IB Science: Sound – Key Exam Points | IB 科学：声 考点精讲

📚 IB Science: Sound – Key Exam Points | IB 科学：声 考点精讲

Sound is a mechanical longitudinal wave that propagates through a medium, causing oscillations of particles along the direction of energy transfer. In IB Physics, sound is examined under topics such as wave properties, standing waves, Doppler effect, and resonance. Mastery of these concepts is essential for answering both conceptual and calculation-based questions.

声音是一种通过介质传播的机械纵波，引起粒子沿能量传递方向振动。在 IB 物理中，声音涉及波的特性、驻波、多普勒效应和共振等考点。掌握这些概念对于回答概念题和计算题至关重要。

1. Nature of Sound Waves | 声波的本质

Sound waves are mechanical longitudinal waves, meaning they require a medium (solid, liquid, or gas) to travel and cause particles to oscillate parallel to the direction of wave propagation.

声波是机械纵波，意味着它们需要介质（固体、液体或气体）才能传播，并使粒子沿着波传播的方向振动。

The regions of high pressure and density are called compressions, while the regions of low pressure and density are rarefactions. These alternating regions propagate through the medium.

高压高密度的区域称为压缩区，低压低密度的区域称为稀疏区。这些交替的区域在介质中传播。

Unlike electromagnetic waves, sound cannot travel through a vacuum. This is a common exam question about why astronauts cannot hear each other in space without radio devices.

与电磁波不同，声音不能在真空中传播。这是一个常见考点，例如为什么宇航员在太空中没有无线电设备就无法听到对方讲话。

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2. Wave Properties | 波的性质

The key parameters of a sound wave include frequency (f), wavelength (λ), amplitude (A), period (T), and speed (v). The wave equation v = f λ relates these quantities.

声波的关键参数包括频率（f）、波长（λ）、振幅（A）、周期（T）和波速（v）。波动方程 v = f λ 把这些量联系起来。

The audible frequency range for humans is approximately 20 Hz to 20 kHz. Frequencies below 20 Hz are infrasound, and above 20 kHz are ultrasound. Ultrasound has applications in medical imaging and sonar.

人耳可听见的频率范围大约是20 Hz到20 kHz。低于20 Hz的称为次声波，高于20 kHz的称为超声波。超声波在医学成像和声呐中有应用。

Amplitude is related to the loudness of sound; larger amplitude corresponds to a louder sound. Frequency determines pitch; a higher frequency gives a higher-pitched sound.

振幅与声音的响度有关，振幅越大声音越响。频率决定音调；频率越高，音调越高。

Note that the speed of sound in a given medium is constant for a given temperature and is independent of frequency and amplitude. Therefore, changing frequency will change wavelength, not speed.

注意，在给定介质和温度下，声速是恒定的，与频率和振幅无关。因此，改变频率会改变波长，而不会改变速度。

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3. Speed of Sound | 声速

In air at 20°C, the speed of sound is approximately 343 m s⁻¹. It increases with temperature and varies with the medium: sound travels faster in solids than in liquids, and faster in liquids than in gases.

在20°C的空气中，声速约为343 m s⁻¹。声速随温度升高而增加，并且在不同介质中不同：声音在固体中传播最快，液体次之，气体中最慢。

The formula v = √(B/ρ) gives the speed in a fluid, where B is the bulk modulus and ρ the density. In a solid rod, v = √(E/ρ) where E is Young’s modulus. These are not usually required in standard-level IB, but may appear in higher-level or data-analysis questions.

流体中的声速公式为 v = √(B/ρ)，其中 B 是体积模量，ρ 是密度。在固体杆中，v = √(E/ρ)，E 是杨氏模量。在标准级别 IB 中通常不要求，但可能在高级别或数据分析题中出现。

When a sound wave passes from one medium to another, its frequency remains constant, but its speed and wavelength change. This is analogous to light refraction.

当声波从一种介质进入另一种介质时，频率保持不变，但速度和波长会改变。这类似于光的折射。

Echoes and sonar make use of the constant speed of sound. The time delay between emission and reception can be used to calculate distance: d = v × t / 2 (for a round trip).

回声和声呐利用声速恒定的特性。发射和接收之间的时间延迟可用来计算距离：d = v × t / 2（往返行程）。

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4. Intensity and Decibels | 声强与分贝

Sound intensity (I) is the power per unit area carried by a wave, measured in W m⁻². It is proportional to the square of the amplitude (I ∝ A²).

声强（I）是波携带的单位面积功率，单位是 W m⁻²。它与振幅的平方成正比（I ∝ A²）。

The decibel scale is logarithmic and compares sound intensity to a reference intensity I₀ = 1.0 × 10⁻¹² W m⁻². The sound level β in decibels is given by β = 10 log₁₀(I / I₀).

分贝标度是对数的，将声强与参考声强 I₀ = 1.0 × 10⁻¹² W m⁻² 进行比较。声强级 β（分贝）由 β = 10 log₁₀(I / I₀) 给出。

A doubling of sound intensity corresponds to an increase of about 3 dB. A tenfold increase in intensity adds 10 dB. Thus, a 20 dB increase means the intensity is 100 times greater.

声强加倍对应声强级增加约3 dB。声强变为10倍则增加10 dB。因此，声强级增加20 dB意味着声强是原来的100倍。

Questions often require converting between intensity ratios and decibel differences. Remember that perceived loudness roughly doubles with every 10 dB increase.

考题常要求在声强比值和分贝差值之间转换。记住，人感觉到的响度大约每增加10 dB加倍。

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5. Doppler Effect | 多普勒效应

The Doppler effect is the change in observed frequency when a source of sound moves relative to an observer. It applies to all waves, including light.

多普勒效应是当声源相对于观察者运动时，观测到的频率发生的变化。它适用于所有波，包括光。

The observed frequency f’ is given by

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