Addition of sound levels
If the noise levels of two machines, such as a fan and its driving motor or two fans, have been measured individually and you want to know how much noise the machines will make when operating together, the two sound levels must be added.
However, when using dBW one cannot add the sound levels directly as the scale is logarithmic and:
~ SWL,
10 To
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Swl2 + 10 10 |
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4. |
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1. 2. 3. 4. |
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Measure the levels of machine 1 and machine 2. Find the difference between these levels. From the bottom of the chart with this difference, intersect the curve, obtaining increment on the vertical axis. Add the value indicated at the vertical axis to the level of the noisiest machine. This gives the sum of the noise levels of the two machines. |
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Figure 14.32 will assist in this calculation, the procedure being as follows: |
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Figure 14.32 Calculation of combined sound level for fan and motor
Teristics, distances from sound sources to point of measurement, etc. They are best calculated by acoustic specialists knowing the fan SWL levels.
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Environment |
NR criterion |
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Concert halls, opera halls, studios for sound reproduction, live theatres (> 500 seats) |
20 |
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Bedrooms in private homes, live theatres (< 500 seats), cathedrals and large churches, television studios, large conference and lecture rooms (> 50 people) |
25 |
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Living rooms in private homes, board rooms, top management offices, conference and lecture rooms (20-50 people), multi-purpose halls, churches (medium and small), libraries, bedrooms in hotels etc., banqueting rooms, operating theatres, cinemas, hospital private rooms, large courtrooms |
30 |
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Public rooms in hotels, etc., ballrooms, hospital open wards, middle management and small offices, small conference and lecture rooms, (< 20 people), school classrooms, small courtrooms, museums, libraries, banking halls, small restaurants, cocktail bars, quality shops |
35 |
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Toilets and washrooms, large open offices, drawing offices, reception areas (offices), halls, corridors, lobbies in hotels, hospitals, etc., laboratories, recreation rooms, post offices, large restaurants, bars and night clubs, department stores, shops, gymnasia |
40 |
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Kitchens in hotels, hospitals, etc., laundry rooms, computer rooms, accounting machine rooms, cafeteria, canteens, supermarkets, swimming pools, covered garages in hotels, offices, etc., bowling alleys |
45 |
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NR50 and above NR50 will generally be regarded as very noisy by sedentary workers. Higher noise levels than NR50 will be justified in certain manufacturing areas. |
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Table 14.10 Recommended noise rating (NR) levels |
The use of empirical “laws" to determine fan noise can be fraught with danger. Even the use of so-called “fan sound laws”, when applied to test data can lead to serious error. In all possible cases, reference should be made to actual tests, and results taken from as near as possible to the same size, speed and installation category.
If the flowrate varies, care should be taken in selecting an appropriate method. The sound output may increase if the ducting resistance has been incorrectly assessed and the fan does not operate at the correct point on its characteristic. Ductwork impedance can determine the fan noise, particularly at low frequencies. The need for good inlet and outlet connections cannot be understated.
Posted in Fans Ventilation A Practical Guide