Whistler edition

In 1995 a sound-reduced version of a large transport refrigeration unit was introduced as one of the quietest options for delivering temperature-sensitive goods to stores and restaurants. It became known as the Whisper Edition. After a couple of years the service department received a few calls about these units whistling. Lab tests found a 5-kHz tone that overwhelmed the 30-hp diesel engine noise. The noise was related to the blower inlet displacement, and a simple modification seemed to resolve the problem. Sporadic reports of whistles resurfaced after a couple more years, and an investigation showed a dimension on the blower inlet needed tighter control. Once those changes were made, recommendations for positioning the blowers in the factory and field solved most problems, and replacing blower inlets addressed the few remaining problems in the field. By now the unit was known in the service department as the Whistler Edition. The problem resurfaced yet again in the factory after a couple more years, and it was there where they recognized a slight tooling ridge on the inlet ring played a role. Subsequent tests showed the ‘‘whistle’’ was actually noise from vibrations in the ring induced by airflow.