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Propagation of AE Wave in the Air that was Occurred in a Window Frame Kunihisa KATSUYAMA, Masahiro SETO National Institute for Resources and Environment 16-3 Onogawa, Tsukuba, Ibaraki, Japan 305-8569 Koichi Hanasaki Kyoto University Yoshidahonmachi, Sakyo, Kyoto, Japan Contact

ABSTRACT

Window frames of an excellent hotel were warmed up by sunshine in the summer. The window frames were stressed by the thermal stress, and frames squeaked because of the sudden changes in temperature. This phenomenon is called one of Acoustic Emission (AE). Many guests could hear those squeaks, but they could not find out the hypocenters. So, the acoustic emissions and the squeaks were measured by an AE instrument and by sound-level meters.
The hypocenters which were occurred in the window frames were decided by the AE instrument. But it was very difficult to decide them by the sound-level meters because of the difference of the propagation velocity of P-wave in the window frames and in the air.

INTRODUCTION

The occurrence of various kinds of phenomena in materials gives rise to the emission of elastic waves. These waves are called acoustic emission (AE) or stress wave emission (SWE). AE technique is used very well for material science, non-destructive test of facilities in industry and predicting fracture.
At the excellent hotel, a lot of complain that squeaking noise could be heard around windows frame came out from many guests. Employees of the hotel listened carefully and investigated where the squeaking noise happened. They could find out rooms where the squeaking noise could be heard, but they could not decide the window frames where they happened. Therefore, AE sensors were set up on the window frames to obtain hypocenters of AE. In this paper, propagation phenomena that AE waves which generated in the window frames propagate in the frames and they went out into the air as sound were discussed.

EXPERIMENTAL PROCEDURES

The photo of the hotel is shown in Fig. 1. As the section of a floor of this cylindrical building is shown in Fig.2, it is divided into 24 pieces. One floor has 16 guest rooms, linen rooms, emergency exit stairs and an elevator room. The sun moves one room in one hour. Two rooms of 9^th floor and 10^th floor were used for measuring AE and squeaking sound. The reason why those two rooms were used was that one window curtain wall which is made of aluminum was used for those two rooms as shown in Fig. 3. Twelve AE sensors were attached at the window frames as shown in Fig. 4. A sound meter and two directional sound meter were used as shown in Fig. 5.

Fig 1: Photo of the hotel

Fig 2: A section of one floor of the hotel

Fig 3: Rooms used for measuring AE

Fig 4: AE sensor attached at the aluminum window frame

Fig 5: A sound-level meter near the window frame


AE measurement apparatus: NF7661
Sensors: 12......AE-901D       Pre-amplifier: 12.....AE-912
PC.....Mac     Data recorder: Jaz disk drive(1GB)
Sound directional sound meter: 2....Sony C-48
Sound level meter: 1.....Rion NA-29
Measurement of temperature : Adachi AM-7052
Measurement of radiant heat.: Eikouseiki MS-801

EXPERIMENTAL RESULTS

Upper figure of the Fig.6 shows temperature of the window frame and lower figure of the Fig.6 shows event number of the squeaking noise. The horizontal axis is time (hour). Mark "­" shows 6 o'clock in the morning in June. The frame rise in temperature from this time. At 2 o'clock , it began to rain. Temperature fell suddenly. The squeaking noise as shown in lower figure of Fig.6 happened when temperature changed suddenly.
Fig. 7 shows typical AE wave forms measured by AE sensor attached at the window frame.

Fig 6: Experimental results of temperature of the window frame and squeaking noise gure

Fig 7: Typical result of AE wave forms

SIMULATION OF AE PROPAGATION AND SOUND PROPAGATION

Fig 8: A force given at mark in Fig. 9 and Fig.11.

The computer used for analysis is IBM·RS6000·SP. Program is ANSYS which is made for a wide use of structural analysis by FEM.
Fig. 8 shows a force given at one point which is assumed to be a hypocenter of AE. Fig. 9 shows the simulation result in the case of sound propagation in the air of the room. The size of the area calculated by FEM is 100 cm×100 cm, and one mesh size is 1cm×1cm. At the center of the left hand side of the air, the force as shown in Fig.8 was given to the air. The sound propagates concentrically. And the sound wave forms at points 1~9 are shown in Fig.10. The amplitudes of the wave forms become smaller with distance from the hypocenter. The coordinates of these points are as follows.
1( 20, 0), 2(10,-20), 3(20,20) , 4(40,10), 5(40,-20), 6(10,15), 7(0,-24), 8(6,7), 9(13,-20)
unit: cm
Fig.11 shows the simulation result in case of with window frame. AE hypocenter happened in the aluminum frame, and AE propagates in the frame at first with the velocity of 6420m/s, then it propagates in the air with the velocity of 340m/s. So, the sound does not propagate concentrically. Fig. 12 shows the sound wave forms at point 1~9. The amplitude of sound wave forms near the hypocenter is not always big.
The coordinates of these points are as follows;
1(20,30), 2(10,10), 3(20,10), 4(40,40), 5(40,10), 6(10,10), 7(0,6), 8(6,37), 9(13,10)

Fig 9: The simulation result of the concentric sound propagation in case of without window frame

Fig 10: Amplitudes of the sound at points 1~9 in case of without window frame

Fig 11: The simulation result of the sound propagation in case of with the window frame

Fig 12: Amplitude of the sound at point 1~9 in case of with window frame

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