The range in frequencies of musical sounds is approximately 20-20,000 cycles per second (cy/sec). Some people can hear higher frequencies khan others. Longitudinal waves whose frequencies are higher than those within the audible range are called ultrasonic frequencies. Ultrasonic frequencies are used in sonar for such purposes as submarine detection and depth finding. Ultrasonic frequencies are also being tried for sterilizing food since these frequencies kill some bacteria. Sound waves of all frequencies in the audible range travel at the same speed in the same medium. In the audible range, the higher the frequency of the sound the higher is the pitch. The term supersonic refers to speed greater than sound. An airplane traveling at supersonic speed is moving at a speed greater than the speed of sound in air at that temperature. Mach 1 means a speed equal to that of sound. Mach 2 means a speed equal to twice that of sound, etc.
Musical sounds have three basic characteristics: pitch, loudness, and quality or timbre. As was indicated above, pitch is determined largely by the frequency of the wave reaching the ear. The higher the frequency the higher is the pitch. Loudness depends on the amplitude of the wave reaching the ear. For a given frequency, the greater the amplitude of the wave the louder the sound. To discuss quality of sound we need to clarify the concept of overtones. Sounds are produced by vibrating objects. If these objects are given a gentle push, they usually vibrate at one definite frequency producing a pure tone. This is the way a tuning fork is usually used. When objects vibrate freely after a force is momentarily applied, they are said to produce their natural frequency. Some objects, like strings and air columns, can vibrate naturally at more than one frequency at a time. The lowest frequency which an object can produce when vibrating freely is known as the object's fundamental frequency. Other frequencies that the object can produce are known as its overtones. The quality of a sound depends on the number and relative amplitude of the overtones present in the wave reaching the ear.
A soprano would probably have a frequency of ______.
A.200 cy/see
B.5,000 cy/sec
C.10,000 cy/sec
D.20,000 cy/sec
第1题
某公司为便于员工在家里访问公司的一些数据,允许员工通过Internet访问公司的 FTP服务器,如图18-1所示。为了能够方便地实现这一目标,决定在客户机与FTP服务器之间采用(1)协议,在传输层对数据进行加密。该协议是一个保证计算机通信安全的协议,客户机与服务器之间协商相互认可的密码发生在(2)。
A.SSL
B.Ipsec
C.PPTP
D.TCP
第2题
某公司为便于员工在家里访问公司的一些数据,允许员工通过Internet访问公司的FTP服务器,如图4-7所示。为了能够方便地实现这一目标,决定在客户机与FTP服务器之间采用(51)协议,在传输层对数据进行加密。该协议是一个保证计算机通信安全的协议,客户机与服务器之间协商相互认可的密码发生在(52)。
A.SSL
B.IPSec
C.PPTP
D.TCP
第3题
A.L2TP
B.PPTP
C.IPSEC
D.SSL
第4题
A.接通阶段
B.密码交换阶段
C.会谈密码阶段
D.客户认证阶段
第5题
A.SSL
B.IPsec
C.PPTP
D.L2TP
第6题
A.L2TP
B.PPTP
C.IPSEC
D.SSL
第7题
A.SSL
B.IPsec
C.PPTP
D.TCP
第8题
A.SSL
B.IP sec
C.PPTP
D.TCP
第9题
A.L2TP
B.PPTP
C.IPSec
D.SSL
第10题
A.L2TP
B.PPTP
C.IPSec
D.SSL
为了保护您的账号安全,请在“上学吧”公众号进行验证,点击“官网服务”-“账号验证”后输入验证码“”完成验证,验证成功后方可继续查看答案!