• Reliable acoustic particle velocity data
• Real time visualization of all relevant acoustic data
• One point methodology
• Full bandwidth
• Large dynamic range
• Low susceptibility to background noise
• Visualization of transients
• Free configuration of measurement grid
• Multi purpose tool
• Intuitive approach

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Reliable acoustic particle velocity data

The PU probe based acoustic camera can be used for acoustic near field measurements. Any sound field is described by two complementary acoustic properties, the scalar value 'sound pressure' and the vector value 'particle velocity'.

In the acoustic near field, acoustic particle velocity is the dominant acoustic property. Only as the sound field reaches the acoustic far field, sound pressure and particle velocity becomes of equal level.

The Microflown sensor is the only transducer available capable of measuring the acoustic particle velocity.
Combined with a 1/10'' sound pressure transducer, a PU probe measures in one single spot both required acoustic quantities.
Thus the PU probe based acoustic camera is the only sound field visualization method that can measure directly these two physical properties close to the surface of the sound source.
Based upon these two measured quantities, acoustic properties like sound intensity, sound energy and acoustic impedance can be computed easily.

All other near field (and far field) sound field visualization techniques try to compute the acoustic particle velocity out of sound pressure data captured at a certain distance from the surface of the sound source. But with sound pressure being a scalar value with low dynamic range and acoustic particle velocity being a vector value with high dynamic range, these problems are inevitably ill posed.

Whilst applying these other techniques, many assumptions have to be made that are often not met in practice. But even under the most ideal laboratory conditions, the reconstruction of acoustic particle velocity out of sound pressure data can result in errors up to 30dB, as proven in a peer reviewed JASA paper.

• Near Field Acoustic Holography with particle velocity transducers, Finn Jacobsen and Yang Liu,  JASA 118(5), November 2005, pp 3139-3144

Real time visualization of all relevant acoustic data

The PU acoustic camera captures in each PU measurement point both the sound pressure and the acoustic particle velocity. From here, acoustic quantities like sound intensity, acoustic impedance and sound energy can be calculated using straightforward algorithms. The low CPU time required for these simply algorithms allows real time visualization.

One point methodology

The PU probe methodology only needs one single measurement point to be taken in order to determine both acoustic properties at a certain position.
Contrary, both near field holography, HELS- and far field beam forming methods intrinsically require upfront a larger number of spatially distributed sound pressure measurement points to be taken, even if the acoustic properties at one certain position are expected to be of interest. Thus the quick PU probe methodology lowers the user entry barriers for troubleshooting.

Full bandwidth

The PU acoustic camera covers the entire audio bandwidth, whereas other methods (e.g. near field holography or far field beam forming) cover only a part of the relevant frequency range. But also infrasonic and ultrasonic frequencies can be measured under certain specific circumstances.

Large dynamic range

The dynamic ranges of beam forming and near field holography are usually not exceeding 5dB and 20dB.
The PU probe based acoustic camera easily exceeds 45dB, the upper limitations still to be explored.

Low susceptibility to background noise

Acoustic particle velocity sensors are far less susceptible to background noise than sound pressure transducers, reducing the need for measurements in an anechoic environment to a very large extent. As described in the Forum Acusticum paper up to 40dB less sensitivity to background noise can be expected. The same can be expected for PU sound intensity measurements that are also a vector value with a figure of eight characteristic. The use of traditional p-p based sound intensity probes is practically impossible due to the intrinsic susceptibility to the prevailing pressure intensity index.

• A particle velocity sensor to measure the sound from a structure in the presence of background noise, H-E. de Bree, W.F. Druyvesteyn, Forum Acousticum 20054

Visualization of transients

The PU acoustic camera allows the analysis of transients, such as doorslams, combining high sample rate data acquisition tools with slow motion post processing techniques.
 

Free configuration of measurement grid

The PU probe methodology allows a completely free configuration of the measurement point positions in the measurement grid. There are no requirements on the number of sensors per frequency wavelength. A finer mesh of PU probes can be placed at the position of interest, e.g. an expected acoustic hot spot, providing the right trade off.

Multi purpose tool

The PU probe based acoustic camera is a multi purpose tool with a reversed system cost structure as compared to other sound source visualization methods. The same PU probes used for the direct visualization of the relevant properties can also be used but for holography with acoustic reconstruction when this still would be required. PU probe based sound field reconstruction techniques require significantly lower number of measurement points to be taken.

• Obtaining the complex pressure field at the hologram surface for use in near-field acoustical holography when pressure and in-plane velocities are measured, M.C. Harris, J.D. Blotter, JASA 2005

The very same PU probes placed in a fixed grid for the acoustic camera can also be used for different types of arrays, e.g. distributed arrays for interior noise problems. Driven by the high value PU probe, the cost structure of the complete system is reversed and lowered. The PU probe methodology requires a relatively low number of data acquisition channels and straightforward (and thus affordable) sound field visualization software.
Near field and far field sound source visualization techniques are based upon large numbered data acquisition channels and complicated (and notably dedicated) software routines. They take the major share of the total system investment costs as compared to the low cost (no class A) electrets microphones are used to stay with budget limitations.

Intuitive approach

The PU probe methodology is highly intuitive; the consequences can be overseen easily without a thorough background in all sorts of assumptions and mathematical routines of other methods. There is direct feedback from the impact of changes in design or operating conditions.

More info

• Background information can be found in Chapter 8: 'Visualization' of the Ebook 
• Scientific papers on the subject of sound field visualisation
• Presentation on the Acoustic Camera can be found at the presentations page
• Video presentation of the 'acoustic camera' can be found at the video page

Key papers

• Near Field Acoustic Holography with particle velocity transducers, Finn Jacobsen and Yang Liu,  JASA 118(5), November 2005, pp 3139-3144
• The Microflown, a novel approach to helicopters interior noise testing, Antonio Vecchio et al, IMTC 2006
• A particle velocity sensor to measure the sound from a structure in the presence of background noise, H-E. de Bree, W.F. Druyvesteyn, Forum Acousticum 2005