SOUND INSIDE A RIGID-WALLED

CAVITY

ANSYS WORKBENCH 15.0

sanjeet kumar singh

Learning outcomes

1. Calculate the undamped natural frequencies of a rigid walled cavity using

ANSYS.

2. Use the FLUID 130 Fluid element in ANYS to model rigid wall cavity.

3. Sound pressure level at a receiver inside a rigid-wall rectangular cavity due to an

acoustic volume velocity source, using the modal summation method.

4. Mode shape of a rigid-wall rectangular cavity

5. Conduct a harmonic response analysis in ANSYS

Rigid-walled rectangular cavity with an acoustic volume velocity source and receiver to

measure the resulting sound pressure

y

x

z

Sound Source

Receiver

Parameters of a Rigid-walled Cavity

Description Parameter Value (m)

Cavity:

Length X

Length Y

Length Z

Lx

Ly

Lz

0.5 m

0.3 m

1.1 m

Acoustic Source:

Location X

Location Y

Location Z

Xs

Ys

Zs

0.15 m

0.12 m

0.0 m

Acoustic Receiver

Location X

Location Y

Location Z

Xb

Yb

Zb

0.3 m

0.105 m

0.715 m

Fluid :

Speed of sound

Density

Number of modes

C0

ρ0

Na

343 m/s

1.21 kg/m3

500 no units

The mess in finite element model will create nodes at the location of the source and receiver, and

use hard key point points in the mess, which will create a key-point at a desired location and can

not be changed. However, mapped meshing is not supported when hard key-points are used, so it

is not possible to create a regular mesh with brick elements.

Undamped natural frequencies of a rigid walled cavity using ANSYS

40 natural frequencies of the cavity

The 18th mode will be (1,1,2) mode. ANSYS does not have an in-built mechanism for

determining the modal indices of acoustic(or structure) responses and therefore these have to

be determined by viewing the response of the system and the user has to recognize the mode

shape and appropriate model index

Harmonic response analysis in ANSYS

1. In harmonic analysis, calculate the acoustic pressure response within the cavity using the full

method and an acoustic mass source.

2. Calculate the pressure response at the receiver location.

Under the harmonic response (B5) branch, click on

the analysis setting branch. Change all the setting

according to show in the figure. Solution intervals

500, which will give a frequency spacing of 1 Hz.

Save MAPDL db to yes as the database is needed for

post-processing of the acoustic results.

Define two named selection objects that correspond to the nodes where the acoustic source and

microphone are located as listed in table in slide no 4.

The worksheet (Model (A4,B4)-insert-Named selection-row scoping method-worksheet) use to

select a node by initially selecting a group of nodes and then filtering the set. After the filtering

operation, one node remaining in the selection set.

The name selection have been defined for the nodes for the acoustic source and

microphone, the acoustic source is defined and acoustic pressure can be calculated at the

microphone.

Sound pressure level at Receiver (microphone )