The "alka-seltzer sound" refers to the fizzing, hissing noise made when an alka-seltzer tablet is dropped into water. Some synonyms for this sound could be bubbling, effervescing, or sizzling. Other descriptors could include crackling, popping, or fizzling. Depending on the context, one could also describe the sound as tingling, sparkling, or even explosive. Ultimately, the "alka-seltzer sound" is a distinctive auditory experience that is familiar to many people, and it offers a useful way of conveying the sensation of effervescence or bubbling.