Reference Frequency

IEC 61260-1 uses 1000 Hz as the reference frequency (band index k=0). All other centre frequencies are derived mathematically from this reference. The choice of 1000 Hz aligns with IEC 61672-1 frequency weighting reference and acoustic calibrator frequencies.

Exact Centre Frequencies (Base-10)

The preferred base-10 system calculates exact centre frequencies as:

fm = 1000 × 10^(k / (3·b)) Hz

Where b is the bandwidth designator (1 for octave, 3 for 1/3 octave) and k is the band number (positive above 1 kHz, negative below). For 1/3 octave bands (b=3):

k	Exact fm (Hz)	Nominal (Hz)
-17	19.953	20
-14	39.811	40
-10	100.00	100
-7	199.53	200
0	1000.0	1000
7	5011.9	5000
13	19953	20000

Base-2 Alternative

IEC 61260-1 also permits a base-2 calculation system where centre frequencies are exact powers of 2 relative to 1 kHz. See frequency ratios for the differences between base-10 and base-2 systems. The two systems produce slightly different centre frequencies — the maximum deviation between base-10 and base-2 is about 0.15% (0.013 dB), which is within all class tolerances.

Standard 1/3 Octave Bands (20 Hz – 20 kHz)

The full audio-range 1/3 octave series comprises 30 nominal bands: 20, 25, 31.5, 40, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3150, 4000, 5000, 6300, 8000, 10000, 12500, 16000 Hz (with 20000 Hz as the 31st if included).

Standard 1/1 Octave Bands

The standard octave band series: 16, 31.5, 63, 125, 250, 500, 1000, 2000, 4000, 8000, 16000 Hz. For ISO 3382-1, the required range is typically 125 Hz to 4 kHz (extended to 63 Hz–8 kHz for precision grade). For IEC 60268-16 STI, 7 octave bands from 125 Hz to 8 kHz are used.

Bandwidth

The bandwidth of each filter is defined by its upper and lower edge frequencies:

Upper edge: f₂ = fm × 2^(1/(2b))

Lower edge: f₁ = fm × 2^(-1/(2b))

For 1/1 octave: the bandwidth ratio f₂/f₁ = 2 (the upper edge is twice the lower edge). For 1/3 octave: f₂/f₁ = 2^(1/3) ≈ 1.26.

SonaVyx Implementation

The RTA mode uses the IEC 61260-1 base-10 centre frequencies for both 1/1 and 1/3 octave display. The SPL meter provides octave-band analysis with the same centre frequencies. See filter design for how these bands are implemented and response requirements for tolerance masks.