Table 1310 Level Adjustments for the NEMA Rating of a Transformer dB Miller 1980

Octave Band Center Frequency, Hz 31 63 125 250 500 1k 2k 4k 8k CT -1 5 7 2 2 -4 -9 -14 -21

through a string tied around the unit), and the correction term, which includes the 10.5 dB adjustment for the power to pressure conversion. Table 13.10 gives the correction term for an unenclosed transformer.

where Lw OCT = octave band sound pressure level (dB) ST = surface area of the transformer (ft2) CT = octave band correction (dB) Over time, transformers can grow noisier as their laminations and tie bolts become loose. Miller cites increases as large as 5 dB at the fundamental and 10 dB in the second and third harmonic frequencies. When transformers are enclosed in small vaults they can induce standing wave patterns in the room, which have the effect of increasing the transmitted power by 6 dB in the same bands.

Transformers that are directly tied to a wall can induce structure-borne noise. Jones (1984) recommends isolation techniques shown in Fig. 13.27 to prevent this. In areas of seismic activity, one or more sway braces may be necessary to provide stability at the top of the unit.

Reciprocating Engines and Emergency Generators

Most large buildings have emergency generators to provide power when the normal sources fail. It is often argued that noise control of emergency generators is unnecessary since they would be used only in an emergency, when noise is a secondary concern. Although this is probably true, generators must be tested periodically, an hour a month, and during these test periods the building functions normally and noise is still a concern. During power outages generators can be needed over longer periods of time.

Generator sets are powered by a diesel, methane, or propane fuel reciprocating engine and radiate sound from their casing, intake, and exhaust. Miller (1980) measured the casing radiated power levels, which followed the relationship

Lw = 93 + 10 log (rated hp) + A + B + C + D (13.36)

where Lw = overall sound power level (dB)

rated hp = engine manufacturer's continuous full load rating for the engine, (horse power) A, B, C, D = correction terms given in Table 13.11 (dB)

Octave-band casing-radiated noise can be obtained from the overall sound power level spectrum by subtracting the levels given in Table 13.12.

Figure 13.27 Vibration Isolation of Floor-Mounted Transformers (Jones, 1984)

Figure 13.27 Vibration Isolation of Floor-Mounted Transformers (Jones, 1984)

Rigid electrical connectors

Keep space beneath transformer and rails clear of foreign objects

3bO loop or grossly slack flexible conduit or manufactured resilient fitting

Rigid electrical connectors

Keep space beneath transformer and rails clear of foreign objects

Manufacturer. r\eoprer\e floor mouhts or r\eoprer\e pads

3bO loop or grossly slack flexible conduit or manufactured resilient fitting

.Steel channel 'mounting rails

Concrete housekeepina

Xz!iad

Manufacturer. r\eoprer\e floor mouhts or r\eoprer\e pads

.Steel channel 'mounting rails

Concrete housekeepina

Table 13.11 Level Adjustments for Engine Casing Radiated Noise, dB (Miller, 1980)

Speed Correction Term, A

Above 1500 rpm 0 Fuel Correction Term, B

Diesel fuel only 0

Diesel and/or natural gas 0

Natural gas only (may have small amounts of "pilot oil") —3 Cylinder Arrangement Term, C

In-line 0

Radial -1 Air Intake Correction Term, D

Unducted air inlet to unmuffled Roots Blower +3

Ducted air from outside the room or into muffled Roots Blower 0

All other inlets to engine (with or without turbochargers) 0

Noise radiated from the inlet is usually the same as the casing radiation unless there is a separate ducted inlet to a turbocharger. In these cases the inlet noise is given by

0 0

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