[Harp-L] Embossing and Compression (was Rick Epping, father of embossing)
There was a time a few years ago when I was doing experiments with side vents and exactly how they change tone. I was doing numerous experiments, over hours and ours over the course of about a year with the hypothesis that they manipulated sound somehow. Somewhere in this process, I started working for Harrison Harmonicas. I was talking this over with Brad. Now, he would never come right out and say what the answer was and his process of telling you would take like 20 times as long, but he trained you to think and solve the problem yourself. He ask specific questions, such as:
"So... you REALLY think that's how it works, huh?"
or if those questions didn't work... And they didn't work with the side vent mystery, so he said:
"You know what your problem is?"
"No, Brad, I have no clue."
"You're not thinking about air flow."
Then, the light bulbs started going off like falling dominos and it was all clear. What changed the tone was air flow and finally the experiments I was doing were confirming things instead of making it all more confusing.
One I'd figured out the basic concept, he started explaining it in detail and showed me the side-vent concept on the B-radical. You can see the diagram of air flow in the B-rad here, there's a pic of it around the reed, but also a great deal of drawings and writing about it in the B-radical patent, which you will also find on this page.
I don't know what this did to tone, but I thought it was very interesting at the time there was a weak vortex around the coverplate screw post.
The best way I can explain the side vent thing is a chance in the proportion of air flow from different directions and you can see this in the drawings linked to above. When air comes in straight up from the bottom of the harp, there is a certain tone. When more air is allowed to come in from the side, that first tone still exists, but there is an addition of treble overtones. I can hear them both. With vents open, the treble overtones kind of lay on top of the tone that is already there.
So, I as I experimented with this, and I still do, it seemed really obvious that ANY change in air flow changes the tone.
Now with embossing,
When people emboss, they emboss at an angle. socket, coin, knife, whatever - and there is a specific angle they go for. It's always something that cuts into the slot, that changes the angle of the edge on the outside of the slot. If embossing were only about bettering tolerances, the angle would be nearly irrelevant. Jason Ricci was the first to show me how to emboss. When he showed me, he showed me how to do it with a knife and he had a specific way of doing it, his way the knife was nearly flat against the reedplate. At the time - I don't know if this is still true - Jason wasn't too crazy about adding too much treble overtone and he was played vented harps that already had a lot of treble, so he didn't use the angle everybody else was. I will explain the significance of that... There are other ways of increasing slot tolerances without cutting the reedplate material at an angle (as is done in normal embossing). Those techniques are not mine and I
don't feel comfortable discussing what they are. But they exist and there is a reason they exist - to increase slot tolerance without increasing treble overtone.
On the Optimized Sessions I build for Hetrick, they are embossed in a super-secret-squirrel 4 stage embossing process. First stage is not so secret - at least now it's not - I cut a certain angle with a certain cutting tool I have. Now, put it on a lightbox and you will notice NO change WHATSOEVER in slot tolerance. BUT you WILL notice a distinct EMBOSSED tone.
Rick said this and it is true:
"But I think most will agree that the closerthe embossing, the more the higher frequencies will be strengthened. There
comes a point when some listeners may find the instrument excessively
bright, so it's a matter of taste."
Now if embossing only works like most people assume it does then a good ear should notice a proportional addition of treble overtones between a harmonica with loose slots and another of the same model with tight ones. I've never noticed that, only noticed the tight ones are louder, but the mix of tones is the same.
Rick also said that "all harmonicas" can benefit from embossing, presumeably even one with acceptable slot tolerances.
Because embossing isn't really about slot tolerances. I know that was the reason behind its modern development. Rick did it to increase that tolerance and it did. But that's only 10 percent of how it works. Embossing with a coin or with a cutting tool alters the slot, it makes what was a 90 degree angle on the outside of the slot a particular slope. This slope does two things:
1) From the reed's perspective, it lowers the reedslot.
2) It dramatically alters the air flow to the reed.
Simply put, instead of coming straight down, after embossing, the air not only comes in straight down as it did before, it comes in from the side. Like with air flow with the side vents, any change in air flow changes tone. That's a very simple way of putting it, but that's how it works. Now, if that slope is rough and burry, the air flow is different from when the slot is clean and so is the tone. More raspy is the best way I can describe it.
Compression has a lot to do with gapping and vice versa. It's like trying to continually blow into a balloon with a hole in it. A high-compression harp is like a balloon with a small hole, a low-compression one would have a large hole. There's a point in gapping where the reed will choke or not respond to soft playing on the other extreme, but beyond that it's a matter of compression. Compression is the amount of backpressure that's in the comb cell when you blow or negative pressure in there when you draw (although science wouldn't call that compression probably).
To me, a too-tight gap is like a restrictor plate the intake of a race car. That's what it feels like to me as it restricts my control. To others, that same compression is desired. They want that restriction, they want that restrictor plate to conserve air.
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