Soak the noodles in hot water to cover and let stand, about 30 minutes, until soft.
Peel the shrimp, reserving the shells. Cut them in half lengthwise, then toss with 1/4 teaspoon salt and the sesame oil, and refrigerate.
Bring the stock to a boil with galangal, garlic, and lemongrass. Turn the heat to low and simmer for 35 minutes. Add the shrimp shells and simmer for another 15 to 20 minutes, skimming if necessary. Strain the stock and bring to a boil. Turn heat to low and simmer with the shallots and chiles. Simmer for 15 minutes and add 2 teaspoons of salt, the fish sauce, and the sugar. Continue to simmer.
Meanwhile, drain the noodles well and divide among four to eight bowls. Then add the shrimp to the simmering broth. Turn off the heat and stir in the fresh herbs and lime juice.
Divide the soup among the soup bowls, sprinkle over the black pepper, and serve.
Recipe courtesy Big Bowl Noodles and Rice by Bruce Cost and Matt McMillin (HarperCollins, 2000); Photograph by Zeva Oelbaum