If you tune your harmonica reeds to concert pitch you’ll probably end up sounding flat.
The premise seems logical — blow on a reed, check the tuner and adjust the pitch to A440. Doing this, however, may not produce A440 results when actually playing. Several factors influence reed pitch.
Breath Pressure and Volume
Blowing hard to achieve high volume on a harmonica tends to flatten the tone, especially on the lower reeds. Any change in breath pressure (however slightly or unconsciously) while holding a note can cause the pitch to fluctuate up or down. The middle and higher end reeds are less affected by high volume playing.
Assess your playing style and decide if you generally blow at soft, moderate, or hard pressure. The pressure you use when actually playing is the same pressure to use when tuning. The closer you are to your natural playing style, the more accurate the tuning.
Cold v. Warm Harmonica
Reeds expand when warm and contract when cold. A cold reed is slightly higher pitched than the same reed when warm. As the harmonica warms up, the tone will drop.
As you tune your harmonica, maintain a temperature close to the inside of your mouth. Blow on a reed only long enough to get a reading on the tuner. Repeated blowing when testing the tuning will generate heat and lower the pitch. If you find yourself blowing on the same reed repeatedly, skip over it, move to the next reed and return to the initial one later when the temperature has normalized.
Reed Surface Deposits
Adding weight to a reed causes the pitch to lower. Blowing on a cold harmonica raises the temperature and condensation develops, increasing reed weight.
To prevent condensation, a harmonica should be warmed up prior to the start of the tuning process. Do not spend too much time on one reed. The more you play the same reed, the more moisture will accrue and affect pitch. Move on to another reed and return later.
Changes in air movement around the harmonica caused by wind savers or cover plates tend to drop the tone. The shape of your mouth and tongue also influences airflow and ultimately alters the reed pitch through bending or overblowing.
When tuning, play as straight as possible, avoiding bends or other mouth manipulation. If you remove wind savers and cover plates, keep in mind the pitch will lower slightly when the harmonica is reassembled. Hold the cover plates in their normal position when testing the tuning.
Avoid removing wind savers if possible.
Hohner recommends tuning your harmonica to approximately A442, in order to counteract any drop in pitch resulting from volume, warming, surface weight and airflow. This modification eliminates the risk of sounding flat and still remains in relative harmonic accordance with other instruments at A440.