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In many applications involving acoustic measurements, the final sensor is the human ear. In other words, acoustic measurements typically attempt to describe the subjective perception of a sound by the human ear. Instrumentation devices usually are built to provide a linear response, but the ear is a nonlinear sensor. Therefore, special filters, known as psophometric weighting filters, are used to account for the nonlinearities.

The following front panel shows the linear frequency response of a microphone.

The frequency response of the microphone is designed to be as flat as possible in the frequency range of 10 Hz to 10 kHz. Compare the frequency response of the microphone with the equal loudness curves shown in the following illustration.

Loudness is a subjective indicator of the perceived noise level expressed in phons. The loudness level in phons is the sound pressure level in decibels of a 1 kHz tone having the same perceived loudness as the tone being evaluated. Thus, a 1 kHz tone with a loudness level of 30 phons is equally as loud as a 1 kHz tone with a sound pressure level of 30 dB referenced to 20 µPa. However, a signal frequency of 100 Hz requires a sound pressure level of 44 dB referenced to 20 µPa to provide the same loudness level.

Psophometric Weighting Filters

The NI Sound and Vibration Measurement Suite provides the following psophometric weighting filters for you to apply acoustic weighting.

• A-, B-, and C-Weighting Filters
• Radiocommunications Weighting Filters
• Telecommunications Weighting Filters

A-, B-, and C-Weighting Filters

The A-, B-, and C-weighting filters are designed for the following uses:

• An A-weighting filter is a highpass filter designed to simulate the loudness of low-level tones. An A-weighting filter progressively de-emphasizes frequencies below 500 Hz.
• A B-weighting filter simulates the loudness of medium-level tones. B-weighting filters are used infrequently.
• A C-weighting filter removes sounds outside the audio range of 20 Hz to 20 kHz and simulates the loudness of high-level tones.

Note  Recent standard revisions introduced the concept of Z-weighting. To accomplish Z-weighting in the Sound and Vibration Measurement Suite, set weighting to Linear in the Weighting and Filtering VIs.

The ANSI S1.4 standard defines the frequency responses of the A-, B-, and C-weighting filters. The following front panel shows the relative attenuation defined for A-, B-, and C-weighting filters.

Note  Each of the A-, B-, and C-weighting filters has a relative attenuation of 0 dB at 1,000 Hz.

Radiocommunications Weighting Filters

ITU-R 468-4 and Dolby weighting filters are bandpass filters you use to emphasize the response to the types of impulsive noise that often couple into audio cables. Typically, you use these filters to measure audio frequency noise in broadcasting, sound-recording systems, and sound program circuits.

The frequency response of the ITU-R 468-4 filter is standardized by ITU-R Recommendation 468-4. The following front panel shows the relative attenuation defined for the ITU-R 468-4 and Dolby weighting filters.

Telecommunications Weighting Filters

CCITT and C-message weighting filters are bandpass filters used to measure audio-frequency noise on telephone circuits. The CCITT (ITU-T) filter is used for international telephone circuits. The C-message filter is typically used for North American telephone circuits.

The frequency response of the CCITT and C-message weighting filters are specified in the ITU-T O.41 standard and Bell System Technical Reference 41009, respectively. The following front panel shows the relative attenuation defined for the CCITT and C-message weighting filters.