49 ACOUSTIC ENERGY LOSSES ON REFLECTION FROM
Transkrypt
49 ACOUSTIC ENERGY LOSSES ON REFLECTION FROM
O C E A N O L O G Y NO 11 (1979) 49 MAŁGORZATA BRZOZOWSKA BARBARA JANUSZEW SKA Polish A cadem y of Sciences Institute of Oceanology — Sopot ACOUSTIC ENERGY LOSSES ON REFLECTION FROM SELECTED FLOORS OF THE SOUTH BALTIC C o n t e n t s : 1. Introduction, 2. Theoretical assum ptions and method of m easure m ents, 3. Experim ental techniques, 4. R esults of m easurem ents and their discussion, 5. Conclusions; Streszczenie; R eferences. 1. INTRODUCTION / In th e p ropagation of sound in a shallow sea, such as th e B altic, a p a ra m o u n t role is played by th e acoustic p ro p erties of th e sea floor. A couple of p a ra m ete rs, such as p ressu re reflec tiv ity , reflection losses, sc atterin g factor, and sc atterin g pow er a re u su a lly em ployed in th e q u a n ti tativ e description of th e reflectiv e and diffusive p ro p erties of th e sea floor. T he m ag n itu d e of reflec tiv itie s fo r n orm al incidence of th e acoustic w ave on th e sea floor d eterm in es th e m ax im u m losses of en erg y to be a n tic ip a ted in th e reflectio n of sound. T he purpose of th is stu d y w as to find th e rela tio n sh ip b etw een th e acoustic re fle c tiv ity on th e sea bed and th e freq u en cy of incid en t acoustic w aves, angle of incidence, and ty p e of sedim ents. T he m easu rem en ts w ere conducted a t frequencies from 2 to 10 kH z (at te rtia ry in te rv als, i.e. for frequencies of 2, 2.5, 3.15, 4, 5, 6.3, 8, and 10 kHz) fo r th re e ty p es of sea bed: fine sand of 0.145-mm m edian, v e ry fin e sand of 0.120-mm m edian, and soil consisting m ain ly of m acrofauna (shells of m olluscs and snails). 2. THEORETICAL ASSUM PTIONS AND METHOD OF MEASUREMENTS The e x p e rim e n ts w e re conducted by th e m ethod of stan d in g w aves, w hich yields th e m ost a ccu rate resu lts [9, 10], T he m ethod consists in th e m easu rem en t of a re s u lta n t acoustic p ressu re a t th e m inim a and m ax im a of a stan d in g w ave th a t re su lts from th e in te rfe re n c e of tw o acoustic signals, th a t d ire c tly from a tra n s m itte r and th e b ed -reflected one (Fig. 1). Fig. 1. Configuration of transm itter (N) and receiver (0) for the incidence angle of an acoustic Wave on the bottom ¡9 ^ 0° n r: hi hs r — d is ta n c e — d is ta n c e — d is ta n c e — d is ta n c e — d is ta n c e tr a v e lle d b y in c id e n t a c o u s tic s ig n a l tr a v e lle d b y r e f le c te d a c o u s tic s ig n a l f ro m b e d to r e c e iv e r f ro m b e d to t r a n s m it te r b e tw e e n r e c e iv e r a n d tr a n s m i t t e r (in h o r iz o n ta l p la n e ) Rye. 1. U sytuow anie nadajnika (N) i odbiornika (0), gdy kąt padania fali akustycz nej na dno & 0° ri r; hi hi r — — — — — d ro g a p r z e b y ta p rz e z s y g n a ł a k u s ty c z n y b e z p o ś re d n i d ro g a p r z e b y ta p rz e z s y g n a ł a k s u ty c z n y o d b ity od d n a o d le g ło ść o d d n a do o d b io r n ik a o d le g ło ść o d d n a d o n a d a j n ik a o d le g ło ść p o m ię d z y n a d a jn ik ie m i o d b io r n ik ie m (w p ła s z c z y ź n ie p o z io m e j) F or a point source of sound, th e p ressu re of th e acoustic w ave tra v e llin g d ire c tly from a tra n s m itte r to 'a receiv er has th e form : P i = P o /ri exp (— ikrj) (l ) in w hich: Po — acoustic p ressu re a t a u n it d istance from th e tra n sm itte r, ri — distance th e acoustic signal tra v e ls d ire c tly from tra n s m itte r to receiv er (Fig. 1), k = 2n/\ — acoustic w ave nu m b er, w hile th e p ressu re of th e b ed -re fle cte d w ave is given by P s = Po/r 2 • exp (— ik r2) (2) in w hich: r 2 — distance tra v e lle d b y th e acoustic signal from tra n s m itte r to receiver, a fte r reflectio n from th e sea bed. T he p ressu re re fle c tiv ity is given b y th e follow ing form ula: V = V0 • exp (icp) (3) in w hich: V 0 — absolute v a lu e of reflectiv ity , cp — phase of reflectiv ity . T he re s u lta n t acoustic p re ssu re of th e point of reception equals P = Ps + Pi (4) A fte r tak in g in to account Eqs. (1, 2) and (3) one o b tains th e follow ing re latio n sh ip for absolute p ressu re: ,= |p h f (^ + 1 -2 V 2o , 2 p 20 V o r 2| cos [k (r2 — ri) — tp] n1 Xr22 * ) ( 4) T his p ressu re w ill be m ax im u m if cos [k (r2 — ri) — <p] = + 1 and m in i m um if cos [k (r2 — ri) — cp] = — 1. U pon s u b stitu tin g consecutive cosine values of + 1 and — 1 in Eq. (4) and solving th e re su ltin g system of tw o eq uations (w ith resp ect to VQ) one obtains P m a x _______ P m in TT rim in v °= rim a x ( 5) --------------- ------- Pmax | Pmin r2min r2max in w hich: r ij2 m ax and ri ,2 m in = d istances ri and r2, determ in ed above, a n d counted to th e receiv er in th e phases of stan d in g -w av e m ax im u m an d m inim um , respectively. g mu I I e cwo *<> Fig. 2. Schem atic diagram of transm itting, receiving, and recording configuration G T N O W, F M — — — — — — p o w e r g e n e r a to r , ty p e PO-21 (K A B ID -Z O P A N ) tr a n s f o r m e r t r a n s m i t t e r : c y lin d r ic a l p ie z o c e ra m ic tr a n s d u c e r r e c e iv e r : 8101 B r u e l- K ja e r h y d r o p h o n e a m p lifie r a n d s e t o f f ilte r s : 2120 B r u e l- K ja e r a n a ly z e r m a g n e tic v o lta g e r e c o r d e r , B r u e l- K ja e r 7003 Rye. 2. Schem at układu nadaw czo-odbiorczo-rejestrującego G T N O W, F M — — — — — — g e n e r a to r m o c y ty p P O -2l (K A B ID -Z O P A N ) t r a n s f o r m a to r n a d a jn ik : c y lin d r y c z n y p r z e tw o r n ik p ie z o c e ra m ic z n y o d b io r n ik : h y d r o f o n 8101 (B rU e l-K ja e r) w z m a c n ia c z z z e s ta w e m f iltr ó w : a n a liz a to r 2120 (B rU e l-K ja e r) r e j e s t r a t o r m a g n e ty c z n y n a p ię c ia 7003 (B riie l-K ja e r) W here th e distance tra v e lle d b y an acoustic signal from th e tra n s m itte r to receiver, is m uch g re a te r th a n th e acoustic w a v e -le n g th (and co n seq u en tly — th a n th e d istance b e tw e en a d ja c e n t stan d in g -w av e m in i m um and m axim um ) one can assum e ri min ~ ri max a n d r 2 min ~ r 2 max. A ccordingly, th e u ltim a te fo rm u łą for th e absolute v a lu e of re fle c tiv ity reads: -y _ _ 1*2 Fl Pm ax P m in P m a x “ I- P m in H ence, th is absolute v a lu e m ay be found for know n ri and ri (from th e geom etrical con fig u ratio n of ex p e rim e n tal environm ent) an d for acoustic p ressu re m easu red a t th e nodes and antinodes of stan d in g w aves. 3. EXPERIMENTAL TECHNIQUES The block diagram of th e m easu rin g system is show n in Fig. 2. The tra n sm ittin g com ponents consisted of a PO 21-type p o w er gen erato r, and a piezoceram ic tra n sd u c e r w ith a conical shield w ith 45° angle of a p e rtu re . T he shield w as n ecessary to elim in ate in te rfe re n c e d u e to scat te rin g of sound off th e u n d u late d sea surface. T he recep tio n com ponents included a hydro p h o n e (type 8100), am p lifier w ith a set of filte rs (type 2120), and m easu rin g tap e reco rd er (type 7003), all in stru m e n ts m an u fac tu re d by B riiel K ja e r. A special s tru c tu re (Fig. 3) to m ove th e h y d ro phone along th e v e rtica l axis w as designed to d e te rm in e th e m ax im a and m inim a of stan d in g w aves. T he positions of th e hydro p h o n e w ere found from indications of a co unter, w hich recorded th e n u m b er of rev olutions of a th read ed rod, along w hich th e m otion occurred. T he accuracy of re ceiver se ttin g a t th e e x tre m u m of th e stan d in g w ave w as 3.5-m m (pitch of rod th read ), w hich w as su fficien t fo r th e w av elen g th e n co u n tered (75... 15 cm). T he su p p o rtin g s tru c tu re of th e hydro p h o n e w as set firm ly on th e bed, w hile th e tra n s m itte r w as low ered on a jib to a d ep th of 1.5... 2 m below th e w a te r surface. A fte r th e au to m atic sw itching-on of th e h y d ro phone (this being controlled by an electric m otor), th e deflection of a po in ter, corresponding to changes in acoustic p ressu re, w as observed on the m easuring am p lifier scale. T he hydro p h o n e w as stopped fo r m axim um and m inim um p o in ter deflections, and a 100-second m agnetic recording taken. F o r each fre q u e n c y th e m easu rem en ts w e re rep e a te d fo r several consecutive m inim a and m axim a. T he v a lu e obtain ed from th e 100-tsecond averaging w as accepted as th e p ressu re fo r stan d in g -w av e m inim um and/ or m axim um . T his p ro ced u re p e rm itte d random in te rfe re n c e and flu c tu ations of th e acoustic signal to be avoided a t th e p oint of reception. T he averaging w as accom plished a u to m atically w ith an in te g ra l system b u ilt in a B riiel K ja e r 2120 analyzer. T he hydroacoustic m easu rem en ts w ere accom panied by sam pling of bed surface sedim ents, th e la tte r hav in g been su b ject to g rain size analysis. T he an aly sis w as perfo rm ed on a set of 13 sieves bounded by 5 an d 0.063 m eshes. Fig. 3. Schem atic diagram of hydrophone structure 1 — h y d r o p h o n e c la m p 2 — e le c tr ic m o to r in w a te r p r o o f c a s in g Rye. 3. Schem at konstrukcji nośnej hydrofonu 1 — u c h w y t d o h y d r o fo n u 2 — s iln ik e le k tr y c z n y w o b u d o w ie w o d o s z c z e ln e j 4. RESULTS OF MEASUREMENTS AND THEIR DISCUSSION \ H ydroacoustic m ea su rem e n ts w ere conducted for th re e types of sea bed. The firs t ty p e consisted of m acro fau n a an d v e ry sm all a d m ix tu re s of fine, as w ell as m edium sand. T he m olluscs M ytilus edulis a n d Cardium Lcimarcki a n d tin y snails 2... 30 m m in size dom inated in th e species com position. The second ty p e of soil w as fine sand w ith a m ax im u m sh are of g rain s of ab o u t 0.16... 0.10 m m , th e m edian being 0.145 m m . The T ra sk so rtin g fa c to r [7, 8] fo r th ese sedim ents w as fa irly high: 1.75. The th ir d ty p e of bed w as c h a ra c te riz e d by th e occurrence of tw o d istin ct lay ers. T he u p p e r lay er, ab o u t 0.2*m thick, of v e ry high w a te r content, consisted of fine a n d m ed iu m sands. The m edian of th is la y e r w as 0.195 m m w hile th e T ra sk so rtin g fac to r rea c h e d 1.70. T he o th er la y e r included h ighly consolidated fine a n d v e ry fine sands. T h eir m ed ian w as 0.120 m m , a n d th e so rtin g fac to r w as 1.37. T he in te g ra l curves obtained fro m the gi'ain size an aly sis of th e soil sam ples a re show n in Fig. 4. Fig. 4. Integral curves of grain size ----------------- m e a s u r in g p o in t 2: fin e s a n d m e a s u rin g p o in t 3: la y e r I (U p p e r), f in e s a n d --------- -------m e a s u rin g p o in t 3: la y e r II (lo w e r), f in e a n d v e r y f in e s a n d Rye. 4. W ykresy uziarnienia gruntu (krzywe kum ulacyjne) ----------------------------------------------- d la p u n k t u p o m ia ro w e g o 2: p ia s e k d r o b n o z ia r n is ty ¿ la p u n k t u p o m ia ro w e g o n r 3: w a r s tw a I (g ó rn a ), p ia s e k d r o b n o z ia rn is ty d la p u n k tu p o m ia ro w e g o n r 3: w a r s tw a II (d o ln a ), p ia s e k d r o b n o - i b a rd z o d r o b n o z ia rn is ty T he ab so lu te values of th e reflec tiv itie s for n orm al incidence of acoustic w aves a t th e sea bed, av erag ed over freq u en cies of 2 to 10 kHz, fo r th e th re e ty p es of soil, w ere as follows: i m acro fau n a V0 = 0.59 fine sand (Md = 0.145 m m ) VQ= 0.55 v e ry fin e sand (Md = 0.120 m m ) V 0 = 0.61. The re su lts o b tained ag ree w ith th e fin d in g s for o th er w a te r bodies u n d e r sim ilar conditions and e x p erim en tal technology [2, 5, 10]. F o r instance, o u r d a ta fall into th e in te rv al of v a ria b ility of reflectiv ities given by L ib e rm an n [5], w ho obtained 0.50... 0.60 for a stony bottom , 0.40... 0.86 for sand, and 0.40... 0.60 for soil consisting of sand and clay. T he d iam eters of th e p a rticles of th e stony floor studied by L ib erm an n coincide w ell w ith th e size of ou r m acrofauna, so th a t th e reflectiv ities in both cases w ere v e ry sim ilar. M oreover, it should be concluded th a t th is coincidence w as fo rtu ito u s, as th e acoustic p ro p erties of th e sea floor a re d e te rm in e d not only by th e size of bottom sedirpents. O u r figures are lo w er th a n th e data given by G ru b n ik [2] for th e C aspian Sea, w ith m ean ab so lu te v alu es of reflec tiv itie s of 0.76 and 0.68 for sand and silty sand, respectively. M ackenzie [6] also detected reflection losses of 6 ... 8 dB for v e ry fine sand, w hich correspond to an absolute m ag n itu d e of reflec tiv ity of about 0.5... 0.4. The dispersion of d ata in d icated by th e above re su lts o b tained by vario u s a u th o rs can be due to b ottom irre g u la ritie s, d iversified in te rn a l stru c tu re , a n d r a th e r in ac c u ra te n o m en c latu re of bed sedim ents (since d iffe ie n t c rite ria of th e classification of sedim ents a re in use, it is necessary to specify th e d istrib u tio n of resp ectiv e sed im en t fractions, to g e th e r w ith th e ty p e of soil, e.g. sand or clay). T he absolute values of re fle c tiv ity found in th is stu d y are show n in Fig. 5 as a fu n ctio n of th e fre q u e n c y of th e acoustic signal. No c le a r dé pendance of th e re fle c tiv ity on th e len g th of th e acoustic w ave w as found in th e stu d ie d in te rv a l of freq u en cies from 2 to 10 kHz, a t te r tia ry spacing. S im ilar re s u lts w ere obtained by G ru b n ik [2], see Fig. 6. H ow ever, a fte r th e re s u lts of ou r ex p e rim e n ts w ere obtained, a stu d y by G oncharenko et al. [4] ap p eared , in w hich th e above rela tio n sh ip w as exposed for tw c lay e r bottom . T h eir findings conform rea so n a b ly w ith B re k h o v sk ik h ’s co m putations fo r a m u ltila y e r bed. T he function of absolute reflec tiv ity v e rsu s fre q u e n c y is illu s tra te d in Fig. 7. T he function VQ(f) is oscillatory, its period changing w ith th e incidence angle of acoustic w aves a t th e sea floor (w ith th e exception of th e case C2 C0, in w hich C2 and CQ are speeds of sound in th e u p p e r floor la y e r a n d in w a te r, respectively). The freq u en cy relatio n sh ip s obtained for vario u s angles of incidence can be used in com putations of the ch a ra c te ristic s of respective bottom layers. T he lack of a freq u en cy fu n ctio n for th e absolute reflec tiv ity , in our Fig. 5. A bsolute valu e of reflecitivity, V0, versus frequency of acoustic w ave f(# = 0°) o d a ta f o r m a c r o fa u n a x d a ta f o r f in e s a n d A d a ta f o r v e r y f in e s a n d Rye. 5. Zależność m odułu w spółczynnika odbicia (V0) od częstotliw ości fa li akusty cznej f (# — 0°) o d a n e d la m a k r o f a u n y x d a n e d la p ia s k u d r o b n o z ia rn is te g o A d a n e d la p ia s k u b a rd z o d ro b n o z ia rn is te g o 0 ___ 1___ I___ 1____I___ 1___ I___ 1___ I--------1--------L 2 U 6 8 10 /[k H z ] Fig. 6. Experim ental absolute valu es of sea-floor reflectivity, V0, versus frequency. D ifferent notations correspond to various m easuring points. According to Grubnik [2] Rye. 6. Eksperym entalne w artości m odułu w spółczynnika odbicia V0 od piaszczyste go dna w funkcji częstości. Poszczególne oznaczenia odpowiadają różnym punktom pomiarowym. Wg pracy N.A. Grubnik [2] Vo f[H z] Fig. 7. A bsolute value of reflectivity versus frequency for a tw o-layer m odel of the sea floor (the case Ca C0, in w hich C 2 , Cc — speeds of sound in the upper layer and ir. w ater, respectively). A ccording to Goncharenko, Zakharov, Ivanov, K irshov [3] O -------- e x p e r im e n ta l d a ta t h e o r e tic a l c u r v e Rye. 7. Zależność częstotliw ościow a m odułu w spółczynnika odbicia dla dw uw arstw o w ego modelu dna (przypadek gdy C2 <^C0; gdzie Ca, C0 — prędkości dźw ięku od powiednio w powierzchniowej w arstw ie dna i w wodzie). Wg pracy B. I. G onczarienko, L.N. Zacharów, W.E. Iwanow, W.A. Kirszow [3] O -------- d a n e e k s p e r y m e n ta ln e k rz y w a te o r e ty c z n a Fig. 8. Experim ental absolute valu es of reflectivity for very fine sand, versus frequency of acoustic w ave, at tw o angles of incidence A d a ta o b ta in e d f o r Si = 0° d a ta o b ta in e d f o r = 25° Rye. 8. Eksperym entalne w artości m odułu w spółczynnika odbicia od piasku bardzo drobnoziarnistego w funkcji częstotliw ości fali akustycznej dla dwóch kątów pa dania A d a n e o tr z y m a n e p r z y fti = 0° j^ d a n e o tr z y m a n e p r z y fit = 25° resu lts, m ig h t h av e been due to too w ide in crem en ts in th e freq u en cy scale. T he v a ria tio n of re fle c tiv ity fo r d iffe re n t angles of incidence of acoustic w aves a t th e bottom is illu stra te d in Figs. 8 and 9 by th e exam ples of tw o angles selected for b oth m acro fau n a an d v e ry fin e sand, i.e. Oj = 0°, 02 — 15°, and i>i = 0°, = 25°, respectively. T he absolute r e flectivities, av erag ed over th e w hole ran g e of freq u en cies fo r th e fo u r angles are: m ac ro fa u n a v e ry fine sand = d2= 0i = = 0° 15° 0° 25° V0 = V0 = V0 = V0 = 0.59 0.80 0.61 0.57 A ccording to th e g e n e ra l th e o re tic a l rela tio n sh ip [2, 9], th e data obtained in th is s tu d y p o in t to th e g ro w th of re fle c tiv ity w ith th e increasing an g le of incidence of th e acoustic w ave. F ro m Figs. 8 a n d 9 it ap p ears th a t th e above rela tio n sh ip is satisfied in th e w hole ran g e of freq u en cies fo r th e m acro fau n a soil and in th e 2... 6.3 kH z frequencies for v e ry fin e sand. Fig. 9. Experim ental absolute values of reflectivity from a sea floor consisting of m acrofauna (molluscs, snails), versus frequency of acoustic w ave, for tw o angles of incidence O • d a ta o b ta in e d f o r fh = 0° d a ta o b ta in e d f o r dt = 15° Rye. 9. Eksperym entalne w artości m odułu w spółczynnika odbicia od powierzchni dna utworzonej z m akrofauny (małże, ślimaki) w funkcji częstości fa li akustycznej dla dwóch kątów padania O d a n e o tr z y m a n e p r z y di = 0° • d a n e o tr z y m a n e p r z y 9« = 15° T he re su lts fo r freq u en cies of 8 and 10 kH z for sand differ from th e rem a in in g findings. To fin d w h e th e r th is deviation is due to a m ore com p lex rela tio n sh ip b etw een re fle c tiv ity and th e angle of incidence, coupled w ith selective absorption in a tw o -la y e r bed, or is caused by e rro rs in m easu rem en ts, is im possible because of th e lim ited n u m b er of e x p e ri m en ta l data. T he m ea su rem e n t of re fle c tiv ity by th e m ethod of stan d in g w aves is one of th e m ost a ccu rate techniques. T he e rro rs caused by m u ltip le reflections from a free su rface and th e sea floor have been estim ated elsew h ere [3]. In ou r e x p e rim e n ts th is ty p e of e rro r does not exceed 5 p e r cent. T he d e te rm in a tio n of th e e x tre m a of stan d in g w aves is also easy, as show n in Fig. 10 a, b, c, d; th e flu c tu atio n s of th e signal received do not exceed 0.1V, w h ile th e differen ce b etw een stan d in g -w av e m ax im a and m inim a reaches 0.4... 0.5 V. Hence, th e accuracy of d e te rm in a tio n of acoustic p ressu re a t th e m axim a arid m inim a of th e field of in te rfe re n c e is basically eq u iv alen t to th e accuracy of voltage rea d -o u t on a v o ltm e te r scale 5. CONCLUSIONS 1. T he fig u res obtained fo r th e sea-floor reflec tiv ity in th e south B altic a re com parable w ith d a ta for o th er w a te r bodies and a re contained w ith in th e lim its given by o th er a u th o rs for sim ilar ty p es of sea floor. 2. In th e case of n o rm al incidence of acoustic w aves th e av erage absolute v alu es of re fle c tiv ity a re 0.55 for fine sand, 0.61 for v e ry fine sand, a n d 0.59 fo r m acrofauna. 3. No d istin ct rela tio n sh ip w as found b etw een th e absolute v a lu e of re fle c tiv ity and th e fre q u e n c y of acoustic w aves in frequencies of 2... 10 kHz. 4. It w as found th a t re fle c tiv ity increases w ith th e increasing inci dence an g le of acoustic w aves on a fla t u n stra tifie d bed, w hich confirm s th e ex istin g th e o ry [2,9], In th e case of a m u ltila y e r bed th is d ep en dence is m uch m ore com plex an d is controlled b y ch a ra c te ristic s of indi v id u al lay e rs a n d acoustic w a v e -le n g th [1,4], 5. In o rd e r to an aly se th e e x p e rim e n tal reflectiv ities fo r vario u s sea floors c o rre c tly not only is a know ledge of th e u p p e r la y e r of th e floor im p o rta n t, b u t also th e in te rn a l s tru c tu re of th e bed, especially if acoustic signals of high in te n sity are em ployed. fe Fig. 10. Exam ples of recorded interference fields, obtained w hen the hydrophone m oved along the z-axis (vertical axis oriented upwards from the bottom). The drawings correspond to various frequencies of acoustic w aves Rye. 10. Przykłady rejestracji pola interferencyjnego otrzymane podczas ruchu h y droforni w zdłuż osi z (tj. osi pionow ej, skierow anej ku swobodnej powierzchni m o rza). Poszczególne rysunki odpowiadają różnym częstotliw ościom fa li akustycznej 5 — O c e a n o lo g ia N r 11 62 O C E A N O L O G IA N r 11 (1979) MAŁGORZATA BRZOZOWSKA BARBARA JANUSZEW SKA Polska A kadem ia Nauk Zakład Oceanologii — Sopot STRA TY EN ER G II A K U ST Y C Z N E J PR ZY ODBICIU OD W YBRANYCH RO D ZA JÓ W DNA PO ŁU D N IO W EG O BA ŁTY K U STRESZCZENIE Przedstawiono w yniki pom iarów w spółczynnika odbicia fal akustycznych od dna Bałtyku dla trzech rodzajów m ateriału osadowego i trzech kątów padania w zakresie częstości 2—10 kHz (w odstępach tercjowych). Dla piasku drobnoziarnistego o średniej średnicy ziaren 0,145 m m w spółczynnik odbicia od dna, uśredniony w ca łym zakresie częstości, w ynosi 0,55, dla piasku o średnicy 0,12 mm—0,61 mm i dla m a teriału złożonego w przeważającej części z m akrofauny (M ytilus edulis, Cardium lamarcki, Macoma balthica) — 0,59. Przytoczone w artości określają m aksym alne straty energii akustycznej otrzymane przy prostopadłym padaniu fali na dno. W zakresie częstości 2— 10 kHz nie zaobser w ow ano w yraźnej zależności częstotliw ościow ej. U zyskane w artości w spółczynnika odbicia zbliżone są do rezultatów otrzym anych przez Grubnika [2] dla Morza K aspij skiego. REFERENCES L IT E R A T U R A 1. B r i e c h o w s k i c h L.M., W olny w sloistych sriedach, Nauka, M oskwa 1973. 2. G r u b n i k N.A., Issledowanije akusticzeskich s w o js tw podwodnogo grunta na wysokich zw u k o w y c h czastotach, A kusticzeskij Zurnał, 6, 1960, 4, s. 446. 3. G o n c z a r i e n k o B.I., Z a c h a r ó w L.N., I w a n o w W.E., K i r s z o w W.A., Ocenka pogriesznosti intierfieriencijonnych m ie tod ow izmierienija koefiicenta otrążenija w słoje żidkosti, A kusticzeskij Zurnał, 22, 1976, 2, s. 214. 4. G o n c z a r i e n k o B.I., Z a c h a r ó w L.N., I w a n o w W.E., K i r s z o w W.A., Czastotn o-u glow yje ch ardktieristiki koefficenta otrażenija zw u k a ot słoistogo dna, A kusticzeskij Zurnał, 22, 1976 3, s. 351, 5. L i b e r m a n n L., Reflection of sound from Coastal sea bottoms, J.A.S.A., 20, 1948, 3, s. 305. 6. M a c k e n z i e K.V., Reflection of sound from coastal bottoms, J.A.S.A., 32, 1960, 2, s. 221. 7. P i e c z k a F., K lasyfik acja współczesnych morskich osadów dennych, Zesz. Nauk. Politechniki Gdańskiej, 1966, nr 93, Bud. Wodne, 9, 55. 8. P i e c z k a F., W skaźniki liczbowe i graficzne stru k tu ry osadów okruchowych oraz reżim u hydrodynam icznego morskiego środow iska sedym entacyjnego, Zesz. Nauk. Politechniki Gdańskiej, 1969, nr 146, Bud. Wodne, 13. 9. S t a s z k i e w i c z A.P., Aku stika moria, Sudostrojenije, Leningrad 1966. 10. Z i t k o w s k i j Ju. Ju., L y s a n o w Ju. P., Otrażenije i rassiejanije zw uka dn om okieana, A kusticzeskij Zurnał, 13, 1967, 1, s. 1.