The One-Note Doorbell

In your building there are eight flats, and every single one has the exact same doorbell: ding-dong. You know the sound too well. It rings, and nobody knows whose door it is, so either everybody gets up or nobody does. Last week your grandmother climbed all the way down the stairs because she thought it was your bell — it was the flat below. She was not pleased. You have decided your family needs its own sound. Not a "ding-dong" like everyone else — one clear note that only your door makes, so that the moment it rings, everyone in the house knows it is for them. Your grandmother should be able to tell it apart with her eyes closed. Here is what makes a note a note. When you pluck something stretched tight — a string, a rubber band, a wire — it shivers back and forth very fast. That fast shivering pushes the air, and the pushed air reaches your ear as sound. Pluck it and you will feel it buzz under your finger. How high or low the note sounds depends on how fast the shivering is — and you can change that. Pull the string tighter, the note climbs. Make the shivering part shorter, the note climbs again. That means a note is not something you are stuck with. It is something you can tune — dial in, exactly, until it is the one you want. So a doorbell that plays one chosen note is really a tuned, plucked string mounted on a box that makes it loud enough to hear from the door. Get the string, the tightness, and the length right, and it will sound your family's note every time — and no one else's. Today, your team is going to build one, and tune it to hit one exact target note on demand.