Acoustic analysis is an essential engineering discipline used to predict sound behavior and interaction within structures and fluids. This blog covers foundational concepts of acoustic analysis, its implementation in Ansys Mechanical, assumptions, limitations compared to CFD-based acoustics, meshing considerations, coupling methods, detailed discussions on modal and harmonic acoustic analyses, damping strategies, and practical industry applications including piezoelectric acoustics.
Acoustic analysis involves the study of sound propagation as pressure waves traveling through elastic media such as air or water. These waves are generated by vibrations in structures or disturbances in fluids. Understanding acoustic behavior is crucial in many engineering applications, from reducing unwanted noise in vehicles and aircraft to optimizing the acoustic performance of speakers and microphones. Acoustic simulations in Ansys Mechanical focus on vibro-acoustic interactions (structure-fluid interaction) and enclosed cavity acoustics. The analyses typically assume linear acoustics (small amplitude waves), inviscid fluid behavior, and homogenous fluid properties without thermal gradients. Key objectives include determining acoustic pressure levels, wave reflections and transmissions, resonance frequencies, and the dynamic interactions between structural vibrations and acoustic pressures.
Ansys Mechanical performs acoustic analyses using finite element methods (FEM). Acoustic domains employ specialized acoustic fluid elements:
FLUID220: Used in purely acoustic or coupled acoustic analyses.
Structural domains use standard elements such as SOLID185, SOLID186, SOLID187.
Mechanical is particularly effective in applications like enclosed cavity acoustics and vibro-acoustic interactions common in automotive, aerospace, and architectural industries.
Acoustic modal analysis identifies natural frequencies and mode shapes within enclosed acoustic cavities or fluid-structure systems. This type of analysis helps engineers pinpoint resonant frequencies at which the system might exhibit significant acoustic amplification or structural resonance. In modal analysis, the dynamic behavior of the structure and fluid cavity is simplified into a set of discrete vibration modes, each having a specific frequency and spatial pattern, known as mode shapes. Unlike purely structural modal analysis, acoustic modal analysis incorporates fluid properties into the equation, coupling structural dynamics with acoustic fluid behavior. Analyzing these coupled modes helps engineers to design quieter and more structurally robust products by avoiding or mitigating resonance conditions. The results from modal analysis are foundational, providing a clear understanding of how the structure and acoustic cavity naturally respond when excited.
The governing equations differ for a purely acoustic cavity and for a fully coupled fluid‑structure system:
Pure acoustic modal (Uncoupled)
Coupled modal (FSI)
Here [Mf], [Cf], [Kf] are the fluid mass, damping, and stiffness matrices. [Ms], [Cs], [Ks] are the structural counterparts, R is the acoustic fluid boundary matrix, and ρ₀ is the mean fluid density. Eigenvalue solvers in Ansys Mechanical (Block‑Lanczos, Subspace, Unsymmetric, Fully‑damped) process these matrices to extract natural frequencies and mode shapes. (Block Lanczos, Subspace, Unsymmetric, Fully damped) solve these coupled equations to find acoustic frequencies and mode shapes.
Harmonic acoustic analysis investigates how structures and enclosed acoustic domains respond to harmonic (sinusoidal) excitations across different frequencies. This type of analysis helps determine how the structure and acoustic environment react at each frequency, revealing which frequencies lead to significant acoustic or structural responses. Engineers use harmonic acoustic analysis to predict steady‑state acoustic pressure, particle velocities, and impedance at specific frequencies, enabling effective noise reduction and acoustic optimization. Unlike modal analysis, harmonic analysis involves applying frequency‑dependent loads and observing the system’s response, making it ideal for assessing radiated sound from vibrating structures or evaluating speaker and microphone performance. Understanding the frequency response allows designers to minimize unwanted noise or tailor acoustic performance for desirable sound qualities.
For steady‑state (harmonic) excitation the system equations become:
Pure acoustic harmonic (Uncoupled)
Coupled harmonic (FSI)
{fF} represents acoustic sources such as imposed pressures or mass sources, while {fS} contains structural loads (forces, enforced motions). Damping matrices [Cs] and [Cf] model energy losses on the structural and fluid sides respectively, and the coupling matrix R transmits interaction forces across the FSI interface.
Damping is the mechanism by which vibration and acoustic energy are gradually converted into heat or other forms of energy, causing amplitudes to decay with time or frequency. In the context of a modal solution, damping determines how quickly each natural mode will die out once it is excited; lightly damped modes produce high, sharp peaks in a frequency response, whereas heavily damped modes appear broad and subdued. During a harmonic analysis the damping terms govern the phase lag between the applied sinusoidal load and the system response, directly shaping magnitude‐vs‑frequency curves such as Sound Pressure Level (SPL) or structural acceleration plots. In practice, most real‑world systems possess a mix of damping sources—internal material friction in metals or polymers, viscous shear losses in fluids, thermo‑viscous losses near compliant walls, and extra dissipation introduced by gaskets, porous liners, or viscoelastic pads. Accurately capturing these mechanisms ensures that simulation results match measured data, particularly around resonance where a few percent change in damping can alter SPL predictions by double‑digit decibels.
Ansys Mechanical offers several straightforward paths to represent damping:
Structural damping via material loss factors or Rayleigh coefficients (α, β) that scale with mass and stiffness, ideal for broad‑band estimates in metals and plastics.
Modal damping ratios applied per mode, useful when test data report quality factors (Q) or half‑power bandwidths for specific structural resonances.
Acoustic (“air”) damping introduced through complex bulk moduli or by assigning a frequency‑dependent impedance/absorption coefficient to cavity walls; this captures viscous and thermal boundary‑layer losses that become significant above a few kilohertz.
Boundary damping and energy absorption by adding Perfectly Matched Layers (PML) or Radiation/Impedance boundaries, eliminating reflections so that outgoing waves exit the model without spurious build‑up.
When setting up a coupled FSI model, remember that the structural damping matrix and fluid damping matrix both feed into the combined equations of motion—removing either term can over‑predict cabin boom or under‑predict panel vibration. As a rule of thumb, start with conservative values (e.g., 1–2 % critical damping for metals, 5–10 % for composite panels) and iterate after comparing to prototype measurements.
Accurate acoustic simulation relies on a mesh that resolves pressure wavelengths:
Element types – Use FLUID220 for acoustic domains and SOLID185/186/187 (or shells) for structures.
Elements per wavelength – At least 6 quadratic or 12 linear elements per shortest wavelength (highest frequency) ensure phase accuracy.
PML and radiation spaces – For exterior noise, add at least three quadratic elements between the radiating surface and start of the PML layer; the PML thickness should be ≥ 0.1 × largest wavelength.
Coupling interfaces – Ensure node sharing (multibody part) or conformal contact to faithfully transmit pressures and velocities across FSI boundaries.
Before Mechanical introduced high‑level coupled‑field analysis systems, analysts had to open the Element Reference manual, scroll through dozens of element types (FLUID30, SOLID65, SOLID226, etc.), and choose an element whose available degrees of freedom (DOFs) matched the physics to be solved. For a vibro‑acoustic simulation you would pick FLUID30 for the cavity (PRES DOF) and SOLID187 for the structure (UX, UY, UZ), then manually merge the shared nodes at the interface so PRES on one side coupled to displacements on the other—a process both time‑consuming and error‑prone. Getting it wrong often meant solver errors about missing DOFs or a silent mismatch that produced meaningless results.
Modern Coupled‑Field Blocks in Workbench automate all of that bookkeeping. When you drop a Modal‑Acoustics, Harmonic‑Acoustics, or Transient FSI system onto the schematic, Mechanical automatically:
Creates the correct element technology under the hood (structural solids, acoustic fluids, coupling matrices).
Exposes physics‑aware objects—Acoustic Region, Structural Region, FSI interface—so you scope bodies instead of thinking about DOFs.
Handles mesh conformity or penalty‑based coupling, depending on whether the bodies share topology.
This greatly streamlines multi‑physics setup compared with stand‑alone Structural Harmonic or stand‑alone Acoustic Harmonic systems where only one physics is present and any cross‑domain transfer must be performed through external data links or manual mapping. The coupled block also manages solver selection (symmetric vs. unsymmetric) and result storage, meaning you get co‑visualized outputs—pressures, displacements, and derived SPL—in a single post‑processing tree.
Tip for legacy users: You can still force a specific element formulation (e.g., FLUID220 vs. FLUID221) via Command Snippets, but in 95 % of cases the automatic choice is optimal.
Weak coupling (one‑way) – Solve structural response first, map surface velocities/pressures to a separate acoustic model. Faster, suitable when acoustic feedback on structures is negligible (e.g., loudspeaker cone driving air).
Strong coupling (two‑way) – Solve structural and acoustic DOFs simultaneously. Necessary when fluid mass or pressure significantly modifies structural dynamics, such as thin panels in water or membrane actuators. Mechanical’s coupled matrices (see equations above) capture these effects automatically when Acoustic and Structural Regions share an interface.
| Sector | Typical Goals | Example Analyses |
|---|---|---|
| Automotive | Cabin noise, intake/exhaust tuning | Modal cabin modes, harmonic speaker studies, muffler TL prediction |
| Aerospace | Fuselage interior comfort, payload acoustics | Coupled vibro‑acoustic panels, engine‑induced cabin SPL |
| Architecture | Room acoustics, sound transmission | Harmonic response of auditoriums, impedance wall treatments |
| Consumer Electronics | Audio quality, microphone crosstalk | Speaker box radiation, enclosure modal damping |
| Industrial Ultrasonics | High‑frequency actuators | Piezoelectric transducer FSI, field directivity |
Pressure & SPL contours – Identify hotspots and nodal surfaces.
Velocity vectors – Visualize particle motion directions, useful for diagnosing standing waves.
Equivalent Radiated Power (ERP) – Structural metric estimating potential acoustic power; great for rapid design ranking.
Transmission Loss & Insertion Loss – Key for mufflers and barriers; compare design iterations in a single chart.
Microphone probes – Place virtual mics to extract frequency spectra at listener positions.
Ansys Mechanical equips engineers with comprehensive tools to model, couple, and interpret structural and acoustic phenomena. By mastering modal and harmonic analyses, appropriate damping, mesh resolution, and coupling strategies, you can predict noise and vibration issues early, iterate designs efficiently, and deliver quieter, better‑performing products across automotive, aerospace, consumer electronics, and ultrasonic domains.