[lupire]

Use double tones/knocks if you don't have long.

[wl]

This is a bad idea. You'll make a dah sound like two or three badly timed dits.

Copying morse code by ear is all about the rhythm. You can hear the rhythm just fine with simple knocks or taps.

[whoooooo123]

Then how do you distinguish between "long" and "two shorts"?

[wrycoder]

Original Morse telegraph was not done by tones.

The sounder emitted two clicks, subtlety different, for each key down/up event.

So, a dit would be two clicks with a short interval, while a dah would be two clicks with a longer (3x?) interval.

[BenjiWiebe]

Timing, I suppose. A double tone with much less than one dit's worth of spacing would be a dah.