Download DESIGN GUIDANCE FOR INTERIOR NOISE REDUCTION IN LIGHT PDF

TitleDESIGN GUIDANCE FOR INTERIOR NOISE REDUCTION IN LIGHT
LanguageEnglish
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Page 44

MIL-HDBK-767(MI)

models, the anticipated noise level for each suspension
component is obtained when predicted dynamic forces from
each suspension component are multiplied by the measured
(or predicted) noise-to-force transfer functions of the hull.
One dynamic force model. original]} designed to predict
suspension components forces in an M 113-sized vehicle. is
called TRAX1ON (Refs. 1 and 2) and was developed by
Bolt. Beranek, and Newman, Inc., (BBN) Cambridge, MA.
Later modifications allowed the model to be used on M1-
sized vehicles. Table 5-1 lists the required input parameters
to TRAXION.

TRAXION assumes several simplifications, such as rigid
suspension, flat ground profile, frequency-independent
parameters, and a two-dimensional analysis. For a given
speed TRAXION creates a time history of the vertical and
longitudinal forces generated at the sprocket, idler, and road
wheel attachments. When subjected to a spectral analysis.
this time history yields a one-third octave band spectrum.
Multiplying this spectrum by [he noise-to-force transfer
function at the hull attachment location gives the predicted
interior noise for each suspension component.

How to measure noise-to-force transfer functions for
existing vehicles is described in subpar. 8-2.3: for new vehi-
cles, however, these functions must be predicted. NOISE. a
computer program developed by FMC Corporation, San
Jose, CA, predicts these functions using normal modes anal-
ysis from a finite element model of the hull to predict the
radiation efficiency of the hull plates and the noise produced
by a unit force at a specified location on the hull (Ref. 3).
Normal modes analysis of the hull structure can be per-
formed using commercially available finite element analysis
programs, such as ANSYS or NASTRAN. Fig. 5-1 illus-
trates the steps involved in predicting intenor noise levels.

5-3 TRACK DESIGN
As described in par, 5-1, the source of suspension system

noise (vibrational forces) can be reduced by appropriately
modifying the track. The elements of track shoe design that
influence noise generation are

1. Shoe mass
2. Pitch (length of shoe)
3. Shoe flexibility
4. Compliance of inner track surface
5. “Flatness”’ of road wheel running surface.

Track system variables that influence interior noise levels
include

1. Vehicle (track) speed
2. Track tension
3. Track age.

The paragraphs that follow describe each of the track
shoe and track system parameters.

5-3.1 TRACK SHOE MASS
It is shown in par. 4-2.1 that chordal action forces on the

sprocket and idler wheel are proportional to the mass of the
track shoe. Thus, as shown in Eq. 4-5, reducing track shoe
mass 50% should lower interior noise about 3 dB. However.
when aluminum track (25% lighter than steel) was used on
an XM800T scout vehicle. interior noise was lowered 3 to 8
dB over speeds of 8 to 64 km/h (5 to 40 mi/h) (Ref. 4)-a
reduction much greater than that predicted by chordal action
theory. One reason for this discrepancy may be that Eq. 4-5
assumed that the strain energy at the shoe/wheel interface
was stored as a linear spring. When a track shoe has a rub-
ber pad, however, the rubber is in compression when it con-
tacts the wheel, which results in a nonlinear, increasing
spring rate. Consequently, the smaller deflection of the rub-
ber pad on the lighter track shoe produces smaller impact
forces, which result in less noise than predicted.

5-3.2 TRACK SHOE LENGTH (PITCH)
Decreasing the track shoe length provides at least three

key benefits:
1. It lowers the impact velocity of chordal action.
2. It raises the tracklaying frequency, as shown in Eq.

4-6, beneficially increasing impedance mismatch between
suspension components and hull structure, thus lowering the
amount of hull vibration.

TABLE 5-1. INPUT PARAMETERS FOR TRAXION
SUSPENSION MODELING PROGRAM

5-2

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