The weakest sound that an average human can hear (reference threshold of hearing) is approximately 0 dB at 1 kHz in a quiet environment.

Introduction / Context:
Decibel (dB) is a logarithmic unit used to express sound pressure level (SPL). By convention, the reference threshold of hearing for an average young adult in a very quiet environment at 1 kHz is set to 0 dB SPL, not zero sound but a standardized reference level.

Given Data / Assumptions:

• Quiet environment, normal hearing, 1 kHz reference frequency.
• We are interested in the conventional threshold used in acoustics.

Concept / Approach:
dB SPL = 20 * log10(p/p0), where p0 is the reference pressure 20 µPa. A level of 0 dB SPL means p = p0, i.e., the threshold reference. Real thresholds vary slightly with frequency and individuals but 0 dB at 1 kHz is the standard baseline.

Step-by-Step Solution:

Verification / Alternative check:
Audiometric curves (equal-loudness contours) show minimal threshold near 2–5 kHz with reference near 0 dB SPL at 1 kHz; testing rooms aim for very low background noise to approach this threshold.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming 0 dB means no sound; forgetting that dB is logarithmic and relative to a reference pressure.

Final Answer:
0 dB