The vroom we know about. It's the regular sound of a turbocharged racing engine producing something not far off a thousand horsepower and running at close to 240mph. But the slap that precedes it is something else, something unique, one of the most evocative sounds in all of sport.
To hear it, you need to stand a hundred yards or so south of the finishing line, on the way to Turn One, in the vicinity of the tall tower that displays the number and position of all 33 starters throughout the race and is the Speedway's principal landmark. From there, the slap reaches you before the car and its accompanying vroom.
It's caused by slick tyres passing at speed over a line of bricks laid in a pattern a yard wide across the finishing line. These bricks are a symbolic presence: a reference to the early days of the Speedway, when its entire surface was covered with bricks - 3,200,000 of them - and it became known, colloquially, as the Brickyard.
All but a few dozen of those 10lb bricks were covered over in the years between 1937 and 1939, when satin-smooth asphalt was laid on the surface of the oval track, first on the four banked turns and then on the straights. But the retention of the yard of bricks on the surface opposite Victory Lane is a sign of Indianapolis's obsessive and endearing pride in its own rich history.
When the track was first mooted, before the First World War, Indianapolis was the centre of the American motor industry. Three of its leading figures bought the 230-acre site north-west of the city centre in 1909, with the intention of building a combined test and race track. Once laid down, the basic configuration was to remain unaltered until the present day: two straights measuring five- eighths of a mile and two of half a mile, linked by four constant-radius turns, each a quarter-mile long. Perfectly symmetrical, the two- and-a-half-mile track was 50ft wide in the straight and 60ft in the turns, which were banked at an angle of just more than nine degrees.
Originally, there were large wooden grandstands and a picturesque pagoda-like structure housing the stewards and the timekeepers. Today, the permanent grandstands are imposing structures made of concrete and steel, providing seating for more than 250,000 people. Even riding the pale grey asphalt carpet at a sedate 20mph in a sightseeing bus, the impression is of a series of tunnels that seem to suck the vehicle forward into the turns, each of which appears to rise up like a ramp until the final approach suddenly reveals its curvature. Speedway is the right appelation for this phenomenon: speed is its only reason for being, and not much is allowed to interfere with the perfect projection of that sole purpose.
Ray Harroun won the first 500-mile race in 1911 at an average speed of 75mph, which meant that it took his eight-litre Marmon Wasp more than six and a half hours to cover the distance. The record for the fastest race is held by Arie Luyendyk, an expatriate Dutchman who averaged 185mph in 1990. Given decent weather - preferably on the cool side, to benefit the turbochargers - and an absence of yellow-flag incidents, that may be broken today, if Al Unser Jr's qualifying average of 228mph is a guide.
The unpremeditated effects of these speeds are contained by a concrete wall, 12 inches thick and betwen 39 and 42 inches high, and by debris fencing almost 20ft high. All of these were revised and rebuilt last year, as part of a programme of safety-minded improvements. Most significantly, the warm-up lanes forming the entry to, and exit from, the pits were separated from the main track by grass verges and rumble strips, which prevented the racers from using the lanes as an extra bit of racetrack. This softened the angle at which out-of-control cars hit the wall.
It also made the track tamer, by discouraging the drivers from taking the turns two and three abreast at 200 mph - a sight that used to be the Indy 500's glory. Now, conscious of the reduced track width, they tend to go through in line astern, leaving enough space to avoid a damaging aerodynamic backwash from the car ahead. Those of us who long admired the 500 from afar but only arrived here with Nigel Mansell have a suspicion that we missed out on something irreplaceable.
Even so, as the half-million gathering to witness today's event will agree, there's nothing else remotely like it. And whatever superficial changes may be made, the presence of those three million bricks just a foot beneath the surface somehow guarantees the lasting integrity of a magnificent sporting ritual, in a setting fit for New World heroes.