Rear Wall Reflection Simulator
Software platforms: Excel | OpenOffice | LibreOffice
This simulation calculates and displays the frequency response of an ideal speaker (speaker cabinet) placed close to a rear wall. One can see how the distance of the rear wall, the baffle size (mainly the width) and the listening distance affects the response. In the model the wall behind the speaker is 100% reflective, the reflection is not diffuse and there are no other effects only the speaker baffle and the rear wall.
At low frequencies where the wavelength is larger than the dimensions of the baffle the speaker radiates evenly in every direction (spherical radiation pattern). At high frequencies where the wavelength is shorter than the baffle dimensions the speaker radiates only in the front half space, and there is no rear radiation (hemispherical radiation pattern). The transition between the two regions is the "baffle step": a 6 dB loss in front of the speaker at low frequencies in free field.
Due to baffle diffraction and other wave propagation reasons, as the frequency increases the spherical radiation gradually turns into hemispherical. The high frequency content of the radiated sound in the rear space is attenuated ("low-pass filtered") by the cabinet. What is very important that both the baffle step and the rear radiation are functions of the baffle dimensions.
In closed spaces the directivity (rear radiation) of the baffle is at least as important as the calculated free field baffle step. The reflected sound waves from the rear wall interfere with the original waves and the result is a bad comb filter response. If the speaker has a radiation pattern close to hemispherical at the first cancellation frequency, then the interaction between the cabinet and the rear wall is minimal.
Comparison of calculated and measured rear frequency responses can be found here.
Speaker placement is also an important factor. A general guide here is to place the speaker as close as possible to the wall or - on the contrary - as far as possible from the rear wall. In the latter case some baffle step compensation is required.
Format: xls (Microsoft Excel 2003 Workbook)
Required software: LibreOffice Calc | Microsoft Excel | OpenOffice Calc