School Bus Sound

July 31, 2005 8:55 PM

Why do _school_ buses sound different as compared to any other type of vehicle?

I asked this question on Google and got a couple of good theories, but no definitive explanation. You can always tell when one is coming, even if you can't see it (over the rise of a hill, for example). Greyhound buses, RVs, motorhomes, dump trucks, semis - school buses don't sound like any of those. It's been suggested that the long exhaust pipe and/or long drive shaft may contribute, but why wouldn't the other "long" vehicles sound like that? I'm sure you all know the sound I'm referring to, it's a particular type of whine (gear noise maybe?) that increases relative to the speed of the bus.

posted by Liosliath to Travel & Transportation (13 answers total)

Are you talking about the engine sound or the tire sound. Much of your typical vehicle sound comes from the tires rubbing against the road. Different tires at different speeds produce different sounds.

As for school buses verse other buses. Most city buses have their engines in the back, while most school buses have them in the front.
posted by dial-tone at 9:11 PM on July 31, 2005

From the way you're describing it, I'm guessing it's the sound of the tires. A drive shaft shouldn't make that much noise if it's working properly.
posted by dial-tone at 9:25 PM on July 31, 2005

I think the sound you're describing is actually the result of the bus driver over revving the engine. I know from my long tenure on a country bus route that the people driving school buses are in a hurry and aren't paying for the gas. They tend to drive the bus like they stole it. They wind out the gears and really rev the engine. That results in a distinctive engine sound. Almost makes me nostalgic...
posted by cosmicbandito at 9:38 PM on July 31, 2005

Best answer: My guess is that you're hearing a large diesel engine that's designed more for speed than torque (like a truck), combined with the fact that school buses are relative high off the ground. Here in NYC, that same school bus sound emanates from the minibuses that service adult commuters. I think these are just large turbojets motors connected to relatively hollow vehicle bodies that transmit sound well.
posted by ParisParamus at 9:39 PM on July 31, 2005

I've always thought it was because school buses are hilariously underpowered, and so always sound like they're going full-out but not really actually moving that fast. Apparently they're 190-250 HP. My car has more than that.
posted by trevyn at 10:01 PM on July 31, 2005

Yeah, I always assumed it's because they were crappy—our school district has been hurting for funding for a long time, and the bus quality reflected that. Remember the green vinyl seats?
posted by jenovus at 10:15 PM on July 31, 2005

TURBODIESELs, Mr. Spell check. I don't buy the underpowered part; makes no sense; new buses sound the same.
posted by ParisParamus at 10:22 PM on July 31, 2005

Best answer: I think I know the sound you're talking about, liosliath, and my entirely non-professional opinion is that the sound you're identifying as 'school bus sound' is actually the characteristic sound of a single dominant GM school bus drive train product line... and the whine of its torque converter running at high revs as trevyn says.

GM seemed to own the school bus business back in the 80's. Nowadays, I doubt this thing would make that characteristic noise.
posted by ulotrichous at 10:24 PM on July 31, 2005

I think he meant "underpowered" as in "deliberately designed to not be physically capable of exceeding 50 mph" and not "decrepit from age". I do know that most school buses have some sort of governer to limit speed, RPM, or both. I'm not sure if that is meant to suppliment or to replace an undersized engine. But it would explain why the transmission would be setup to wind out all the gears since with so little power available you'd have no choice.
posted by Rhomboid at 12:13 AM on August 1, 2005

Torque converter sounds right to me too. I remember that the sound pulsates at about 3Hz sometimes with a hard shift or when going uphill (wheeen-wheen-wheen), and that is what I would expect a torque converter to do under heavy stress -- slip and catch smoothly.
posted by clord at 12:25 AM on August 1, 2005

Something else to consider: A French Horn sounds very different from a Trumpet, even when playing the same note, because of the long tube along which the sound must pass in the former. A school bus has a very long exhaust pipe compared with other diesel powered vehicles, so its exhaust note would be more "mellow".
posted by RMALCOLM at 5:36 AM on August 1, 2005

Aforementioned Google thread.
posted by cribcage at 12:22 PM on August 1, 2005

Best answer: You're not going to hear converter whine over a hill, over your own car.

No, I'm buying the "long exhaust pipe" story, myself.

The school bus is the only large front-engined diesel powered vehicle that *requires* a rear-mounted exhaust (to avoid killing the kiddies, though sometimes that might seem like a pretty neat idea). Semi's have short vertical stacks, and most straight trucks exhaust down right behind the cab. And, as noted, most transit and OTR buses have pusher engines, as do most big RV's.

School buses also have *substantially* larger exhaust pipes per engine size than most other diesels, IME.
posted by baylink at 4:44 PM on August 4, 2005