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Dirty-South Blues Harp forum: wail on! >
Sound waves and reed movement
Sound waves and reed movement
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Tom585
50 posts
Sep 08, 2017
9:31 AM
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Am I correct here?
When we play a note on the harmonica, we move air over the reed, the reed move back and forth through the slot, which pushes the air creating a sound wave.
When the sound wave reaches our ear, we perceive pitch based on the speed of sound wave, i.e., how many sound waves come at us over a fixed time period. Hertz is the number of waves in one second.
For example, 440 sound waves per second gives us the pitch of the note we call "A" (leaving out how harps are sometimes tuned a bit sharp). Does that mean that the reed moved back and forth 440 times for each second that it was played?
I'm trying to understand this in as simple language as possible so I can explain it to students in a simple manner. I started teaching a class two years ago called "the harmonica and the blues" at the university where I teach. There's nothing like teaching to see the gaps in one's own understanding of the topic!)
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the_happy_honker
276 posts
Sep 08, 2017
1:13 PM
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Anything that vibrates in a medium at 440 cycles per second will produce A, a bee's wing, a singer's vocal chords, a plucked string, a moving reed.
But a reed is not moving the air, it is the air stream moving the reed and getting chopped into 440 pieces per second by the reed opening and closing a slot.
But the air stream is not the sound. The air stream is moving far slower than the speed of sound and is quickly attentuated as soon as it leaves the slot. Note: a continuous stream of air through a slot makes little to no sound.
Chopping the air stream transfers energy to the medium in the form of pressure waves, which is what our ears register as sound.
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MindTheGap
2334 posts
Sep 08, 2017
11:22 PM
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Tom, yes that's right.
Although you say 'speed of sound wave' you clarify that you mean 'how many sound waves come at us over a fixed time period'.
For explaining it to others, I'd steer clear of 'speed of sound' in case it's misinterpreted. 'Speed of sound' is important to music and acoustics, but not so much to the bit you're explaining here.
I expect the aficionados will not be happy with 'reed pushes the air' but that's good enough.
Last Edited by MindTheGap on Sep 08, 2017 11:30 PM
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Tom585
51 posts
Sep 09, 2017
2:56 PM
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Thanks for the help. This is helpful and I'm glad I'm not too far off!
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