Intercom additional speaker box

インターホンの増設用スピーカーの箱です
This is a box for an additional speaker for an intercom.

アイホンのIER-2に相当する機能の箱です
This box has the same functions as Aiphone's IER-2.

参考＞https://sasukedog.hatenablog.jp/entry/2021/05/30/235755

なお、IER-2互換に必要な回路は
２０Ω57mm径スピーカー
直流カット用コンデンサ(10V3.3μ無極性等）
ボリューム調整用の抵抗（無くてもOK)だけです
The only circuit required for IER-2 compatibility is a 20Ω 57mm diameter speaker, a DC cut capacitor (10V 3.3μ non-polar, etc.), and a resistor for volume adjustment (optional). All you need to do is connect these in series.