Shattering Rear Window

[CFU LUZ]

Does anyone know of, or maybe had this happen as a result of too much bass. My rear hatch is cracked in too places underneath in the fiberglass, and now iv'e noticed one of my tail lights is now cracked. I'm not that concerned with these, as i am the possibility of bustin that big rear window. It does flex alot(about 1 inch) when i really pound the stereo. But will it actually bust. This has to be a common question amongst Camaro owners.

[megabass]

Of course it can happen. I've heard of guys buing plexiglass replacements.

[Camaro Bob]

It's not the frequency of the sound waves that will break the glass....bass has nothing to do with it. It's the air movement that creates the pressure on the glass. The only way you will break the glass in your rear hatch with bass, is if the force of the airpressure inside your car over comes the air pressure outside and the bonding forces of the glass itself. It could be possible to dismount the window, but it would take some serious woofers to move that much air.

[eZD]

In my old Jeep Cherokee, the rubberized sealing around my rear window came off due to some huge pounding bass and the window feel out, I felt proud!

[zlexiss]

Don't forget the great effects that resonance can have. Frequency _may_ have almost everything to do with it. I don't know if something in/on/near the rear hatch has a resonant node in the bass freq's, but if it did, small inputs can lead to large outputs..

(Seen the Tacoma Narrows bridge collapse video lately? 42mph wind (not too bad) brought down a brand spankin new suspension bridge..)

-Jeff

<blockquote>quote:</font><hr>Originally posted by Camaro Bob:
It's not the frequency of the sound waves that will break the glass....bass has nothing to do with it. It's the air movement that creates the pressure on the glass. The only way you will break the glass in your rear hatch with bass, is if the force of the airpressure inside your car over comes the air pressure outside and the bonding forces of the glass itself. It could be possible to dismount the window, but it would take some serious woofers to move that much air.<hr></blockquote>

[Camaro Bob]

The tacoma narrows bridge fell because the frequency of the resonance of the bridge's structure was matched by the wind....not sound waves. Also, the natural frequency of automotive glass would be such a high frequency, bass would have NOTHING to do with it. You can only hear up to 20,000 HZ, it would take such an extreme sound pressure level at an extremely high frequency, you'd have to have a Mad tweeter setup. You start feeling bass around 100 Hz.....I've personally seen competition setups run over 170 db, and the windows do dismount, but I've never seen one shatter.

[zlexiss]

Keep in mind that it may not be the glass itself that was resonating when it cracked. The inside of the car itself is a chamber with resonant frequencies that could have amplified something. Nearby body panels or the hatch could be vibrating and transmitting energy through the car.

The difference between a dismounted window and one that shatters is how (and how well) it was attached to the body.

That said, I do agree low frequency resonances are unlikely to cause anything like this to happen - carmaker try to design out anything that would cause such severe NVH. But as a car ages and tight seams start to loosen, there's more opportunities for undesirable natural frequencies to start appearing.

[xdamxincx]

i have not seen or heard of a rear hatch in a camaro breaking due to the stereo loudness. i used to have a different box in my car and in the rear hatch area i was hitting 155 DB and my rear hatch glass would flex but not break...but it is possible with higher SPL

[CFU LUZ]

Cool, you guys eased my mind. I don't really understand all that freq stuff and what not, i just know the window flex's enough to worry me. But if it is a fairly difficult and rare thing to happen then i'm sure i'll be fine. THANX.

[Camaro Bob]

zlexiss, Thank you for bringing up the tacoma narrows bridge, most people have no idea what they are talking about when it comes to resonant frequencies. The reason I "reasoned out" frequency, is because the natural frequency of glass (automotive bonded glass) is somewhere between 40,000 and 70,000 Hz. This doesn't mean that a sound wave at this frequency would make your windshield shatter, otherwise a lighning strike would be all that it took. We can't hear these frequencies, but if the spl (amplitude) of these waves were high enough, it would cause the glass to resonate within itself and separate...in simple terms, it would shatter. Bass frequencies are way too low.
eZD, the reason your window "fell out" was because of air movement. Woofers create sound by moving air, they simply move as fast as the speed of sound. The moving air creates a pressure force on the glass that pushed it out of it's rubber seam and the bonding force of the adhesive holding the window, in combination with the air pressure force and gravity, was over powered by the force of the air pressure created by the moving woofers. This is common at competition.
zlexiss, I'm not trying to be a dick, and I have utmost respect. When I was explaining that the air movement can dismount a window, I agree that a window could be broken. Loosened seems and stronger "sun cooked" adhesives could cause a window to break in the situation you explained. If the pressure levels overcame the forces holding the window in the car, but the window was still bonded exrememly hard in a small point, the glass could break due to the forces in certain areas. It's like, if you glued a toothpick to a table top, and pryed on the unglued portion, it would break in half, given that the strength of the adheisives are stronger than the internal bonding forces of the thin wood.
automotive glass is weird. You can hit it with a hammer and not break it, but hit it with an ice pic and it will poke a hole right through it.

[zlexiss]

Bob, no offense taken. Just having fun brainstorming unusual causes for events - need to keep on my toes working in the missile business now where my boss never accepts the obvious answer off the cuff :)

And tell me about auto glass. Spent six months up in Vermont during a break from engineering school molding polycarbonate windows for cars under a contract with GE Plastics.

Most of the time the automakers (their glass suppliers mainly) wouldn't give us real molds, so I ended up using actual glass for the molds.

Broke a few dozen Dodge Caravan backlights due to a combination of the sheer size of the glass, the spherical curvature (vs simpler cylindrical cuts), and the fact that the differences in thermal expansion caused the polycarbonate to expand a full 3/4" more than the glass, literally pulling the glass apart. Fancy teflon slip sheets be damned.

Interesting note for those of you looking for the best optical quality polycarbonate: GE Plastics actually started supplying us with a competitor's sheets instead of their own Lexan(tm), since Sheffield Sheet was making better stuff.

-Jeff

[Camaro Bob]

Auto glass is strange, I've never made it, but I've worked in a GM dealership as a tech for a couple years. From what I know there are two types of automotive glass.One is Tempered Glass. Tempered glass is a single layer of glass that is heat treated to increase impact resistance. It is heated to around 1100 Degrees(F) then rapidly cooled. It is made to size and shape, as it cannot be cut after manufacture.
The other is Laminated Glass. Laminated glass consists of two panes of glass permanently bonded together with a tough interlayer of plastic under heat and pressure into a single piece. (much like the caravan windows you speak of)
Windshields are made up of two layers of tempered Glass and a central layer of Plastic. These are laminated together to form a visual and Structural unit in your car. This is why it is so hard to fracture with a tool such as a hammer, yet a centralized force such as that behind a forced ice pic will puncture the glass.
I'm a double engineering major in college, I'll be a senior in EE and ME next year, and I have a math major as well as a minor in engine mechanics and deisel tech under my belt. I've been working tech at a GM dealership for a couple years while paying my *** off in college. Physics is something I love, and something I'm good at. I'm glad you posted your last response...you should give me your IM or email....I'm beefyboobies@hotmail.com, or beefyboobies as a screen name on AOL IM. I always like a good conversation, and I like picking peoples brains....You should tell me something about auto glass. Both being engineers, maybe we can share some info. Take it easy.

-Bob

[Camaro Bob]

Oh by the way....missile buisness???? Military of course, but what field??? Or do you work for Martin or something? Just curious, I studdied under a professor that worked for a military branch in Martin for years. They designed Anti Tank missiles that traveled over 7000 ft/s....the catch...they weighed in at over 45 pounds....can you imagine the firing mechanism? I was shown a detailed Schematic drawing and the theories and engineering principles behind the project. It was a failure, but the launch speeds were accomplished. Man, thats a fast moving piece of explosive.....especially considering that the average high velocity rifle bullet travels around 3000 ft/s.

[silvachris1]

I've heard of a similar situation happening in a 3rd Gen F-Body. Supposedly, his system (dunno if it was the bass, frequencies, air movement, whatever the heck) popped the glass right out of the hatch. It broke completely free from his hatch. Dunno how true it is, but I've heard the same story more than twice and it was exact down to the location of the event, car type, how it broke loose. Stories were almost identical. Who knows for sure, but I'd say don't worry about it unless you've got over 10 Grand in your system. lol

[zlexiss]

I work down at Raytheon Missile Systems (Missiles 'R Us, about half the world market), on an Air Force cruise missile system. Systems engineering, a good mix of desk work and hands-on.

Missile speed is a definate advantage over bullets. The trusted Phalanx system on Navy ships (that sweet 20mm gatling gun) is being supplemented by the RAM missile, due to its high speeds and increased engagement distance over the gun.

Oh, and check out
http://www.raytheon.com/newsroom/briefs/082902.html

for our version of the high-speed AT round (plus cool pic).

-Jeff