What Is The Public Response To Transformer Audible Sound?

The basic objective of a transformer noise specification is to avoid annoyance. In a particular application, the NEMA Standard level may or may not be suitable, but in order to determine whether it is, some criteria must be available.

One such criterion is that of audibility in the presence of background noise. A sound which is just barely audible should cause no complaint.

Studies of the human ear indicate that it behaves like a narrowband analyzer, comparing the energy of a single frequency tone with the total energy of the ambient sound in a critical band of frequencies centered on that of the pure tone.

If the energy in the single-frequency tone does not exceed the energy in the critical band of the ambient sound, it will not be significantly audible. This requirement should be considered separately for each of the frequencies generated by the transformer core.

The width of the ear-critical band is about 40 Hz for the principal transformer harmonics. The ambient sound energy in this band is 40 times the energy in a 1-Hz-wide band.

The sound level for a 1-Hz bandwidth is known as the “spectrum level” and is used as a reference. The sound level of the 40-Hz band is 16 dB (10 log 40) greater than the sound level of the 1-Hz band. Thus, a pure tone must be raised 16 dB above the ambient spectrum level to be barely audible.

The transformer sound should be measured at the standard NEMA positions with a narrow-band analyzer. If only the 120- and 240-Hz components are significant, an octave-band analyzer can be used, since the 75- to 150-Hz and 150- to 300-Hz octave bands each contain only one transformer frequency.

The attenuation to the position of the observer can be determined. The ambient sound should be measured at the observer’s position.

For each transformer frequency component, the ambient spectrum level should be determined. An octave band reading of ambient sound can be converted to spectrum level by the equation

S = B - 10 log C

where B = decibels octave-band reading, C = hertz octave bandwidth, and S = decibels spectrum level.

Example. Consider the following case:

Transformer sound at 120 Hz by NEMA method = 72 dB
Transformer-sound attenuation to observer = 35 dB
Ambient sound at the 75- to 150-Hz octave band = 36 dB
72 35 = 37 dB at the observer’s position
36 10 log (150 75) = 17.3-dB ambient spectrum level

The 120-Hz transformer sound at the observer’s position exceeds the ambient spectrum level by 19.7 dB. This is 3.7 dB greater than the 16-dB differential which would result in bare audibility; thus the transformer sound will be audible to the observer.

When transformer sound exceeds the limits of bare audibility, public response is not necessarily strongly negative. Some attempts have been made to categorize public response on a quantitative basis when the sound is clearly audible (Schultz and Ringlee 1960).

For a case where specific knowledge of transformer- and ambient-sound-level frequency composition is not available, some more general guidelines are useful. Typical average nighttime ambient-sound levels for certain types of communities have been established.

These are 30 dB for a “quiet suburban,” 35 dB for a “residential suburban,” and 40 dB for a “residential urban” community. All sound levels are based on the A scale of weighing.

Calculations for typical transformer frequency distributions have been made to determine the nighttime transformer noise which will be audible 50% of the time in these communities. The results are 24 dB for quiet suburban, 29 dB for residential suburban, and 34 dB for residential urban.

The NEMA standard sound level can be corrected for attenuation with distance to the nearest observer and checked against the above guides for audibility.

The broadband sound from fans, pumps, and coolers has the same character as ambient sound and tends to blend in with the ambient.

While the noise from cooling equipment may be audible to a
neighboring observer, it will seldom, if ever, cause a complaint.

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