Internal Combustion Engines
The IC engine is a major source of noise in transportation and industrial use. The intake and exhaust noise can be effectively silenced. However, the noise radiated by engine surfaces is more difficult to control. In gasoline engines, a fuel–air mixture is compressed to about one‐eighth to one‐tenth of its original volume and ignited by a spark plug. In diesel engines air is compressed to about one‐sixteenth to one‐twentieth of its original volume and liquid fuel is injected in the form of a spray; then spontaneous ignition and combustion occurs. Because the rate of pressure rise is initially more abrupt with a diesel engine than with a gasoline engine, diesel engines tend to be noisier than gasoline engines. The noise of diesel engines has consequently received more attention from both manufacturers and researchers. The noise of IC engines is discussed in detail in Ref. [66].
The noise of engines can be divided into two main parts: combustion noise and mechanical noise. The combustion noise is caused mostly by the rapid pressure rise caused by ignition, and the mechanical noise is caused by a number of mechanisms, with perhaps piston slap being one of the most important, particularly in diesel engines. The noise radiated from the engine structure has been found to be almost independent of load, although dependent on cylinder volume and even more dependent on engine speed [67]. Priede has given a good review of IC engine noise with an emphasis on diesel engine noise [68]. Reference [14] also includes a detailed discussion on various aspects of engine noise. Further discussion and worked examples on automotive engine noise can be found in Chapter 14 of this book.
Electric Motors and Electrical Equipment
Examples of electrical equipment that cause noise and vibration include motors, generators, and alternators [5, 13], transformers, relays, solenoids, and circuit breakers. Electric motors are used widely in appliances, vehicles, and industry in a variety of types and sizes. They may be commutated, synchronous, or induction types. Electrical energy is converted into mechanical energy, and in the process some heat is produced. Fans are often provided to remove the heat and are the main sources of noise in electric motors. Because of the requirement for most motors that they should operate in either direction of rotation, they are usually provided with axial or tubular centrifugal fans, which can be quite noisy (see Section 11.2.3).
The sources of noise and vibration in electrical equipment are mostly aerodynamic, mechanical, and electromagnetic in nature. They are summarized in Table 11.3. Electric motors and the noise they generate are discussed in detail in Ref. [69].
Table 11.3 Main sources of noise in electric motors [18].
| Mechanical | Excessive bearing clearance |
| Nonround bearings | |
| Rotor unbalance | |
| Rotor eccentricity | |
| Crooked shaft | |
| Brush and brush holder vibration | |
| Misalignment | |
| Loose laminations | |
| Electromagnetic | Magnetostriction |
| Torque pulsations | |
| Air gap eccentricity | |
| Air gap permeance variation | |
| Dissymmetry | |
| Sparking or arcing | |
| Aerodynamic | Fan blade‐passing frequency |
| Turbulence | |
| Noise due to airflow path restrictions |
Some empirical studies have been developed to give a rough estimate of the sound pressure level of different types of electric motors [70, 71]. For totally enclosed, fan cooled (TEFC) small motors, the following equation can be used to give a conservative estimate of the overall sound pressure level at 1 m:
(11.11)
where N is the motor speed in rpm, WR is the motor power rating (in kW), and A and B are constants that depend on the electrical power of the motor. For motors under 40 kW, A = B = 17. For motors over 40 kW, A = 28 and B = 10. It has been observed that the sound pressure level produced by Drip‐Proof (DRPR) motors is approximately 5 dB lower than the sound pressure level produced by TEFC motors. A TEFC motor with a quiet fan is likely to be 10 dB quieter than the overall value estimated by Eq. (11.11). The one-octave band sound pressure levels can be determined by subtracting the adjustments listed in Table 11.4 from the overall values given by Eq. (11.11). The estimation of the noise levels produced by large electric motors is discussed in Refs. [70, 71].
Table 11.4 One‐octave band level adjustments (dB) for small electric motors [71].
| One‐octave band center frequency (Hz) | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Type of motor | 31.5 | 63 | 125 | 250 | 500 | 1000 | 2000 | 4000 | 8000 |
| TEFC | 14 | 14 | 11 | 9 | 6 | 6 | 7 | 12 | 20 |
| DRPR | 9 | 9 | 7 | 7 | 6 | 9 | 12 | 18 | 27 |
EXAMPLE 11.6
Estimate the overall sound pressure level produced by a small 30 kW TEFC motor operating at 2000 rpm at a receiver located at a distance of 10 m from the motor. Both the motor and the receiver are situated on hard ground in open space. For simplicity consider that the motor radiates sound as an omnidirectional source.
SOLUTION
We use Eq. (11.11) with A = B = 17 to estimate the overall sound pressure level produced by the motor at 1 m. Thus, Lp = 17 + 17 × log(30) + 15 × log(2000) = 92 dB. If the motor radiates only to a half space and no sound power is absorbed by the hard ground, then we can use Eq. (3.52) to determine the sound power level Lw of the motor. Rearranging Eq. (3.52) we get Lw = Lp + 20 × log(r) + 8 dB = 92 + 20 × log(1) + 8 = 100 dB. Therefore, the sound pressure level at 10 m is: Lp = Lw − 20 × log(r) − 8 dB = 100 − 20 × log(10) − 8 = 72 dB. Alternatively, since the sound pressure level at 1 m is known, the level at 10 m can be calculated directly from the inverse square law without using the sound power level. The difference in levels at 1 and 10 m is 20 × log(10/1) = 20 dB. Therefore, the sound pressure level produced by the motor at 10 m is: 92 − 20 = 72 dB.
EXAMPLE 11.7
Estimate the sound pressure level produced at 1 m for each one‐octave band from 31.5 to 8000 Hz, by a 50 kW DRPR motor running at 3000 rpm.
SOLUTION
Electric motor noise is normally controlled by passive means (use of enclosures, sound‐absorbing materials, vibration isolation, etc.) The pure‐tone vibration and noise produced at twice line frequency and multiples can also be reduced by active control methods, although active control of electrical equipment so far has received limited attention. One exception is the active control of the vibration and noise of large electrical transformers, which has been successfully reduced by active vibration control approaches
First we use Eq. (11.11) to estimate the overall sound pressure level at 1 m with A = 28 and B = 10. Therefore, Lp = 28 + 10 × log(50) + 15 × log(3000) = 97 dB. Now we subtract the values given in the last row of Table 11.4 from 97 dB for each one‐octave band. Then, the sound pressure levels at 1 m, in the nine one‐octave bands from 31.5 to 8000 Hz are: 88, 88, 90, 90, 91, 88, 85, 79, and 70 dB, respectively.
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