referring now to fig1 and 2 , the presently preferred embodiment of the invention is depicted in a cigarette 10 , only the relevant portion of which is shown in the drawings . cigarette 10 includes a generally cylindrical rod 11 of smokable material contained in a wrapping material , such as cigarette paper 12 , to which a mouthpiece 15 is affixed in any conventional manner . at the draw end of the mouthpiece 15 there is arranged a conventional fibrous filter 30 through which the smoker draws the smoke from the smokable material in rod 11 . the rod 11 is referred to hereinafter as a &# 34 ; smokable rod &# 34 ; or a &# 34 ; tobacco rod &# 34 ; comprising tobacco or other smokable material , such as shredded pieces or strands of tobacco , processed tobacco , tobacco substitutes or blends thereof wrapped in cigarette paper to form a rod . preferably , another fibrous filter 21 is positioned in the mouthpiece 15 at the end thereof abutting the tobacco rod 11 and in a plane perpendicular to the longitudinal axis of the cigarette . the filter 21 prevents tobacco particles from entering the mouthpiece and is also useful to extinguish the lit cigarette should it burn down to the mouthpiece as when allowed to burn out in an ashtray . the mouthpiece 15 includes a rigid or resilient tube 17 of tipping paper , for example , which has a circumferentially aligned row of perforations 20 therein lying in a plane transverse to the longitudinal axis of the cigarette . the perforations , which are made in the course of manufacture of the cigarette either before or after assembly thereof , allow ambient ventilation air to enter the mouthpiece . thus , in this embodiment the tipping paper 17 and perforations 20 constitute air ventilation or air dilution means for the cigarette . of course , the smokestream which flows through the tobacco rod and the mouthpiece arises from suction as the smoker draws on the mouthpiece tip of the lighted cigarette , and thereby entrains air and smoke from the tobacco at the lit end . however , as used herein , the terminology &# 34 ; ventilation air , 38 &# 34 ; dilution air ,&# 34 ; and words of similar import describe air which enters the flow path of the smokestream downstream from the lit end of the cigarette and which serves to dilute the smokestream . the magnitude of &# 34 ; air dilution &# 34 ; or &# 34 ; air ventilation &# 34 ; is defined as the ratio ( generally expressed as a percentage ) of the volume of air drawn through the air dilution or air ventilation means to the total volume of air and aerosol drawn through the smoking article and exiting the draw end of the smoking article . secured within the tube 17 of mouthpiece 15 is a flexible membrane 22 in the form of a tube or duct having a curved sidewall such that the diameter of the membrane duct continuously decreases from either end 24 , 25 to the center thereof in a manner similar to a venturi tube . preferably , the membrane duct has a length at least twice the inner diameter of rigid tube 17 . the ends 24 , 25 of membrane 22 are adhesively or otherwise sealingly attached about their circumference to the inner wall of the rigid tube . in the condition in which there is no suction on the mouthpiece tip , the membrane is in a relaxed position or state as shown in fig2 with the portion between each circumferential fixed end free to move inwardly and outwardly toward and away from the longitudinal axis of the cigarette relative to the inner wall of the rigid tube . the membrane 22 may be composed of any suitable nontoxic flexible material , such as latex , polymeric film or paper . in the central region of the membrane , that is , in the portion encompassing the smallest diameter of the duct , a plurality of longitudinal slits 27 is provided in a circumferential array in the side wall thereof . the slits may also be formed as flaps by cutting the membrane along a non - linear line or intersecting lines , e . g ., a curved line or an intersecting pair of straight lines in a v - shape . the slits 27 are made prior to assembly of the mouthpiece , and are situated preferably to lie in close proximity to the perforations 20 in rigid tube 17 when the membrane is secured to the tube . in the normal , relaxed position of the membrane shown in fig1 slits 27 are substantially closed . referring now to fig3 when the smoker draws on the mouthpiece tip , the suction creates a pressure differential across the sidewall of the membrane 22 thereby causing the sidewall to distort or shrink inwardly throughout its free length , and principally along the central region thereof , thus constricting the opening constituting the flow path for the smokestream . this , in turn , causes the slits 27 to distend and to act as vents for admitting ventilation air , which enters the cigarette mouthpiece via the perforations 20 , into the flow path of the smokestream . it will be observed that although the sidewall of the flexible membrane 22 forming the flow path in its portion of the mouthpiece is constricted in this activated position relative to the relaxed position shown in fig2 there remains a substantial duct opening and , hence , there is no significant impediment to the smooth flow of the smokestream . this is quite different from the smoke control techniques and devices generally found in the prior art , where , apart from the filtering of the smoke by the tobacco itself and by any synthetic filters in the tip , solid obstructions often are interposed in the flow path which are impenetrable by the smokestream except via small holes therein . such techniques and devices make it difficult for the smoker to achieve a smooth draw , with a consequent lessening of smoking enjoyment . the present invention has no such disadvantage . moreover , smoking articles according to the present invention produce no interruption , abrupt or otherwise , in the smoker &# 39 ; s normal puff . the ventilation air introduced into the flow path via the perforations 20 and vents 27 serves to dilute the smokestream , but does not shorten the normal puff . in contrast , many of the prior art techniques and devices employ or operate as closure valves which produce a measured &# 34 ; puff &# 34 ; without regard to the smoking habits of the individual smoker , and thereby further lessen the smoking enjoyment . it will be noted that the flexible membrane 22 together with its vents 27 serves as valve means in one sense , to control the dilution level of the smokestream with ventilating air . within limits which are determined at least in part by the dimensions of the flexible membrane and the vents , and the material of which the membrane is composed , the greater the pressure differential across the wall of the membrane , the greater the distortion of the membrane and the enlargement of its vent openings . accordingly , the smoker is able to control the dilution level of the smokestream at will by the depth of his or her draw , that is to say , by the degree of suction on the mouthpiece tip . unlike many of the prior art devices for smoke control , the smoker need not draw deeply to achieve full - flavor smoke , and if he chooses to increase the length or depth of a puff the dilution level is increased . therefore , it will also be clear that the flexible membrane together with its vents serves as air ventilation means in combination with the tube and perforations therein , or as an adjunct to the air ventilation means . while a presently preferred embodiment of the invention has been depicted and described herein , it is to be emphasized that this is by way of illustration only and that variations and modifications of the illustrative embodiment may be made by persons of ordinary skill in the field to which the invention pertains without departing from the spirit and scope of the inventive principles . for example , the number and location of the perforations in the tube and / or the slits in the membrane may be varied , as may the dimensions of the membrane relative to those of the mouthpiece or other components of the smoking article . similarly , the invention is not restricted to use in cigarettes but may be advantageously employed in other smoking articles , including , for example , cigarette holders , pipes , and tipped cigars . accordingly , the invention is to be limited only to the extent required by the appended claims and applicable rules of law .
a variable ventilation mouthpiece for a smoking article includes first and second sets of vents , the first set serving to admit ambient air into the mouthpiece , and the second set adapted to variably introduce the admitted air into the smokestream for varying the level for dilution of the smokestream in relation to the suction applied by the smoker to the mouthpiece .