Sliiiiiiiiiiiinky - Audio signals!

Perhaps the world's longest acoustic chirp.

The Sliiiiiiiiiiinky is suspended on 418 elastic threads so it is free to move in three dimensions. 

Sliiiiiiiiiiiinky at Eartcore

Back to the main page for the 21 metre (71 foot) long Slinky - including photos and videos.
Back to the First Principles site for all sorts of fun-and-games, show-and-tell, DSP, etc

Last update 14 April 2002.  (Link added 2010-05-15.)

Copyright Robin Whittle 1999 - 2002.

I created Sliiiiiiiiiiiiiiiinky primarily to explore waves in the 0.5 to 5 Hz region - to be made by hand in three dimensions and to watch them travel along.  However, before I built it, I also hatched a plan to mount nine piezo pickups along the spring, one for each pair of frame supports, and to have an amplifier and speaker at each leg.  Then, impulse sounds and wind-generated vibrations would be audible in 9 glorious channels of sound.  This remains on the To Do list!

Firstly, some pretty rough sound recordings with a crude piezo pickup on one end of the Sliiiiiiiiiinky with wind noise, rattles from large waves making the coils touch or the piezo pickup rattle or touch adjacent coils and in particular a chirp sound from an impulse at one end.  Following that, some stereo two-pickup recordings of a wider range of sounds.

Here are links to the sound files, and then some discussion, with graphic frequency analyses of the chirp portion of the recording.

All sounds copyright Robin Whittle 1999 - 2002.  Please contact me at rw@firstpr.com.au if you want to use them for any commercial purpose.

Please download these files to your computer and play them from
there, rather than repeatedly downloading them from my server.  

To do this, hold your shift key down and click on the link.  

Then save it somewhere to your hard drive and use WinAmp or
whatever to play it from there.

Mono sound samples 1999 - wind and chirp

Stereo samples April 2002 - wind, chirps, bangs and BANGs!!!

In April 2002, I recorded to DAT with two piezo-pickups spaced about 1 metre apart near one end of the Sliiiiiiiiiinky.  The results are subtle and dramatic.   These recordings are best on good headphones or a proper high quality sound system.  They would sound pox on the shitty speakers commonly found on computers and the plasticy boom-boxes found on most mass-market consumer audio gear these days.

I made the piezo pickups from some ~ 2" diameter brass/ceramic piezo speakers.  

The following pieces are all MP3s in stereo (not joint-stereo) at 256kbps.  Like those above, they were encoded to MP3 with LAME, using the RazorLame Windows front-end:  http://www.dors.de/razorlame/

The wind noise

There is an eerie noise as gentle wind blows through Sliiiiiiiiinky - not necessarily at the same rate and direction for all its length.  It seems the central frequency and volume both rise with wind intensity, much like black-body radiation.  I guess the sound varies a lot with the speed of the wind, and its angle of incidence with the length of the Sliiiiiiiiiiiiiinky

There are various clacks and clangs in the mono recording too.  The piezo pickup was a little round brass disc from some cheap telephone ringer, with a small alligator clip soldered onto one end and a blob of Blutack (as used to stick posters onto walls) on the free end, to provide some inertia which would cause twists in the clip alignment to flex the disk.  The pickup system is bound to have some resonances, which I think can be seen as horizontal bands of lesser or greater intensity in the spectrum plots below.

Chirp

Chirp means a frequency sweep - in this case a sweep from high to low.  Chirps are used in many radar, sonar (natural by bats and man-made) and other systems, for a variety of reasons.

My interest in chirps here is the way that the stiff steel material of the Sliiiiiiiiiiiiiiiiinky causes vibrations of different frequencies to travel at different speeds.   It seems that in a steel (or some other stiff material) wire, where the stiffness of the wire is significant compared to the tension it is held taut with, the higher frequencies travel faster than the lower frequencies.   This is audible when a coin is hit on a fence-wire or the pantograph of a tram hits an irregularity in its catenery wire.  There is a distinctive zeow sound.  

A good way to hear this is to hold an ordinary steel Slinky by one end, with the last turn hanging from near the end of your little finger.  Now stand on a chair and let the other end dangle freely, and settle down.  Pop the end of your little finger into your ear, and tap the Slinky with a coin.  Bzzeoowwww!!  It is a distinctive sound, but I never thought much about it until I recorded Sliiiiiiiiiiiinky

Fence wires are well known for making this zeeowww sound.  Also, I think that the stiffness of steel strings in guitars, pianos etc. causes the higher harmonics to be sharper than pure harmonics of the first harmonic.  This, I think, is because the first harmonic seeks a restoring force which is primarily caused by the string's tension, rather than the innate stiffness of the string.  This is because the first harmonic only bends the string once gently over its length, and twice acutely at each end.  But the 8th harmonic bends it 16 times, plus the bends at the end.  So if the string is stiff, then there will be a higher restoring force working on the same string inertia, and therefore a higher resonant frequency for 8 waves travelling the length of the string than would be expected by multiplying the first harmonic by 8.

Here, I suspect, I have made the world's most dramatic physcal chirp system.    Chirp is visible when an impulse wave is sent by moving one end of the Sliiiiiiiiiinky by hand.  By the time the impulse has moved towards the other end, the high frequency components (with wavelengths of 20 to 50 cm) can be clearly seen to be ahead of the lower frequencies (with wavelengths of 1 or 2 metres).

At audio frequencies, the chirp seems to be much more intense.

The "chirp" sound in the above mono recording (and many of the stereo recordings) was made by tapping one end with a coin, whilst picking up the vibrations at the other end with a piezo pickup.  This recording does not indicate exactly when the other end was struck.  It is probably a second or so before the start of the chirp-mono recording.  In April 2002, we visually compared the time between us seeing the person at the other end tap the Sliiiiiiiiiinky and us hearing the chirp, and I guess it was a second or so before we heard the highest tones first arriving.

The coin tap puts energy at all frequencies (say 1 Hz to 20 kHz) into one end of the wire for a very short time, say 10 msec.  Recorded at that end, all that would result would be a click.   But due to the frequency-dependent transmission speed, by the time the vibrations have travelled along about 645 metres of wire, the high freuqencies arrive well before the low.   Your ears tell you this very clearly.  

The frequency plots are from Cool Edit http://www.syntrillium.com/ .  They show time and frequency linearly, and energy in a logarithmic fashion from black and blue at low intensities to orange and yellow at high.

The first one shows frequencies from 15kHz - and it can be seen that the chirp starts just above 14 kHz.

The second one focuses on the frequencies below 5.5 kHz.

For each of these images, you can see full-size versions in .jpg or .gif by clicking the appropriate links.

By ear, the chirp lasts for at least three seconds.  Looking at the frequency analyis, it can be seen that:

Yet at 5 Hz, it appears the time delay is about 20 seconds.

Perhaps the system behaves pretty much as a 645 metre long piece of wire above some frequency, say 100 Hz, and as a shorter piece of wire, with much greater flexibility, for lower frequencies.  But perhaps this is a false way of thinking about it altogether.  It is late, and I will leave further analysis as an exercise for the reader.


0 to 15 kHz

15kHz

Full size: .jpg 89 kbytes .gif 216 kbytes      


0 to 5.5 kHz

5.5 kHz

Full size: .jpg  103 kbytes .gif 239 kbytes

If anyone analyses this, please let me know.  Next time I will try to record the time of the impulse.  It is difficult to see a person tapping a coin at the other end, but I think that there is about a 1 second delay before audible signals arrive at the other end.   This contrasts with a transit time of around 20 seconds for signals of about 0.5 to 5 Hz.



Links

New link 2010-05-14: http://wiredlab.ning.com  - a purpose built long-wire in the country for recording sounds, by Alan Lamb (who pioneered this field) and others.

Does anyone know of suitable links or birds-of-a feather sites?  

Here is some search-engine bait:  wire chirp, physical chirp, acoustic chirp.  I did a bit of Googling in early 2002, but couldn't find anything directly relevant.

Maybe the Open Directory department:  http://dmoz.org/Science/Technology/Acoustics,_Ultrasound_and_Vibration/

Surely, this would be a topic worthy of a learned dissertation in JASA - the Journal of the Acoustic Society of America.  http://ojps.aip.org/jasa/   In this fascinating journal, established in 1929, you can read about the latest discoveries about the molecular structure of the hair-cells in some obscure species of flea, and turn the page to read about the frequency response of the entire Pacific Ocean.  The Acoustical Society of America is at: http://asa.aip.org/

Here is a site with some visual demonstrations of wave propagation, including dispersion, in which different frequencies travel at different velocities. http://www.gmi.edu/~drussell/Demos.html

Dispersion is the single biggest limiting factor in long distance optical fibre communication, now that erbium doped fibre amplifiers can be used to create fibre links spanning thousands of kilometres.

Here is the Google page for Usenet newsgroup alt.sci.physics.acoustics. http://groups.google.com/groups?hl=en&group=alt.sci.physics.acoustics  


Please email me if you want to know more!  email: rw@firstpr.com.au .

- Robin Whittle


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