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International Journal of Basic & Applied Sciences IJBAS-IJENS Vol:12 No:06

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Assessment of Acoustic Performance of Anechoic Chamber at Acoustic Laboratory in Syiah Kuala University Zulfian and Lindawati Abstract — This paper describes acoustic performance of anechoic chamber of acoustic laboratory in University of S yiah Kuala. The chamber has a free space of 6.0 meters in length, 4.5 meters in width, and 3.1 meters in height with a lower cut off frequency of 125 Hz. The assessment demonstrates that the performance of the chamber is consistent with the free field characteristics within the tolerance allowed by IS O 3745; it also has sufficient sound insulation for low noise level. The background noise level is 23.9 dB (A-Weighting).

Index Term— Anechoic chamber, acoustic laboratory, a lower cut-off frequency.

1. INTRODUCTION Anechoic chamber is a room designed to be echo free field, which is completely absorb reflections of sound. Sound absorption is obtained by lining the wall, ceiling and floor of the room with absorbent material [1-2]. In general, the acoustic performance of anechoic chambers is tested and quantified in accordance with ANSI S12.35 and ISO 3745, standards to determine Sound Power Level (SPL) of noise source in a chamber [1-4]. The degree at which the SPL measurements follow the inverse-square law indicates sound free-field condition of anechoic chamber. In practice, the required inverse–square law condition is achieved by attaining energy absorption of 99% incident sound, corresponding to 0.1 pressure reflection of boundary surface. The frequency at which the energy absorption drops below 99% or the pressure reflection exceed 10% is known as the low cut–off frequency. Acoustic laboratory of Syiah Kuala University has one fully anechoic chamber, one reverberation chamber, and one control room. The anechoic chamber is constructed from IAC Metaldyne anechoic wedges with type of fiber glass materials. It is intended to be used for the following:  Measurement of sound power level of direct sound from noise source  Testing and calibration of sound devices and electro– acoustic instrument (microphone, speaker, warning sirens )  Measurement of radiation character of sound source  Experiment of sense of hearing and psycho – acoustic test  Experiment of acoustic signal processing. In order to achieve these objectives of the anechoic chamber, it was decided to design the acoustic treatment to limit cut-off

frequency of 125 Hz with sufficient sound insulation for low noise level. The evaluation of anechoic chamber performance must be made in order to quantify the measurement error. In this paper the assessment of acoustic performance of anechoic chamber at acoustic laboratory in Syiah Kuala University is presented. II. STRUCTURE OF A NECHOIC CHAM BER 2.1 Dimension of chamber The dimension of anechoic chamber is 7.8 meters in length, 6.3 meters in width, and 5 meters in height. The whole anechoic chamber is double wall structure with vibration insulation to reduce both air and structure borne noise transmission into the chamber. The outer wall is brick wall and the inner wall is IAC Moduline single wall of perforated metal panel system. The chamber sits on the isolated mount with 6Hz natural frequencies achieved in the building. 2.2

Wall and Ceiling The anechoic chamber wall and ceiling consist of noise– lock metapanel system. The inner walls, floor and ceiling surface of anechoic chamber are fully covered with metadyne anechoic wedges with a clear dimension of 2.98 meters in length, 3.71 meters in width, and 2.08 meters in height. The fully anechoic chamber meets the specification of ISO 3745. The sound insulation performance between the control room and the anechoic chamber designed with three layers of doors is 96.8dB at 500 Hz. The dimension of the chamber measured from tip to tip of wedges is 6.0 meters in length, 4.5 meters in width, and 3.1 meters in height. 2.3 Doors The anechoic chamber is accessed through two acoustic doors one is mounted on the interior door and other on the external door. The interior door is combined with sound wedges, which are installed at the interior wall and swing into anechoic chamber. Its dimension is 1.26 meters in length and 2.17 meters in width. The external door is installed at the outer wall structure and swings outward into the control room. The dimension of this external door is 1.27 meters in length and 2.17 meters in width.

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International Journal of Basic & Applied Sciences IJBAS-IJENS Vol:12 No:06 2.4

Floor The wall and ceiling of the chamber is built on an IAC insulated door. The floor surface is covered continuously with metadyne wedges, while the working floor is provided with steel grafting floor. 2.5

Ventilation The anechoic chamber is furnished with ventilation system, which is remote from the control room. Ventilation system consists of in fate and exhaust silencers with flow box. The air glow is calculated to meet the requirement of the chamber when then ventilation system on, the background noise level in anechoic chamber is 23.9 db (A). 2.6

Lighting and Electrical System Lighting is provided by 6 x 80 watt incandescent light with low noise emission light and 500 lux at illumination level. Electrical system provides a minimum of acoustically reflective surfaces. Power is brought by other to central terminal box mounted outside the chamber III.

A SSESSM ENT OF A COUSTIC PERFORM ANCE 3.1 Background Noise Level The background noise measurements were conducted at three different times of the day (10am, 2pm, and 5pm) with the air-conditioning both on and off. The measurement was done in accordance with ISO 3745 standard. Instrumentations e.g. Microphone Larson Davis Laboratories 2541, PreAmplifier Larson Davis Laboratories 900B, and Larson Davis Time Analyzer & Sound Level Meter 2900 were utilized in the measurement. One-third octave band levels were measured ranging from 50Hz to 10kHz. 3.2

Sound Insulation Performance Sound insulation tests were performed in order to assess the sound insulation performance of anechoic chamber. These tests were conducted inside and outside of the chamber. One microphone inside the chamber recorded the incident sound pressure level, while another outside the chamber recorded the transmitted sound pressure level. Pink noise was used as source for the measurements . The sound insulation measured is presented in noise isolation class (NIC) term by plotting the difference of the source and the receiver measurement (corrected for ambient) against a family of NIC curves in order to determine the overall NIC value. Inverse – square Law Inverse–square law is the most commonly used in assessing the free field characteristic of anechoic chambers . 3.3

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Pink noise generated in the Nors onic N840-2 was used as source for the inverse square law measurements . IV. RESULTS AND DISCUSSION 4.1 Background Noise Level The background noise levels measured are tabulated in Table I and Table II. T ABLE I BACKGROUND NOISE LEVEL (Air Conditioning off) Frequency (Hz) 10am 2pm 5pm

125

250

500

1k

2k

4k

8000

dB/A

0.4 1.5 1.8

2.8 4.4 4.6

0.6 2.1 0.3

2.6 2.1 2.4

4.1 3.9 4.2

5.6 5.7 5.8

6.4 6.5 7.3

16.7 16.9 15.3

T ABLE II BACKGROUND NOISE LEVEL (Air Frequency 125 250 500 1k (Hz) 10am 1.0 -0.4 -0.2 2.6 2pm 1.9 4.4 0.4 2.2 5pm 2.6 4.2 0.6 2.8

Conditioning on) 2k 4k 8000

dB/A

4.1 3.9 4.4

17.0 17.0 19.3

5.7 5.7 5.8

6.5 6.6 7.4

As seen in Table I and Table II, the background noise levels measured are very low for an anechoic chamber. There is no significant increasing when the air conditioning in turned on. The background noise level for anechoic chamber stated in ISO shall be at least 6dB (more preferably more than 12dB) with minimum source requirement of 23.9 dBA (minimum) and 29.9 dBA (preferred) are substantially lower than most common noise sources[5]. The background noise levels inside the anechoic chamber are lower than most of common noise source. Thus, the background noise levels inside the anechoic chamber are acceptable for assessing noise level. 4.2 Sound Insulation Performance The results for noise isolation class are summarized in the Table 3. Although sound insulation requirement for an anechoic chamber is not included in the ISO 3745, the noise insulation achieved by the anechoic chamber building element is satisfactory for low noise level [6]. T ABLE III NOISE ISOLAT ION CLASS (NIC)

Anechoic Chamber Door Element

NIC Rating

All three doors closed Int. doors open / ext & middle door close Int. & middle doors open / ext doors close Ext & int. doors open / middle doors close Ext & middle door open / int. door close

NIC – 76 NIC – 73

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NIC- 37 NIC – 43 NIC – 60

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International Journal of Basic & Applied Sciences IJBAS-IJENS Vol:12 No:06 4.3 Inverse – square Law According to the inverse – square law, the sound pressure level of noise sources will loss 6 dB per double distance. The maximum allowable variation of Sound Pressure Level in the ISO 3745 is shown in Table IV. T ABLE IV MAXIMUM ALLOWABLE SOUND PRESSURE LEVEL Frequency 125 250 500 1k 2k 4k (Hz) Allowable ±1.5 ±1.5 ±1.5 ±1.0 ±1.0 ±1.0

8k ±1.5

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university since 2001. Besides, he is an advisor of the University Development Project. Lindawati was born in Lhokseumawe, Indonesia, on 20 March 1986. She attended Syiah Kuala University, Banda Aceh and obtained her bachelor degree in Physics in 2008. In 2009, she pursued her master degree in University of Tun Hussein Onn Malaysia (UT HM) and awarded Master of Mechanical Engineering in 2012. After graduated in 2008, she worked at Acoustic laboratory of Unsyiah as Laboratory Assistant. She was also a Laboratory Instructor at Noise and Vibration Laboratory of University of T un Hussein Onn Malaysia (2010 2011). Today, she is a research assistant at Acoustic Laboratory of University of Syiah Kuala. Besides, she is teaching physics at University of U’budiyah, Banda Aceh.

The results obtained show that all of paths measured in 1/1 octave band in the frequency range of interest which demonstrate that the anechoic chamber’s performance is consistent with that of a free field and all within the tolerance allowed by the ISO 3745. This means that any points of anechoic chamber achieve free field properties. III. CONCLUSION The anechoic chambers performance is fully complian t with the free field inverse–square law requirement as specified in the ISO 3745. The noise insulation performance is satisfactory for low – noise level. The background noise level is 23.9 dB (A) which is minimum source requirement specified in ISO 3745 criteria, at the same time, is also sufficient thought in the anechoic chamber.

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Panchouly, M., Chhapgar, A.F., and Mohanan, V., “ Design and construction of an Anechoic Chamber at the National Physical Laboratory of India,” Applied Acoustic, 1981, 14, 101-11. Valis, A.G, Giulino, H.G.,and Mendez, A.M. “T he anechoic chamber at the Laboratory de Acoustica y Luminotocnia CIC,” Applied acoustics, 1995, 44, 79 – 94. Biesel, V.B. and Cunefare, K.A. “A T est System for Free-Field Qualification of Anechoic Chambers,” Sound and Vibration Megazine, May 2003. Sun, D., Jin, C., Schaik, V.A., and Cabrera, D. “T he Design and Evaluation of an Economically Constructed Anechoic Chamber,” Architectural Science Review,2009, 52(4), 312-319. ISO 3745: 1977, Determination of Sound Power Level of Noise Source–Precision Method for Anechoic and Semi–Anechoic Camber. Jonshon, S.B, Noise Qualification Report for Reverberation Room and Anechoic Chamber at Universitas Syiah Kuala, Banda Aceh. Singapore Report no: 902530, 2000.

Zulfian was born on in Banda Aceh, Indonesia on 9 December 1951. He attended Institute Technology of Bandung (IT B) and obtained his bachelor degree in physic engineering in 1977. In 1989, Zulfian attended Institute of Sound and Vibration (ISVR) of University of Southampton, United Kingdom, to complete his master in noise and vibration. He is a Lecturer at University of Syiah Kuala (Unsyiah), Banda Aceh, since 1981 until present. He is also a head of Acoustic Laboratory at the same

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