Units of Measurement of Noise

The noisiness of a sound source could be expressed in terms of its Sound Power (Watts). However, the range of values found in practice (from a soft whisper at 0.000000001W to a Saturn rocket at 40,000,000W) make this impractical. Noise is therefore expressed by its Sound Power Level, a ratio which logarithmically compares its Sound Power with a reference power, the Picowatt (10^-12 Watt).

The unit of Sound Power Level (Lw) is the Decibel (dB) re 10^-12 Watts.

Figure 1 clearly shows how the logarithmic scale compresses the unacceptably wide range of possible Sound Power to Sound Power Levels having a practical range of 30-200dB.

Sound Power (Watts)	Sound Power Level (dB)	Source
25 to 40,000,000	195	Saturn Rocket
100,000	170	Ram Jet
10,000	160	TurboJet engine 3200kg thrust
1,000	150	4 propeller airliner
100	140
10	130	75 piece orchestra
1	120	Large chipping hammer
0.1	110	Blaring radio
0.01	100	Car on motorway
0.001	90	Axial ventilating fan (2500 m³/h)
0.0001	80	Voice - shouting
0.00001	70	Voice - conversational level
0.000001	60
0.0000001	50
0.00000001	40
0.000000001	30	Voice - very soft whisper

Frequency

The unit of measurement of frequency is the Hertz (Hz). This has generally replaced the older ‘cycle per second’ (cps or c/s).

To better describe the intensity and quality of a noise, the frequency scale is split into bands and a Sound Power Level quoted for each. Octave Bands are generally used for this purpose (bands of frequency in which the upper limit of frequency is twice that of the lower limit), and are specified in terms of their ‘mid-band’ frequencies. These would commonly be 63, 125, 250, 500, 1000, 2000, 4000 and 8000 Hz.

The Sound Power Level of a fan can be compared to the Power Output of a heater in that both are measure of the energy being fed into the environment surrounding them. Neither, however, will tell us the effect that will be experienced by a human being in that surrounding space. In the case of the heater, the temperature that we feel is dependent on the surroundings. The effects of distance, volume of space, absorbing and reflecting surfaces, other heat sources, etc., will combine to determine the resulting Temperature at any point.

In a similar way, the Sound Pressure Level that we hear is determined by the surroundings and the acoustic engineer must take this into account when specifying noise levels.

Sound Pressure Level (LP) is also measured on a logarithmic scale but the unit is the Decibel (dB) re 2 x 10-5 Pa

In-duct, Open - inlet/outlet, Spherical radiation, Hemispherical radiation.

A fan manufacturer can publish noise information for various installation conditions.

It is important to consider the use or installation conditions of a fan when checking a manufacturer’s noise data. The information given may not always be appropriate and it is as well to check. As is the case with Sound Power Levels, Sound Pressure Levels must be quoted or each Octave Band if a complete picture of the effect of the noise on the human ear is required.

Sound Pressure (Pascals)	Sound Pressure Level (dB)	Typical Environments
200.0	140	30mm from military aircraft at take-off
63.0	130	Pneumatic chipping and riveting (operator's position)
20.0	120	Boiler shop (maximum levels)
6.3	110	Automatic punch press (operator's position)
2.0	100	Automatic lathe shop
0.63	90	Construction site - pneumatic drilling
0.2	80	Kerbside of busy street
0.063	70	Loud radio (in average domestic room)
0.02	60	Restaurant
0.0063	50	Conversational speech at 1m
0.002	40	Whispered conversation at 2m
0.00063	30
0.0002	20	Background in TV and recording studios
0.0002	0	Normal threshold of hearing

THE ENGINEER MUST CLEARLY DISTINGUISH AND UNDERSTAND THE DIFFERENCE BETWEEN SOUND POWER LEVEL AND SOUND PRESSURE LEVEL. HE MUST ALSO APPRECIATE THAT dB RE 10-12 WATTS AND dB RE 2x10-5 Pa ARE DIFFERENT UNITS WITH NO ABSOLUTE FORMULAE CONNECTING THEM.

It is impossible to measure directly the Sound Power Level of a fan. However, the manufacturer can calculate this level after measuring the Sound Pressure Levels in each octave band with the fan working in an accepted acoustic test rig.

What he cannot do is unequivocally state what Sound Pressure Levels will result from the use of the fan. This can only be done if details of the way the fan is to be used, together with details of the environment it is serving, are known and a detailed acoustic analysis is carried out.

A disadvantage of the Decibel scale is that values cannot be added or subtracted using the normal arithmetic rules. Decibel values must be converted back into absolute units of power (Watts), when they can be added or subtracted directly before re-converting back into ecibels. However, this tedious process can be avoided by using the following simple but approximate method. Column 2 shows how much must be added to the higher of two sound powerlevels to obtain the equivalent combined level, when the dB difference between the two levels is shown in Column 1.

Difference between the two levels dB	Quantity to be added to the higher level
0	3
1	2.5
2	2
3	2
4	1.5
5	1
5	1
6	1
7	1
8	0.5
9	0.5
10 or more	0

As an example, if one fan has a sound level of 50dB, and another similar fan is added, (the sound level is doubled). The combined sound level of the two fans will be 53dB. If two more fans are added, i.e. the sound source is doubled again, the resultant sound level from all four fans will be 56dB.

If the first fan had a sound level of 50dB and another, larger fan with a sound level of 55dB was added, the difference between the two is 5dB, so 1dB is added to the higher figure, giving a combined level of 56dB.

To estimate the effect of distance from a sound source the rule of thumb calculation is very simple. Sound normally weakens by 6dB each time the distance from the sound source is doubled. i.e. a sound level measured as 60dB at 1 metre will be 54dB at 2 metres and 48 at 4 metres.

Smaller fan sound levels are sometimes measured at 1m, whereas the industry standard is becoming standardised at 3m, for comparison purposes. The difference between 1m and 3m is therefore 9dB. (41dB@3m) and 50dB@1m are in effect the same).

dBA (dBB, dBC)

This is a single number, weighted sound index. It is obtained by subtracting different values from each of the frequency bands in an attempt to approximate the response characteristics of the human ear. The resultant values are then added together to obtain a single number sound level. However, too much information is lost in this process to allow this one figure to be of any use for calculation.

Unfortunately, the majority of fan manufacturers indicate the noisiness of their products by Sound Pressure Levels expressed in dBA (or dBC). These figures refer to the Sound Pressure Levels which would be experienced by an observer at a distance of 3m (or occasionally 1m or 3 x fan diameters) from the fan if both were suspended in an infinitely large and empty volume (technically called Free Field). To say the least, this is a very unlikely set of conditions and these dB values must only be used to compare the noisiness of similar types of fan!

It cannot be stressed too often, that on no account must the engineer be tempted to assume that the Sound Pressure Levels quoted in manufacturers’ catalogues will in anyway be similar to those achieved in practice. Depending on circumstances, they can be substantially exceeded or reduced.

Thought of objectively, using dBA to describe the noisiness of a fan is as absurd as stating the output of a heater in terms of the temperature it would produce three metres from its face if it were suspended in space! It will be better for all involved when only Sound Power Level information is used to specify fan noise. In the meantime, engineers must always be careful to check how noise information is expressed.

Noise Criteria Curves (NC)

It is obvious that the combination of a single figure index such as dBA, with more information on the shape of the frequency content would be useful. Noise Criteria curves (NC) were evolved to meet this need.

NC curves consist of a family of octave band spectra, with each curve marked with its own NC rating number. The octave band spectrum of the noise being analysed is plotted on the same grid and the NC rating of that noise corresponds to the highest NC curve touched by the noise spectrum.

Figure 3 shows a set of NC curves together with a table indicating recommended levels for various environments. The spectrum of a noise with a NC rating of 35 is also shown on the grid.

NC ratings are particularly suitable for selecting and assessing suitable background noise levels for ventilating and air conditioning systems.

It is pointless specifying Sound Pressure Levels or NR ratings to a fan manufacturer unless adequate information about how and where the fans are to be used is also provided. In view of the considerable amount of work involved in calculating Sound Pressure Levels and NR criteria, engineers will be well advised to check whether a particular manufacturer includes this analysis work in his service or whether acoustic engineers must be employed.

Environment	NR criterion
Concert halls, opera halls, studios for sound reproduction, live theatres (-500 seats)	20
Bedrooms in private homes, live theatres (+500 seats), cathedrals and large churches, television studios, large conference and lecture rooms (50 people)	25
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
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
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
Kitchens in hotels, hospitals, etc., laundry rooms, computer rooms, accounting machine rooms, cafeterias, canteens, supermarkets, swimming pools, covered garages in hotels, offices, etc., bowling alleys	45

NR50 and above

NR50 will generally regarded as very noisy by sedentary workers but most of the classifications listed under NR45 could just accept NR50. Higher noise levels than NR50 will be justified in certain manufacturing areas: such cases must be judged on their own merits.

Recommended NC levels for various environments
Environment	Range of NC levels likely to be acceptable
Factories (heavy engineering)	55 - 75
Factories (Light engineering)	45 - 65
Kitchens	40 - 50
Swimming baths & sports areas	35 - 50
Department stores & shops	35 - 45
Restaurants, bars, cafeterias & canteens	35 - 45
Mechanised offices	40 - 50
General offices	35 - 45
Private offices, libraries, courtrooms & schoolrooms	30 - 35
Homes, bedrooms	25 - 35
Hospital wards & operating theatres	25 - 35
Cinemas	30 - 35
Theatres, assembly halls & churches	25 - 30
Concert & opera halls	20 - 25
Broadcasting & recording studios	15 - 20

Noise from Fan Systems

The noise resulting from a fan system can be caused in several ways and can enter an area by more than one route.

A	The noise energy generated by a fan will be red into the duct system, forced to pass along its length, and a proportion will enter the area being served by the system. The balance of original noise energy not reaching the ventilated area will be absorbed by the system.
B	Noise will also be generated by the airflow as it is obstructed and turned in its passage along the duct system. Sound energy will therefore, be introduced into the system at bends, dampers, heater batteries, etc., and again a proportion will pass into the ventilated area. And noise produced as the airflow passes through the terminal devices themselves will be radiated directly into the ventilated area.
C	Some of the noise absorbed by the duct system is in fact lost through the duct walls and can be a nuisance in those areas through which the ducting passes. This break-out noise can even reach the ventilated area itself. Airborne noise from a plant room, and vibration energy from the fan, can also be transmitted through walls, floors and ceilings into adjacent areas.

If the Sound Power of a fan is too large, and no other selection is possible, attenuation must be introduced immediately after the fan to prevent unacceptable noise passing along the system.

Attenuation can be provided by lining the ducts with absorptive material or by insertive proprietary attenuator units. Duct lining, especially at bends, is extremely satisfactory providing sufficient length of duct is available and thick enough lining is used.

System Design

The engineer should remember that noise generation within an air distribution system is caused by aerodynamic turbulence. If, therefore, he conforms to the codes of recommended design practice, paying special attention to those areas where turbulence is like, both aerodynamic and acoustic efficiencies will improve. (An example of the correlation between aerodynamic and acoustic efficiency has already been given).

FIGURE 7 shows designs which will cause turbulence (and hence noise) and how they can be improved.

Care must be taken not to exceed recommended outlet velocities at terminal devices because even small excesses over the recommended levels can cause appreciable increases in noise.

When the airflow generated noise is unacceptably high engineers should always first attempt to reduce the air velocities (airflow generated noise is proportional to the sixth power of the velocity!). In practice, airflow generated noise can be ignored when velocities are below 7.5m/s in the main duct and 3m/s in branch ducts.

If this is not possible then secondary attenuators, inserted as near to the outlet as possible, must be used to absorb the unacceptable noise energy. FIGURE 7 shows one example of the use of an attenuator.

Obviously attenuators cannot be fitted after terminal devices and therefore engineers must rely absolutely on the selection of the correct outlet itself; perhaps oversizing it from the aerodynamic point of view, in an effort to avoid noise generation at a point where it cannot be subsequently eliminated.

WARNING
Noise, not produced by the ventilating system, can enter through the ductwork and subsequently be a nuisance in the ventilated area. This situation can be thought of as break-in and must be treated in a similar manner to break-out. In the same way outside noise (e.g., aircraft noise) can enter the system through the external louvre and must be attenuated if unacceptable.

1.	It may be acceptable to reduce the airflow through the system. A 20% reduction in airflow will reduce the noise level by approximately 5 dB.
2.	The system may be aerodynamically re-balanced so that duct branches responsible for feeding excessive amounts of noise into the area have their airflow (and hence their noise energy carrying capacity) reduced. However, care must be taken that the dampers etc., used to re-balance do not cause significant noise increase or alternatively as in 5 below additional silencing may be required.
3.	Attenuation, either in the form of duct lining or proprietary units, may be introduced into the system.
4.	Terminal devices may be exchanged for models which produce less noise, or for larger size devices where, because of the resulting lower air velocity, noise generation is reduced.
5.	Dampers, and other adjusting and balancing devices, may have to be relocated away from outlets, and secondary attenuation inserted between their new positions and the outlets.
6.	Quiet fans may have to be installed in place of the original units.

If none of these measures reduce the noise level sufficiently, then it may well prove necessary to attempt to alter the acoustic characteristics of the area being served in an effort to absorb more of the sound energy discharging from the fan system. However, depending on circumstances the reduction achieved by this method may be somewhat limited. An acoustic engineer should be consulted if such a stage is reached.

However, everyone must be aware (especially clients!) that reducing noise levels below reasonable values is incredibly expensive. In fact, the total cost of a system increases exponentially as the NR rating is lowered. For this reason, the target noise level resulting from a fan system must be chosen with care after studying the guides and the conditions special to the contract.

Remember - there is no justification in paying for a fan system which would generate a background level of only NR25, if the noise from an adjacent road would make NR35 more appropriate!