referring to the drawings wherein like numerals correspond to like elements throughout , attention is first directed to fig1 wherein a gas turbine engine nozzle 10 embodying the present invention is diagrammatically shown . hot gases of combustion are expanded through a turbine ( not shown ) in a manner well known in the art and exit through exhaust nozzle 10 in the direction indicated by arrow 12 to generate a propulsive thrust in the opposite direction . the nozzle is shown to include a rigid centerbody 14 symmetrical about axis 16 and a generally coannular nacelle 15 comprising an external cowl 17 and a duct wall 18 whereby centerbody 14 and wall 18 cooperate to partially define a hot gas passage 20 therebetween . centerbody 14 is provided with a cavity 21 into which an acoustically treated structure 22 is received to further define passage 20 and to suppress noise propagating therein similar acoustic treatment 24 is received within a cavity 25 associated with wall 18 for the same reasons . referring now to fig2 and 4 wherein the duct acoustic treatment 24 associated with wall 18 is shown in greater detail , there is provided a high frequency acoustic panel 26 and a low frequency acoustic panel 38 , both of which are mounted concentrically within cavity 25 . the high frequency panel consists of an inner sheet 28 , perforated with a plurality of small diameter holes 30 , and an outer nonperforated sheet 32 sandwiching therebetween a cellular core 34 of the honeycomb variety . this pair of sheets is bonded to the honeycomb core by any of several methods such as brazing or diffusion bonding . each cell 36 is aligned with one of the holes 30 to provide communication between the cell interior and flow passage 20 , the cells functioning as helmholtz resonators to tune out noise within a frequency band which is related to the cell size . this high frequency acoustic panel covers the low frequency acoustic panel and further defines the hot gas flow path 20 . as used herein , the terms &# 34 ; low frequency panel &# 34 ; and &# 34 ; high frequency panel &# 34 ; are relative terms , it being well understood that larger chambers are required to suppress lower noise frequencies . the low frequency acoustic panel includes a plurality of resonating chambers 40 which may be formed in a variety of ways but are here formed between a cylindrical inner facing sheet 44 and an outer annular stepped facing sheet 46 separated radially from sheet 44 by a plurality of upstanding partitions 48 . the facing sheets and partitions may be attached as by welding or brazing to form a rigid structure which , in turn , is connected to wall 18 as at 49 . in fact , in the embodiment shown , stepped facing sheet 46 comprises a plurality of rings 50 between adjacent pairs of partitions 48 . each chamber 40 is provided with a hollow tube 42 which is integral with facing sheet 44 and which penetrates into the chamber . the high frequency panel 26 has a plurality of apertures 52 extending completely therethrough , the apertures being in general alignment with , and of a larger diameter than , the tubes . accordingly , the tubes pass through , but are not connected to , the high frequency panel and thus provide communication between the hot gas passage 20 wherein the noise to be suppressed is located and the resonating chambers 40 . the resonating chambers vary in size ( volume ) and the tubes vary in length to provide for wide - band noise suppression as is taught and described in u . s . pat . no . 3 , 819 , 009 , previously noted herein . as discussed earlier , differential thermal expansion exists relative to the ow and high frequency panels . this condition is due to the direct exposure of the high frequency panel , particularly facing sheet 28 which partially defines hot gas passage 20 to the exhaust gas flow through the passage . the low frequency panel 38 , on the other hand , is shielded from the hot gases by the presence of the high frequency treatment and , thus , remains substantially cooler . this temperature differential is most pronounced during transient operation such as during engine start - up . to provide for this relative thermal expansion , expansion means such as slip joints 54 are incorporated to connect duct walls 18 and high frequency panel 26 thereby permitting the high frequency panel to expand as a whole relative to the cooler outer structure . while the expansion means are shown to connect the high frequency panel to wall 18 , it will be appreciated that such expansion means can be provided anywhere between the high frequency panel and the low frequency panel in order to compensate for differential thermal growth therebetween . as shown in fig2 facing sheet 44 is recessed within cavity 25 by an amount sufficient to create an annular pocket 55 between facing sheet 44 and the plane of wall 18 , and the high frequency panel is nested within this pocket utilizing the slip joints to permit both axial and radial thermal growth with respect to the low frequency panel 38 and wall 18 . the tubes 42 are attached to the cooler low frequency panel rather than the high frequency panel since it is desirable from an acoustic point of view to have chambers 40 totally sealed with the only access to the interior thereof provided through the hollow tubes . if the tubes were attached to panel 26 , a slip fit would be necessary between the tubes and facing sheet 44 to accommodate thermal expansion , thus presenting the possibility of an acoustic &# 34 ; leak .&# 34 ; to ensure against interference between the tubes and the high frequency panel due to thermal expansion of the latter , a clearance 56 is established in the apertures encircling the tubes . preferably , the apertures are not placed concentrically about the tubes during fabrication when the entire structure is cold . instead , since high frequency panel 26 will expand generally uniformly from its center outwardly when heated , each such clearance 56 can be preset such that each tube will be concentric within its aperture when the panel is heated and expanded . this ensures against interference during engine operation when vibrations and aerodynamic loading could tend to distort the nozzle . note also that an annular gap 58 has been provided between the low and high frequency panels , not only to provide for relative thermal expansion , but also for the purpose of reducing conductivity of heat to the low frequency panel which clearly assists the duct wall 18 in functioning as the load - bearing structure in this portion of the nozzle . it is clear , therefore , that the thermal growth problem between the low and high frequency panels has been overcome . the remaining concern is with respect to intrapanel thermal stresses , particularly in the high frequency panel . since the inner - facing sheet 28 will tend to expand at a much greater rate than outer - facing sheet 32 , stresses will be imposed on the honeycomb core sandwiched and brazed therebetween which will tend to lean , and to possibly buckle , the honeycomb core . the effect will be most pronounced on the panel ends since the panel can be expected to expand rather symmetrically from the center outwardly . accordingly , and as shown in fig3 facing sheet 32 has been scored by axial and circumferential slots 60 and 62 , respectively , into a plurality of rectangular mosaic - like pieces 64 . preferably , the pieces should be made as small as possible to minimize the end effects without excessively sacrificing the structural integrity of the panel or its acoustic properties . ( note that the scoring &# 34 ; unseals &# 34 ; some of the high frequency chambers .) thus , the pieces have relative freedom of expansion in all directions . for a typical gas turbine engine nozzle , slots having a width of about 0 . 01 - 0 . 02 inch will significantly reduce the thermal stresses without seriously affecting the noise suppression effectiveness . although the sandwich - like structure is not relied upon primarily for its structural strength and stiffness but rather for its acoustic properties , a certain degree of structural rigidity is required . because the axial slot 60 may reduce the hoop stiffness to the point that it could not withstand anticipated vibrational and other loadings , it may become necessary to attach a plurality of thin ring stiffeners 66 to the backside of the inner - facing sheet 28 adjacent the honeycomb core . such a stiffener does not suffer thermal stress problems since its excellent heat conduction capability limits the temperature gradient in the stiffener . it will be obvious to one skilled in the art that certain changes can be made to the above - described invention without departing from the broad inventive concepts thereof . for example , while the present invention has been directed to annular panels for use in gas turbine engine exhaust nozzles , it is clear that the invention is equally applicable to any acoustically treated hot gas flow path regardless of its shape , and the use of the word &# 34 ; panel &# 34 ; is not meant to be limited to any particular shape . while the present invention has been described with particular emphasis on its incorporation into nacelle 15 , it is to be noted that a similar structure may be incorporated into rigid centerbody 14 and that incorporation into nacelle 15 is merely illustrative of one possible use of the present invention . similarly , as noted earlier , the terms &# 34 ; high &# 34 ; and &# 34 ; low &# 34 ; frequency are merely relative terms as used herein and do not limit the scope of the invention to any particular band or bands of frequency .