In common with several other first generation computers, the CSIR Mk1 had a built-in speaker, a Rola 5C mounted on the console frame. The speaker, or 'hooter' as it was known, was an output device used by programmers to signal that a particular event had been reached in the program. It was commonly used for warnings, often to signify the end of the program and sometimes as a debugging aid.
The speaker on the CSIR Mk1 was built into the computer in such a way that it was a destination for data, effectively on a register of the machine and it received the raw pulse data off the 'bus'. This would work to create some sort of sound, but it would require much more effort on the part of the program to get a predictable sound out of it as a single pulse would barely make a click. Therefore, multiple pulses would be required to achieve an audible result, as a short loop of instructions. The timing of the loop of instructions would have caused a change in the frequency of the sound from the hooter. Any programmer with an interest in sound or music would immediately see the potential available.
Although the musical developments with the CSIR Mk1 were accomplished in isolation and with no precedence, it was not unique at that time. The Ferranti Mark I had a hooter arrangement very similar to that on the CSIR Mk1 and it was used to play music in 1951. The music played by the Ferranti Mark I was recorded by the BBC in September 1951. The BBC disk of this music has unfortunately been lost. Luckily the (British) Computer Conservation Society is the custodian of a disk made by the BBC at the same time for the computer operator, Frank Cooper, which has been transcribed to DAT by the National Sound Archive (archive number H3942).