In sound waves, which physical quantity primarily determines the pitch, that is, how high or low a sound is perceived by the human ear?

Introduction / Context:
This question checks your understanding of the basic characteristics of sound waves and how human hearing interprets them. In physics, a sound wave can be described by its frequency, amplitude and wavelength. In everyday life we describe sounds as high pitched or low pitched. To answer correctly, you must connect the subjective idea of pitch with the correct physical parameter of the wave.

Given Data / Assumptions:

• We are dealing with ordinary audible sound waves in air.
• Pitch refers to how high or low a sound seems to the listener.
• Amplitude, frequency, loudness and wavelength are given as options.
• The ear is assumed to be normal, with a typical audible range of about 20 Hz to 20000 Hz.

Concept / Approach:
Pitch is the sensation that allows us to order sounds on a scale from low to high. In physics, pitch is mainly linked to the frequency of the sound wave. A wave with higher frequency produces a higher pitched sound (like a whistle), while a wave with lower frequency produces a lower pitched sound (like a drum). Amplitude is related to the intensity of the sound and is perceived as loudness, not pitch. Wavelength is inversely related to frequency, but pitch is defined more directly in terms of frequency. Therefore, frequency is the correct choice.

Step-by-Step Solution:
Step 1: Recall that frequency is the number of complete vibrations or cycles per second and is measured in hertz (Hz). Step 2: Understand that when the frequency increases, the sound is perceived as higher in pitch, and when frequency decreases, the sound is perceived as lower. Step 3: Recognise that amplitude measures the maximum displacement of particles from their rest positions and is linked to sound intensity and loudness. Step 4: Note that loudness is a psychological response mainly dependent on amplitude, not on frequency, although the ear sensitivity also plays some role. Step 5: Observe that wavelength and frequency are related by the formula v = f * λ for a given medium, but our ears respond directly to frequency as pitch. Step 6: Conclude that frequency is the physical quantity that primarily determines pitch.

Verification / Alternative check:
Think of a musical instrument such as a guitar. When you tighten a string, its frequency of vibration increases and the note becomes higher pitched. The loudness does not necessarily change unless you pluck harder, which affects amplitude. Different musical notes in a scale are distinguished mainly by their frequencies. This everyday experience supports the idea that pitch depends on frequency, not directly on amplitude or wavelength.

Why Other Options Are Wrong:
Amplitude is associated with sound intensity and loudness, so a larger amplitude produces a louder sound, not necessarily a higher pitched one. Loudness is a perception related mainly to amplitude; it describes how strong a sound feels, not how high or low it sounds. Wavelength is linked to frequency through the speed of sound, but pitch is defined more directly in terms of frequency; different media can change wavelength without changing pitch if frequency stays constant.

Common Pitfalls:
Students sometimes confuse loudness and pitch because both are subjective qualities of sound. A louder sound can feel more intense but is not necessarily higher in pitch. Another mistake is to choose wavelength because of its mathematical relation to frequency, without remembering that the ear responds to how many cycles reach it per second. Focusing on the definitions helps avoid these errors: pitch corresponds to frequency, and loudness corresponds primarily to amplitude.

Final Answer:
The physical quantity that primarily determines the pitch of a sound is its Frequency.