What causes a harmonica note to drop in pitch (AKA bend)? I understand how I can lower the pitch of any note on my trumpet, regardless of the volume, by slowing the rate of vibrations of my lips. But how is it that I can lower (AKA bend) the pitch of a reed on my harmonica, regardless of the volume, by altering the way I draw air through my throat? 
Can you suggest one or more household experiments that I could conduct to demonstrate the harmonica note bending phenomena?
I have read past information relating to this subject (below). I was wondering if I was somehow creating a change in air pressure, verses velocity, within the resonant tube of the harmonica reed chamber and or my oral cavity.
Additional information that may be helpful:
I did not want to over complicate the question. But yes, regardless of key, the low notes in holes 1-4 and 6 can be bent down using a unique inhaling technique and the higher notes in holes 8-10 can be bent down using a unique exhaling technique. The note in hole 5 does not respond well to either technique. 
However these observations are only true with blues harmonicas (AKA Richter tuned).  Harmonicas other than common blues respond differently. So there is a contributing factor at work here that I was not pursuing.
To keep this simple: I have conducted my own experiment and found that I can bend down the pitch of most any harmonica reed when it is the sole reed present in a single reed chamber. The reed chamber naturally includes one reed slot for said read. 

Related past posts:
The basic model for the harmonica is an open tube as a resonator and an oscillating free reed as a driving mechanism.
The free reed could be modeled in the first (and pretty good) approximation as an oscillating cantilever beam. Which opens and closes the tube - hence the pressure and velocity variations. The frequency (or frequencies - there would be more then one peak in frequency domain) of such an opening mechanism is given by the cantilever paratemetrs such as stiffnes, mass, surface, length etc. (see the link) and intensity of the excitation mechanism: the velocity of blown air.
These frequencies are provided to the resonant open tube in which some of them are attenuated and some amplified based on tube parameters (where the length and cross section area are the most decisive parameters).
By a combination of many free reeds and resonant tubes the diatonic harmonica is given. More to that: these instruments usually posses a posibility to chose between two resonant tubes: one for the inspiration and one for the exhalation.
Note: In fact, almost all of that is valid for all reed pipsef with free reeds (e.g. for some parts of organ as well), i.e. for all the Hornbostel-Sachs class 422.3
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answered Dec 12 '15 at 9:35
Victor Pira
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Each tube in the harmonica is an open tube. The frequency that is produced in any open tube depends on length, speed of sound and n, using f=(nv)/(2L). The base note is where n= 1, which is the usual note you hear. If you blow a bit harder, you can hear the first harmonic, where n = 2. Here the frequency is double what was heard earlier (when n=1), and sounds an octave higher. There are more, higher harmonics that can be produced, so the answer is, yes, the physics calculations do predict higher sounds.
 A: In general, when playing a wind instrument the sound is not exclusively down to the (internal) shape of the instrument, your mouth is involved as well; the amount and shape of the space inside your mouth/throat/etc. is a contributing factor.
Vivid example: pucker whistling.
The constraint from the puckered lips sets up that some turbulence of air is caused. In itself that turbulance is random, but the presence of a resonant cavity (your mouth cavity) gives a feedback loop that favors a sound at the resonant frequency. To whistle at a lower pitch you enlarge the space inside your mouth, for higher pitch you decrease the space inside your mouth.
Conversely, if the shape of your mouth cavity is ambiguous, if multiple frequencies kinda fit in that cavity then you don't get that feedback loop going, and you don't get the whisling sound. You need a resonant cavity with a very specific response. The more specific the response the stronger the response.
The direction of the air flow makes some difference, but not a lot. Inhaling with exactly the same mouth position produces whistling too. The reason for that is that the speed of sound in air is much faster than the speed of the air flow itself. So the turbulance at the lips is in acoustic contact with the mouth cavity regardless of the direction of the air flow.
Back to wind instruments  
Part of the subconscious learning process of becoming proficient with a wind instrument is that you learn to change the shape your mouth cavity to assist the pitch of the note that you are playing. You just try and try and try. Over time you develop a muscle memory for things that work for you.
I don't play a wind instrument myself (I'm a singer) but the following seems plausible to me:
To bend a note when playing a harmonica the crucial factor is resonance. I assume that in non-bending play you tune the resonance of your mouth cavity to the natural frequency of the reed. But if you adjust your mouth to be a resonant cavity for a slightly lower pitch then the feedback from that can be sufficient to make the reed vibrate at a lower frequency. I assume that a strong resonant response is necessary for that, which means the resonance needs to be very specific.
(I suppose that with the changed frequency of the reed the air needs to be at a slightly different velocity or pressure, but those are secondary adjustments, the crucial factor is the resonance.)
I'm sure the same applies in the comparison case that you mentioned; trumpet playing. The vibration of the lips is passive just as in the case of a harmonica reed. When you start learning the trumpet it is hard to sustain the vibration of the lips. The reason for that: when the resonance in the mouth cavity is not well tuned to the note you are trying to play you don't have a good feedback loop going. The better the feedback loop the easier it becomes to sustain the vibration of the lips. Of course all the elements need to play together: the tension of your lips, the resonance, pressure, flow. But the resonance inside your mouth is for sure a crucial factor.
[Later addition]
After some internet search:
In 2013 a harmonica instructor and an opthalmologist collaborated to assemble a team to perform MRI scans of the throat anatomy during note bending. For the sake of completeness an instrument maker was commissioned to construct two harmonicas using materials that do not magnetize, so that they could be used inside an MRI scanner. The harmonica instructor is David Barrett, and his account of the findings is titled 'Bending process on the 10-hole diatonic harmonica with the visual aid of MRI' 
A: Great thread!
But there's more to it with harmonicas (and I don't know the full details). You can see this because different holes bend more than others...and the amount of possible bend is related to the pitch of the other reed at that hole.
Hole 2: Draw/inhale, can bend 2 semitones. Hole 2 blow/exhale is 3 semitones down.
Holes 4, 6: Draw can bend 1 semitone. Holes 4, 6 blow are 2 semitones down.
Same goes for blow bends in holes 7-10, in reverse.
Also, hole 5 does bend on the draw, but only about a quarter tone, as hole 5 blow is only one semitone down.
So it's not just that the reed is vibrating slower. There's an interplay with the other reed on that hole which matters a lot. The article cited earlier has some info on it.
P.S. Then there's overblowing and overdrawing, which are two other ways to produce notes... :)  :)
