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COMSOL Multiphysics User Case Study

Krix Loudspeakers Horn Loaded Subwoofer

Krix Loudspeakers is a South Australian company with a near fifty year heritage of design and manufacture of loudspeakers. Starting out with Hi Fi loudspeakers, Krix have broadened their range to include home cinema, concealed audio, high end cinema, and commercial cinema. Krix now supplies speakers to most of the major commercial chains in Australia and exports to over 30 different countries.

An important aspect of sound systems for commercial cinema is control of the directivity of the loudspeaker system, and Krix systems have very good behaviour in this characteristic. Krix have been designing acoustic horns using Finite Element Analysis techniques since 1999 to achieve exemplary directivity performance. Since 2008 Krix has used COMSOL, from 3.5 to the present version.


Challenges and Objectives:

Loudspeaker systems for commercial cinema deploy acoustic horns for midrange and high frequencies, large direct radiator 300 litre enclosures for low frequencies, and even larger direct radiator 500 litre enclosures for the sub woofers. Behind the screen are left, centre and right horn loaded loudspeaker systems and sub woofers, and on the side and rear walls are multiple surround loudspeakers. 

Over the last 20 years Krix has developed a range of acoustic horns, wave guides and low frequency enclosures to supply appropriate acoustic outputs to almost any size of cinema. The largest so far has been in Deira, Dubai, the screen being 31 metres wide, the cinema depth being 38 metres! Because of the width it used five screen systems.

In the good old bad old days, acoustic horn designers would select a throat size, a mouth size, a lower cut off frequency and calculate a suitable exponential profile. A prototype could be laboriously fabricated and acoustically measured to ‘verify’ the design. There was usually a passing resemblance between calculated and measured results, so a redesign and another laborious fabrication was required. Accurate fabrication, especially for large format horns of mouth size 700 mm by 800 mm and depth 750mm, consumed several hundred hours of work.

Krix purchased FEA software, developed a suite of macros to draw a horn shape, draw air in and around it, precariously mesh the volume, and do a frequency run. The results were saved and post processed to obtain polar responses in horizontal and vertical planes, and graphs of the beam angles (-6dB angles) versus frequency. Turn-around time using overnight simulations was two or three days, much faster than before.


Figure 1; basic floor plan of venue showing dimensions in metres
"This software has more advanced meshing algorithms, has near field to far field transformation and calculation of polar responses and beam angles. Turn-around time from CAD model to simulated results is typically less than half a day, enabling many more design iterations to be explored."
Figure 2; COMSOL Multiphysics model showing off-set manifold and horn in corner of room.
Figure 3; Side view of subwoofer manifold drawn in SolidWorks CAD.
Figure 4; COMSOL Multiphysics simulation showing wave fronts negotiating the corners in the manifold and emerging substantially planar at 50Hz.
Figures 5 & 6; Photographs showing 3 @ 18” loudspeakers assembled into manifold, and testing the completed manifold (black studded box) into a temporary rectangular plane wave tube filled with absorbent white tontine.
Figure 7; Frequency response measured at mouth of manifold ie start of rectangular plane wave tube

Figure 8; venue before showing quarter round stage

How COMSOL has helped:

COMSOL, the Acoustics and CAD Import modules are a later generation of FEA software. This software has more advanced meshing algorithms, has near field to far field transformation and calculation of polar responses and beam angles. Turn-around time from CAD model to simulated results is typically less than half a day, enabling many more design iterations to be explored.

An interesting case study:

A venue in Bondi Junction Sydney presented an interesting challenge: 

  • The room was 22 metres by 19 metres by approx. 2.5 metres high, with a corner stage. 
  • The client’s brief wanted a sound system with the full frequency range and extremely high Sound Pressure Levels (SPLs) for music bands and late night discos. To achieve that with conventional 500 litre subwoofer enclosures the stage area would have to be filled, leaving no room for musicians and bands. 
  • The house loudspeaker system would be Krix 3-way cinema loudspeakers.
  • The proposed innovative solution was to use an under-the-stage horn loaded subwoofer, with a manifold at the rear of the stage providing the acoustic source. The client accepted this solution.
  • An initial horn design was estimated and refined using other software, which assumes a round shape of horn and mouth. The parameters for the loudspeaker drivers were entered and horn size, length, and mouth area were explored. The actual horn was required to have a wide flat rectangular mouth, acoustically different from round. 
  • The room and rectangular horn were drawn up in CAD. The quarter round stage was offset from the corner and the manifold was accordingly located off set to enable a symmetrical horn. The internal structure of the manifold had a three sided truncated pyramid on which was mounted three 18” subwoofer loudspeakers. The acoustic path had two corner folds before the exit into the throat of the horn. 
  • The CAD geometry was imported into COMSOL, boundary conditions set up, and the model meshed and solved for a frequency range from 20Hz to 160Hz. 
  • Photographs of the construction and sound pressure distribution results are shown in the following figures.

Figure 9; venue showing construction of under stage horn
Figure 10; showing mouth view of completed subwoofer horn
Figure 12; COMSOL Multiphysics Sound Pressure Level – pattern at 25Hz
Figure 13; COMSOL Multiphysics Sound Pressure Level – pattern at 35Hz
Figure 14; COMSOL Multiphysics Sound Pressure Level at 50Hz
Figure 15; COMSOL Multiphysics Sound Pressure Level at 71Hz
Figure 16; COMSOL Multiphysics Sound Pressure Level at 100Hz (above working range)

COMSOL Multiphysics, especially the acoustics module, has become progressively easier to use; likewise the post processing to obtain frequency responses on and off-axis, and polar and beam angle graphs to assess the performance of the horn design before prototyping. 

Good agreement between simulated and measured results has enabled tooling, successful production and a speedy time to market. 


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