Influence of background noise on non-contact vibration measurements using particle velocity sensors Daniel FERNANDEZ COMESAÑA 1 ; Fan YANG 1,2 ; Emiel TIJS 1 1 Microflown Technologies, the Netherlands 2 University of Stuttgart, Germany ABSTRACT Structural vibrations are usually measured using accelerometers or laser Doppler vibrometers. However, the weight of the accelerometers can influence vibration patterns and lasers can have problems with non-reflective or fibrous materials. Alternatively, vibration can also be measured acoustically using particle velocity sensors positioned near the structure. Although the capabilities of this approach have been demonstrated in several studies, the influence of background noise on the results has been considered insufficiently thus far. This paper explores how non-contact vibration measurements using particle velocity sensors are affected by external sound sources; results from both simulations and measurements are presented and analysed. Keywords: particle velocity, non-contact vibrations, background noise. I-INCE Classification of Subjects Number(s): 72.2.2 1. INTRODUCTION Accelerometers and laser vibrometers are common transducers used in most vibro-acoustic problems. Both sensors give reliable results under favourable conditions. However, attaching sensors to a vibrating structure is not always possible and accelerometers add a mass load that may significantly influence the panel surface vibration. On the other hand, a non intrusive approach using laser vibrometers may seem more attractive. Optical non-contact solutions, such as Laser Doppler Vibrometry (LDV) (1), enable the fast acquisition of a large number of measurements with good spatial resolution. On the other hand, the high price, setup complexity and the fact that non-reflecting or fibrous materials may cause difficulties during the measurement process, limit the use of LDV in many applications. Alternatively, acoustic particle velocity sensors have also been proven suitable for performing non-contact vibration measurements (2, 3). Under specific conditions, surface velocity is proportional to particle velocity allowing for the acquisition of vibrational information with an acoustic transducer. Several studies have revealed the potential of particle velocity sensors for characterising structural vibrations (4, 5, 6, 7, 8, 9), but thus far the presence of noise has not been assessed in detail. This paper studies the impact of noise during the data acquisition process upon spectral estimations of structural vibrations from a theoretical and experimental point of view. 2. THE VERY-NEAR FIELD THEORY The following derivation for relating the velocity of a vibrating surface and the particle velocity above it follows the work of de Bree et. al. introduced in (4, 5). It begins by studying the definition of the Helmholtz wave equation in terms of velocity potential Ψ(x), i.e. ▽ 2 Ψ + k 2 Ψ = 0 (1) where ▽ 2 is equivalent to the Laplace operator and k is the wave number (2π f /c 0 ). A description of the sound field near a vibrating surface can be obtained by evaluating Equation (1) with the following boundary conditions:        u n = ∂ Ψ/∂ n if x = 0 Ψ∝ e jkx /x if x → ∞ (2) 1 Inter-noise 2014 Page 1 of 6