Did You Know They Used a Trombone to Make the Teacher's Voice?

Did you know they used a trombone to make the teacher's voice in the classic Charlie Brown cartoons? They would the line said by the teacher, then trombonist Scott McGuire would try to imitate the voice inflections using his trombone. To simulate adult voices they used a plunger mute opening and closing on the bell.

If they did this today, we would perhaps hear something closer to "blah, blah, blah".