Trane TRG-TRC007-EN manual Octave Bands, Fundamentals of Sound, period one

period one

Fundamentals of Sound

notes

Again, a pure tone has a single frequency. If a sound in a narrow band of frequencies is significantly greater than the sound at adjacent frequencies, it would be similar to a tone. Tones that stand out enough from the background sound can be objectionable. Many of the sounds generated by HVAC equipment and systems include both broadband and tonal characteristics.

Octave Bands

octave	center	frequency
band	frequency (Hz)	range (Hz)
1	63	45 to 90
2	125	90 to 180
3	250	180 to 355
4	500	355 to 710
5	1,000	710 to 1,400
6	2,000	1,400 to 2,800
7	4,000	2,800 to 5,600
8	8,000	5,600 to 11,200

Figure 8

Octave Bands

Because sound occurs over a range of frequencies, it is considerably more difficult to measure than temperature or pressure. The sound must be measured at each frequency in order to understand how it will be perceived in a particular environment. The human ear can perceive sounds at frequencies ranging from 20 to 16,000 Hz, whereas, HVAC system designers generally focus on sounds in the frequencies between 45 and 11,200 Hz. Despite this reduced range, measuring a sound at each frequency would result in 11,156 data points.

For some types of analyses, it is advantageous to measure and display the sound at each frequency over the entire range of frequencies being studied. This is called a full-spectrum analysis and is displayed like the example shown in Figure 7.

To make the amount of data more manageable, this range of frequencies is typically divided into smaller ranges called octave bands. Each octave band is defined such that the highest frequency in the band is two times the lowest frequency. The octave band is identified by its center frequency, which is calculated by taking the square root of the product of the lowest and highest frequencies in the band.

center frequency = lowest frequency × highest frequency

The result is that this frequency range (45 to 11,200 Hz) is separated into eight octave bands with center frequencies of 63, 125, 250, 500, 1,000, 2,000, 4,000, and 8,000 Hz. For example, sounds that occur at the frequencies between 90 Hz and 180 Hz are grouped together in the 125 Hz octave band.

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