mono headphone , through to a triple plug noise cancelling type with either internal or external noise cancellation circuitry and still provide the expected result . manufacturing tolerances and minor specification differences in the headphone plugs where the plug is two or three pin may be coped with by the floating construction of one of the jack sockets . the sensing circuitry may rely merely on detecting which of the jack sockets supplied have plugs entered by detecting which of the socket switches are closed or open , or it may additionally detect the presence of various impedances or supply voltages across certain of the plug tips , rings and sleeves . while the sensing circuitry is described as being electronically implemented it is possible to implement it with either simple logic circuitry or with programmable software controlled circuitry which may be updateable remotely . this will allow an already installed system to cope with variations in headsets as the specifications for these change from time to time . while the sensing circuitry is described in relation to headphones it is equally applicable to headsets containing headphones and microphone . finally various other alterations or modification may be made to the foregoing without departing from the scope of this invention .