Egg boxes most definitely do not work for sound isolation, don't waste your time (or binge eat eggs!). Very marginal absorption of high frequencies and a diffusion effect, but they have close to zero mass hence no isolation/blocking capability.
Perhaps people think they help because the profile resembles that of contoured acoustic foam, which is shaped as such to increase effective absorptive area, but this would be misguided too as we're dealing with isolation, not absorption. The latter is mostly only effective as reverb control, unless you have several feet of foam as per an anechoic chamber. This would then be sufficient to isolate too.
Perhaps people think they help because the profile resembles that of contoured acoustic foam, which is shaped as such to increase effective absorptive area, but this would be misguided too as we're dealing with isolation, not absorption. The latter is mostly only effective as reverb control, unless you have several feet of foam as per an anechoic chamber. This would then be sufficient to isolate too.
