CHAPTER 22. Generation Characteristics of Wave Sounds as a Factor of Beach Amenity

Transcription

1 CHAPTER 22 Generatin Characteristics f Wave Sunds as a Factr f Beach Amenity Hitshi Murakami 1,Sadahik Ith 2,Yshihik Hsi 3, Hide Araki 4 and Tsuyshi Kyabu 5 Abstract This study deals with the cmfrtableness f wave sunds frm physilgical and psychlgical aspects. Firstly, the difference f wave sund generatin mechanisim between plunging and spilling breakers is discussed n basis f the variatin characteristics f sund pressure level and the vlume f air bubbles in water. Secndly, the cmfrtableness f wave sunds is investigated based n the genaratin characteristics f the brain wave and the result f questinnaire surveys f auditry test. Intrductin Many artificially nurished beaches have been created as part f the imprvement wrks related t castal envirnment. The visual,auditry and lfactry factrs are cnsidered imprtant factrs f beach amenities. Recently, much imprtance has been attached t the rles f desirable sunds in envirnment. The term "Sund Scape " named by R.M.Schafer(1977) has been used in much the same way as the term "Land Scape ". Mre attentin must be paid t the sund envirnment f the castal zne. As a matter f fact, the sensry tests n psychlgical respnses t wave sund l.fac.f Eng.,The Univ.f Tkushima, Minami-jsanjima,Tkushima 77,Japan. 2,Fac.f Eng.,Kyt Univ.,kyt 66-1,Japan. 3.Fac.f Eng.,Tttri Univ., Kyamach-minami,Tttri 68,Japan. 4.Fac. f Integrated Arts and Sciences., The Univ. f Tkushima, Minamijsanjima, Tkushima 77,Japan. S.Nikken Cnsultants Inc., Higashi-gtanda,Shinagawa-ku,Tky 141,Japan 275

2 276 COASTAL ENGINEERING 1996 have shwn that the auditry factr such as breaking wave sunds is ne f the mst imprtant factrs affecting the amenities f the beach envirnment just as much as visual and lfactry factrs( Nadaka et al,1991a,b. Murakami ea al, 1991). Authrs have als investigated the characteristics f breaking wave sunds in labratry tests and field bservatins(murakami et al,1993). Hwever, the exact physical mechanism f sund generatin due t wave breaking remains under debate. The purpse f this study is t investigate fundamental characteristics f the wave sund generatin and t examine the difference f the sund genaratin mechanism in tw types f wave breaking; the plunging and spilling breakers. Furthermre, the relatinship between wave sund and its cmfrtableness is discussed frm physilgicaland psychlgical aspects. Characteristics f Wave Sund Generatin f Air Bubbles in Wave Breaking 1) Experimental Methds A wave tank with a pendulum-type wave generatr at ne end, which was 3m lng, lm wide and.9m deep, was used as shwn in Fig.l. The bttm slpes 9 were 1/1 and 1/15, and 45 kinds f waves which had different surf similarity parameters were induced. Wave sunds were recrded by a sund level meter at the upper pint 3cm frm still water surface. The vide pictures f breaking beach slpe tan wave generatr v *Bl> 35 l Cm wave perid T (sec) 3 cm i sund level meter ^^1: m Fig. 1 Experimental Apparatus Table 1 Experimental Cnditins equivalent deepwater waveheight H ' (cm) surf similarity parameter 1/ / waves were taken simaultaneusly at the distance f 1.5m frm the side wall f wave tank t bserve air bubbles in water induced by wave breaking. The water depth h was kept at a cnstant f 35 cm. The wave perid T was changed frm.8 t 2. sec.

3 WAVE SOUNDS 277 Experimental cnditins f wave breaking were shwn in Table 1. The surf similarity parameter as a breaker type index is defined as fllws(battjes, 1974): ^= tan //-(H '/L) Spilling breaker,if <.5. Plunging breaker,if.5< "-3- Surging breaker, if 4 >3.3. In which, H '; equivalent deepwater wave height, L ; deepwater wave length. in this study are in the range f This means the experiments include bth spilling and plunging breakers. 2) Characteristics f Generatin f Air Bubbles and Wave Sunds Fig.2 shws an example f the wave breaking prcess and air bubble generatin during a wave perid fr the plunging and spilling breaker, respectively. Nndimensinal phase t/t= means the nset f wave breaking. Generally, the behavir f wave breaking is divided int the fllwing fur phases; breaking, plunging, run-up and backwash. Fr the plunging breaker, the wave begins t break and flds ver air at thebreaking phase. At the plunging phase, a large quantity f air bubbles are generated. (1) plunging breaker (2) spi)ling breaker (H '/L =.1, tan 6 =1/15) (1^/1^=.12, tan 6 =1/15) Fig.2 Breaking Prcess and Air Bubble Generatin

4 278 COASTAL ENGINEERING 1996 Subsequently air entrainment ccurs and the air bubble clud reaches the sea bttm. After that, the air bubble clud spreads ut gradually t create the patch f fam at the run-up and backwash phases. On the ther hand, fr the spilling breaker, the air bubble clud induced by the frmer wave breaking remains withut appearance ver a wide area, even when a new wave begins t break at the wave crest in the breaking phase. The air bubble clud near the wave crest spreads radially in every directin at the breaking and plunging phases. Finally, it cmbines with the frmer air bubble clud. Frm this experiment, the difference f generatin and disappearance mechanism f air bubbles fr the tw types f wave breaking was revealed t <r* CD.i 1 "1" "1.8 L tan 61 i/isj T(sec) 1.6 O ^ - i J I < 2 H b (cm) i (1 ) plunging breaker ->.8 ra CO Fig.3 Relatinship between B av and Hj, H b (cm) ( 2 ) spilling breaker O A Fig.4 Relatinship between P.8 B (m 3 ).1 andb'

5 WAVE SOUNDS 279 sme degree. The vlume f air bubbles, hwever, depends n the breaking wave height. In additin, the wave sund must be related t the vlume f air bubbles. Fig.3 shws the relatinship between the average vlume f air bubbles per ne wave Bav and a breaking wave height Hb in the case f the bttm slpe 1/15. It is difficult t measure directly a vlume f air bubbles. The vlume f air bubbles B at an arbitrary phase f wave is cnveniently defined s that B can be btained by multiplying a bubble clud crss sectin area prjected n the glass wall f wave tank with a wave tank width. An average vlume f air bubbles Bav was btained frm the average value at the 5 different wave phases in a ne wave perid. It can be seen that Bav increases with an increase f Hb. Hwever, it shws a partially ppsite tendency in the Q OU r ~^nem J TJ Jbreaking %, run-up m,, spilling breaker's lng perid wave. The relatinship between an average sund pressure level Pav and Bav is shwn in Fig.4. Pav increases with an increase f Bav regardless f the breaking type. The rate f increase f Pav, hwever, is mre distinct with a steeper bttm slpe. The sund pressure level is als clsely related t the vlume f air bubbles. Furthermre, let us cnsider the crrelatin between.$t t/t (1) plunging breaker(h 7L =.1,tan e =1/15) 65 6Q =.21 P(dB) Q- -B(m 3 ).8' L.6.8 t/t (2) spilling breaker(h 7L =O.12,tan in =1/15) - Fig.5 Tempral fluctuatin f P and B

6 28 COASTAL ENGINEERING 1996 the variatin f sund pressure level P and the vlume f air bubbles B in time in rder t clarify the sund generatin mechanism. Fig.5 shws the tempral fluctuatins f P and B fr tw types f wave breakers. Let us first cnsider the variatin characteristics f P fr the plunging breaker. P takes the maximum value in the middle f breaking and plunging phases. P decreases rapidly at the run-up and backwash phases. On the ther hand, B takes the maximum value in the middle f run-up and backwash phases. It is significant that the maximum sund is nt genarated when the vlume f air bubbles is at a maximum. It means a dminant sund is generated nly in the shrt time when a water wave plunges n t the water surface, hwever, air bubbles are nly slightly generated. Subsequently, even if the vlume f air bubbles increases gradually after breaking, these bubbles are nt an imprtant factr in the generatin f dminant sunds. Secndly, let us cnsider the characteristics f the spilling breaker. P des nt fluctuate s nticeably, thugh it increases slightly at the breaking phase in cmparisn with ther phases. The fluctuatin f B is smaller than that f P. It can be recgnized that the mechanism f wave sund generatin fr the spilling breaker is attributed t the strng turbulence and bursting f air bubbles in the vicinity f the water surface. Cmfrtableness f Wave Sunds due t Auditry Tests 1) Experimental Methds In rder t investigate the cmfrtableness f wave sunds, the brain wave test and questinnaire surveys were carried ut simultaniusly with 8 students f the University f Tkushima being the subjects. Fig.6 shws an utline f brain wave measurement. First f all, a subject listened t varius kinds f recrding tapes f wave sunds n the headphnes. The brain \headphnes waves at the central zne Cz and ccipital ne Oz were measured and recrded in the FM data recrder whilst the subject's eyes / / bran waves meter, J V spectrum ( y 7_ analyzer / / 7 FM data rerder / / tape recrder were pen fr 3-6 min. The brain Fig.6 Outline f Brain Wave Measurement /

7 WAVE SOUNDS 281 sund factr strength wave perid sune tne Table 2 Experimental Cnditins f Auditry Tests wave sund kind field bservatin sunds artifical sund field bservatin sunds cnditin Pav(dB) 54,62,74 T(sec) 4,6,8,1,12 r 1/2, r 1 subject students male4,femalel male7 male8 wave spectrum was btained by a spectrum analyzer. In additin, the questinnaire surveys were cnducted t find the mst desirable sund by the same subjects. The experimental cnditins used in the brain wave tests and questinnaire surveys are shwn in Table 2. Field bservatin wave sunds and artificial sunds made frm white nise by FM radi were used as wave sunds in the tests. The effects f sund strength, sund wave perid and sund tne as physical sund factrs n the cmfrtableness f wave sunds were examined. The sund tne spectra at the backwash phase used here is shwn in Fig.7. One is having minus 1/2 pwer f frequency f at the higher frequency regin. The ther is having minus 1st pwer f frequency f. w 1 e g 1 5 r 1 4, Q2,,, CD «1,<HZ) (Z) sw2 Fig.7 Sund Tne Spectra at Backwash Phase,<HZ)

8 282 COASTAL ENGINEERING ) Effects f Wave Sund Strength n Cmfrtableness Fig.8 shws the relatinship between the average pwer f a -wave(8-13hz) Pw and an average sund pressure level SPav ver 6 min. Sign N in the transverse axis means nn-sund. Pw decreases slightly with an increase f SPav at the central zne Cz. On the ther hand, Pw at the ccipital zne Oz has the maximum value fr the 62 db sund. Especially, it shuld be nted that the pwer f a - wave fr the 74 db sund is smaller than that fr the tw thers. Fig.9 shws the result f a questinnaire survey n the relatinship between the O Q _ I -b 2. X 1-5 CL O N SPJdB) (1) C (2) O z Fig.8 Relatinship between P w and SP av N SP (db) cmfrtableness and the average sund pressure level Pav. The vertical axis indicates the number f persns, wh gave the mst suggestive respnse frm the three sunds. Frm this test, the 62dB sund is the mst cmfrtable f the three which crrespnds t as the tendency f the brain wave tests at Oz. N ne felt any cmfrtableness frm n the 74dB sund t the subjects. The 74dB sund seemed t be felt as if it were strmy sea. CO c CO & ) Q. - -L O X -a SP (db) Fig.9 Effect f SP n Cmfrtableness

9 WAVE SOUNDS 283 3) Effects f Sund Wave Perid n Cmfrtableness The authrs have already pinted ut frm the questinnaire survey f the 67 students that the subjects feel the cmfrtableness fr 6-8 sec f the sund wave perid, but d nt feel the cmfrtableness under 6 sec f the wave perid. It has been pinted ut this desirable value is nearly equivalent t the perid f ur breathing(murakami etal,1992). Fig. 1 shws the effect f the sund wave perid n the brain wave. The pwer f a -wave Pw has the muxmum value in 8-1 sec f wave perid in bth figures. In the case f the wave perid being under 6 sec, Pw has less than the pwer at nn-sund T(sec) (1 ) C Fig. 1 Relatinship between P w and T T(sec) (2) O Fig. 11 shws the results f the pints btained frm the questinnaire survey f the five grade estimatins with a scre f 5 t 1 pints fr each wave perid. The tendency in this figure is similar t the result f brain wave tests, thugh the tendency f the tw in 12 sec f the wave perid is different. *-* c Q _ I 1_ L_ I I T(sec) Fig. 11 Effect f T n Cmfrtableness

10 284 COASTAL ENGINEERING N sw1 sw2 ' " ' N sw1 SW2 (1 ) C (2) O z z Fig. 12 Effect f Pwer Spectrum Shape n Cmfrtableness 4) Effects f Sund Tne n Cmfrtableness The subjects listened t wave sunds which had tw kinds f sund tne spectra as shwn in Fig.7. Fig. 12 shws the effects f wave sund tne n the a-wave pwer. As a result, it can be seen that the pwer n sw2 is nly slightly bigger than that n swl. It is widely nted the a -wave pwer increases in the case f sund tne with the minus 1st pwer Fig. 13 Cmfrtableness f Sund Tne f frequency at a higher frequency regin. Fig. 13 shws the cmparisn f the cmfrtableness f tw kinds f sund tne n the basis f the questinnaire survey. The sund tnes with sw2 and swl are in the rati 6:2. This is the same tendency as the result f the brain wave test. Cnclusin The generatin mechanism f wave sund fr tw types f wave breakers was discussed n the basis f measurement f sund pressure level and vlume f air bubbles. The effects f sund factrs n cmfrtableness were examined frm the physilgical and psychlgical aspects. As a result, the wave sunds shuld be emphasised as being ne f the mst imprtant factrs f beach amenities. The

11 WAVE SOUNDS 285 resuls are summarized as fllws. 1) The difference f the sund generatin mechanism between the plunging and spilling breakers was clarified. It was cnfirmed the behavir f air bubbles generated by wave breaking was clsely cnnected with the characteristics f wave sunds. 2) It was prved that the cmfrtableness f wave sunds was affected by the factrs f sund pressure level, sund wave perid and sund tne and s n thrugh the pwer f (X -wave in the brain wave test and questinnaire surveys. Acknwledgement The authrs wuld like t acknwledge the assistance f Mr Kazut Iida and Mituhir Sat wh cntributed their effrts fr the accmplishment f many f the experiments. Reference 1) Battjes,J.A.: Surf Similarity,Prc. 14th Cnf. n Castal Eng.,ASCE,pp , ) Murakami,H. Hsi,Y. andy. Kzuki: Wave Sunds as a Factr f Castal Envirnment and Their Generatin Characteristics, Envirnmental Sanitary Engineering Research, Vl.6,N.3, pp , 1992(in Japanese). 3) MurakamiJH. Ith.S. Hsi,Y. Ogawa,Y. and T.Kyabu, Cnsideratin n Generatin Characteristics f Breaking Wave Sunds, Prc. f Castal Engineering, Vl.4, pp ,1993(in Japanese). 4) Nadaka, K. and T.Tkumi: Sunds f Breaking Waves as a Factr Cnstituting Amenity in Beach Space, Castal Engineering in Japan, Vl. 34, N. 1,pp , ) Nadaka,K. and K. Tamashima: Auditry Characteristics f Wave Sunds n Beaches, Castal Engineering in Japan, Vl.34,N. 1, pp , ) Schafer,R.M.: The Tuning f the Wrld, Arcana Editins, Ontari,1977.