IEC 61260-1

IEC 61260-1 specifies the performance of octave-band and fractional-octave-band filters, the filters behind the 1/1 and 1/3-octave spectra that sound analysers display. It is a separate standard from IEC 61672-1, with its own class 1 and class 2 limits, its own pattern-evaluation tests (Part 2) and its own periodic tests (Part 3). A sound level meter’s class says nothing about its filters, and a datasheet that states only “class 1” has not told you which standard it means. ANSI/ASA S1.11-2014 is the American adoption.

Editions

The current edition is IEC 61260-1:2014. It replaced IEC 61260:1995, which defined three classes, 0, 1 and 2. The 2014 edition dropped class 0, so an instrument whose datasheet claims IEC 61260 class 0 is citing the old standard; its filters are not worse for that, but the statement should be read in context. Several instruments in the database cite both editions because their filters were designed to the older one and later re-stated against the new.

Band designations

Filters are identified by their nominal midband frequency and bandwidth designator: 1/1 for octave bands, 1/3 for one-third octaves, and so on to 1/12 and 1/24 for the fine-resolution filters a few analysers offer. The standard allows two systems for computing exact midband frequencies: base 10, in which a decade is divided into exactly 10 one-third octaves and which the standard prefers, and base 2, in which an octave is exactly a factor of 2. The differences are small but accumulate across the range; a datasheet should state which is used. The preferred nominal frequencies are the familiar series 31.5, 63, 125, 250 Hz and so on for octaves, and 100, 125, 160, 200, 250 Hz for one-third octaves.

What it specifies

• Relative attenuation limits: a tolerance mask for the filter shape, defining how flat the passband must be and how steeply the filter must cut off outside it. Class 1 masks are tighter than class 2 masks.
• Reference attenuation at the midband frequency and its tolerance.
• Linearity of the filter output with level, and the operating range over which it holds.
• Summation: the requirement that the energy sum of adjacent bands reconstructs a broadband signal within a stated tolerance, which checks that filters neither overlap excessively nor leave gaps.
• Real-time operation: for filters running simultaneously in parallel, the maximum time between a signal arriving and the filter output being available.
• Environmental effects on filter performance, paralleling the requirements of IEC 61672-1.

Why the filter class matters

Building acoustics measurements to ISO 16283 and ISO 3382 require class 1 filters, as do most product noise standards. Environmental tonality assessment by the one-third-octave method of ISO 1996-2 compares the level in one band with its neighbours, so the steepness of the filter skirts affects the result directly. Hearing protector selection by the octave-band method of ISO 4869-2 is less sensitive and class 2 filters, as found in some dosimeters, are adequate for it. For general source identification either class serves.

Digital filters and FFT

All current instruments implement the filters digitally, and the standard is written to apply equally to analogue, digital and FFT-synthesised filters: what is specified is the response, not the method. Some analysers derive octave-band levels by summing FFT lines; that is permitted provided the result meets the masks across the operating range, which is harder at low frequencies where few lines fall in a band. FFT analysis as such is not covered by IEC 61260-1 or by any instrument standard.

Instruments stating conformance

The table lists every instrument whose manufacturer-published standards list mentions IEC 61260, quoting the statement as the manufacturer makes it, including the class and edition where given. Filters are an option on many instruments; the instrument page shows whether the manufacturer states them as standard or optional. The category tables can be filtered to show which instruments offer 1/1, 1/3 or finer bands.