You mean I've been humping around all 29kg of my Wharfedale Sand Filled Baffle for nothing!
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Don't fret, I expect the frequency response will be just as erratic with the sand as it would have been without it. You have not lost a daughter, you've gained a so....erm - some sand.You mean I've been humping around all 29kg of my Wharfedale Sand Filled Baffle for nothing!
In American barnyard vernacular, I suspect sand-filling an open baffle serves about the same function as putting lipstick on a pig.
Many years ago I read - and thoroughly enjoyed - a dusty copy of Gilbert Brigg's 1948 book on loudspeakers, which I found buried among the shelves at the local branch of the British Council Library. If memory serves me, the book included descriptions of a wide array of fairly impromptu loudspeaker enclosures, and Briggs even tried to convince the reader that double-mono (two speakers wired in parallel and fed a mono signal) sounded almost as good as (much more expensive) stereo.
What I liked most about the book was that Briggs seemed to be thoroughly enjoying himself trying every possible idea he could come up with ("What if we point the speaker up and suspend an ice-cream-cone reflector above it? What if we mount a speaker in a closet door? What if we mount a speaker in a big wooden storage-bench in the living room?")
It would seem that this was the Wild West era of speaker "design", with no rules, no mathematical models (Thiele-Small being decades away), no computer simulations, and negligible, if any, measurement equipment or systematic quantitative experimentation. Just tinkering, listening, and more tinkering, purely for the fun of it.
-Gnobuddy
One thing missing. You need enough mass that the pressure developed on one side not push the baffle toward the other. This would be more for bass frequencies. A sheet of paper won't do, card stock might even be a little light. Cardboard? You get some stiffness from the corrugated paper and hopefully it stops the two layers of paper from deforming too much.
I have a copy of Briggs' Loudspeakers book, and it is one of my most treasured possessions!Briggs even tried to convince the reader that double-mono (two speakers wired in parallel and fed a mono signal) sounded almost as good as (much more expensive) stereo.
One has got to remember that, when this book was published, stereo was yet in its infancy. The vast majority had mono systems and needed some convincing to spend more money on a stereo cartridge, an additional amplifier and another loudspeaker enclosure.
What Briggs actually said was "The excellent results obtained from stereo are, to some extent, due to the use of two loudspeakers which break up resonance; therefore similar room control can be achieved with single-channel output. I would not say that this is half way to stereo, but I think we could label it 'demi-semi stereo'." Briggs had a great sense of humour!
The 'Questions and Answers' chapter did throw up some amusing questions on speaker positioning - most of which were dismissed in the answers. These included knocking a hole in the floor from inside an existing enclosure to achieve an infinite baffle, mounting a 15" bass speaker in a brick pantry wall and putting a 12" speaker in an enclosure which occupied 20 cubic feet. Note that none of these were Briggs' ideas!
Yes, fun days when experimentation was all the rage and Theile & Small had not yet come along to say "you can't do that"!
P.S. The attached photo shows Gilbert Briggs with some of his "negligible, if any, measurement equipment"!
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Actually, two speakers playing the same sound much better for guitar - and four even more
I think it is because of the driver interaction. The "boxy" sound can definitely be a good thing in some cases.
I use 12" speakers exclusively for guitar, but I need to try my K15 clone with a nice tube amp and guitar and bass guitar. I bet it will sound nice, but is impractically big and heavy compared to a classic V30 equipped 2x12.
I think it is because of the driver interaction. The "boxy" sound can definitely be a good thing in some cases.
I use 12" speakers exclusively for guitar, but I need to try my K15 clone with a nice tube amp and guitar and bass guitar. I bet it will sound nice, but is impractically big and heavy compared to a classic V30 equipped 2x12.
Evidently time has dimmed some of my memories of the book. Mea culpa.P.S. The attached photo shows Gilbert Briggs with some of his "negligible, if any, measurement equipment"!
-Gnobuddy
There is certainly appreciable pressure inside a sealed box at low frequencies, or in a ported box at certain frequencies near the port tuning frequency.
But this is because the speaker cone acts like a piston, and the box acts like the cylinder, so that the speaker actually compresses the air behind it, trapped in the box.
For an open baffle, none of this applies. There's no "cylinder" to trap the air, which just whooshes away instead (i.e. radiative coupling efficiency from speaker cone to air is tragically low, particularly at low frequencies.) Certainly there is some pressure difference, but it's tiny.
But all these comments about the need for heavy or rigid baffles got my curiosity going, though, so let's do some math and work out some actual numbers to see what's what.
0 dB SPL - the quietest sound a healthy human ear can detect - corresponds to a pressure of 2 x 10^(-5) pascals - that's two over one hundred thousand, or twenty parts per million of one pascal. That's the pressure reference level used to calculate SPL in decibels.
Since SPL is calculated with the usual decibel formula (SPL = 20* log(P/P_ref), that means an extremely loud 100 dB SPL corresponds to a sound pressure of only 2 pascals.
(I've solved the logarithmic equation 100 dB = 20*log(P/2x10^(-5)) to find P. All logarithms are to the base 10.)
In North America people are more likely to be familiar with pounds per square inch as a unit of pressure than with pascals, so let's convert 2 pascals to PSI; you do this (reasonably accurately) by dividing by 6895, so 2 pascals corresponds to a pressure of 2.9 x 10^(-4) PSI, or two hundred and ninety millionths of a pound per square inch.
This is a very small pressure indeed!
Clearly a pound is too big a unit to make sense here, and so is an ounce, so let's convert to a horribly mixed unit of grams per square inch to see if that's any easier to comprehend; one pound is about 453.6 grams, so 2.9x10^(-4) PSI corresponds to about 0.132 grams per square inch. In other words, a very loud 100 dB SPL corresponds to a very small pressure of a wee bit more than one-eighth of a gram per square inch.
You mentioned paper, so let's compare the force per square inch due to the 100 dB SPL with the weight per square inch of paper; this is harder than it seems because a truly bizarre set of units is used to measure paper weight in North America ( Paper Weight Chart | Thickness of Paper Explained | Printi ), but apparently brown Kraft paper has a weight of about 100 grams per square metre, which translates to about 0.065 grams per square inch.
It seems typical printer paper weighs about the same as brown Kraft paper - 100 grams per square metre ( Printing Terms: What does GSM mean? Which GSM or paperweight is right for me? - Latest News-
StuPrint.com )
Remember, 100 dB SPL corresponds to a pressure of 0.132 grams per square inch, while the weight of the same square inch of paper is about 0.065 grams, or half as much. Sticking that into Newton's equation f=ma, we find that the sheet of paper would be accelerated at approximately 2g.
From memory, the cone of a loudspeaker generating 100 dB SPL experiences much stronger accelerations, in the ballpark of 50 g. So the sheet of paper, even unsupported, would accelerate very little compared to the speaker cone itself. (If you know the speakers BL constant, its moving mass (m), and the peak current (Ip)you need to get 100 dB SPL, you can calculate cone acceleration in metres/second squared: acceleration = BL*Ip / m. )
The paper would vibrate most at low frequencies, where it has the most time to move before the pressure reverses direction each half-cycle. I made a rough estimate that paper would move less than three thousandth of an inch even at the low frequency of 100 Hz. The speaker cone itself would be moving more than a hundred times as far.
(Remember, a small open-baffle speaker is never going to be capable of 100 dB SPL at 100 Hz, so I'm being extremely conservative with these approximate calculations.)
So it turns out that a sheet of paper actually would work fairly well for our infinite baffle, if only it would stand up under its own weight. There is not enough force on it from the acoustic pressure generated by the speaker to move it significantly, even at a ear-splitting 100 dB SPL! (And a more realistic SPL for noodling on the couch at home - the OPs intended usage - is 70 dB SPL or less, the typical loudness of a household vacuum cleaner. That's about thirty times less pressure, thirty times less force on the paper baffle.)
So paper would work. And cardboard? Obviously it's way heavier than paper (per square inch). Card stock is already 400 grams/square metre, four times the weight of Bond paper. Corrugated cardboard weighs even more. Perfect baffle material...
-Gnobuddy
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"for noodling on the couch at home - the OPs intended usage" - THAT explains the image; I thought it should be more like a small crowd with at least one audience member body surfing!
I've oft wondered what hex pickup - hex amplifier - hex speaker would sound like? With each driver in a 1/4 wave tube, tuned to the center frequency of the corresponding string.
I was thinking of putting a box around my 18" OB woofers - was told they need to cross below 300 Hz to avoid exciting the cavity. I can see how that wont work for guitar...
I have to believe the standard open back combo amp cabinet was designed purely for practical transport of the amp / speaker - giving both some protection as the amp is shoved into the back of the van. Wonder what a "practical" OB combo amp would look like?
I've oft wondered what hex pickup - hex amplifier - hex speaker would sound like? With each driver in a 1/4 wave tube, tuned to the center frequency of the corresponding string.
I was thinking of putting a box around my 18" OB woofers - was told they need to cross below 300 Hz to avoid exciting the cavity. I can see how that wont work for guitar...
I have to believe the standard open back combo amp cabinet was designed purely for practical transport of the amp / speaker - giving both some protection as the amp is shoved into the back of the van. Wonder what a "practical" OB combo amp would look like?
Open back speakers have the air compress in the back of the speaker as it does in front of the speaker. Otherwise we would not have any sound.
What is the displacement of air for a given SPL?
Of course, but...the compression is not a mathematically perfect zero, but it IS zero for all practical purposes.
In my previous post I calculated the pressure at 100 dB SPL to be only 300 millionths of one pound per square inch - 0.0003 PSI. Since 1 atmosphere is about 15 psi, another way to look at it is that a very loud 100 dB SPL generates a pressure of 0.00002 atmospheres! Not a perfect mathematical zero, yes, but certainly zero for the practical purposes we're considering here.
That's for 100 dB SPL. A more realistic living-room SPL of 70 dB reduces the pressure to 0.00000063 atmospheres. That's six zeros after the decimal point!
The pressure and velocity of air are connected by Bernoulli's theorem. The displacement will vary with frequency, with more displacement at low frequencies (same SPL => same velocity => greater displacment if there is more time per cycle of sound => greater displacement at lower frequencies).
The pressure is easily calculated, since SPL (dB) = 20* log10(P/2e(-5)). Solve for P, in Pascals. One pascal is one newton per square metre.
To get everyday North American units, divide the pressure pascals by 6895 to get PSI.
Back to your question about (air) movement, there are a few ways to calculate it. Acoustics textbooks like Beranek contain formulae linking air pressure (SPL) to air velocity. But to make life easy for ourselves, I found a nice easy online calculator here: Sound pressure level SPL 400 particle velocity characteristic specific acoustic impedance Z sound intensity acoustic power sound energy formulas acoustics sound units intensity acoustic characteristic impedance dB SPL calculations pascal audio calcul
I calculated earlier that a ear-shatteringly loud 100 dB SPL corresponded to a pressure of 2 pascal, so I put that number into the calculator, and the result is that the air velocity is 0.005 metre/second. That's five millimetres per second, or less than a quarter of an inch per second.
Slugs and snails can reach 0.013 m/S when they're in a hurry, and slow down to 0.003 m/S when they're completely unmotivated. So at 100 dB SPL, the air is moving at about the speed of a leisurely snail.
The speaker cone is moving a lot faster, of course; as I said earlier, the coupling from speaker to air is extremely poor, so all that frantic motion from the loudspeaker doesn't result in much air movement. In technical terms, radiative coupling efficiency from a typical loudspeaker to air is extremely poor. That's why horns make speakers so much more efficient - better coupling to the air.
Okay, we've found air velocity. How about air displacement? Let's assume that the velocity above is the peak velocity, and the velocity is sinusoidal, v = 0.005 exp(i 2 pi f t). To find the displacement, integrate the velocity wrt time; this gives us displacement = 0.005/(2 pi f) exp(i 2 pi f t). Sinusoidal, with a peak value of 0.005/(2 pi f) metres.
At f = 100 Hz, this means peak air displacement is only eight millionths of a metre - 8 microns! (This is for 100 dB SPL, @ 100 Hz).
As a rough sanity check, ignoring the sinewave motion and just using middle-school dynamics: f=100 Hz; one cycle is 10 mS - one half-cycle is 5 mS, or 0.005 second. Distance covered = velocity x time; 0.005 m/S times 0.005 second gives us 25 microns. This is the same order of magnitude as the 8 micron distance we calculated using more accurate mathematics, taking the sinusoidal motion into account.
So, unless I've made huge errors in both memory and calculations throughout this thread, we're back to the same conclusion as before: feel free to make your open baffle of the lightest material you can find that will actually stand up under its own weight. There is negligible pressure or force on it due to the sound waves from the speaker.
The physical vibration of the speaker "basket" itself is certainly the dominant force on the baffle, a big mechanical shaker mounted directly to it. That's probably why Briggs filled his baffle with sand.
However, I can't think of a single good reason why the speaker needs to be mounted rigidly to an open baffle. I think it would be better to mount the speaker with a very soft mount, perhaps some closed-cell plastic foam or something like that. There will be less mechanical vibration transfer to the baffle.
I've known for a long time that air pressures and velocities due to sounds are very small, but it's been interesting to calculate them and find out just how small. A good thing, too, otherwise our glass windows would shatter every time someone spoke nearby - they certainly wouldn't withstand a few PSI of pressure difference across them!
And its interesting what happens when the air cannot whoosh away from the "speaker" - when an aircraft goes supersonic. Now the pressure increase due to the moving rigid body (aircraft) is much bigger, and windows have indeed been known to shatter in the flight path of a low-altitude supersonic aircraft.
-Gnobuddy
What happens when you play chords or single-note solos, though, and shorten several of the strings? The speaker tuning wouldn't be right any more.
I guess it depends on your preferred playing style. There is no "dusty end" on any of my guitar fretboards, so I'm the wrong person to try this particular idea!
I've wondered for a long time about the first part of your idea - hex pickup, hex amplifier. This would let you distort the notes from each string without the enormous intermodulation distortion from feeding two different notes (strings) into the same nonlinear circuit. Would it just create a guitar that sounded like a keyboard? Or would it allow you to have distorted guitar and still play complex chords, instead of being stuck with two-note dyads ("power chords")?
How about some hinged "wings" that fold flat over the front of the speaker for transport, and open up to give you a much bigger baffle once the speaker is placed on the floor at your performance venue? Maybe a box that Velcro's onto the back to protect the rear of the speaker in transit (you pull it off when ready to play?)
Build the centre panel with 1/4" plywood, the folding wings with something even lighter (1/8" ply, maybe), and it should end up very light, and reasonably compact.
-Gnobuddy
So the paper is moving 3/1000" but the actual air is moving 25 microns?
Also, if a sheet of paper was enough to stop bass from canceling then that sheet of paper makes for a good sound barrier. If that were the case then we could just put sheets of paper between people in offices and they will not be able to hear each other. Saves on fancy acoustical media.
Also, if a sheet of paper was enough to stop bass from canceling then that sheet of paper makes for a good sound barrier. If that were the case then we could just put sheets of paper between people in offices and they will not be able to hear each other. Saves on fancy acoustical media.
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For the paper movement estimation, I didn't include air mass, or the differential equations that describe wave propagation in air, I just paper mass and middle-school kinematics equations to get a crude (and therefore too-high) estimate.
The second calculation I did - air movement - uses the equations of fluid dynamics. It doesn't look at how much paper might move, only air molecules. Unless I made a mistake, those numbers should be more accurate - assuming the online calculator I used is accurate.
So how badly too-high was my crude estimate of paper movement? (3/1000)" is 76 microns. Actual air movement using more rigorous math turns out to be 8 microns. They're within one order of magnitude, not bad for a quick back-of-the envelope calculation.
More importantly, 80 microns is as negligibly small as 8 microns; when the answer is "The movement is negligible", it doesn't matter if its eight microns or eighty.
One more practical bit of evidence: how far does a microphone diaphragm move when recording a loud sound? It's not unknown to place a condenser mic a few inches from a screaming guitar loudspeaker, and the mic diaphragm is spaced extremely close to the backplate in order to have sufficient capacitance for the mic to be usable at audio frequencies. SPL may be 100 dB at 1 metre, therefore, its considerably higher than that at the microphone location, usually within an inch or two of the speaker cone.
Evidently the microphone diaphragm vibrates even less than the tiny distance to the backplate, otherwise its movement would be clipped, and the sound heavily distorted. And condenser mic diaphragms are made from a thin film of Mylar a few microns thick, with much lower areal density than paper. The mic diaphragm moves only a tiny, tiny distance; a sheet of paper would move even less.
Exactly how close is the microphone diaphragm to the back-plate? I couldn't find hard numbers, but I did find typical capacitances for a condenser mic are 5pF - 100 pF. Knowing that, you can calculate the distance to the backplate using the formula for capacitance of a parallel-plate capacitor; plugging in plausible diaphragm areas, I come up with diaphragm-to-backplate distances in the range of 10 microns to 200 microns. Ergo, the ultra-light diaphragm of a condenser mic exposed to extremely loud SPLs is still less than 200 microns.
You've being quite nit-picky - what is your point? Feel free to do your own calculation, or to find numbers from a trustworthy source, and by all means correct my errors if I made any.
The bottom line won't change, I'm confident of that: there isn't enough pressure difference between the front and back of an open baffle at tolerable SPL levels to matter in any mechanical sense.
-Gnobuddy
Let's look at the flip side of that. If a sheet of paper was not enough to stop bass cancelling, why doesn't the bass from the front of the speaker go through the paper cone and cancel the bass from the back of it? According to your argument, no speaker with a paper cone can produce any bass output!
-Gnobuddy
I knew we'd get somewhere if I only beat my head on a wall for long enough.Now you're talking!
-Gnobuddy
Sorry for sounding nit-picky. It is just that paper is not a good isolator of sound, even though we use them as loudspeakers. If all we needed was cardboard just think how light horn loaded low frequency loudspeakers would be. Cardboad baffles would flap in the breeze so to speak. What is to stop it from vibrating like a drum? It probably can be braced with struts to stop it from vibrating, some hotmelt and more cardboard, but as a baffle on its own I see it vibrating and partly canceling out. If it was a viable material do you not think it would have been used rather than guys buying sheets of 3/4" plywood?
I have no question of your knowledge and ability related to this hobby. I just do not buy cardboard being a material I would use. And I have used cardboard as a quickie 'enclosure/baffle' to get a feeling what a speaker sounds like. I wish I had access to large sheets of cardboard as I once had. It would be fun to build a cardboard A7 enclosure and see how it fairs.
Gnobuddy - "There is no "dusty end" on any of my guitar fretboards, so I'm the wrong person to try this particular idea!"
I meant to tune the pipes at the middle frettted frequency for each string, so they'd resonate best for chords played up toward the 1st octave fret...maybe it makes more sense for them to be designed at the open string frequency, or a little higher but "karlsonated" making them somewhat more wideband. I spose the trick would be to pick these "cabinet" tuning features to give a certain sound to the amp/speaker combo - hopefully a desirable one!
What you said about IM for non-power chords is spot on. Still curious why this hasnt caught on with anyone; hex pickups having been commercially available for, oh, 40 years. They seem to be relegated to synth pickup duty, which makes perfect sense.
Perhaps in this day and age with chip amps having internal DSPs per channel, such a beast may be more easily realizable without being so "fat" circuit wise. Imagining a "hex" potentiometer for tone control ortherwise...
There's probably some way to have the amp chassis be part of the OB design, where you fold it open as you suggest for performance, fold it closed for travel. That's kind of a neat idea ;')
I meant to tune the pipes at the middle frettted frequency for each string, so they'd resonate best for chords played up toward the 1st octave fret...maybe it makes more sense for them to be designed at the open string frequency, or a little higher but "karlsonated" making them somewhat more wideband. I spose the trick would be to pick these "cabinet" tuning features to give a certain sound to the amp/speaker combo - hopefully a desirable one!
What you said about IM for non-power chords is spot on. Still curious why this hasnt caught on with anyone; hex pickups having been commercially available for, oh, 40 years. They seem to be relegated to synth pickup duty, which makes perfect sense.
Perhaps in this day and age with chip amps having internal DSPs per channel, such a beast may be more easily realizable without being so "fat" circuit wise. Imagining a "hex" potentiometer for tone control ortherwise...
There's probably some way to have the amp chassis be part of the OB design, where you fold it open as you suggest for performance, fold it closed for travel. That's kind of a neat idea ;')
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