Question 1

What is the difference between A-weighting and C-weighting?

Accepted Answer

A-weighting (dBA) mimics the frequency sensitivity of human hearing, emphasizing mid-high frequencies (1–4 kHz) and attenuating low and very high frequencies. It is the most widely used weighting in environmental noise regulations. C-weighting (dBC) is nearly flat at low frequencies, making it more suitable for evaluating impulsive or low-frequency dominated noise such as explosions. When LCA ≈ LCC, the noise has little low-frequency content; when LCC ≫ LCA, low frequencies dominate.

Question 2

What is an NC curve?

Accepted Answer

NC (Noise Criterion) curves define acceptable octave-band SPL limits for indoor environments. NC-35 is typical for offices and conference rooms; NC-25 is used for broadcast studios and bedrooms. A room meets the NC-XX rating when all octave band levels stay below the corresponding curve. NC curves are the dominant standard in North America, while NR (Noise Rating) curves are common in Europe.

Question 3

How are sone and phon related?

Accepted Answer

Phon quantifies loudness level in terms of the equivalent 1 kHz pure-tone SPL. Sone is a linear perceptual scale where 40 phon equals 1 sone, doubling every 10 phon: S = 2^((P−40)/10). For example, 50 phon = 2 sone, 60 phon = 4 sone. Sone values can be added arithmetically for combined sources, making them useful for engineering design.

Question 4

How is the overall SPL calculated from octave bands?

Accepted Answer

Octave band levels are combined using energy (logarithmic) summation: L_total = 10 × log₁₀(Σ 10^(Lᵢ/10)). For example, two 80 dB bands sum to 83 dB (+3 dB). Simple arithmetic averaging is incorrect — decibels represent acoustic energy (proportional to pressure squared) and must be summed in the linear domain.

Octave Band Noise Spectrum Analyzer

Theory Notes