You know that woman whose baby screams in a public place while she seems not to hear it? I am that woman.
As you can see from this photo of Alder in his carseat, he sometimes isn't too happy to be in there. Usually it's the first five minutes of him being in it and then he goes to sleep, but sometimes he persists with screaming for a longer period of time (for example, the entire way home from the hospital)! The truth is, there's no use in trying to comfort him: shushing, rocking, offering a pacifier...we've tried everything, and nothing works. You just have to give him a few minutes to scream, and then he's fine. 
Today at Alder's speech therapist appointment (no, he doesn't stutter -- she's helping him learn to nurse) I had my hands full: car seat, diaper bag, baby. After changing his diaper I decided to go ahead and buckle him in the car seat before paying at the front, so that I wouldn't be trying to hold him while I dig out my wallet, sign the receipt, and so forth. I knew he would cry, but this is a pediatric dysphagia clinic; they hear crying all the time, right? I didn't really feel like I had a practical choice.
I got up to the counter with my screaming baby in his carseat, set him down right next to me on the floor, and handed the lady behind the counter my credit card. Within about 20 seconds, one of the nurses came out and sweetly started talking to Alder, trying to get him to stop crying. Then a few more seconds of screaming and a second lady came out, perhaps an office assistant, who noticed that there was a pacifier in Alder's car seat. She picked it up and offered it to him persistently. (I was of course standing just about two inches from Alder's carseat, seemingly unaffected by his screaming, I'm sure.) I told her the pacifier wouldn't work, but she kept on trying. Finally they were both there in his face, rocking the seat, talking to him, obviously trying to spare themselves or me the stress of hearing a baby wail in such agonizing tones (and at agonizing volume)! He just kept on screaming, oblivious.
Finally I was done paying. I had patiently awaited an itemized receipt so I could file with my flexible spending account, and I calmly paraded out the door carrying my 7-pound screamer. Anybody just observing me would probably think I was deaf, to be able to seemingly ignore my baby's cries so completely. But I'm not deaf. I've just gotten to know my baby a little, and I'm old enough not to care what anybody thinks of us. :)