Load "roar" from MTA buses?

[nashvillwill]

This is a random question, but we have some smart people from all walks of life here, so I figure it can't hurt to ask. Two part question.

Part 1;

MTA #28 runs directly in front of my house. There is a designated stop across the street. Often, as the bus slows for the stop, the is a very loud "vacuum cleaner like" noise associated with the slowing. My mechanical understanding tells me that it is the sound of regenerative braking. I'm thinking that as the brakes are applied, an electic motor resist the momentum, winds up, and creates a burst of electrical charge. The resulting noise is what I hear. Can anyone confirm?

Part #2.

I only hear this noise during the day, even though buses pass my house in the evening. I also don't really notice the noise from buses when I am downtown. At first i thought "well maybe it's only a few hybrid buses that happen to pass my house during the day". But it's gotten to the point that it's too predictable to be coincidence. So, I guess my other question is this. Do drivers have the ability (and the directive) to turn this feature "off" at certain times of day, and/or in certain parts of town to avoid disturbing people?

It's a pretty loud noise. In the daytime, it's just a minor momentary disturbance, but if it happened while I slept, it would certainly wake me up and I would be making some nasty calls to MTA. I imagine if it happened downtown among buildings, the sound would echo for blocks. So, something tells me they have some sort of "plan" for controlling this. Can anyone explain?

[nashvillwill]

I'm sorry, I meant to put this in the coffee house. Can someone move it for me?

[whatwhat]

I'm not sure what it is but I've been caught walking down the breezeway between the R2D2 building and parking garage as southbound busses are hitting the breaks to stop at the North side of 4th & Commerce & the noise echoing in that canyon will about blow you away.

[captainwjm]

the noise you hear is only from the hybrid buses. It is part of the recharging system, same as in the Prius, where the braking mechanism uses those forces to turn a flywheel.

[nashvillwill]

Ok. But do they have the ability to disengage that?

[captainwjm]

Why would they?  Would you disconnect it on a hybrid car?

[nashvillwill]

I hope my original question was clearly stated.

It's very loud. It's an audible nusciance. No, I wouldn't disengage it on a hybrid car, but hybrid passenger cars aren't making 90 decibels either.

[captainwjm]

No, your question was clear.  Interesting that you mention 90 decibels, which is sometimes described as the level of rush-hour traffic at street level.  At that level, the bus sound could hardly be described as a nuisance.  I'd guess that the bus sound is about that level.  I say your reference to 90 decibels is interesting, as Metro Code says that recorded music played outside by restaurants can't be above 89 decibels when heard at street level some distance [i think 100 feet] from the property edge.  Also, of interest is that Metro has no decibel limit for live music.

[nashvillwill]

Well, to be fair, I pulled that particular number out of my back side. I have no idea what the actual sound output is, but in a residential neighborhood it appears louder than standard traffic (at least from my perspective).

I know what you mean about "why would they disengage it". I mean really, it would be a lot of trouble. But like I said, the time of day it does/does not happen just seems too predictable to be coincidence.

If I get bored enough, I'll sit on my porch for a while and try to video it.