Sound

Close particles do not explain the whole result: two solids can have different sound speeds because they resist deformation differently. A disturbance spreads as one region exerts forces on the next, so the material’s restoring response matters. Greater density alone does not automatically mean faster sound. In an IGCSE comparison, explain the effective interactions in solids while recognising the question’s stated materials. Use a supplied value for calculations rather than treating the speed of sound in air as a constant for every medium.

The division by two follows from the journey, not from a rule applied to all sound measurements. The pulse travels from the transmitter to the reflecting surface and returns to the receiver. If the receiver or reflector geometry differs, the path must be analysed accordingly. A short pulse makes its departure and return easier to separate. The sound speed can depend on water conditions, so real instruments need suitable assumptions or corrections; a school question usually provides the value to use.

A trace twice as tall is not automatically twice as loud to a listener, because perceived loudness is not a simple linear reading of graph height. At this level, use amplitude to compare the signal under controlled conditions. For pitch, find the period from horizontal spacing and calculate f = 1/T when needed. Distortion or complex waveforms can contain several frequencies, so a simple single-tone diagram is a model. Matching instrument scales are essential before a visual comparison of two traces is valid.

Mark the first detection and the corresponding second detection of the same pulse. The delay is the time to travel from one microphone to the other, not the total time since the source was activated. Position the microphones along the direction of travel, since a sideways separation alone may not give the sound-path difference assumed by the formula. Repeating with several separations can reveal a consistent delay trend. Electronic timing avoids the large human reaction uncertainty associated with very short manually timed sound journeys.

Ultrasound still consists of mechanical disturbances in a material and cannot cross a vacuum. Its short wavelength can help distinguish smaller details in suitable imaging applications, while reflections provide information about boundaries. The word “ultra” refers to its place beyond the audible frequency range; it does not turn the wave into electromagnetic radiation. Human hearing limits vary, especially with age and hearing conditions, so 20 kHz is a conventional boundary rather than a promise that every person hears exactly up to that frequency.

A bell in a jar can illustrate the distinction: as air is removed, less sound is transmitted through the air even while the bell remains visible. Some vibration can still reach the surroundings through solid supports, so a real demonstration may not become perfectly silent. Describe the intended transmission route rather than claiming every remaining sound disproves the model. The explanation also distinguishes a vacuum from merely thin air: reducing the amount of gas changes the transmission conditions, whereas an ideal vacuum contains no particles to carry ordinary sound.