Sonic Vista – An Art Installation

For a few months now, I’ve been acting as an acoustical consultant to Bruce Odland and Sam Auinger on their project “Sonic Vista,” that opened on October 2nd for a 5 year long installation.  Bruce and Sam have been making publicly installed sound art pieces in North America and Europe for over twenty years.  I first heard their work at MassMOCA  in 2006, and met Bruce while he was putting the finishing (soldering) touches on “Harmony in the Age of Noise” at Tufts University in 2008.  Sonic Vista is installed in Frankfurt, Germany on a train & pedestrian bridge connecting two sections of Frankfurt’s greenway, located here.

Sonic Vista is a real-time sound art installation.  It gathers all the airborne noise from the industrial cityscape, filters it acoustically, and plays the resulting harmonious sound back through two giant, brightly colored spherical speakers placed directly over the heads of listeners as they walk along the pedestrian bridge.  Walking along the bridge literally becomes a consciousness raising event as one enters the sonic environment of the piece.  Noise from the cityscape (backhoes, earth movers and jackhammers from the nearby bank construction site, trains, dogs, humans, jets, boats, busses, and other city roar) is captured by two 4” diameter, 18 foot long (B0) and 12 foot long (F#1) tubes, each containing one microphone.  The sound reaching the microphones is naturally filtered by the harmonic overtone series of acoustic modes supported by the long tubes.  These microphone signals travel directly to the speakers, and are not processed with any electronic effects (such as reverb or chorus).  To hear Sonic Vista, follow this link to SoundCloud.  The exact position of the microphone in each tube affects the harmonic balance it detects, as shown by the following plot:

I aided in the design of the 1 meter diameter spherical speakers, first in solving the directivity problem: How can the speakers be designed so that listeners walking along the pedestrian bridge hear a uniform sound pressure level as they walk?  The problem with overhead speakers is that they typically send most of their sound straight downward – so it is much louder directly under them than it is off to the side.  The problem is made more challenging by the wide bandwidth — the lowest note is 60 Hz, and the highest notes are near 3000 Hz.

I’ll skip all the math and acoustics theory, and just state the solution: a 12 inch diameter woofer needs to be positioned behind a 4 inch diameter hole that all the sound must exit through.  This greatly improves the radiation pattern for the highest notes.  The heights and horizontal spacing of the two speakers also play an important role in setting how loud the sound will be when one is in between the two speakers.  Interestingly, I was able to balance these factors and the system as built achieved a pressure that was uniform within a few dB as listeners walk along the bridge underneath the speakers.  The plot below shows the loudness as a function of position along the bridge at the highest and lowest pitches, along with the 35+ dB soundfield variation that would have resulted without this hole to fix the radiation pattern.  The speakers are located at 50m and 64.7m, the two loudest locations along the bridge.

The second problem arose due to the solution of the first:  That the speaker radiates through a 4” diameter, 4″ long port created an undesirable resonance that I correctly predicted through the use of a computational acoustics model.  This resonance was actually eliminated by simply equalizing the speakers electrically.

This front cavity does lead to the loss of almost half of the sound output, but it’s a high sensitivity, 300 Watt maximum input driver, so it is still more than capable of being loud enough.  This is a small price to pay for the tremendous improvement in radiation pattern and listener experience.  To hear Sonic Vista, follow this link to SoundCloud.

Rubens Tube Update!

The Rubens tube I made a while back puts on a fairly impressive show when its speaker is driven with music or a noise box, as was done during the Milwaukee Makerspace Grand Opening.  The story is somewhat different when it is used to display the acoustic standing wave pattern inside the tube.  When a single tone (sine wave) at a resonance frequency of the system is played though the speaker, the heights of the flames map out the sinusoidal shape along the length of the tube. There are two very important variables whose values determine how well this will work.  These are the acoustic pressure in the tube, which is set by the speaker and its input voltage, and the propane gas pressure, which is set by a regulator or the valve on the propane bottle.  Even a professionally made Rubens tube has a relatively small range of these two pressure settings that create a nice sine wave distribution of flame heights.

After running my Rubens tube for a short while, I realized I’d made a few design choices that make the already small range of good operating parameters even smaller.  First, I didn’t actually use a gas regulator, I only used the valve on the propane nozzle.  Note that the propane flow rate is highly affected by the temperature of the nozzle, so the propane tank must be kept in a water-filled bucket to prevent the outlet valve from freezing up.  Just like a gas pressure regulator can be used, so could an acoustic pressure regulator (i.e. a compressor).  This could help prevent the flames from extinguishing during particularly dynamic musical passages.  Alternately, some type of pilot light system could be devised so that the flames automatically relight – perhaps glowing red nichrome wire could be added in a moderately safe way.  I also spaced the fifty 0.043” diameter holes apart by only 0.9 inches.  Having this many holes reduces the amplitude of the resonances, making them more difficult to ‘find’ by simply listening to the amplitude when adjusting the frequency input to the speaker.   Better performance would be achieved by having fewer holes spaced further apart.  Lastly, I’ve used a pipe whose inner diameter is only 2.5 inches.  A larger diameter pipe would further increase the amplitude of the resonances.

I put an electret microphone inside the Rubens tube at the end opposite the speaker, and measured the pressure inside while electrically driving the speaker with pink noise.  I did this with both air and propane inside the tube.  The following graph shows the low amplitude of the tube’s resonances with propane inside — between 4 and 7 dB.

The other interesting bit of data one can find from this graph is the speed of sound in propane.  Knowing the sound speed, one can calculate either the length of pipe needed to have a particular fundamental resonance frequency (n=1) or if a particular speaker has a resonance frequency low enough to excite the fundamental resonance of a particular length Rubens tube.  The resonance frequencies of a tube having uniform cross sectional area and two rigidly closed ends are given by: Fn = nC/(2*L), where n is the nth mode, C is the sound speed, and L is the length of tube.  The Rubens tube doesn’t have two closed ends, it has a paper cone speaker at one end.  It also doesn’t have uniform area – it has a small open volume in front of the speaker.  Both of these will change the “effective” length of the tube.  Don’t worry though, we can use the measured resonance frequencies with air in the tube to calculate the effective length: Leff = nC/(2*Fn).  Knowing C=343 m/s in air, we can use the measured resonances of the first three modes  (144 Hz, 262 Hz and 380 Hz) to find that the (averaged) effective length is 1.28m.  Using this effective length and the lowest three resonance frequencies (108 Hz, 205 Hz, and 300 Hz) with propane in the tube, Fn = nC/(2*Leff) predicts the sound speed to be ~265 m/s.

Makerspace members or any other folks near Milwaukee should feel free to stop by (on Tuesdays or Thursdays evenings) and fire up the Rubens tube.  Just use a small amplifier so you don’t put more than 30 Watts into the 4” speaker!  For some scholarly information about Rubens tubes, check  out the series of articles in the journal “The Physics Teacher:”  M. Iona (14), p325 from 1976; T. Rossing (15), P260 from 1977; R Bauman (15) p448 from 1977; and G. Flicken (17) p306 from 1979.

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.

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!

Tea Box Radio

Someone bought me a gift assortment of teas last Christmas and it came in this decorative wooden box.  When finished the teas, I couldn’t bring myself to throw it out so I started turning it into an iPod dock.  The speakers and electronics came from a set of computer speakers.  I took some measurements and made a template in AutoCAD to lay out the parts.  I hope to have the panel milled out of basswood using a CNC machine shortly. 


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