Fan noise
The principle source of noise in any air moving system is the main fan. Rules for determining fan noise and noise-producing mechanisms are covered as well as a review of the sound laws. If the ducting resistance has been incorrectly assessed, the fan noise can be significantly affected.
This Chapter points out some of the pitfalls in the selection of ductwork of the ventilation system which contribute to the addition of unforeseen noise.
A prime source of noise in any air moving system is the main fan. It has the ability to direct its duct-borne noise to the farthest corners of any occupied space and can be a major irritant. The problem can, of course, be magnified by the addition of system generated noise. To the humble fan engineer, it seems remarkable from a noise point-of-view, therefore, that so little apparent attention is given, in the design of a ventilation system, to the correct selection of the fan. To this must be added the often less than ideal ductwork connections to the fan, which can result in an additional unforeseen noise.
It is the intention of this Chapter to point out some of the pitfalls and to suggest that the requisite information be obtained from a reputable manufacturer at the earliest possible time. Unfortunately this is not always possible, as the fan supplier will only be chosen late in the building programme when much of the design has been “frozen”. It would be beneficial, however, to conduct a feasibility study using results obtained from experiments beforehand.
The user’s primary aim is to ensure that the fan will satisfactorily perform its duty. That is to say, it will handle the required volume flowrate at the system pressure and for the stated power. Even more important, however, is what nuisance will be caused, by its noise, to operators of the plant, to neighbours, or to inhabitants of the conditioned area. So many misconceptions, half-truths, and errors have been propagated in the field of acoustics, that one might imagine it had replaced alchemy as the “black art” of 20th century man.
This Chapter is not intended to be a textbook of noise measurement, and those who wish to know more are referred to the references in Section 14.15. However, in orderto give meaningful information, it is worth reminding the user of some of the terms employed and their values and underlying concepts.
Noise may simply be defined as:
Sound undesired by the recipient.
Sound may be defined as any pressure variation in a medium — usually air — that can be converted into vibrations by the human eardrum, causing signals to be sent to the brain. As with all other sensations, the result can be pleasant or unpleasant.
To vibrate the eardrum it is necessary for the pressure variations in the medium to occur rapidly. The number of variations per second is called the frequency of the sound, measured in cycles per second or Hertz. The human ear can detect sounds from about 20 Hz to 20,000 Hz — the lowest and highest sounds respectively. As a guide, the lowest note on a piano has a frequency of 27.5 Hz, whilst the highest note is at 4186 Hz.
The noisiness of a fan can be expressed in terms of its sound power (the number of watts of power it converts into noise). It is unusual to do this, however, as the range of values found in practice would be very large. Fan noise can be measured by its sound power level, a ratio which logarithmically compares its sound power with a reference power, the Pico Watt (10-12 watts). The unit of sound power level is the decibel.
Sound power level may be defined as:
SWL =10 log——————————— Equ 14.1
W0
Where:
SWL = sound power level in decibels (re 10-12 watts)
W = sound power of the noise generating equip
Ment (watts)
W0 = reference power (re 10’12 watts)
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Table 14.1 shows how the logarithmic scale compresses the wide range of possible sound powers to sound power levels having a practical range of 30 dBW to 200 dBW.
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Table 14.1 Sound powers expressed as sound power levels
The sound power level of a fan is comparable to the power output of a heater. Both measure the energy (in one case — noise energy, the other — heat energy) fed into the environment surrounding them. However, neither the sound power level nor the power output will tell us the effect on a human being in the surrounding space.
In the case of a heater, the engineer, by considering the volume of the surroundings, the materials of the room, and what other heat sources are present, can determine the resulting temperature at any point. In a similar way, the acoustic engineer, by considering very similar criteria, can calculate the sound pressure level at any point. (Remember, it is sound pressure that vibrates the eardrum membrane and determines how we hear a noise.)
Sound pressure levels are also measured on a logarithmic scale but the unit is the decibel re 2 x 10‘5 Fa. There is another advantage in using the decibel scale. Because the ear is sensitive to noise in a logarithmic fashion, the decibel scale more nearly represents how we respond to a noise.
SPL = 20 log — Equ 14.2
Po
Where:
SPL = sound pressure level in decibels (re 2 x 10 5 Fa)
P = sound pressure of the noise (Pa)
P0 = reference pressure (= 2 x10-5 Pa)
It should be realised that in specifying a sound pressure level, the distance from a noise source is implied or stated. In Table
14.1 The position of the observer relative to the source is indicated.
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Sound pressure Pa |
Sound pressure level dB |
Typical environment |
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200.0 |
140 |
30 m from military aircraft at take-off |
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63.0 |
130 |
Pneumatic chipping and riveting (operator’s position) |
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20.0 |
120 |
Boiler shop (maximum levels) |
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6.3 |
110 |
Automatic punch press (operator’s position) |
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2.0 |
100 |
Automatic lathe shop |
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0.63 |
90 |
Construction site — pneumatic drilling |
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0.2 |
80 |
Kerbside of busy street |
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0.063 |
70 |
Loud radio (in average domestic room) |
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0.02 |
60 |
Restaurant |
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0.0063 |
50 |
Conversational speech at 1 m |
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0.002 |
40 |
Whispered conversation at 2 m |
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0.00063 |
30 |
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0.0002 |
20 |
Background in TV and recording studios |
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0.00002 |
0 |
Normal threshold of hearing |
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Table 14.2 The position of the observer relative to the source |
Note: 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 2 x 10’5 Pa are different units.
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 standard 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.
Noise usually consists of a mixture of notes of different frequencies, and because these different frequencies have different characteristics a single sound power level is not sufficient in itself to describe the intensity and quality of a noise.
Noise is therefore split up into octave bands (bands of frequency in which the upper frequency is twice that of the lowest) and a sound pressure level is quoted for each of the bands. The octave band frequencies universally recommended have mid-frequencies of 63, 125, 250, 500, 1000, 2000, 4000, and 8000 Hz.
It is now becoming an increasing requirement for data at 31.5 Hz and 16000 Hz to also be included, although for a number of reasons the former is exceedingly difficult to measure with any degree of certainty.
The noisiness of a fan is specified by a number of sound power levels (in decibels re 10-12 watts), each corresponding to an octave band of frequencies. For research and other purposes it is also possible to measure the noise in more precise bands e. g. y3 octave or at so-called discrete frequencies.
As with sound power levels, sound pressure levels must be quoted for each octave band if a complete picture of the effect of the noise on the human ear is required.
The effect of a sound source such as a fan on its environment can be likened to dropping a pebble into a pond. Ripples will spread out uniformly in all directions and will decrease in height as they move from the point where the pebble was dropped. Normally the ripples will be circular in shape unless affected by some barrier. See Figure 14.1
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Sound Source
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Absorbed ♦ »Transmitted
Figure 14.1 Sound in a free field (above) and sound incident on a surface (below)
It is just the same with a sound source in air. When the distance doubles, the amplitude of the sound halves, and this is a reduction of 6 dB, for using equation 14.2:
Reduction = 20 log — = 20 log 2 = 6 dB
Pi
But the power of the sound source and therefore the SWL is unchanged.
To summarise, if you move from one metre from the source to two metres, the SPL will drop by 6 dB. If you move to four metres it will drop by 12 dB, eight metres by 18 dB, and so on. But this is only true if there are no objects in the path of the sound, which can reflect, or block.
Ideal conditions where the sound can spread unhindered are termed “free field". If there is an object in the way, some of the sound will be reflected, some absorbed, and some transmitted right through. How much is reflected, absorbed, or transmitted depends on the properties of the object, its size, and the particular wavelength of the sound. Generally speaking an object must be larger than one wavelength to have an effect.
I xu Speed of sound =340/s
Wavelength = ———————-
Frequency Hz
For example
340
Sound of 8K Hz : wavelength 340 =- = 0.425 m
8×1000
Sound of 63 Hz: wavelength = = 5.4 m
63
Hence for a high frequency noise even a very small object will disturb the sound field and absorb or isolate it. But low frequency noise, whilst less objectionable, is more difficult to block.
Source
/
AA/WvVWVWVWW:
Figure 14.2 Sound in an anechoic chamber
Sound reflecting or reverberation chambers
This is the opposite of the anechoic chamber. All surfaces are made as hard as possible to reflect the noise and all the walls are made at an angle to each other so that there are no parallel surfaces. Thus the sound energy is uniform throughout the room and a “diffuse field” exists. It is therefore possible to measure the SWL, but the SPL measurements in any direction will be meaningless due to the many reflections. Such rooms, see Figure 14.3, are cheaper to build than anechoic chambers and are therefore very popular.
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If we wished to make measurements in a free field without any reflections, then the top of a very tall but small cross-section flagpole in the middle of the Sahara desert (after it had been raked flat) would probably be ideal. Obviously there are difficulties and an anechoic room is a reasonable alternative. Here the walls, ceiling and floor are covered in a highly sound absorptive material to eliminate any reflections. Thus the SPL in any direction may be measured. See Figure 14.2. WWWTOWWV Sound 5 |
Should be made. Sometimes, however, conditions are so reverberant or the room so small, that a free field will not be present. Afan in a “real room” is shown diagrammatically in Figure 14.4.
Relationship between sound pressure and sound power levels
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SPL = SWL+ 10 log |
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Equ 14.3 |
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Where: SPL SWL R Qe Rc |
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(m2 |
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Figure 14.3 Sound in reverberation chamber
In practice we usually wish to make measurements in a room that is neither anechoic nor reverberant, but somewhere in between. It is then difficult to find a suitable position for measuring the noise from a particular source.
When determining noise from a single fan, several errors are possible. If you measure too closely, the SPL may vary considerably with a small change in position when the distance is less than the wavelength of the lowest frequency emitted or less than twice the greatest dimension of the fan, whichever is the greater. This is termed the “near field” and should be avoided.
Other errors arise if measurements are made too far from the fan. Reflections from walls and other objects may be as strong as the direct sound. Readings will be impossible in this reverberant field. A free field may exist between the reverberant and near field and can be found by seeing if the level drops 6 dB for a doubling in distance from the fan. It is here that measurements
The relationship between SPL and SWL is given as:
47ir
Sound pressure level dB (re 2 x 10’5 Pa)
Sound power level dBW (re lO’12 W)
Distance from the source (m)
Directivity factor of the source in the direction of r
Saa
Room constant =
S = total surface are of the room (m2)
Aav = average absorption coefficient in the room
The first term, within, the brackets is the “direct” sound, whilst the second term is “reflected” sound.
The value of the average absorption coefficient aav can be calculated.
If we have an area S, of material in the room having an absorption coefficient a-i, and area S2 with absorption coefficient a2,
And so on, aav = — (S^ +S2a2 +S3a3 +etc)
S
A not only varies with the material, but also differs according to the frequency of the noise. It is therefore necessary to calculate the SPL from the SWL in each frequency. Some typical values of absorption coefficient a can be found in Table 14.3.
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— SWL -10 log 2nr |
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SPL = SWL + 10 log |
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4nr |
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-10 |
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-20 DB -30 |
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0 |
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High reverberant sound pressure level to be built up. When this Understanding the difference between sound power level and It is inconvenient to quote a series of sound values for each ap- To obtain basic sound pressure level, re 2 x 10 5 Pa under free Weighted sound pressure levels А, В, C, and D noise levels are an attempt to produce single +10 |
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-50 |
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Figure 14.9 Weighted sound pressure curve С |
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+10 |
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0 |
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-10 |
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-30 |
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-40 |
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О О о о со о |
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ООО ООО О о ю СО о N |
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Hz |
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-50 |
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HP |
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Figure 14.10 Weighted sound pressure curve D Theoretically dBA values apply up to levels of 55 dB only, dBB for levels between 55-85 dB only and dBC for higher levels only. dBD is reserved for special noise, e. g., aircraft. However dBA is now used almost exclusively whatever the level. Engineers should check what weighting curves have been used by manufacturers and, if necessary convert them to a common base before comparisons are made. A, B, C and D weightings are useful for making initial assessments (inexpensive sound level meters are available which measure directly on these scales). Unfortunately too much information is lost in combining all the data into one figure for it to be of use for calculation and design work. Most noise control depends on frequency analysis. Posted in Fans Ventilation A Practical Guide |