Can't figure out how to see my posts on the resonator box group, so I'm reposting here.


I've been testing my hand built box with a tuning fork, looking for bridge placement sites.  I have found a few areas that vibrate very well with nice tone.  Next question would be, where to place the sound holes, since some of these placement sites are mirrored on the opposite end of the box.


One spot is 1.5" away from the longitudinal edge.  The surprising part is that there are two bands that vibrate well, both at almost exactly 1/3 of the box length, so, one at 1/3, and one at 2/3, with sound production better towards the ends than at the center.


When I place the tuning fork at any of these positions, the other points vibrate well.  At the center of the box is sort of dead.
Question is, would these vibrating bands also be a good place for a sound hole, or, should I let them resonate and add to the sound, and place the sound hole offset from there.


Box is 8x12x3, so, bands are at 4" and 8" of length.
Must be a standing wave going on.  I used an A turning fork.  Hope other note values don't shift the optimal points.
Please see attached photo.

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Here's the answer I always hate:  Try it and see what happens.

 

I just did it, and I can hear fuller ranger/more bass out of the hole than I can from the face plate.

Andrew,

You're getting there, maybe. I think many people believe that the hole acts like a speaker cone. One thought I've had is that the hole is actually there to improve frequency response (per David's hated empirical analysis above); in other words, it is acting like a bandpass filter. Proof of this concept can be shown by varying the hole sizes and observing the frequency response; a highly generalized observation of this is bigger hole = more highs, smaller hole = more bass. However, it is also interesting to note (pun intended) that measurable puffs of air can be detected coming through the sound hole when the air in the box is compressed by the soundboard being driven by string vibration. Maybe this is why people think the hole is acting like a speaker cone. The reality, I suspect, is that the soundboard itself is acting like a speaker cone.

 

Does that make sense? Mark, your turn.

 

Fine, just lob the ball back over here then! My simplified answer just doesnt do it for you? LOL!

I think the correct answer is sometimes......

One thing to remember is that you are producing multiple frequencys simultaniously and that the various components are vibrating at different frequencys and amounts, in different places, all at once. There is no static status quo. Sometimes there is opposing air movement, sometimes not so much.

At certain frequencys and amplitudes there probably is no need for any port or hole. at others a small hole, at others a larger hole and the "Ideal" location, is likely to be different in each of those situations as well. The very best comprimise would be a nice balance somewhere within the useable operating range.

The analogy I have used in discussing this in the past is speaker enclosures, but I find many people today are too young to remember a time when the idea and engineering behind speaker enclosures was to accurately reproduce sound, not have the loudest thumping "dung" horn on the block!

But if you can relate, the useful observation here would be that (properly engineered) enclosures intended for different frequency ranges have very different enclosures. Size, sealed, ported, even open back, every design parameter has a purpose. We used to experiement with "hard" and "soft" surfaces, placement. One common design theory used about 45 years ago involved "choking" the speaker with an opening somewhat smaller than the speaker itself!

Modern trickery by Bose and others aside, traditionally the real limitation was always found to be size. You just couldnt fit the ideal cabinets into the intended area unless you had a theater or arena stage to place it all.

Anyway, what I take away from this observation is that given the size, materials and inherent limitations of what we are working with here, there is way to much effort trying to analyse something thats just not going to have that great of an effect in the end. These things are great fun, but will never have a great deal of acoustic output. Just think along the lines of making the best of what you have. Lets not try to reinvent the wheel here.

Trust me on this, I share some of the desire to experiment with this and get all I can out of one of these silly things, but I also accept the reality of it. Until further rnotice, thats the best answer I can give.....

Maybe it's then time for an analogy from another sport besides tennis: golf

A good game for me, given that I play so rarely, is one or two good shots per 18 holes. If I practiced like crazy, took lessons from pros, and really worked hard on improving my mechanics, I'd find that 7 times out of 10, the ball would actually make it past the windmill arm, hit two bumpers, and land in the cup.

CBGs are like golf: it's more important to hit the hole (with a good song) than to redesign the course from scratch every time you pick up your niblick...which gives me a neat idea for a neck-through.

Cool to hear that you tested it like that, now if you have the guts, how about cutting a few nice f holes in the top and see if it changes any more? My unscientific tests have only been box to box, made simullar. After I made my Punch with the side holes I made another just like it but with f holes, both sounded very nice, I think the F holes one was better but it could have been a better neck or bridge placement etc. etc, so not a conclusive test. Keep us posted if you choose to make a guinnie pig out of you cbg, it is in the name of cbg science after all.

David Ford said:

put a couple of small holes today in the side that faces up towards me (where did I get that idea, Michael?).

 

Anyway, I had to lean close to hear any difference over the whole sound of the box.  What I did hear was a more.....don't know how to say it.....acoustic.....sound.....free air environment sound. Real sound.  The sound of just the box top, while leaning close, seems constrained in comparison.  It was like the difference between listening to a cd and then listening to someone play live.  Though the quality was very indistinct without leaning in.

They'll all be experiments of sorts.  As long as I have at least one to plunk on.

 

Still trying to find the right ergonomic configuration for me.  the second one has improvements in neck angle, scarf joint, acoustic body, etc, but, with each hack job, I'll improve.  working again full time, so that took some energy out of my hobbying.

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