The most obvious way to make a harmonica louder is to breathe harder — to push or pull air through the reeds at a faster rate. To accommodate the increased airflow, the reeds have to make wider swings as they vibrate, which makes louder sound.
However, harmonica reeds are tiny, and when you push them too hard, the sound they produce is distorted and hard to control. Also, you’ll wear them out sooner — they’ll literally crack and break.
You can increase a reed’s capacity to move large amounts of air by increasing its gap, the angle by which it sticks up from the reedplate. However, high gapping has some trade-offs.
The receding listener exercise
To work on enlarging your sound, try playing for a listener who keeps getting farther away. You can do this with an actual person (if she’s sympathetic and patient) or with an imaginary listener. Here’s how you do it:
Stand at one end of a long hallway or in the corner of a large room.
Have your listener (real or imaginary) stand facing you about a foot away.
Start playing this harmonica exercise at a fairly quiet sound level.
As you play, imagine the sound that your listener is hearing as she stands facing you.
After a few cycles of breathing in and out, let your listener take a few steps back so that she’s a little farther away.
You want to still deliver the same level of sound to her ears as when she was standing right in front of you, so you let the sound get a bit louder.
After a few more breathing cycles, let your listener move a few more paces back from you.
Now you have to play a bit louder still to keep delivering a consistent level of sound to your listener’s ears.
After a few cycles of increasing your playing volume as your listener recedes into the distance, you’ll discover a limit to how loud you can get without distorting the sound coming out of the harp. That’s where you should stop and back off slightly to a level that’s loud but still sounds good.
The sleeping baby exercise
Getting louder is great, but getting softer is also important. You can develop your abilities to gradually get quieter with the sleeping baby exercise.
Imagine that someone is standing far away, and he’s holding a sleeping baby in his arms. You don’t want the baby to wake up and start crying, but at first you’re so far away that the baby can’t hear you, so you can play at the loudest volume you can reach, like you did at the end of the receding listener exercise.
Try playing the harmonica exercise again, this time as loudly as you can.
But then the person holding the baby starts slowly walking toward you. As he gets closer and closer, you continue to play. However, you have to keep reducing the volume of your sound so that you don’t wake the baby. Finally, the baby, still sleeping peacefully, is right in front of you, and you’re playing very softly.