Subwoofers and bumpy roads
Thursday, August 21 2008 | Comments (1)
This week Adam and Jamie try to destroy a car with a subwoofer. Kari, Grant and Tory test if driving fast on a dirt road reduces the vibration in the car.
Adam and Jamie decided to test whether a car's subwoofer could be so powerful it would destroy a car. A subwoofer works by switching on and off an electromagnet, which causes the the cone to move. The question was if the resulting pressure could be strong enough to cause the car to basically fall apart.
"I say we just right into it," Jamie said. "Just go big."
So, the guys decided to skip a scale model and go right to the big guns. The average subwoofer is about 10 inches in diameter, they decided to fill up the entire inside of a car, about 51 inches. To start, they had to clear out the car - seats, steering wheel, etc. . . had to go.
A normal speaker uses an electromagnet to move the cone. That allows the speaker to vary amplitude and pitch, creating music. For this experiment, they didn't need sound, only the air pressure the speaker would create. Instead of using a magnet, they simply replaced the drive shaft with crank shaft and "powered" the speaker from the engine. When they put the car in gear, the shaft would spin. The shaft was attached to a push rod, which was attached to the cone of the speaker, so as it turned it would move the cone up and down. This created raw volume, instead of any sound.
To find out what kind of decibels (Db), the amount of pressure, a speaker would have to put out to do damage, they sent Tory to the Db Drag Race in Florida. This is a yearly contest where contestants are rated on the amount of Db they can make a car stereo system put out.
Tory met up with Wayne, one of the heads of the contest. He told Tory that he's seen cars crack frames, shatter windshields and other minor damage, but never anything powerful enough to destroy the car.
"It takes about 160 decibels to shatter a windshield," Wayne said.
To put things in perspective, Adam and Jamie shared a few facts about decibels - 140 Db could damage human ears, 165 Db is equivalent to a jet engine with 15000 pounds of thrust, 198 - 200 Db could kill you and 240 Db is close to what the atomic bombs dropped in Japan put out.
The cars in the contest add special specs to avoid damage, like concrete, steel, Plexiglas insulation and four-inch thick windows.
Adam and Jamie's speakers were made of pie-shapped steel, welded into a cone. Then they attached the inner tube of a tractor tire to the outside of the cone, to allow it to flex. Next, they created a wooden frame to house the speaker and attached the inner tube to it.
"That's not half bad," Jamie said.
With the speaker built, it was time to explain a little more science. There would be no sound pumped into the speaker, but it would created sound, because oscillating pressure creates waves. The sound created, however, would be subsonic, meaning it might not be audible to the human ears.
To measure the decibels, they brought Wayne in from Florida. Wayne attached a sound pressure meter to the inside of the car.
Before testing the pressure, Adam wanted to make sure the speaker would work. They pushed the car out to the parking lot and fired it up. They opened the doors, so the pressure wouldn't build up, and put it in gear. The speaker worked perfectly, which meant it was time to try and blow the car up.
They closed the doors and fired it up again. The speaker worked again, but the pressure didn't even blow out the windows. It did, however, destroy the speaker. Upon closer inspection, Wayne discovered the sun roof had popped off the track. Once the sunroof popped, it allowed the pressure to escape and therefore kept any other damage from occurring.
Wayne reported that the speaker did put out 161.3 Db, which was pretty impressive from a 16 hertz speaker.
"I'm not saying we cannot smash glass with a subwoofer," Adam said, "but there is no way we could destroy a car."
The problem is that once any opening occurs, the pressure escapes.
For the next myth, Grant, Kari and Tory tested to see if traveling faster on a bumpy road would make for a smoother ride. This myth came from the backwoods of Australia, where drivers claimed that speeding up allowed them to avoid the discomfort of dirt roads. The theory was that when traveling slower, the wheels have time to drop into the cracks, while traveling faster keeps the tires skimming across the top of the bumps.
To measure the ride, each member of the build team attached their own gauge to the 1978 Oldsmobile Cutlass.
Tory created a suspension deflection gauge, which was a needle attached to the suspension and run up through the hood. Mounted on the hood was a panel with lines on it, one inch apart. As the car travels over the bumps, the needle would move up and down on the panel, measuring the distance the suspension moves.
Grant decided to attach a accelerometer, which electronically measured the deflections.
And Kari decided to try a more visual test. She created a pyramid of glasses, filled with water. For her test, they would measure the amount of water that was displaced from the glasses.
Before heading to the dirt roads, they decided to take a test run across the parking lot with speed bumps. The test worked, so it was off to the state's recreation center - this was the closest dirt track to the city. The track is used by All-terrain vehicles, four-wheel drive trucks and the like, for a fun off road experience and it was the closest thing they had to an actual dirt road.
They decided to try two runs, one at 15 miles per hour (mph) and another at 45 mph.
On the first run, the water splashed all over and the suspension deflection gauge measured three inches of deflection. They had to wait until they got home to find the results from the accelerometer.
"Your pants are all wet," Tory told Grant. "Is that water?"
"Why don't you come over and find out," Grand said.
They tried again at 45 mph.
"Ready for your 45 mile per hour run," Tory said. "The paramedics are nowhere to be found."
The results: at 20 mph (the car idled a little higher than 15 mph), the deflection was three inches and they lost three and half liters of water; at 45 mph, they had six inches of deflection and lost four and a half inches of water.
Despite the results, Grant made an observation, "That was a lot smoother than I thought."
Watching Grant drive made Tory a little antsy, however, so he decided to take over and try the test again at 70 mph. Again, the ride was smoother, despite what the gauges said.
When they got back to the shop, Grant plugged in the accelerometer and the results were somewhat confusing. The water and deflector gauges said the faster they went, they rougher the ride, but the data from the accelerometer showed the faster speeds meant a smoother ride.
The myth was looking plausible, but Adam said they should try another test. The myth was talking about washboard roads, or chatter bumps for us country folks. Adam suggested they create a replica road to test the the difference on the roads in question versus a plain dirt road.
To create the road, the gang welded together 4000 pounds of steel angle-iron, creating a virtual, 100-foot road. The bumps were four inches tall and eight inches apart.
For the last test, they only used the glasses and the accelerometer.
The results: at 5 mph, lots of water was displaced and the high-speed camera showed the tires sinking into the gaps: at 40 mph, more water spilled out, but the tires seemed to skim over the top: at 70 mph, much less water spilled and again the tires skimmed over the bumps. This time the accelerometer confirmed the data.
MYTH CONFIRMED - although Kari pointed out that at 70 mph on a dirt road, one pothole could ruin the suspension.