4.3 Noise shield measurements
4.3.1 Measurement procedure
When the noise shield panel is folded up, the loudspeaker array is po-sitioned in front of the noise shield. Fig. 4.9(a) shows the final position of the loudspeakers, which was reproduced for every measured con-figuration of the noise shield. The distance between the loudspeaker membranes and the surface of the noise shield is adjusted to be ap-proximately 0.26 m everywhere along the loudspeaker array surface.
Several types of excitation signals are available to evaluate the sound
(a)Noise shield with loudspeaker array in position.
400 mm
400mm
(b)Microphone array for mapping the sound level inside the cabin.
Figure 4.9: Photographs of the measurement setup with the excitation sys-tem in front of the noise shield and the microphone array inside the cabin.
transmission through the noise shield for different incident sound fields.
A broadband pink noise signal (bandwidthf = 60. . .1000 Hz) is used to characterize the noise shield over a wide range of frequencies. To emulate typical noise fields generated by CROR engines, multi-tonal noise fields with arbitrary trace wave numbers imprinted on the noise shield are employed.
The different sound fields on the surface of the noise shield, as gen-erated by the loudspeaker array, are characterized using the moveable microphone array between the fuselage and the loudspeakers. The mi-crophones allow the measurement of the sound pressure field at 240 points distributed on a rectangular grid across the surface covered by the loudspeaker array. Fig. 4.10 shows the measured sound pressure level and phase angle fields for the pink noise excitation at a represen-tative frequency off = 100 Hz. It can be seen that the sound pressure level distribution is nearly uniform with LP ≈ 85 dB, except for a considerable sound level reduction near the vertical center line of the loudspeaker array. The explanation for this particular inhomogeneity in
0 0.25 0.5 0.75 1 1.25 1.5 1.75
Figure 4.10: Measured spatial distribution of the sound pressure level and phase angle atf = 100 Hz for the pink noise excitation.
the pink noise sound field can be found in the phase angle distribution shown in Fig. 4.10(b). The left and right halves of the loudspeaker array exhibit a phase difference of 180◦ which leads to cancellation effects at the vertical center line. This phase difference is caused by the internal circuitry of the two amplifiers driving each half of the loudspeaker ar-ray and could not be compensated for the pink noise fields at the time of the measurements. Therefore, when discussing measurement results obtained under pink noise excitation, this particular characteristic of the pink noise field needs to be accounted for.
An example for one of the multi-tonal noise fields at a frequency of f = 100 Hz is shown in Fig. 4.11. The imprinted trace wave propagates inx-direction (i.e. along the fuselage axis) with an axial wave number component of kx = 5.5 rad/m, as can be seen in the phase angle dis-tribution in Fig. 4.11(b). The sound pressure level is mostly uniform atLP ≈88 dB across the loudspeaker array (see Fig. 4.11(a)). Several wave numbers ranging from 2.5 rad/m (long wavelength) to 8.5 rad/m
0 0.25 0.5 0.75 1 1.25 1.5 1.75
Figure 4.11: Measured spatial distribution of the sound pressure level and phase angle atf = 100 Hz for the multi-tonal excitation with an imprinted trace wave number ofkx= 5.5 rad/m.
(small wavelength) as well as wave directions (axial kx, circumferen-tialky, and diagonalkxy) are investigated. In case of these multi-tonal sound fields, the phase mismatch of the two amplifiers could be com-pensated.
Inside the cabin of the demonstrator fuselage, the resulting sound pressure field is measured using a microphone array carrying 43 1/2”
diffuse field microphones in a grid-like pattern with a regular micro-phone spacing of 400 mm, as shown in Fig. 4.9(b). This array allows the simultaneous measurement of a whole cross-section through the cabin and can be traversed along the fuselage axis to capture the whole cav-ity. For every measurement, the array is moved to 15 stations with a uniform spacing of 533 mm along the fuselage axis (equal to the frame spacing), so that the total number of measurement points inside the cabin is 645. For the evaluation of the measurement results, this large number of data points is condensed to a single value per frequency by calculating the average sound pressure level (SPL) inside the cabin, defined as
hLPicabin= 10 lg
1 Vcabin
Z Z Z
Vcabin
100.1LP dVcabin
, (4.1) whereVcabin≈37 m3 is the measurement volume inside the cabin and LP is the sound pressure level. Since LP is measured only at discrete points, the integral in Eq. (4.1) is calculated numerically using the trapezoidal rule. In order to obtain a measure for the noise shield trans-mission loss, the spatially averaged noise reduction hNRi is employed.
This quantity is closely related to the TL of the noise shield and is defined as
hNRi=hLPiexc− hLPicabin, (4.2) wherehLPiexc is the surface average of the excitation field sound pres-sure level [36]. hLPiexc is obtained from the measured spatial sound
pressure level distributions (as for example shown in Fig. 4.10(a)) us-ing an integration procedure over the loudspeaker array surface, similar to Eq. (4.1) for the interior cavity.
In order to identify the acoustic performance of the MAM elements integrated inside the noise shield structure, a total of three different noise shield configurations and, as a reference, the bare fuselage struc-ture without a noise shield are investigated. These configurations are listed in Table 4.4 with the estimated total surface mass densitym00totof each configuration. Configuration A corresponds to the reference case where the cabin sound pressure levels without any noise shield structure are measured. A photograph of noise shield configuration B is shown in Fig. 4.12(a). This configuration represents a conventional double wall design, where the cover sheet of the noise shield is filled with glass wool packages. The resulting total surface mass density is nearly twice as large as that of the fuselage alone. Configuration C basically is the same as the fully assembled MAM noise shield (configuration D), how-ever with the MAM elements removed (see Fig. 4.12(b)). The rubber strips are kept inside the noise shield for this configuration in order to act as dummy masses. Thus, by comparing configurations C and D, the influence of the MAMs on the sound transmission properties of the
Table 4.4: Descriptions and estimated total surface mass densities of the measured noise shield configurations.
Conf.: Description: m00tot(kg/m2)
A No noise shield (fuselage only) 10.3
B Cover sheet filled with glass wool (double wall) 19.7 C Noise shield without MAM elements, but rubber
strips as dummy masses
21.9 D Noise shield with MAM elements and rubber
strips
25.5
(a)Double wall configuration. (b) Noise shield without MAM ele-ments.
Figure 4.12: Photographs of the additionally measured noise shield config-urations B and C, as listed in Table 4.4.
noise shield can be evaluated. The total surface mass density of con-figuration C is 11 % higher than that of the double wall (concon-figuration B). With the MAM elements integrated, the surface mass density of the noise shield is 29 % larger.