JANDS NEWS
JBL CBT Calculator
01 September 2010
JBL Recently released a software array calculator application for their Constant Beamwidth Technology (CBT) series of line array products.
With four products in the range (CBT50, CBT100, and CBT70J & CBT70JE) most with selectable beam width and voicing, the CBT series has quickly become a favourite of contractors, consultants and system designers due to its configuration flexibility. However with increased flexibility comes increased choice and sometimes it can be difficult to determine the best configuration for your specific application without resorting to high level acoustic modelling applications such as EASE.
The JBL CBT array calculator application has been designed to assist by simulating the coverage of CBT arrays in venues. It maps predicted sound pressure level (SPL) over user defined listening planes (maximum 4), and plots frequency response at specific microphone positions (maximum 6).
"Ok, big deal” I hear you say. "I’ve been installing loudspeaker systems for years and have always managed so far without the assistance of a computer application”. That may be true, but the CBT array calculator does more than just paint pretty colours on the screen based on the coverage angles of the selected loudspeaker systems. It calculates and displays the complex coverage distortions resulting from interactions between multiple sources and allows the operator to tweak position, coverage, delay, gain & equalisation of individual loudspeakers to achieve optimum coverage. For example;
Figure 1 shows a fairly typical style auditorium consisting of large floor seating area and a single rear balcony. The modelled sound systems consists of an upper array (JBL CBT70J and CBT70JE LF extension set to "narrow” mode) to cover the balcony, and a lower array (CBT70J set to "wide” coverage) to cover the stalls.
At first glance, without the aid of frequency response probes, over all coverage seems good. However what we can’t see is distortions caused by destructive interference between the two loudspeaker systems.
In figure two we have added a few probe (microphone) positions. The resultant frequency response at each position can be seen in the lower right. It is now immediately apparent that we have issues, particularly at position 4 (purple) and 3 (green). The substantial dips in frequency response are caused by comb filters caused by time arrival differences from each array. Left unrectified, this system would deliver disappointing results.
Figure 3 shows the same physical configuration with a small amount of time delay added to the lower array which significantly improves the frequency response.
Figure 4 shows another useful function of the CBT array calculator, the ability to place probes on the stage and thus gain a valuable indication of available gain before feedback. Something often overlooked in system design, but critically important if you intend to run wireless lapel or lectern microphones on stage.
But the best thing about the CBT Array calculator has to be the price. The JBL CBT Array calculator is available as a FREE, click here to download.
Article written by Jeff Mackenzie, Jands TRG Australia, jands.com.au
