Why do big diesels pulling heavy loads make a throbbing noise when heard from a distance?
Huge displacement together with relatively low RPMs.
The low “tones” carry further than the higher frequencies as well.
Another factor may be the same phenomenon you get with piston aircraft engines - you can sense the difference in RPMs between the motors by that same sort of “throbbing.” Odds are all of the Diesels in the locomotive consist aren’t running at exactly the same RPM.
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If you’re talking about an increase in volume that occurs a time or two per second (or less), that’s a mismatch in running speed of the engines. If you have one engine running at, say, 900 rpm, and the other at 875 rpm–again, this is most noticeable on EMDs–the result will be a “beat” frequency that represents the difference in frequency between the two. In this case, it would be 25 beats per minute, or one every two seconds or so. It will also fluctuate.
The E8, with it’s twin 567’s, would produce a really cool harmonic throb when running (under load sufficient to maintain speed) in the 5th notch. Unfortunately, in suburban service they rarely were operating at anything other than idle or run 8.
I think that’s it. My hearing is excellent, in the lower ranges, not so good in the upper (wife) ranges. I can usually feel a train coming long before it shows up. I told my wife that I have a 6th sense like Radar on MAS*H.
Why doesn’t this rhythmic throbbing, combined with the scenery of running through cornfields this time of year, put the engineer to sleep?
I think that thobbing was a little louder with earlier diesels. I was told by a C&NW employee that their GP35s would sometimes break windows of depots.
From some of the accidents that have happend - it does.
That was just from all the parts that were falling off of the old equipment. It was called the Cheap & Nothing Works for a reason…
Man this thread brings back pleasant memories of pacing Milwaukee Road GP40s powering the daily freight from St. Paul, MN to Duluth, MN along the former NP Skally Line north through the many small towns along Old US Hwy 61 on summer evenings. The train was normally powered by trios or quartets of GP40s with the tracks on the left side of the highway as far as Rock Creek, MN. I would be driving slowly with my window cranked down matching the trains speed of 20 mph as it came out of each town and the Engineer would throttle up to track speed which was no more than 40 mph. With the distance between train and my car being no more than one lane of highway plus the drainage ditch, the effect of throb, especially with a quartet, was incredible. It produced a visceral feeling as your whole body would be vibrating from the sound waves, as well as any loose parts on the car, and then the locomotives would fall in and out of sync. I normally had my scanner on to know when it was time to leave my friends house “To Catch my Train” in the Twin Cities. I really miss those nights.
I wish I could travel back in time to do what you did. Now that line ends on the south side of Hugo and is a bike trail 20 to 40 miles until its tracks again. A few years ago I bought some riding mower wheels and tires from a guy about half way to Duluth. Instead of taking I-35 I took 61 just to see where the RR tracks used to go. It sure was a lot more interesting drive than I-35.
At a typical max engine speed of 900 to 1050 RPM (for a Dash 9 - 44CW, see: http://www.alkrug.vcn.com/rrfacts/dash9.htm ), that’s 15 to 17.5 cycles per second, which is right at the lower level of most normal human hearing (15 to 20 Hz). Also: “Inaudible infrasonic sound waves can be felt by humans through physical body vibration in the range of 4 to 16 Hz.” (from: http://en.wikipedia.org/wiki/Hearing_range ) See also: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/earsens.html#c5
Could it also be related to the governor ‘cycling’ from a higher to a lower setting and back again to try to match a specific RPM/ power demand that’s midway between them, even if the locomotive’s throttle is set firmly in a particular notch (say Run 8) ?
- Paul North.
Norris, that’s one reason I didn’t want to work on actual trains as I got older…I knew that the droning/throbbing/whatever of the locomotives would make me drowsy.
I believe it’s the beat frequency you hear that sounds like axthrib.
Errr… Throbbing,
Two engines running at slightly different engine speeds will make a very low frequency beat frequency
Is there a way, or is it even neccesary, to have all the engines in a train running at the same speed ?
Murhy Siding, That very thing was one of my Father’s biggest complaints. After riding along a time or two, I have to agree. Older units had little or no sound insulation, so the cab was a very noisy place and that throb and vibration could put you to sleep in a very short time. Night time was worse. dad had several “the engineer fell asleep” stories. Including melted gibs, driver tire separations and emergency applications. Scary stuff.
Tim
Multi-engine propeller driven aircraft have had that capability since the 1930’s, the synchronizers were overlaid on the constant speed prop governors. Keep in mind that typically the engines were all identical and had a fixed relationship with each other. Something similar could be done with diesel locomotives, if the locomotives were all of the same make and model (i.e. same rpm settings for each notch of the throttle) and MU connections were modified to provide synchronization signals. There wouldn’t be any point if the lash-up was a combination of EMD’s and GE’s.
- Erik
I know nothing is perfect, but how much can two diesel locomotives deviate in engine RPM when in M.U.? I thought they would be quite close to running at identical RPM at each throttle setting.