Blues Harp
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Questions about blues harp physics

  • Why "Physics of the Blues harp"?
  • Questions for physics
  • Are there any practical applications for instrument making?
  • What are self-excited oscillations?
  • What is the role of the slits between the reeds and the reedplate?
  • The pressure force pushes, the elastic force pulls back - is this true?
  • Do we hear our blues harp playing unlike our audience hears it?
Why "Physics of the Bluesharp"?
Physics shapes our lives with countless technical applications. But physics can also be fun! To identify contexts, to get to the root of things, to explain many phenomena by a handful of laws, all this satisfies the basic human need to want to understand the world.
In this sense, works on the physics of the blues harp want to reduce the complex interaction of the player with the instrument to a few and as simple as possible physical laws. However, such research does not have to remain an end in itself, but can also have an inspiring effect on the optimization of playing technique and instrument construction.
Questions for physics
How is it possible for gentle inhalation or exhalation to make metal reeds oscillate? Where does the rich sound of a blues harp come from? MRI scans show that the oral cavity looks much the same when bending as it does when speaking the vowels [i] or [u]. How can this cross-connection to speech production be explained? Why can one instrument be played better than the other?
Are there any practical applications for instrument making?
Instrument making and customizing live from many years of practical experience. The better you understand your instrument, the more purposefully you can try to improve it. Of course, knowledge of the physical background can also be helpful. For example, if you know how important pressure fluctuations in the channel are for bending and overbends, you will pay attention to the greatest possible air density between reedplate and comb.
The natural frequency of a reed can be calculated for different materials and dimensions using physical formulas. Finally, let’s have a look into the future: The air flow through the blues harmonica  is way too complex to be  processed by today's computers. One has equations (the Navier-Stokes equations), but one cannot solve them. Perhaps one day, with the help of  much better computers, it will be possible to construct better instruments by specifically changing the shape of the reed channel  or the geometry of the reed.

What are self-excited oscillations?
When playing the blues harp, a constant flow of air comes out of the lungs (blow note) or a constant flow of air flows into the lungs (draw note). On the other hand, in the oral cavity, in the reed chamber and in the surroundings, the air oscillates (that's why we hear a tone). And the reeds in the instrument also oscillate vigorously. How do these vibrations arise?
In the blues harp, the fluctuating airflow and the oscillating reeds influence each other: airflow acts on reeds, reeds act on airflow. As a result, the pressure fluctuations in the air stream and the reed vibrations can blow each other up. The term "self-excited oscillations" for this process means that airflow and reeds convert the constant airflow from the lungs or into the lungs " by themselves" into oscillations.
What is the role of the slits between the reeds and the reedplate?
With a blow note, air flows from the outside into the reed channel, with a draw note, air flows from the channel to the outside. In doing so, the air has to pass through the slits between the reeds and the reedplate. It accomplishes this by flowing faster inside the slits. The necessary acceleration comes from overpressure (blow note) or underpressure (draw note) inside the channel. The slit size, the air velocity in the slit and the pressure in the channel are therefore correlated.
On the one hand, this is important if one wants to understand the emergence of self-excited oscillations of reeds and airflow. On the other hand, the sound emitted by the instrument is generated by the fluctuations of air velocity, which in turn are related to the fluctuations of air pressure in the reed channel and in the vocal tract.
The pressure force pushes, the elastic force pulls back - is this true?
For blow notes, the pressure in the reed channel is higher, for draw notes it is lower than the ambient pressure. The elastic force is a force with which the reed "defends" itself against deformation. The crucial thing now is that the reeds of a blues harmonica have a kind of "life of their own".
Does the pressure force push the reed toward the reed plate, and does the elastic force pull it back?
This concept is wrong for several reasons. To argue the point, let's imagine that the blues harp sounds at full volume (this makes everything easier). The blues harp reeds are thus supposed to vibrate powerfully already.
1. Each of the two  reeds in the channel oscillates almost by itself. It is primarily the elastic force and the inertia of the reed mass that alternate in their influence on the reed movement: Once the reed moves, it "wants" to keep moving. It has "momentum", with which it can run against elastic braking forces.  
2. If you take the blues harp away from your mouth, you will hear the reeds keep oscillating for a short time. If a reed in channel #4 of a C-harp continues to vibrate for half a second, it has moved back and forth about 250 times - completely without the influence of a pressure force. From a physical point of view, the reed continues to oscillate for such a long time because only small energy losses occur during the oscillations. Only these energy losses have to be replaced by the pressure force.
3. Elastic force and pressure force do not act alternatively, but both fluctuate around an average value.
4. The elastic force of a reed is approx. 30 times greater than the pressure force during a normal draw note. In the case of a draw bend, it is still about 10 times as great. Elastic force and pressure force are therefore by no means equally strong "partners".
5. It is the true role of the pressure force to compensate for the small energy losses of the oscillating reeds. A blues harp reed almost vibrates by itself, but only "almost".
6. The influence of the alternating pressure force on the reed movement should be thought of more as a kind of "balance". While a reed oscillates back and forth one time, it is - taking everything in account - accelerated more than it is slowed down by the pressure force. The pressure force adds more energy to the reed movement by accelerating it than it takes away by decelerating it. The energy balance is positive!
7. Since the reed requires only a small amount of energy, a comparatively small compressive force is sufficient.
Do we hear our blues harp playing unlike our audience hears it?

As with speaking, playing the blues harp creates a "hellish noise" in the mouth.  To demonstrate, I used a small microphone to record my blues harp sound directly in front of the harp as well as in my mouth. The photos show the volumes indicated by a mixer in decibels (decibels are a measure of how loud we perceive sound): It's much louder in the mouth than in front of the harp!

When speaking, a large part of the sound is reflected at the mouth opening (a fact for which, unfortunately, there is no simple explanation). Only a small part of the sound in the oral cavity radiates outward through the open mouth when speaking. With the blues harp, everything is even more complicated. First, the pressure fluctuations in the channel cause the air in the slots between the reeds and the reedplate to move back and forth at high speed. This pulsating airflow then creates the sound pressure fluctuations in the outer space. Since the outer space is much larger than the channel, these pressure fluctuations are much smaller than inside the channel.

Why do we hear our voice differently from our surroundings (as evidenced by sound or film recordings of us)? Part of the sound in the mouth travels through the bones of the skull to the inner ear, where it is perceived without the eardrum being involved. During bone conduction, the high-frequency sound components are largely absorbed. What we "hear" as a whole is a mixture of the sound radiated into the environment and our "bone sound". Due to the low-frequency bone sound, our voice sounds "fuller", "richer".

It is the same when we hear ourselves playing the blues harp. We hear more low frequencies than our surroundings. This is especially noticeable on low-pitched harps. What do we do with this realization? For one thing, we should enjoy the sound perceived by our own playing. On the other hand, we should strive all the more when playing for a "good" sound, which will then also be perceived positively by the environment.
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