You are here: NI Home > Support > Product Reference > Manuals > Sound and Vibration Measurement Suite 6.0 Help

This topic discusses the ANSI, IEC, and JIS standards regarding midband frequencies, the nominal frequencies returned by the Octave Analysis VIs, and compliance with ANSI, IEC, and JIS standards by the Octave Analysis VIs.

Note  The Octave Analysis VIs use the base 2 system for midband frequencies instead of the base 10 system. Using the base 2 system means that the ratio of two midband frequencies is a fractional power of 2 instead of a fractional power of 10.

Midband Frequencies

According to the IEC 1260:1995 and the ANSI S1.11:2004 standards, the midband, or center, frequency of the bandpass filter is defined by the following equations.

f_i = 1000 · 2 ^ib for 1/N octave filters when N is odd

for 1/N octave filters when N is even

where

f_i is the center frequency of the i^th band-pass filter expressed in hertz

i is an integer when i = 0, f₀ = 1 kHz, which is the reference frequency for the audio range

b is the bandwidth designator and equals 1 for octave, 1/3 for 1/3 octave, 1/6 for 1/6 octave, 1/12 for 1/12 octave, and 1/24 for 1/24 octave

Nominal Frequencies

The exact midband frequencies are used to design the filters for fractional-octave analysis. However, all the Octave Analysis VIs return the nominal midband frequencies, also called the preferred frequencies. In the case of octave and 1/3 octave analyses, the nominal frequencies are calculated in accordance with the ANSI and IEC standards. In the case of 1/6, 1/12, and 1/24 octave analyses, the nominal frequencies are calculated in accordance with the Annex A (informative) of the IEC 1260:1995 and the ANSI S1.11:2004 standards.

Compliance with ANSI, IEC, and JIS Standards

When combined with any National Instruments DSA device, the appropriate microphone, and proper signal conditioning, the filters produced by the Octave Analysis VIs comply with the following standards:

• ANSI S1.11-2004: Specification for octave-band and fractional-octave-band analog and digital filters, class 1
• IEC 1260 (1995 - 07): Electroacoustics - Octave-band and fractional-octave-band filters, class 1
• JIS C 1514:2002: Electroacoustics - Octave-band and fractional-octave-band filters, class 1

For octave filters, choose a sampling frequency at least three times the exact center frequency of the highest frequency band. For fractional-octave filters, choose a sampling frequency at least 2.5 times the exact center frequency of the highest frequency band.