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

Swl2 + 10 10






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.

Figure 14.32 will assist in this calculation, the procedure being as follows:

Addition of sound levels Addition of sound levels
Addition of sound levels
Addition of sound levels
Addition of sound levels

Addition of sound levels

(SWL2 — SWL,) dBw







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.


NR criterion

Concert halls, opera halls, studios for sound reproduction, live theatres (> 500 seats)


Bedrooms in private homes, live theatres (< 500 seats), cathedrals and large churches, television studios, large conference and lecture rooms (> 50 people)


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


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


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


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


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.

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