Audio Switch Boxes!

I was recently comparing several powered speaker systems, but didn’t like the audible *Pop* I heard when plugging and unplugging the 3.5mm headphone jack into each of them.  I made a switchbox that eliminates this pop by switching the one input (my mobile phone) to any of the four outputs.  I didn’t have the right rotary switch at the time I built it, so it actually has two knobs that select where the input signal goes, meaning I can have two systems playing at the same time.  Alternately, the box can be used to switch either of two inputs selected with one knob to either of two outputs selected with the second knob.  Even more alternately, the box can be used to switch four inputs to a single output.

I few days later, after I purchased(!) the correct rotary switch, I built a second switch box.  This one has stereo logarithmic-taper potentiometers wired as voltage dividers, effectively acting as independent volume knobs on each channel.  With these volume controls, the playback levels of speaker systems with different gains can be matched, for ease of comparison.

Giant, Ominous Wind Chimes

A while back I bought five 4.5 foot long aluminum tubes because the price was so low that I couldn’t resist.  They are 3.25 inches in (outer) diameter, and have a wall thickness of 0.1 inches.  Recently, I decided to make them into the longest and loudest wind chimes I’ve ever heard.  The longest tube rings for over a minute after being struck by the clapper.  After thinking for a while about which notes I should tune the tubes to, I found that fairly large chimes are commercially available, but they are tuned to happy, consonant intervals.  I consulted a few musically savvy friends (Thanks Brian and Andrew!) to gather some more ideas for interesting intervals on my chosen theme of “Evil & Ominous.” I ended up with quite a few ideas, and with Andrew’s help, I sampled the sound of the longest tube being struck, and recorded mp3’s of each set of notes to simulate the sound of the chimes ringing in the wind.  I ended up with something delightful: D4, G#4, A4, C#5 and D5 (which are 294 Hz,  415 Hz, 440 Hz, 554 Hz, and 587 Hz).  That’s right, there are two consonant intervals (octave and major 5th), but look at all those minor seconds and tritones: Delightfully Ominous!

Then the science started:  How to determine the tube lengths to achieve the desired notes?  How to suspend the chimes so they sound the best, and are the loudest?  Where should the clapper strike the chimes in order to produce the loudest sound or the best timbre?

Wind chimes radiate sound because they vibrate transversely like a guitar string, not because they support an internal acoustic standing wave like an organ pipe.  Pages 152 & 162 of Philip Morse’s book “Vibration and Sound” show that the natural frequencies, v, of hanging tubes are given by the following expression:

Pretty simple, right?  One only needs to know rho and Q, the density and Young’s modulus of aluminum, l, the length of the tube, a & b, the inner and outer radius of the tube, and the beta of each tube mode of  interest.  Don’t worry though, there is a simpler way.  If all of the tubes have identical diameter and are made of the same material (6061-T6 Aluminum!), the equation indicates that the natural frequency of a hanging tube scales very simply as the inverse of the tube length squared.

Using the above relationship (frequency ~ 1/(length*length)) to compute the ratios of tube lengths based on the ratio of frequencies produces:

Length of D4 tube = 1.000 * Length of D4 tube

Length of G#4 tube = 0.841 * Length of D4 tube

Length of A4 tube = 0.817 * Length of D4 tube

Length of C#5 tube = 0.728 * Length of D4 tube

Length of D5 tube = 0.707 * Length of D4 tube

The longest tube is 133.1 cm (52.40 inches) long, so all the tubes were scaled relative to it.  Note that the frequencies are slightly different than the notes I was aiming for, but absolute pitch is only a requirement when playing with other instruments.

~D4 = 293.66 Hz = 133.1 cm = 280.3 Hz

~G#4 = 415.3 Hz = 111.9 cm = 396.4 Hz

~A4 = 440.0 Hz = 108.7 cm = 420.0 Hz

~C#5 = 554.37 Hz = 96.9 cm = 529.1 Hz

~D5 = 587.33 Hz = 94.1 cm = 560.6 Hz

How accurately do these tubes need to be cut?  For example, how important is it to cut the tube length to within 1 mm?  This can be calculated simply, using the above equation.  A length of 108.7cm gives 420.0 Hz, whereas a length of 108.8cm gives 419.23 Hz.  This spread is 0.67 Hz, which is a fairly small number, but these small intervals are often expressed in cents, or hundredths of a half-step.  This 1 mm length error gives a frequency shift of 31cents.  Does this matter?  Well, the difference in pitch of a major third in just and standard tuning is 14 cents, which is definitely noticeable.  It is preferable to be somewhat closer than this 1mm, or 2/3 Hz to the target interval.

The tubes were rough-cut to 2 mm longer than the desired length on a bandsaw to allow the ends to be squared up in case the cut was slightly crooked.  The resonance frequency was then measured by playing the desired frequency from a speaker driven by a sine wave generator with a digital display.  I then struck the tube and listened for (and counted) the beats.  If two beats per second are heard, the frequency of the tube is 2 Hz different than the frequency played through the speaker.  With this method using minimal equipment, I quickly experimentally measured the resonance frequency to less than 0.5 Hz (one beat every two seconds), which is ~10 cents.  I then fine tuned the tube length using a belt sander, and measured the resonance frequency several times while achieving the correct length.  In reality though, if I missed my target lengths I’d only be adding a little more beating and dissonance, which might have only added to the overall ominous timbre.

How to suspend the tubes?  Looking at the mode shapes of the tube for guidance, I suspended the tubes by drilling a hole through the tube at one of its vibrational nodes, and running a plated steel cable through it.  Check out the plot below from Blevins’ New York Times Bestselling book “Formulas for Natural Frequency and Mode Shape.”

This plot shows a snapshot of the tube’s deflection as a function of position along the tube.  Imagine that the left side of the tube is at 0, and the right side of the tube is at L.  This plot shows the first three mode shapes of a “straight slender free-free beam,” which my 1.33 meter long, 83mm diameter tube qualifies as.  Just like a guitar string, this tube has multiple overtones (higher modes, or harmonics) that can be excited to varying degree depending where the clapper strikes the tube.  The guitar analog of this is the timbre difference one hears when picking (striking) the string closer to or further from the end of the string (the bridge).  This plot also shows where the tube should be suspended – from the locations where the tube has no motion in its first, fundamental mode.  Those two places, a distance of 0.221L from the tube’s ends, are circled in red.  When striking the tube suspended from either of these locations, the tube rings the loudest and for the longest time duration (as compared with any other suspension location).  Similarly, when striking the tube in the location noted by the red arrow (the midpoint of the tube), the tube rings the loudest.  I won’t get into more math and fancy terms like “modal participation factor,” but it is true that suspending the tube from the circled red locations also results in the lack of excitation of the third mode (which has a motional maximum at this location).  Similarly, striking the tube at its midpoint results in the lack of excitation of the second mode, due to its motional minimum at this location.

Thanks to David for the Ominous Photo.   An Ominous Chime video will soon follow.

Making Carbonated Mineral Water

I really like the refreshing taste of San Pellegrino, but dislike that this water is bottled in Europe, shipped over water and delivered to me in Milwaukee, where we also have water.  San Pellegrino costs about $1.75 per liter, and comes in recyclable bottles. The homemade version I’ve been making for the last four months costs less than one penny per liter, and is made in my kitchen in reusable bottles.   The cost of the equipment was less than $150, which paid for itself after I’d carbonated my first 100 liters of water.

The equipment required is relatively simple: An aluminum tank that contains 5Lbs of CO2, a gas regulator, a hose ending with a locking Schrader air chuck, a plastic bottle, a bottle cap with a Schrader valve stem mounted in it and two hose clamps.  All of these items are visible in the photos below.

Carbonation Caps With Fittings

The aluminum tank and gas regulator are available locally at restaurant or homebrew supply stores, or online from places like or  Prices at these latter two places are $85 – $100 for the pair.  I filled the CO2 tank for $9 at a local beer retailer.  I purchased the locking chrome plated air chuck, the stainless steel hose barb connected to it, the hose clamps, and the steel wire reinforced hose from a local hardware store for $15.  The Schrader valve stems were purchased from a local auto parts store – they are fully chrome plated, and are sold as replacement car tire valve stems for $2 each.

I initially used standard industrial air hose fittings instead of Schrader valves, but ran into several problems.  Only one side of this type of fitting seals when the mating fittings are disconnected.  This means that after a liter is carbonated and the hose is detached from the plastic bottle, either all the CO2 in the hose leaks out, or some of the CO2 leaks out of the bottle.  Also, inexpensive industrial air fittings are either made of steel or bronze and begin to corrode due to exposure to the carbonic acid formed when the water is carbonated.  Chrome plated Schrader valves have neither of these problems, and are even less expensive than industrial air fittings.

The carbonation process is also simple.  I fill a plastic San Pellegrino bottle 80% to 85% full of Brita filtered water chilled to ~36 degrees (standard refrigerator temperature), I squeeze all the air out of the bottle and tighten the plastic cap with the Schrader valve onto it.  I fully open the CO2 tank valve, set the gas regulator valve to 55 PSI (typical commercial waters are carbonated to about 20 PSI), squeeze the locking Schrader air chuck, and lock it onto the bottle.  CO2 immediately begins to flow, and inflates the bottle instantly.  An audible hiss continues as the CO2 pressurizes the bottle, which I shake vigorously for 20 to 25 seconds, after which time the CO2 hiss has stopped.  The hose is then disconnected from the bottle, and the water is carbonated!

All these details are important to successful carbonation.  The empty space in the bottle (the 15% to 20% of the bottle that doesn’t contain water) is critical to allowing the CO2 to get and stay in suspension.  The amount of CO2 that is soluble in water increases with colder temperatures.  Squeezing out all the air allows for more CO2 to fit in the bottle.  Shaking the bottle increases the rate at which the CO2 dissolves in the water.  All of these factors make for more fizzy water (which is the goal, right?)

The taste of San Pellegrino can be more accurately replicated with the addition of minerals.  With the addition of 1/8 tsp of Magnesium sulfate (Epsom salts) and 1/8 tsp of calcium chloride, one achieves the 210mg/L of Calcium and 60mg/L of Magnesium that San Pellegrino has!  Both of these minerals are wine/beer brewing additives, and can be purchased from local homebrew supply stores.  Check here for more mineral additive possibilities, and the book “The Good Water Guide” for the mineral composition of most commercial waters on Earth.  I find that carbonating to 55 PSI rather than a more reasonable 20 to 25 PSI makes for so much more joy that I (and my kidneys) don’t miss the extra minerals.

If you want to make this setup at home, please follow these safety guidelines.  There are several which are very important, as a gas cylinder is somewhat dangerous, as its internal pressure is between 700 to 800 PSI, depending on temperature.   Carrying the cylinder by its valve is a bad idea.  The tank should be secured at all times so it doesn’t tip over and damage the valve.  When it is transported, it should always be upright and it shouldn’t be left in a car sitting in the sun, as the internal pressure will increase hundreds of PSI.  The regulator you purchase should have a pressure safety valve which releases at ~60 PSI to vent excess pressure and prevent your plastic bottle from exploding.  Similarly, your hose should be rated for higher than the pressure you intend to carbonate to.  You should never carbonate in glass bottles.

I measured the pH of my 55 PSI carbonated water, and found it to be 4.6, whereas the pH of Coke is a much more acidic 3.2, as shown below.  The pH of my water prior to carbonation was a perfectly neutral 7.0.

Basic and Premium StrobeTV

Do you ever want to watch a movie at home, but can’t decide between your favorite two?  Maybe your favorite TV show is about to start, but you’re still watching a film on DVD?  Perhaps you and a friend can’t agree on which film to watch? Well, you need not fret if you’re an early adopter of StrobeTV – a new device that enables the viewing of TWO films simultaneously on a single television by simply alternating between them!  The basic version features two knobs for fine-tuning your experience.  One sets “impatience,” which is how long you’ll view and listen to one film before switching to the second film.  This can be set from once every10 seconds and mere fractions of a second! The second knob controls “preference” or the relative amount of time spent watching the first or second film – perhaps you want to view the first film 60% of the time, and the second 40%.  Or, might it be 25% first and 75% second?

If you’re the proud owner of the Premium StrobeTV, a world of customization is achievable using the eight three position switches and two knobs.  The upper row of switches configures the experience you’ll enjoy for an amount of time set by the upper knob.  When this time expires, your experience is set by the lower row of switches for the amount of time selected by the lower knob.  Naturally, the process repeats, endlessly alternating between your two chosen forms of entertainment – be that TV, DVD, Netflix, gaming consoles, etc.  Perhaps you feel you’ll miss out on important action while you’re watching one film? Premium StrobeTV allows you to listen to the left and right audio channels of the second film, while you view the image from the first film!  Maybe you’re more comfortable reserving the left speaker for playing the left channel audio for the film you’re not currently viewing?  Or maybe the audio alternates? With StrobeTV, the possibilities are virtually endless.  Premium StrobeTV has an enhanced switching range, from once per 15 seconds to over forty times per second, ensuring you won’t miss a single detail!

Premium StrobeTV features an ingenious 2 meter long  cable that allows the controls to be at your fingertips, while the wiring remains hidden away.  Note that Premium StrobeTV, like its more economical sibling, allows for the switching of four signals: right audio, composite video, left audio, and a forth signal of your choice!


Redwood Audio Amplifier

A few months ago I started collaborating with Jordan Waraksa on a project that is currently on display at the Haggerty Museum of Art, in a show running until the end of December.  He sculpted a pair of wooden acoustic horns called Bellaphone 5 & 6 out of walnut.  Each horn rests on a redwood base that houses a small speaker.  If you visit the Haggerty, you’ll hear the speakers playing songs by Jordan’s band, The Vitrolum Republic.  If you really love the Bellaphones and amplifier, know they are for sale.

I developed and built the electronics for the horn’s amplifier, whose chassis Jordan also sculpted from redwood.  Because the chassis is wood and has no vents (for aesthetics), electrical efficiency became a high priority in order to prevent overheating.  I chose to use a Maxim 98400A class D amplifier driven by a high efficiency switching 15V DC power supply.  I added a digital signal processing chip from Analog Devices to increase the bandwidth of the horn and smooth out its frequency response (i.e. to improve the audio fidelity).  Analog’s ADAU1701 is a remarkably powerful chip – it is more than capable of these tasks.  In addition, the 1701 prevents bass notes (frequencies below 100 Hz) from reaching the small speakers (which are incapable of reproducing these low notes), which would otherwise emanate from the horns as distortion.  Finally, the 1701 also adds a small amount of compression, which prevents distortion at the loudest output levels.   The 1701 is actually real-time programmable via a USB connection to a computer running Analog Device’s free SigmaStudio software.  It’s a tremendously user friendly GUI environment with drag and drop audio processing blocks.

Check out the inside of the amplifier as it was being assembled, prior to the addition of many ferrite beads which eliminate the audible noise from the three high efficiency switching power supplies:  One powering the amp, one powering the DSP board, and one continuously charging a Motorola Cliq XT handset playing songs from the Vitrolum Republic.

Here are two more close ups, one of the amplifier:

And one of Bellaphone five and six:


I’ve recently made several audio synthesizer / noise generating boxes.  The most recent is the Cacophonator, whose circuit description and board layout are easy to find on the web.  Thanks to Adam for showing me how to etch my own circuit boards – something I can now do quite easily at the Makerspace.  I modified the board layout, drawn in DipTrace, so I could add SIP sockets and pin headers to tidy up the inside of the project box.  The sockets also allow the board to be easily removed, despite the 20 wires running between the board and other components inside the project box.  The  photos below were taken before I modified the circuit by adding a momentary power switch to slowly recharge the giant power supply capacitor if held for a few seconds.  I also added a LM317 adjustable voltage regulator so the power supply voltage can be reduced all the way down to 1 V DC and below.  The sonic complexity of this circuit is surprising considering its small part count, and lowering the power supply voltage further adds to the chaotic behavior of the cacophonator.  Careful inspection of the board will show additional components not associated with the original cacophonator, but are discussed on  These are used for inputting audio signals (music, for example), which come out the RCA jack quite cacophonated.


Makerspace Eight Speaker Super Surround Sound System

The day has finally arrived: The Makerspace Eight Speaker Super Surround Sound System (MESSSSS) is fully installed, wired and amplified.  This morning MESSSSS reproduced its first 8 channel audio piece, authored using adobe audition and played back by four android devices through four 1/8″ stereo cables feeding the bank of amplifiers.  More integration work remains to be completed (a dedicated computer with an 8 channel sound card plus a patch bay) but the MESSSSS is up and running!

Bay View Gallery Night is Friday, June 3rd at the Makerspace

Friday, June 3rd is Bay View Gallery Night: The Milwaukee Makerspace is hosting our own artists, and artists from Bay View Arts Guild to show their work. We have over 20 confirmed artists, plus two bands and more!  Stop by the Makerspace between 6pm and 10pm!

BVAG Participants:
Janet Falk / fiber
Karen Costello / jewelry – recycled stuff
Terry Nichash / photography
Ian Pritchard / photography
Donna Pogliano / jewelry
Cali Thomas / paintings
Anita Burgermeister – paintings
Stan Doty – wood carving
Erich Ebert – poetry & installation

MM Participants:
Brent Bublitz – sculpture
Jackie Steffen – illustrationsc
Kevin Bastyr – sound sculpture
Pete Prodoehl – drawing robots
Matt Neesley – sculpture
Amanda Endries – collage
Jason Gessner – interactive audio
Matt Gauger – interactive audio
Ross Oldenburg – interactive audio

Community Participants:
Dani Schmidt – Vase Sculpture
Bill Arthur – Painting
Lisa Sim – jewelry

We also have two bands playing: Makerspace’s own Ross Oldenburg in his band The Plane to Lisbon, followed by Steve Cohen.

Interested in Milwaukee Makerspace Membership or Information?

Curious about Milwaukee Makerspace Membership after attending our Open House on Saturday? Feel free to attend the weekly meeting held at our space at 3073 S. Chase Ave Building 34. The meeting starts every Tuesday evening at 7 P.M. and lasts about an hour. After the meeting all types of Making will be happening. The Milwaukee Makerspace is also open for prospective-member visits on Thursdays after 7pm on our weekly electronics night – its BYOEP (Bring your own electronics project). Please do stop by to meet the Makers! We hope to see you on an upcoming Tuesday or Thursday night!