Particle tracing solutions

Particle tracing is a technique that complements standard ray acoustics (that uses only beam tracing) by also accounting for diffuse reflections and late reverberations.

Thus, complementing ray acoustic analysis with particle tracing can account for absorbed, reflected, diffracted specular energy. Also, because of the particle tracing simulation, it can account for diffused specular energy and capture the effects of late arrivals in enclosures.

Typically, you use particle tracing for analyzing interior problems, for example, complex room or interior cabin acoustics with point sources, or sound distribution in large aircraft cabin.

Working principles of the particle tracing simulation

To estimate the acoustic diffusion phenomena, the particle tracing simulation does the following:

1. Launches a large number of particles randomly from each acoustic source.

2. Computes the trajectories of all particles by using a time integration method to account for:Reflected, transmitted, or absorbed particles.Both specular and diffusion phenomena.

3. Lets each microphone capture some of these particles carrying a quantum of acoustic energy.

4. Because particle tracing is a wide-band simulation, for each wide band, the software obtains the acoustic pressure time evolution for each microphone.Note: Beam tracing is a narrow-band simulation.

5. Hybridizes the beam tracing narrow band with the particle tracing wide-band results to obtain a complete impulse response, when frequency-to-time-domain conversion was also requested.Note: When frequency-to-time-domain conversion was not requested, the software outputs only the Fast Fourier Transformation of the hybridized impulse response.

The following plots for an auditorium with an approximate length of 25 m show a comparison of impulse response results from a beam tracing solution and a particle tracing solution (that also contains frequency-to-time-domain converted results):

Beam tracing impulse response results

This plot shows the results from the Beam Tracing - Time response function on a linear scale and shows acceptable results until around 0.6 seconds. The maximum time of around 0.6 s is calculated from the length dimension of the enclosed space, maximum reflection order, and speed of sound. In the above case, a maximum reflection order of 8 was used, which corresponds to an approximate maximum ray path of 25 x 8 = 200 m, and with a speed of sound of 331 m/s (dependent on environmental temperature and pressure) results in 200 m / 331 m/s = 0.6 s.

Hybrid (beam and particle tracing) impulse response results

This plot shows the results from the Total - Time response function on a linear scale. These results are similar to the Beam Tracing - Time impulse response results until around 0.6 s. However, a close comparison reveals the effect of diffuse reflected energy (computed with the particle tracing solution) superimposed over the specular energy obtained from the beam tracing solution. Compared to only the beam tracing solution, the particle solution also shows the late reverberations after 0.6 s.

Sound quality criteria

In addition to the output requests (acoustic pressure results at microphone nodes, ray paths, echograms, and so on) in a beam tracing solution, you can also request output of sound quality criteria: reverberation times, clarity (C80), direct arrival times, unitless definition (D50), early decay time, and speech transmission indices (STIs). You can post-process those results with the Data definition in the Contour Plots scenario.

The software computes all sound quality criteria using hybrid impulse responses as follows:

• For all criteria except for the STI criteria, the software filters the entire band hybrid impulse response (also known as total time response) on each wide band defined in the particle tracing computation to obtain a series of wide-band impulse responses. These wide-band impulse responses are used as input data for the computation of the criteria. Hence, the criteria are frequency dependent. That is, the criteria are dependent with respect to the central frequencies of the wide bands.

• For the STIs, the software computes on the entire band impulse response. Consequently, the STI criteria are frequency independent.The non-dimensional indices range from 0 to1, where 0 stands for the lowest and 1 the highest intelligibility rating.

Note:

As prerequisite for computing the acoustic criteria, you must specify the following for the narrow-band frequencies using the Forcing Frequency modeling object:

• A constant frequency step.

• A frequency that starts at 0 Hz and ends at the desired maximum frequency {f_{\max }}. Also, for the STI criteria, {f_{\max }} must be greater than 11360 Hz.

Post-processing of beam and particle tracing results

You can post-process the particle tracing results with the Function Plots post-processing scenario using the Scenario Based Data-Visualization Navigator. For example, when the particle tracing solution creates a data source that contains both beam transfer and particle tracing results and where the data was:

• Converted from frequency to time domain, the following Response Functions are available:BeamTracing - Frequency—Plots the beam tracing narrow-band frequency response accounting only for the specular energy.BeamTracing - Time—Plots only the beam tracing contribution (non-hybridized) impulse response corresponding to the IFFT of the narrow-band frequency response.ParticleTracing—Plots the particle tracing results, which are the acoustic pressure time functions for each microphone and wide band.Total - Time—Plots the impulse response computed by combining or hybridizing both the:Beam tracing narrow-band frequency response.Particle tracing results, which contain both specular and diffusion energy and late reverberation results, which are not captured by the beam tracing computation.Total - Frequency—The FFT of the hybridized impulse response.

• Not converted from frequency to time domain, only the following Response Functions are available:BeamTracing - Frequency—Plots the beam tracing narrow-band frequency response accounting only for the specular energy.ParticleTracing—Plots the particle tracing results, which are the acoustic pressure time functions for each microphone and wide band.Total - Frequency—Plots the FFT of the hybridized impulse response.

Note:

You can request conversion from frequency to time domain by using the Convert Results to Time Domain check box in the Solution dialog box.

Where do I find it?

Enabling particle tracing

Application	Pre/Post
Prerequisites	A Simulation file as the work part and displayed partSimcenter 3D Ray Acoustics as the specified solver
Simulation Navigator	Right-click the active solution or solution step node→Edit
Location in dialog box	Particle tracing group

Enabling conversion of results to time domain

Application	Pre/Post
Prerequisites	A Simulation file as the work part and displayed partSimcenter 3D Ray Acoustics as the specified solver
Simulation Navigator	Right-click the active solution node→Edit
Location in dialog box	Time Domain Conversion group

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Particle tracing solutions, Simcenter 3D 2021.1 Series

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