Sound Waves Between Two Walls

Version: 1.03 (2019-08-12)
Software platform: Microsoft Excel

As the title says, this software calculates the sound field of a point source placed between two parallel walls.

Software features:

The simulation is based on the image source model (ISM). Only the first eight reflections are computed (per wall).

simple acoustic simulator: reflections, interference patterns, standing waves between two walls, waterfall plot

This software is suitable for studying the following cases:

Version: 1.03.
Format: xls (Microsoft Excel 2003 Workbook)
Required software: Microsoft Excel 2003 or newer
License: freeware

New in version 1.03.:

  • Improved resolution of the frequency response graph.
  • Added: Save frequency response as 'frd' file.

Examples, case studies

(Modal analysis, room modes)

Wall distance is 3 meters, sound source x coordinate and y coordinate are zero (source is on the left wall), microphone y coordinate is zero. Reflection coefficient is one.

A simple formula for calculating the frequency of the first mode (F0):

F0 = 343 / 2L

L is the distance between opposite walls in meters, F0 is the frequency of the first mode in Hz. In our case the first mode is at 57 Hz.

We will see that the excitation of room modes depends on the location of the source and microphone.

1. Microphone is on the right wall. All modes are at full amplitude:

2. Microphone is located in the middle between two walls. Only even order modes:

3. Microphone is 3 meters from the left wall, but the reflection coefficient has been changed to 0.71 (equivalent to an absorption coefficient of 0.5). Reducing the reflection coefficient has a huge effect on the level and decay of room modes. Decay increases from 22 dB / 50 ms to 40 dB / 50 ms.

4. Vertical room modes. For this, we have to rotate the side view of the room by 90 degrees: the left wall will be the floor, the right wall will be the ceiling. Since the typical ear height while sitting is approx. 1.2 meters, it's worth setting the distance of the microphone from the left wall to this value. The vertical offset is the distance from the speaker. We change the distance of the source from the left wall from 0 meter to 1.2 meter.

Acoustical simulation (all software)