The Science of Small-Scale Sound

Model train sound systems face a challenge in accurately reproducing a giant machine's sounds through a tiny speaker. To better understand the physical constraints on digital sound from a small source, I talked to Dr. Jamie White, chair of the Juniata College physics department.

Just becasue someone has a Ph.D doesn't mean they have to be hard to impress. Dr. Jamie White of Juniata College was quite intrigued by my Kato E8A and its MRC 1645 sound decoder.

I found Dr. White in an electronics lab, of all appropriate places, and explained that I wanted to ask him about the limitations of small speakers. I set out my first sound-equipped locomotive, a Kato E8A with MRC sound, and ran it down the track powered by a nine-volt battery. I had Dr. White's interest from the very start, and I think his comments will be of interest to you readers as well.

Model train sound has many strikes against it in overcoming the physics behind clear and loud sounds. "[A speaker] is not going to be very effective at getting a wavelength that’s longer than its width," Dr. White said. "Or at least, there’s going to be some dramatic falloff."

Looking at a little 10 mm speaker, Dr. White ran a few calculations in his head involving the width of the speaker, the wavelengths it would effectively produce and how those wavelengths of sound would translate to the human hearing range. "Anything lower than the highest frequencies you can hear aren’t going to be very effective," he concluded. "Then of course you’re not moving much air. So if you’re not going to move much air, you’re not going to get much volume.”

By that analysis, there isn't much hope for small-scale sound to ever really reproduce the sounds of a real locomotive. "If I wanted to do this, I wouldn’t put the speakers in the train," Dr. White said. "But of course that way, the sound doesn’t come from the actual train."

My conductor's hat sitting next to a modern airhorn at the Rockhill Trolley Museum (top), and next to an N scale locomotive with sound (bottom). The size difference is staggering, and it is a bold, if not monumental, undertaking to attempt to reproduce sound on such a reduced scale.

From there, however, we began to uncover the little things modelers can do to improve sound, and perhaps even the saving graces of sound decoders in the struggle to overcome physics. When shown an oval speaker from ESU, Dr. White said "That should definitely bring out the lower frequencies a little better. That should at least double your accessible frequency range." The added length of the speaker's external moving surface, or diaphragm, adds to the lengths of sound waves it can effectively produce.

Also, using your locomotive's body shell or tender as a resonator can increase both the volume and the frequency range. "That volume being so small, it’s not going to make a very large difference. But in general it’s going to be better than the speaker itself."

Additionally, the size of the opening in a sealed resonator like a steam engine tender will determine whether you boost the frequency range more or the volume more. A smaller hole will amplify lower frequencies, while a larger one will allow more volume of sound to escape.

There are a few aspects of model train sounds that work around the physical limitations to deliver a good illusion of train sounds. Dr. White explained that the physical limitations mostly affect the pure pitch of a sound, but the pure pitch is not as important as the progression of changing sounds. An Alco diesel sound with the correct pattern of engine clatter will sound like an Alco, whether it is actually hitting the 100 hertz low-end sound or not.

John Sipple offered a modelers' perspective on the science of model train sound. John has worked with many scales of sound-equipped model train, and he also has experience as a musician.

"All the sound we’re playing with is playing snippets of recordings," John said. "That makes it a secondary sound source. If I picked a string on a bass, that’s a primary sound source. The transient highs and lows are staggering compared to a secondary sound source. So you don’t need as much frequency response in a sound decoder as you’d think."

John also mentioned that new super-capacitors could bring new levels of sound quality to the same old speakers. "A capacitor works by storing electrons and using them later as a power source," John said. "Most capacitors are rated in thousandths of a farad. But one of these new super-capacitors with a third of a farad could keep the lights on in a model caboose for seven minutes after the track power is cut off."

The fact that a sound decoder is a sound circuit integrated in with lighting, motor control and processing functions means that a capacitor is necessary to provide power for sudden peaks in demand. Throttling up a locomotive from a dead stop will often draw extra current from a capacitor. Improving this circuit could be the best way to improve small-scale sound without fighting against the physics of speakers.

"You can use [capacitance] to enhance the sound," John explained. "It can add frequency response."

"In any of this, there’s going to have to be some willingness in the listener to go with the illusion," Dr. White said. "You can think, 'It's a model. Look at how many of the real thing's characteristics we can put into it.' And you accept the illusion. And I think sound works in the same way."

The illusion of realism, and the illusion of sound... do you believe?

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