Based on sound installation articles and recommendations, I see reference to an air-tight enclosure for the speaker. I installed a Tsunami decoder and built an airtight box for the speaker, a 1.3" oval (HO Y-6b) and I’m really impressed with the sound quality. I even sealed the hole for the wires with a dab of RTV. Soundtraxx says to make an air-tight box and their manual gives approximate volume sizes.
SO, my question is… why are all of the commercial sound installations that I’ve seen so far just have the speaker sitting in some form of retainer but do not have any kind of enclosure?
My BLI Alco RSD-16 has two speakers back to back but is in no way “airtight”… For another example of what I mean, look at the Intermountain FP7 review on Pg. 94 of the Feb. MR. You see the speaker just snapped into a ring above the gear tower. Its open on the backside. Would it be that difficult for manufacturers to make a decent enclosure to improve sound quality?
Wouldn’t the sound quality be better with some kind of enclosure? Has anyone tested various speakers with or without enclosures???
There are whole books on speaker enclosure design but I’m not sure that it’s scalable. For what we’re doing a sealed enclosure enhances the bass frequencies which are the weak section of small speakers. It would be worthwhile sealing the enclosures on all the sound equipped locos even if it just means sealing the body apetures as you did. The box should normally be as rigid as possible to stop deflections. It would be interesting if someone would experiment with bass reflex ports to see if that would help in the sizes we work with but that’s really for the manufacturers to deal with. J.R.
…the problem is that air pressure waves from the rear of the speaker diaphragm can mix with pulses from the front of the diaphragm if the enclosure is not airtight. These impulses are 180 degrees out of phase with each other, and by the time they travel around from the back to the front, they are unknonw, variable, and dynamically out of phase with the impulses from the front of the cone. You get a comb or notch filter effect that simply sounds wrong…however…
The fidelity of the speaker and sound samples are so low to begin with, that you usually don’t make it sound much worse, airtight enclosure or not.
Ports are good thinking but not much help in actual practice. What a tuned port does is artificially enhance bass response in such a way that the cabinet’s natural low end rolloff is delayed, giving a smooth frequency response at lower frequencies, (or a massive thump in automotive systems) From memory, a tuned port for a roughly two cubic foot cabinet requires aboput 100 cubic inches in the port…odds of finding that much space in HO are slim.
For further reference, a horn is necessary to produce true low end, again from memory, something like 66 feet of horn is required to get a true 20 cycle tone. In theaters, they usually place this under the stage. In HO? Good luck.
This is way over simplifying a very complex issue. The real answer is no. An “air tight” enclosure does not necessarily produce the “best” sound. An improperly sized air tight container can actually reduce the sound quality (fidelity). Sound quality is dependent on so many factors that choosing just one of those and saying, “do this” is missing all the other elements of the equation. What the terms ‘sound quality’ or ‘best’ sound mean are even open for debate.
Personally I believe the ideal speaker for model trains will be one that someone develops just exactly the right chamber, baffel, and port size to resonate the exhaust sound. In this imaginary world that means a different speaker would be “ideal” for each prime mover. A different speaker would be ideal for Alco w/turbocharger than for and EMD 1st generation, and definitely different for a steamer.
Some Factors Effecting Sound Quality
Speaker
a. size
b. shape
c. impedance
d. size and strength of magnet
e. distance cone travels
f. coil windings
all of the above will be determining factors for frequency response and distortion
Sound Processor
a. power output
b. frequency responce
It’s quite complicated, but the simple answer is you want the front and back of the speaker(s) seperated by an airtight barrier, so the back-side sound waves don’t cancel out the front side ones. With BLI’s back to back speakers, the two back sides are enclosed. In steam tenders, usually the tender itself is the back side enclosure with the front of the speakers left open.
An enclosure - air tight or not - in HO is generally too small to produce bass so the best that can be accomplished is to separate the (pulling) back wave from the (pushing) front wave by as much distance as feasable. The furter apart, the better the bass response.
Try a test with a small sheet of plastic using a 3/4" speaker over a 1/2" hole 3" from the corner - compared to one in the center. This will show how much the amount of back/ front separation affects the sound.
Folding this sheet into a ‘car’ shape still keeps the wave separation. A ‘ring’ or bottle cap enclosure while sealing off the back wave creates an undesirable high pitched resona
Bruce at Litchfieldstation.com has a very nice PowerPoint presentation on some speaker installation theory that he has presented at NMRA events. You can find it by following the link at the bottom of the follwing page.
