Replies to This Discussion

Permalink Reply by Michael Recchione on May 28, 2013 at 7:27am

No, I'm not looking for an argument, if that's what you mean - not at all.  It's just that there are a lot of sort of half-truths and urban legends floating around, that get repeated until we've heard them enough times to consider them "true".

For example, placing the sound hole on a CBG in the upper bout.  Why?  I suspect the original reason was that, in through the body neck designs, putting it the center looked terrible because you could see the neck through the hole.  So people found a convenient spot to put it where it would look ok, which had to be on one side or the other of the neck.  When it was closer to the player, the instrument sounded louder to the player, so that became the sweet spot.  After that, a whole bunch of extra stuff grew up around that placement.

Anyway, I'm really not trying to be argumentative.  I just suspect that placement of the soundhole isn't all that critical.  I do believe that having a soundhole is better than not having one, most of the time, for an instrument that's to be played acoustically.  But even that's not always true - it really depends on the sound you're going for.  I just completed a copy of an Anglo-Saxon lyre, that used a cedar top, no bracing, with no soundhole at all, and it sounds very sweet, and plenty loud, even with nylon strings.  (I left the top thick near the bridge, and tapered it out to the edges...)  Also, the Turkish saz is usually made with no soundhole at all, and it sounds ... well,... like a saz.

I've been trying to understand this all better myself - maybe a little obsessed.  In any case, I apologize if I came off too strongly.  I think questioning and challenging one another, as long as it's done with politeness and respect, is a great way for all of us to learn more than we came to the discussion with...

Permalink Reply by wormil on May 28, 2013 at 2:22pm

Urban myths aside, here is one you can reason through... The sound comes from the top and back vibrating, if you cut out the vibrating portion of the top, you'll have less mass vibrating. So on a CBG, generally a square box, there will be natural nodal points near the corners since it's supported on two sides and the diagonal will be in tension so that's where I cut sound holes. You can get into resonant frequencies and all that but it seems overkill for a box with only 100 or so cubic inches of volume.

Permalink Reply by Michael Recchione on May 28, 2013 at 2:59pm

I'm not following your reasoning.  Of course there are nodal points where the top is fixed - the anchor points of a vibrating plate are always nodes - they're fixed and can't move.  But the middle is not the same - it can be a node or an anti-node, (or neither), depending on the frequency of excitation.  One thing that I've found when I've used the "tuning-fork test" for bridge placement, i.e. finding the spot that makes the loudest sound when I hold an A440 tuning fork against it, is that the middle is typically the spot that will allow the sound to be loudest, i.e. it's an anti-node for that frequency.  I don't think I'd want to put a hole there.

Now, as far as mass is concerned, the thing that makes the sound coming off the top isn't the mass of the top, it's the surface area - the surface area moves air, which creates pressure waves in the air, which conduct the sound to your ears.  Adding a hole removes some of that surface area, but it's compensated by the fact that the vibration of the top and back is also moving air through the hole.  So, starting from no sound hole, the total sound volume increases as you increase the size of the sound hole - up to a certain point, at which point increasing the size of the sound hole starts to decrease the sound volume.

I believe you were right earlier, (I think it was you), when you said that ideally the hole would be at a place where there was no movement at all.  Other than the place where the top attaches to the sides, there are no points like that on the top for all frequencies, so any placement is a compromise.  This is why a lot of guitar makers have moved to the idea of putting the sound hole, or sound port, on the side of the instrument instead of on the top - usually on the side that faces up towards the player.  In this way, they get the maximum energy transfer from the top, as well as getting the benefit of a sound hole ...

So maybe the real answer to the question that started this thread is to put the hole on the side that faces up when you play...

Permalink Reply by Sean Laffey on May 28, 2013 at 10:44pm

I did a quick and dirty empirical test on this theory. I took an old six string guitar and a Vintage brand ukulele. (Same basic shape,same cheapo wood). 

Here how to repeat my test, you'll need a quiet room::

Strike a string and  as the note resounds rub your finger over the sound board. For the guitar  I could  hear the note dying as my finger dampened the resonance in the plate, i could also get it to wow and flutter too, .With different strings  I  used this method to find the hot spots. The uke was dead before I ran my finger over it.  So first conclusion  the bigger top plate of a guitar does transmit a large proportion of its sounds as it vibrates.

OK we all know that.

So next up I took a very thin piece of clear  plastic (from a CD stack.. the protective disc that comes with the  CDs) and  placed it over the sound holes of the uke and the guitar. This would add negligible mass to the top but prevent air escaping from the hole.

The results were:  the uke was very much quieter with the plastic cover. The guitar was a bit more complex, hardly any change in volume  but the treble was less complex, the top E string had less sparkle and gave a warmer sound. I then moved the disc around the sound hole of the guitar , partially covering it, and the result was with just a fraction of the air hole left open the top 3  treble strings  brightened again. It seemed to have no noticeable affect on the 3 bass strings

Next I made a floating bridge from a scrap of wood and  stuck it under the strings on the uke. Repeated the test with the sound hole cover and the sound was brighter (OK it wasn't in tune as the intonation  was off, but I was only sampling the volume)r.  So my conclusions are 

1)small bodies instruments  such a uke or CBG need the sound hole  because the top is too stiff to project  enough sound energy on their own.

2) a floating bridge will help drive the small stiff top.

3)) the bigger the instrument. the more of the top plate can flex, so the importance of the sound hole decreases.But as the higher tones comes from the smaller , narrower, stiffer upper bout of the guitar a sound hole helps to develop their character.

As a CBG is by definition  box , there is no upper or lower bout*, so you should have far more choices for a sound hole size and position. Of course how and where you join the neck will affect the sound greatly.

 *On the guitar I tested. I calculated the air volume in the upper bout to be about 2 litres , the lower  at least 7 litres. I'd guess because  the neck attachment is at the upper bout this would make that part t of the guitar much stiffer.The maximum width of the upper bout, ie side to  to the finger board  was only 10cm,again adding stiffness to that part of the guitar.

Permalink Reply by Michael Recchione on May 29, 2013 at 4:02am

That's a really cool experiment.  A couple of observations: 

The top of a CBG, assuming it's wood, can be thinned and tapered so that it's more responsive and flexible, so, while it's never going to be other than a small top, it doesn't necessarily have to be an overly stiff top.  I like to leave it thick at the point where the bridge is going to go (I always use floating bridges on CBGs), and taper it as it gets close to the sides.  I'm really curious to see how a sound port on the side of the box, rather than a soundhole on the top, would sound if the top were thinned and tapered so that it was more flexible.  Also, how using some better wood for the top would sound.  I have a Western Red Cedar half-top from Stew-Mac left over from my lyre project ... (it was half of a bookmatched set).  I'm considering using it to make a couple of CBG tops, to see how that influences the sound.  The small soundbox doesn't have to be a total handicap - there are a number of instruments with a full, loud sound, that have really tiny, acoustically dead boxes, e.g. the Greek Tzouras and the Greek Baglamas. 

I've had a lot of travel for work over the last few months, which has really slowed things down as far as building is concerned.  Mostly, all I can do is "thought experiments" while sitting on the train from NY to DC.  Sigh.

Permalink Reply by Sean Laffey on May 29, 2013 at 11:13am

I've a couple of boxes I might try thinning on the next build or so, might take a while as my next project is a ukulele body dulcimer strum stick. Don't get me wrong I'm not hung up on the dulcimer. I play 4 string banjo and bouzouki in Irish tad sessions here in Tipperary. Mandoline in a country folk band and some Mumford and Son style banjo with a pub ballad ban.

I went down the dulcimer route coz we can't get them easily here unless by mail order from Germany. I'm 20 miles away from a music store, bummer when it comes to replacing a broken string

We have a house full of stringed instruments mandolines, madoloas, fiddles, banjos, bouzoukis, ukes, acoustic and electric guitars, basses some CBGs,...drums, bodhrans , brass and keyboards, whistles a flute that nobody can play, pipes,and a xaphoon which we all gave up on

I'm thinking of building a whamola for my son,, and I really like the sound of the Baroque Guitar....

Permalink Reply by wormil on May 29, 2013 at 7:37am

the thing that makes the sound coming off the top isn't the mass of the top, it's the surface area

Surface area is a measurement, mass vibrates.

I should clarify what I meant by center; I was referring to the upper center near the neck joint and where the fretboard overlaps the soundboard. That area will be less free to vibrate.

A sound hole facing up will benefit the player but not the listener. If the instrument is amplified then it doesn't matter. As I said, always compromises.

Permalink Reply by Sean Laffey on May 29, 2013 at 11:26am

Snip the thing that makes the sound coming off the top isn't the mass of the top, it's the surface area.

It is both, the energy from the strings has to drive the top, the lighter the  top the more energy goes into vibrating it, in practice that means a thin top should vibrate more easily than a thick one. Once the top has begun to vibrate,  it then  moves the air inside the box and outside it, both are surface area effects. There will also be an effect at the base of the bridge as the sound runs through  the saddle. That's how a skeleton instrument works when fitted with a bridge piezo

Permalink Reply by Michael Recchione on May 29, 2013 at 5:42pm

It's true that mass vibrates.  But do this thought experiment.  Imagine two objects with the same mass, but one of them is an extremely thin plate, and the other is a sphere.  If you drive them with the same energy - like a vibrating string - which one do you think will transfer more of that energy to the air to create pressure waves that you hear as sound?  There's a reason that fan blades are blades, and not fan balls. 

Anyway, I think Sean response covered this.  It would be cool to be able to get the benefit of a hole without reducing the area of the top, like when you put a sound port on the side facing the player, but to have it facing forward, towards the listener - maybe by using an elbow or a horn going from a sound port on the upper side, but facing forward.  I think there was a fiddle made that way ...

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