The firm was fined £1,000 on each of three charges of non-compliance and ordered to pay £2,500 in costs to the Vehicle Certification Agency (VCA).
STM managing director Simon Tullett said he was disappointed with the prosecution as he felt his company had rectified the problem and co-operated fully with the VCA, but he admitted the machinery had failed the noise legislation.
He told HW: "I don't think it's unfair because we were supplying machines that weren't compliant, but we were working on it."
He added that all irregularities were corrected immediately. "At no time did STM intentionally not adhere to any of the regulations regarding sound power levels.
"We co-operated fully with the VCA and rectified all irregularities, which we felt were a result of the lack of initial assistance, explanation and guidance from the VCA."
The firm was prosecuted at Caerphilly Magistrates' Court on 26 November.