Page 285 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

262 BIOMECHANICS OF THE HUMAN BODY

10

0

–10 W k

Frequency weighting, dB –30 W d W h
–20

–40

–50

–60

–70

–80
0.1 1 10 100 1000 10000
Frequency, Hz
FIGURE 11.1 Frequency weightings for whole-body (W and W ) and hand-transmitted (W ) vibration. W and W are for the z direction
h
d
k
k
d
and x and y directions, respectively, and are applicable to seated and standing persons (see Fig. 11.2). W is for all directions of vibration
h
entering the hand. The filters are applied to acceleration-time histories a(t). (ISO 2631-1, 1997; ISO 5349-1, 2001.)
/ 12
T
Ea , ) , 22 = ⎡ ⎣ ⎢∫ 0 [ a t)] 2 dt ⎤ ⎦ ⎥ (11.4)
(
(
T
w
w
For an exposure continuing throughout a working day, T = T = 28,800 s, and Eq. (11.4) can be
(8)
written [using Eq. (11.2)]:
12
/
⎡ 1 T ()8 ⎤ ⎤
/
/ 12
T
(
Ea , ) , 22 = T ( ) ⎢ ∫ [ at)] 2 dt ⎥ = T 12 a RMS(8) (11.5)
(
w
8 ()
8
w
⎣ ⎢ T () 0 ⎦ ⎥
8
where a RMS(8) is the 8-hour, energy-equivalent, frequency-weighted RMS acceleration.
A second function, used for exposure to whole-body vibration, is the vibration dose value, VDV,
for which F(a (t)) = a (t) and m = r = 4. The function is thus:
w w
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T
VDV = Ea , ) , 44 = ⎡ ⎣ ⎢∫ 0 ( a t)] 4 dt ⎤ ⎦ ⎥ (11.6)
(
(
T
w
w
which is more influenced by the large amplitudes in a fluctuating vibration than the energy-equivalent
exposure. A related function, the severity index for which F(a (t)) = a (t), m = 2.5, and r = 1, is
w
w
sometimes used for the assessment of head impact, though it cannot be applied to continuous
acceleration-time histories owing to the value of m.