Basses, bottles, fish & reinforcement of low frequencies...

[swingingoodtime]

Ok, so here's a interesting tale for all of you out there who've ever held a passing interest in Helmholtz Resonators, ported cabinets, f-holes & bass bodies etc.

My neighbour recently asked me if I could come & have a look at his stereo, to check out a 'loud hum' problem. Having heard it everytime I walked passed his door, and even through the apartment walls late at night, I assumed an earth loop somewhere.

When I walked into his place, I realised just how LOUD a hum it was; deep, bassy & constant. I knew it was lound from outside, but inside, you could literally feel the air in the room vibrating ever-so-slightly in sync with it as the room nodes picked up on it. Trouble is, his stereo wasn't on.

I walked around his apartment for a minute & eventually traced the source to somewhere around his very large fishtank in the corner. Being a 500L tank, I thought it might be the tank itself. Nope. Checked the two rather large filters in the supporting cabinet. Ah, ha: they were vibrating at exactly the right frequency. However, lifting them up off the cabinet base made no difference. Hmmm. Opened the cabinet doors: nope. Closed them: still no joy. Moved the hoses around in the tank. Nope. Moved the hoses around in the cabinet. Nope. Pressed hard against the doors/walls of the cabinet to suppress vibrations; nope. That vibration persisted, loud & intrusive.

At this point, I'd pretty much run out of options until I looked at the last thing in the cabinet that I'd not as yet touched; a large plastic water cooler bottle, about 45-50L in capacity with a 2" mouth that he used to change the water with. I suddenly became rather thoughtful…

I picked it up; the noise vanished. Other than residual noise from the filters (which are pretty darn big to begin with), the noise reduction was amazing. I put it back down on top of the hoses that it had been resting on; bingo! The noise came back.

My neighbour was perplexed; what was causing the loud hum? he asked. Not answering, I gently blew across the mouth of the bottle and got the same low, deep hum.

At this point, the geek in me was blown away by the coolness of this situation; the filters were vibrating at exactly the resonant freqency of the air mass in the bottle & with respect to the mouth dimensions. As the air mass vibrated, the mouth was then acting like a Helmholtz Radiator, amplifying the sound of the resonant frequency significantly. Seriously cool. Also reminded me why volume (L) = bass volume (dB) & depth (Hz) in loudspeakers etc (artificial bass boosting amplification & high excursion drivers aside; I'm talking conventional boxes here).

Reflecting on this, it's made me realise a thing or two about our DB's that I'd never really thought about; although the strings vibrate the top & back to generate sound, these plates will be of too low a compliance to generate deep acoustic bass on their own. But when youl also vibrate the large air mass inside the instrument then, dependent on the size & shape of the f-holes, there would have to be natural reinforcement of certain frequencies. Given the shape of the f-holes, I suspect a spread of frequencies, rather than just a narrow notch; kind of reminds me of a speaker design that I saw years ago which had a large port in front of a chamber in front of the speaker cone (yeah, I know; sounds weird) that was the same shape as a french-horn bell when viewed side on. The theory was that the irregular and non-linear change in port dimensions prevented a single resonant peak from occurring. Can't for the life of me remember what the design was called, though.

Then I started thinking; if this is the case, then how often are the f-holes on a bass actually cut to compliment the air-mass in the bass and the frequencies that we want reinforced, rather than just cut to a pretty shape & dimension?

Anyone else willing to let their inner-geek comment…?

Luthiers, I'd love to hear your thoughts on this (I'm assuming that, although it may not involve maths per se, you guys would take this into account when designing/building basses)

[Matthew Tucker]

As you know paul, I fill each bass I make with rice to calculate the volume of the air cavity for exactly this reason. However I find I often have to enlarge the F holes somewhat to get the last of the rice out, as it tends to stick to the sides. Very nice, but after that I have to go and lie down for a while to digest everything ...

[swingingoodtime]

Originally Posted by Matthew Tucker

As you know paul, I fill each bass I make with rice to calculate the volume of the air cavity for exactly this reason. However I find I often have to enlarge the F holes somewhat to get the last of the rice out, as it tends to stick to the sides. Very nice, but after that I have to go and lie down for a while to digest everything ...

Hey Matt, great to hear from you; hope you've been well.

That's a fair whack of rice...! Ah, but do you wash it first? If you don't, it'll go gluggy...

Seriously, do you try to tune the cavity to any particular frequency(s)? I've found that my bass' top resonates at about A/Bb, but I'm not sure if that's just the top (& back via the soundpost, I guess) or if the air mass has any bearing on that...

[Matthew Tucker]

no, and i don't know how i'd go about it even if i wanted to. what frequency would you want? if you're interested in all the research on air resonance and wood resonance etc the late Carleen Hutchins is the goddess. She's all over google and tomes have been written and argued over. Traeger is a disciple and covers it in his book.

I'm keeping an open mind about everything.

[VitaminC]

I have a book, "Stradivari's Genius." In this book the author talks about his life and all that jazz, but I do remember in one section of the book it talked about the F-holes. It said that Stradivari put on F-hole slightly higher than the other in order to increase the maximun sound output.

[swingingoodtime]

Originally Posted by VitaminC

I have a book, "Stradivari's Genius." In this book the author talks about his life and all that jazz

Stradivari played jazz...? (Sorry; it was there to be said!)

but I do remember in one section of the book it talked about the F-holes. It said that Stradivari put on F-hole slightly higher than the other in order to increase the maximum sound output.

How the heck does that work...? (I'm sure it does, I just can't think how)

Then again... perhaps having too much symmetry is a bad thing; introducing variables into the top may reduce its susceptibility to nodes & anti-nodes from forming across the dimensions which may interfere with sound propagation. Kind of like how too perfect a speaker cone actually promotes standing waves to form within its structure (assuming a flexible material as most are) & therefore distortion (although I'm assuming too imperfect would create other problems like weight imbalance).

[swingingoodtime]

Originally Posted by Matthew Tucker

no, and i don't know how i'd go about it even if i wanted to. what frequency would you want? if you're interested in all the research on air resonance and wood resonance etc the late Carleen Hutchins is the goddess. She's all over google and tomes have been written and argued over. Traeger is a disciple and covers it in his book.

I'm keeping an open mind about everything.

I don't think any one frequency is desirable; that's why I suspect the f-holes may be used: to promote a spread of frequencies.

I'll try to check those books out; sounds like interesting reading (now all I need is the time to read!)

[Maaaven]

I did an experiment where I placed a subwoofer near the
flat back of a reclining bass, and then did frequency scans with a test disk. There was a clear area of resonance where the air was rushing in and out of the f-holes. I seem to recall it being around B/C area and somewhat broad, with gradual transitions into and out of the region, perhaps over one third.
I did not try to change it, just noted it's location.

[Andrew McGregor]

A resonator has two variables: the resonant frequency, and the 'Q factor' which measures both how strong the resonance is and how wide it is in frequency (those who know some audio engineering will have heard of Q from the controls of a parametric equaliser).

So for a stringed instrument, you want a fairly low Q so that the instrument doesn't have one note much louder than the ones either side. Q depends on the shape and dimensions of both the cavity and the ports (f-holes). Now, it happens that if a cavity has two ports, and the ports have different areas, you have an effective Q that is quite a bit lower. Also long skinny ports lower the Q.

I think, whether they knew this or not, the luthiers that evolved the violin and viol families created something with close to perfect resonant properties as an instrument.

[gbaker]

Originally Posted by Andrew McGregor

A resonator has two variables: the resonant frequency, and the 'Q factor' which measures both how strong the resonance is and how wide it is in frequency (those who know some audio engineering will have heard of Q from the controls of a parametric equaliser).

So for a stringed instrument, you want a fairly low Q so that the instrument doesn't have one note much louder than the ones either side. Q depends on the shape and dimensions of both the cavity and the ports (f-holes). Now, it happens that if a cavity has two ports, and the ports have different areas, you have an effective Q that is quite a bit lower. Also long skinny ports lower the Q.

I think, whether they knew this or not, the luthiers that evolved the violin and viol families created something with close to perfect resonant properties as an instrument.

The f-hole is acoustic, mechanical and decorative all wrapped together. You pointed out the acoustic properties, also the long cutout loosens the plate and allows the bbar to pump it more efficiently, the rounded ends spread the stress to prevent cracks, and it looks pleasing to the eye. Evolution (or Deism) at work.

On a DB the Q is flat enough (the resonance is spread) so that the trick of blowing compressed air across the port, such as can be done with a guitar or speaker cabinet, to excite the cavity at the box resonance frequency (fB) won't work.

Wonder what the average fB of a bass and how it reflects quality?

[Jake deVilliers]

Originally Posted by gbaker

The f-hole is acoustic, mechanical and decorative all wrapped together. You pointed out the acoustic properties, also the long cutout loosens the plate and allows the bbar to pump it more efficiently, the rounded ends spread the stress to prevent cracks, and it looks pleasing to the eye. Evolution (or Deism) at work.

On a DB the Q is flat enough (the resonance is spread) so that the trick of blowing compressed air across the port, such as can be done with a guitar or speaker cabinet, to excite the cavity at the box resonance frequency (fB) won't work.

I regularly use my blower nozzle to blast shavings etc out of the corpus and its pretty easy to get a note by directing the air across the f-holes...you just need a bigger compressor!

[drurb]

Thanks, swinging, for pointing this out. Indeed, if it were all or mostly in the plates, then one wouldn't lose much by making an extremely thin DB. Reducing the depth of the ribs would not change the surface area much (relatively speaking) but sure would radically change the cavity volume! That would sure alter the sound. Your point, and the posts from others also serve to help understand why simply making the ribs deeper and deeper is not necessarily a good thing. The volume has to be the "right" volume to match the other variables.

[swingingoodtime]

Originally Posted by drurb

Thanks, swinging, for pointing this out. Indeed, if it were all or mostly in the plates, then one wouldn't lose much by making an extremely thin DB. Reducing the depth of the ribs would not change the surface area much (relatively speaking) but sure would radically change the cavity volume! That would sure alter the sound. Your point, and the posts from others also serve to help understand why simply making the ribs deeper and deeper is not necessarily a good thing. The volume has to be the "right" volume to match the other variables.

No worries; glad it stimulated some very interesting & informative posts! And thanks to all who contributed (and may yet do so); it's been a great read so far!