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Episode 58: Shattering Subwoofers and Rough Road Driving

  • Shattering Subwoofer: a car speaker system can blow up a car. busted
  • Rough Road Driving: driving faster on a rough road is smoother. confirmed

Update: rough road driving revisited in More Myths Revisited (holding your hand up against a windshield to prevent it from shattering)

"Inexplicably, Adam is wearing chain mail" - Adam

Shattering Subwoofer

Myth: a car speaker system can blow up a car

Previously: Breaking Glass

Adam and Jamie discussing this myth at the Encinal High Benefit:

Adam: " For any sound geeks out there, this single speaker is going to have about 9 cubic feet of displacement --"

Jamie: " -- and it's supposed to blow the car apart... You know that's just another day in the life of MythBusters: whirlpools and gigantic speakers that explode cars."

The MythBusters went straight to big: build the most powerful subwoofer they could to see if they could replicate the myth.

db Drag Racing competition

They sent Tory down to Florida to check out the "Spring Break Nationals" car speaker contest: "dB Drag Racing". Tory's mission was to see if any of the speaker systems were capable of destroying the cars they were installed in. Tory interviewed Wayne Harris, President of dB Drag Racing, to find out if he had witnessed the myth. Harris mentioned windshields, lights, and panels breaking, but never a car exploding. At about 160 dB, windshields can shatter.

Tory next interviewed Mike Bartells, Extreme Audio/IASCA champ. Bartell's vehicle is 11,000lbs with 50 bags of concrete and windows up to 4" thick. The reinforcing is two-part: it keeps the vehicle from falling apart, and it also helps maintain the air pressure for higher dB. One end of the vehicle is capable of generating 177dB.

Decibels chart

  • 140dB: severely damage hearing in a short amount of time
  • ~160dB: shatter windshields
  • 165dB: jet engine with 16,000lb of thrust
  • 198-201dB: shockwave fatal to humans
  • 248dB: atomic bombs on Hiroshima and Nagasaki

The MythWoofer Build

  • Car: they stripped out the interior of an old diesel Mercedes to make as much room for their subwoofer as possible. A typical subwoofer has a cone size of 10", but the MythBusters were going for a 51" speaker cone.
  • Speaker cone: they built a large 51" speaker cone out of metal pie-sections approximating a cone shape. They then used contact cement to attach a rubber gasket and mount it to a wooden housing inside of the car. Foam was used to seal the gaps in the housing.
  • Diesel-powered speaker: Speaker systems are normally powered by an electromagnet, which can rapidly generate variations in amplitude to produce music. As the MythBusters were only interested in raw power and not music, they decided to power their speaker off the engine instead, i.e. a diesel-powered speaker. They attached their own crankshaft to the driveshaft. The crankshaft moved a pushrod up and down to power the speaker cone.

"That's not half bad. It's a little off, but I think it will sort of seat as we kind of turn on the diesel. The diesel will make it fit." - Jamie

"It's like the world's largest wok" - Wayne Harris

The speaker they made generates a sound wave, but it is too low to be heard by human ears. A sensor on the windshield captured the dB measurements.

"We've got three melodies. It's called 1st gear, 2nd gear, and 3rd gear." - Jamie

The Test

Wayne Harris (President of dB Drag Racing) was on-hand to measure the decibels generated. Harris' biggest concern was that the cone of the subwoofer wouldn’t be able to hold together. After the car passed a test run without shaking itself apart, they set to blowing it all apart.

They sealed up the car and pulled the trigger for the actual run. The speaker pumped violently but, other than a side-view mirror that dropped off, their was no broken glass. Adam was initially upset ("I don't see no cracked glass"), but then they realized that the speaker managed to blow the sunroof off of its tracks. They sunroof released the pressure inside the car and prevented the glass from shattering.

Harris' measurements showed that the speaker generated at 161.3 dB at 16hz, which he felt was an impressive amount of power -- almost equivalent to a jet engine.

They didn't bother doing another test run as they had the result they needed -- the car wasn't going to explode, because as soon as a window or sunroof blows out, there isn't enough pressure anymore.

busted. You can blow out windows (or a unhinge a sunroof), but you can't blowup a car.

Rough Road Driving

Myth: If you are driving on a rough road, it is better to drive fast as it smooths out the ride. At slow speeds, the wheels have time to fall down into the pits, but if you drive fast enough, the wheels skip across the top.

Update: rough road driving revisited in More Myths Revisited (holding your hand up against a windshield to prevent it from shattering)

This myth comes from Australia, where washboard roads are apparently more common.

The Build

The build team installed three measurement systems on a 1978 Oldsmobile Cutlass Supreme to gauge the roughness:

  • Tory's suspension gauge: a rod was stuck through the hood of the car and welded to the suspension. A simple gauge on the hood measured how far the suspension was traveling.
  • Grant's 3-axis accelerometer: ShockLog similar to the system installed in Buster's head. Can load data into computer.
  • Kari's wine (chardonnay) glass pyramid: wine glasses were filled with liquid and arranged on the passenger side. After each run, they measured how much liquid spilled out of the glasses.

Test 1: Prairie City SVRA

After Grant (with Tory in the backseat) took the car on a test drive on a road with speed bumps, they drove out to a suitably rough access road at Prairie City State Vehicular Recreation Area (SVRA).

  • 15mph-20mph test: 3" deflection on suspension gauge. 3.5L of water lost.
  • 45mph run: 6" deflection (max) on suspension gauge. 4.5L of water lost. Despite the data, Grant actually thought the run was smoother.
  • 70-75mph run: Tory took it for a spin at 70-75mph. They didn't provide water and suspension measurements, but they implied that this run was worse than 45mph.

Accelerometer data and Grant's subjective opinion were in conflict with the water and suspension measurements. Grant felt that it was smoother at higher speeds and the accelerometer showed that when road was more uniform, there was a smoother ride as they drove faster.

Tory: "I'm noticing something. You're pants are wet. Now is that water?"
Grant: "Why don't you come over and find out?"

They had enough data to indicate that the myth might be plausible, but Adam wanted to test a Australian-like 'washboard' road to get data that matched the conditions implied in the myth.

Washboard road test

Washboarding is created by reaction of car suspensions to alternatively accelerating, breaking, or hitting pot holes. These conditions create oscillations in suspension that create the washboard-like road surface.

They went out to Alameda and built their own washboard road out of welded steel:

  • 4000lbs of steel angle iron laid
  • 8" increments
  • 4" tall

They based their replica on Australian washboard roads.

They did three test runs with Grant driving:

  • 5mph: "that's pretty bumpy" - Grant.
  • 40mph: The high speed cameras showed that the wheels skimmed across top of bumps, Grant felt it was smoother. More water was lost, though.
  • 70mph: The high speed cameras showed that the wheels again skimmed across the top, and Grant again felt it was smooth. Less water was lost.

Driving faster on a rough road will give you a smoother ride.

confirmed

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