Chapter 14 SOUND
Chapter 14 SOUND
Sound waves(声波) 波的几何描述: Wavefronts oit sourd Ray(波线) Wavefronts (波振面) Spherical waves Planar waves (球面波) 平面波)
Sound waves（声波） Planar waves （平面波） Wavefronts Ray（波线） S Wavefronts （波振面） Spherical waves （球面波） Point source 波的几何描述：
The speed of sound Sound waves are longitudinal mechanical waves that can travel through solids, liquids, or gases. The speed v of a sound wave in a medium( having bulk modulus b(容变弹性模量)and density p)is: B 纵波在媒质中的传播速度) △ Where: B p (Ap is change of pressure) △/
Sound waves are longitudinal mechanical waves that can travel through solids, liquids, or gases. The speed v of a sound wave in a medium (having bulk modulus B（容变弹性模量） and density ) is: B v = V V p B / Where: = − The speed of sound (p is change of pressure) （纵波在媒质中的传播速度）
interference S and s, are point sources, they are in phase, and same wavelength. when they travel P through point P, the path length difference: △L To relate phase difference to path length difference AL, we recall that a phase difference of 2nt rad corresponds to one wavelength. So: △L △L 2丌 2 h
interference S1 and s2 are point sources, they are in phase, and same wavelength. when they travel through point P, the path length difference: L= L2 -L1 s P 1 s2 L2 L1 To relate phase difference to path length difference L, we recall that a phase difference of 2 rad corresponds to one wavelength. So: L = 2 2 L =
△L (1)Whenφ=±2mπ,(m=0,1,2… p==2丌 we called fully constructive interference(相长王 YBE). The two waves are in phase at p △L=±m (2)Whenφ=±(2m+1)π,(m=0,1,2.) we called fully destructive interference(相消 王涉). The two waves are out of phase at P. △L=±(2m+1) Of course, two waves could produce intermediate interference when AL is other value
(1) When =±2m, (m=0, 1, 2….) we called fully constructive interference（相长干 涉）. The two waves are in phase at p. L = m (2) When =±(2m+1), (m=0, 1, 2….) we called fully destructive interference（相消 干涉）. The two waves are out of phase at P. 2 (2 1) L = m + Of course, two waves could produce intermediate interference when L is other value. 2 L =
18-5 Intensity and sound level(声强和声强级) (1)Intensity and variation of intensity with distance p dE g y AA·t2 Density of medium For spherical waves: /=.p 4r For planner waves: /=a constant 声强:声波的能流密度 能够引起人们听觉的声强范围: 015M5→丁N
18-5 Intensity and sound level（声强和声强级） Density of medium For spherical waves: 2 4 r P I s = (1) Intensity and variation of intensity with distance: For planner waves: I =a constant 2 2 2 1 m avg avg v y A dt dE A P I = = = 声强：声波的能流密度. 12 2 2 10 W/m → 1 W/m − 能够引起人们听觉的声强范围：
(2) TThe decibel scale(声强级): Consider l=101W/m2(人能听到的最小声音) is a standard reference intensity, because it is near the lower limit of the human range of hearing, the sound level(声强级) defined as B=10g The unit of sound level is decibel(dB分贝). ForI=l02β=0 声强级:人们规定声强0=10Wm2(即相 当于频率为1000Hz的声波能引起听觉的最弱的声强) 为测定声强的标准如某声波的声强为/,则比值/ 的对数,叫做相应于/的声强级
(2) The decibel scale（声强级）: Consider I0=10-12W/m2 （人能听到的最小声音）is a standard reference intensity, because it is near the lower limit of the human range of hearing, the sound level（声强级）is defined as: 0 10lg I I = For I = I0 , = 0 The unit of sound level is decibel (dB 分贝). 声强级：人们规定声强 （即相 当于频率为 1000 Hz 的声波能引起听觉的最弱的声强） 为测定声强的标准. 如某声波的声强为 I ， 则比值 的对数，叫做相应于 I 的声强级 . 12 2 0 10 W m − − I = 0 I I
几种声音近似的声强、声强级和响度 声引炮铆 源 声强(Wm2)声强级b 引起痛觉的声音 120 钉 声机 120 102 100震耳 交通繁忙的街道 103 70响 通常谈话 10-6 60正常 耳 语 10-10 20轻 树叶沙沙声 1041 10极轻 引起听觉的最低声音10 有的地方规定户外噪声不得超过100分贝
引起痛觉的声音 炮 声 铆 钉 机 交通繁忙的街道 通 常 谈 话 耳 语 引起听觉的最低声音 树 叶 沙 沙声 声 源 声强（W m-2） 1 1 10-2 10-5 10-6 10-10 10-11 10-12 声强级db 震耳 响 正常 轻 极轻 120 100 70 60 20 10 0 120 有的地方规定户外噪声不得超过100分贝 几种声音近似的声强、声强级和响度
人耳的平均听觉范围( the ear' s response) Iw/m2↑Ldb 痛阈 threshold 100120 02100 10 80 10 60 语言区域 10840 10 20 闻阈 threshold ofhearing 10 0 H 20 1005001000500010000 人耳听觉的范围为20~2000Hz
人耳的平均听觉范围（the ear’s response） 人耳听觉的范围为20~2000Hz. 20 100 500 1000 5000 10000 0 20 40 60 80 100 120 10-2 10-4 10-6 100 10-8 10-10 10-12 I/W/m2 L/db HZ 痛阈threshold 闻阈threshold of hearing 语言区域