How do blow and draw reeds work?
My education is in engineering, and I've never been able to figure out a couple things. First, why don't both reeds produce a sound when blowing and drawing? And second, I'm kind of confused on the blow and draw reeds. The blow and draw reeds sit closer to the higher pressure. ie, the blow reeds sit below the reed plate, and the draw reeds sit above the reed plate. When blowing, wouldn't the air just take the path of least resistance, and just move the draw reed slightly and leek out from there? Why does the air push the blow reed all the way past the reed plate to make the reed vibrate?
Hello Philosofy,
Have you read this white paper yet? This may answer your query.
http://www.turboharp.com/cmspage/JASA_download2.pdf
Your Harpsmith, Kinya
if you imagine a cut away drawing of a chamber in a harmonica with your blow reed and draw reed, and follow the air pressure into the chamber, the air pressure will build up in all directions. Several things are happening here that favor the blow reed moving into a vibration cycle before the draw reed.
1. The blow reed is inside the chamber where the air pressure is being fed by your breath. The draw reed is on the outside of the chamber and the air from your breath has to travel through the slot to get to the reed to move it. So the blow reed is closer to the source of the air pressure than the draw reed.
2. The sound cycle starts after the reed closes the slot, passes through it until the spring action of the reed counter acts the momentum of the reed and the air pressure to send the reed back through the slot.
3. The air pressure is roughly equal on the blow and draw reeds, but the draw reed would first have to swing away from the slot in response to the air pressure before it could come back and then pass through the slot to start the vibration cycle.
4. The blow reed will already be in business before the draw reed has a chance to start the cycle.
5. In the case of the draw reeds, the air pressure is coming into the chamber from both reed plates, but the same basic thing is happening - the draw reed goes into the slot first and starts the cycle of vibration. The blow reed has to move first away from the slot and then back before it even has a shot at starting the vibration cycle.
The gist of it here is that the reed that gets the air pressure first and closes the slot first has the first shot at getting the cycle of vibration happening. Until you get to the point where the air pressure and the bending process bring in other factors that influence the rate of vibration of the reeds, the reed closest to the source of air that pushes on the reeds will tend to vibrate first.
That's the way I visualize it. Hope this helps!
Richard Sleigh
http://www.rsleigh.com
http://www.hotrodharmonicas.com
Hi Richard,
Thanks for the description of how the reeds work, easy for me to understand. It also makes it clear to me why a smaller gap increases response of the reed - it has less distance to go before starting to vibrate.
Steve
Thank you Richard,
Joining us this year at David Barrett's 20th Anniversary Harmonica Masterclass was a bonus for our attendees and myself. Your knowledge of all things harmonica did not disappoint! The harmonica community looks forward to having you back in the Bay Area ASAP.
Your friend and Harpsmith, Kinya
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