QSI Weighs In on HO Sunset 4-8-4 Sound Baffling

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Fred Severson from QSI responds to our news item concerning the poor sound output of the QSI equipped HO Sunset 4-8-4 NP.

Dear Tony,

I was aware of the problems with the HO Sunset Northern when we received preproduction samples to do the speed and motor calibration. At that time the sound was even worse than your reported production units. We sent Sunset an extended document on acoustically designing rounded tenders. They or their Korean locomotive manufacturers did not understand or use this information correctly. I was also disappointed in the sound quality when we received production samples. Unfortunately, we have no control over the manufacturing process.

You are quite right; the problem is that the cavity for the back of the speakers was not sealed. This allowed the front wave and the back wave of the speakers to interact and cancel each other. Generally, brass tenders have good sound but unless the back wave is completely sealed from emanating from the tender, the sound will have poor volume and poor bass. I have attached our acoustic design manual (PDF) that shows a number of ways that speaker(s) enclosure(s) can been designed for optimal sound quality.

However, all these methods or suggestions are easier for a locomotive manufacturer that has fabrication options for acoustically designing the tender that are not usually available to the end user. Your suggestion of adding the small plastic enclosures to the backs of the speakers and using high-bass speakers is an excellent approach, especially as an aftermarket solution that is relatively easy to accomplish.

Fred Severson
QSI Inc.