示波器的使用及声速的测量

保持刚才的接线，水平显示选择 保持刚才的接线，水平显示选择X-Y，当改变 ， 换能器之间的距离时…… 换能器之间的距离时
Experimental equipments
• Ultrasonic Velocimeter(声速测量仪) 声速测量仪) 声速测量仪 • Function generator (函数发生器) (函数发生器 函数发生器) • Oscilloscope (示波器) (示波器 示波器)
The answer is triggering. After drawing one curve (sinusoidal, triangular, square…) on the screen the electron beam returns back to its starting point, but it does not start drawing another curve, before the voltage reaches some preset value – trigger level.
v = fλ
Purpose
• 学会用不同的方法测定空气中的声速。 学会用不同的方法测定空气中的声速。 • 掌握数字式函数发生器、示波器的使用方 掌握数字式函数发生器、 法。 • 学会用逐差法处理测量结果，并对结果的 学会用逐差法处理测量结果， 不确定度进行分析。 不确定度进行分析。
Experimental methods
2、利用共振驻波法测量声速
将换能器的接收端接入示波器的CH1通道，当改 通道， 将换能器的接收端接入示波器的 通道 变换能器之间的距离时…… 变换能器之间的距离时
Phase method
(1) traveling wave method
沿传播方向上的任何两点，其振动状态相同（ 沿传播方向上的任何两点，其振动状态相同（相 位差为2 的整数倍） 位差为2π的整数倍）时，两点间的距离应等于波 的整数倍， 长λ的整数倍，即:
trigger level
Lissajous Figures
Vertical intersection
fy
nx 1 = = f x ny 2
垂 直 线
Horizontal intersection
李萨如图线与频率比
水平线
Function generator
空气中声速的测量
Measurement of Sound Velocity in Air
• The most familiar acoustic phenomenon is that associated with of sound. For the average young person, a vibrational disturbance is interpreted as sound if its frequency lies in the range of about 20 to 20000Hz. However in a broader sense acoustic also includes the ultrasonic frequencies above 20000Hz and the infrasonic frequencies below 20Hz.
• The measurement of speed of sound is of interesting for understanding the main property of sound. One can easily obtain the velocity of sound through the following expression:
Resonance frequency
• 当一个振动系统受到另一个系统周期性的 激励时， 激励时，若激励系统的激励频率与振动系 统的固有频率相同时， 统的固有频率相同时，振动系统将获得最 大的激励能量，此现象称为共振（谐振）。 大的激励能量，此现象称为共振（谐振）。 • 实验中使用的超声压电陶瓷换能器具有固 有谐振频率， 有谐振频率，当换能器系统的工作频率等 于谐振频率时，换能器处于谐振状态， 于谐振频率时，换能器处于谐振状态，发 射器发出的超声波功率最大， 射器发出的超声波功率最大，是最佳工作 状态。 状态。
l = nλ
(n=positive integer)
3、利用行波法测量声速
保持刚才的接线，同时将发射波接入CH2, 选择触 保持刚才的接线，同时将发射波接入 发与发射波同步，当改变换能器之间的距离时…… 发与发射波同步，当改变换能器之间的距离时
(2) Circular trace method
• Incidence signal • Reflected signal • Compositional signal
x = A1 cos(ωt + ϕ1 )
y = A2 cos(ωt + ϕ 2 )
x 2 y 2 2 xy ( ) +( ) − cos(ϕ 2 − ϕ 1 ) = sin 2 (ϕ 2 − ϕ 1 ) A1 A2 A1 A2
φ2 −φ1 = 0
π
4
π
2
3π 4
π
5π 4
3π 2
7π 4
2π
4、利用李萨如图法测量声速
• • • • Resonance frequency Resonance Method (amplitude method) Phase method Lissajous Figures method
Ultrasonic velocimeter
Piezoelectric -ceramic transducer Digital ruler
The Lab' center of physics
Introduction
• Acoustics may be defined as the generation, transmission, and reception of energy in the form of vibrational waves in the matter. As the atoms or molecules of a fluid or solid are displaced from their normal configuration, an internal elastic restoring force arises. Examples including the tensile (张力 张力) 张力 force produced when a spring is stretched. It is this elastic restoring force, coupled with inertia (惰性 of 惰性) 惰性 the system, that enables matter to participate (参与 参与) 参与 in oscillatory vibrations and thereby generate and transmit acoustic waves.
Experimental contents and procedure
• 用示波器观察接收器输出的电信号，并调 用示波器观察接收器输出的电信号， 节出压电陶瓷的谐振频率 • 振幅法测波长求声速 • 相位法测波长求声速 • （1）行波比较法 • （2）李萨如图法
1、谐振频率的调整
改变信Байду номын сангаас发生器的频率时，看到…… 改变信号发生器的频率时，看到
Standing wave
| cos kx |= 1
x
kx =
2π
λ
x = nπ
n = 0,±1,±2,...
当两个换能器之间的距离, 当两个换能器之间的距离,等于半波长的整数倍时 将发生共振，产生共振驻波现象，波幅达到极大。 将发生共振，产生共振驻波现象，波幅达到极大。
Fluorescent screen Fluorescent screen
Circuit
display principle of Oscilloscope
Vx
t0
0
t1
t2
t
t0
t1
Vy
0
t
sinusoidal wave
saw tooth wave
• Even rotation of the drum – even motion of the electron beam along the x-direction. • Motion of the pen in y-direction – deviation of the beam in y-direction proportional to the measured voltage. • A question: The electron beam moving along xdirection reaches the end of the screen at some point and has to jump back and start writing again from the beginning of the screen. How come that you normally only see one curve on the screen at any time? By the way, how fast does the beam go from left to right?
示波器的使用 Application of Oscilloscope
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vertical deflector oscilloscope tube electron gun electron beam