Boom

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Craig Bumgarner
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Boom

Post by Craig Bumgarner » Tue Feb 14, 2017 1:03 pm

My guitars have been getting louder but along with that has come a boomy sound I'd like to reduce. I build Selmer style, ladder braced. The smaller sound hole option of the petite bouche helps but still some boominess. Air resonance is typically 90-110 us. Peak monopole 230-240hz. Reading the Book, I gather boominess is attributed to the 100-200hz range. As my peak resonances are outside or maybe at the extremes of this range, my guess is it is just part of the overall volume of the guitar.

If I wanted to reduce the boominess, what can I do? The only thing I can figure is lower the volume. That would not be too bad if I can maintain good mid and high range. Thicker top would tend to reduce bass and accentuate the highs, yes? Am I correct in thinking thicker top but keep deflection and T(1,1)2 high would be a possible solution?
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Trevor Gore
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Re: Boom

Post by Trevor Gore » Tue Feb 14, 2017 11:39 pm

So you are getting boominess on both the D hole and the petite bouche, with the main air resonance ranging from 90-110Hz? That would make the boominess pretty well independent of the main air resonance, which is unusual.

As "tone" is about balance, are you missing out on mids and trebles? If that's the case, stiffening up the bracing a bit should shift the whole tonal spectrum a little higher in frequency, giving you more mids and trebles (and a bit less bass). It will also knock the monopole mobility down, too.

Craig Bumgarner
Blackwood
Posts: 181
Joined: Thu Apr 11, 2013 10:28 pm
Location: Drayden, MD, USA

Re: Boom

Post by Craig Bumgarner » Wed Feb 15, 2017 1:21 am

Thanks Trevor!

The bigger the sound hole, the more the boom characteristic is heard. I've progressively closed off sound holes with a bit of top wood and a clamp several times and the result is dramatic. If the hole approaches being completely closed off, the sound gets thin and weak.

As expected, the frequency of the air resonance goes up with the larger sound hole ~110-115hz) down with the smaller hole (87-100hz), but the perception of the player is more bass to the point of booming with the larger hole ("raging base" one player called it). I assume this is because the player is hearing more of the air resonance via the larger hole.

The boom characteristic seems to have a range of only a meter or so, the audience does not hear it nearly as much as the player. I tell myself what the audience hears is more important, but I doubt many players would agree. Ideally, the player should hear exactly what the audience hears, yes? Not always possible for many reasons, but a good goal I think.

After thinking about this overnight, it seems boom in my guitars is 1) a function of sound hole size, smaller hole, less boom and 2) the strength of the air resonance as opposed to the frequency of the peak. So, keep the hole small and find a way to weaken the air resonance or strengthen higher resonances relative to the air. Strengthening the higher peak resonances would seem to be preferable so that overall volume is not lost but the composite of tones is shifted in favor of the highs.

For what it is worth, here is a shot of the resonance curve on my most recent petite bouche. I see now there is a bit of a bump around 200hz which I suspect is the octave of the air resonance at 100hz. It may well be a contributing boom factor. T(1,1)2 is 233hz and t(1,1)3 is 296hz. Not sure what the bump between 233 and 296 is though I see it fairly often.
IMG_8950.JPG
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Trevor Gore
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Re: Boom

Post by Trevor Gore » Wed Feb 15, 2017 10:00 pm

If you're happy with the projection you're getting now and you basically have a near field problem, you might consider a sound port "monitor" for the player.

Craig Bumgarner
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Re: Boom

Post by Craig Bumgarner » Thu Feb 16, 2017 1:45 am

Unless I misunderstand you, I think I need to go the other way. My experience is sound ports accentuate the lower register to the player, I'm trying to reduce it.
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Trevor Gore
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Re: Boom

Post by Trevor Gore » Thu Feb 16, 2017 8:10 am

Sound ports can have varying results, for sure, and I've not done many of them. The ones I've done have raised the main air resonance, which generally cuts down the bass and may help with boom on your guitars.

Al Curruth has done a lot more experiments and is pretty reliable. Here are a couple of his posts where he reckons a sound port helps the player hear the high frequencies: Post 1), Post 2), (penultimate paragraph).

Craig Bumgarner
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Posts: 181
Joined: Thu Apr 11, 2013 10:28 pm
Location: Drayden, MD, USA

Re: Boom

Post by Craig Bumgarner » Thu Feb 16, 2017 8:17 am

Okay, I see what you are saying. Yeah, the added area of the sound port raises the air resonance. But, to me as a player, the larger the sound hole area, the more of that air resonance I hear. Whether it is 90hz, 100hz or 120hz, it is still very bassy. I've done sound ports and abandoned them for that very reason, too much bass. Sound ports are seductive, at first it is WOW! After a while not so much.

I fully admit the requirements of the style I build has different characteristics and requirements than most other guitars. Thanks for your thoughts, always helpful!
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Bumgarner Guitar Blog

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