Sound Intensity has long been used as the near ideal tool for translating scalar sound pressure quantities into a virtual vector; What stands-out with this tool and its approach is its ability to both calculate sound powers and as well to identify sound sources, both with a refined degree of accuracy. This is the convincing reason why standards like ISO 9614 have backed and certified this method as a precise procedure for sound power determination.

As for the sound source assessments, many newer tools & techniques have come into practice. Of them are the acoustic holography and beam forming techniques that have gained good traction among the engineering community due to their fast deployment and quick visualization and interpretation of results. Let us dwell into each of these approaches to know their merits and limitations.

BEAM FORMING

Essentially, beam forming technique are used in multiple application areas and is in practice for a good time in radar and other special uses where the sources of specific noise were to be identified. The approach of using a set of array sensors to measure and assess time delays between the signals arriving at each sensors are then characterized to point to their sources. The re-adaption of this methodology is effectively used in acoustic source localization in many of developmental and research activities.

Beam forming technique

 

ACOUSTIC HOLOGRPAHY

This method also uses an array of microphones to register the sound pressure values at an incident time in space. Acoustic holography identifies noise sources by measuring sound pressure at a specific plane around a source, then mathematically reconstructing the sound field to visualize the exact location and strength of the noise origin. The term holography comes from the recombination of calculated data to form a 3D image of the sound source distribution; this is particularly useful for pinpointing the precise location of noise emissions from complex machinery or structures.

Holography technique, observe the back propagated holographic plane.

 

Both beam forming and holographic methods are limited by their high computational load, lag or loss in real-time imaging, spatial resolution issues owing to frequency / wavelengths, complex setup requirements including large hardware channel counts and their associated costs. In addition, boundary conditions influencing the actual measurements also limit their usage.

SOUND INTENSITY

Relatively, Sound Intensity approach is straight forward; a set of phase matched engineering grade microphones are configured in a specific physical arrangement as part of the noise measuring apparatus. The distance between the 2 microphones makes the incidence of sound pressure values from a designated source in a sequence of small time delay and this delay generates the vector information of magnitudes and phase.

Sound pressure data gathered over a matrix of test plane combines all the sequential data to derive magnitudes and their relative phase/s to determine the sources of noise. With few initial checks on the pressure, phase and residual index, the sound intensity tool is up and ready for deployment for both sound power determination and to identify noise sources.

Planar sections of sound pressures, intensity gradients and ranking of sound sources are all generated as a standard output.

Essentially, sound intensity is simple both by configuration & usage, as well effective and reliable. These can be deployed in a variety of real life situations wherein multiple sound sources are present and sound fields are not well defined.

Notwithstanding this, the theoretical understanding of how Active and Reactive intensities work, the relevance of residual intensity values and understanding the grid spacing and spatial resolution to arrive at good quality results put some challenges for the user initially.

The other known drawback of intensity hardware is that the phase matched microphones are supplied as a set; any of the microphones or their pre-amplifiers deviating from their performance will call for changing of both the microphones and their pre-amplifiers together, which are expensive.

Overall, all the methods discussed have their merits and limitations, given the long standing credibility of sound intensity approach, it is still the preferred tool of choice both for sound power determination and more importantly for noise source identification.

NV Dynamics uses sound intensity as its key resource in many of the industrial noise source identification activities. Our newly acquired Intensity hardware from G.R.A.S specifically built for SEIMENS test platform has taken this to next level of application with its remote controlled one-person operation to conduct the entire testing campaign.