SRF18-cb_2tpc-lt is the circuit board of a Sony SRF-18, a small radio/external speaker, which produces CrackleBox-like sounds when touched with bare fingertips, and to which 2 telephone pickup coils are connected, “sniffing” the electromagnetic waves produced by a laptop and translating them into sound. the following is a guide to SRF18-cb_2tpc-lt.
 

sniffing a laptop

[photo by Kostas Tataroglou]

a telephone pickup coil is, basically, a long and thin copper wire wrapped around an iron slug and connected to an audio cable. it is sensitive to changes in the electromagnetic field around it, enabling therefore the recording/monitoring (picking up) of telephone conversations. rather than picking up the electromagnetic waves produced by a telephone, however, in SRF18-cb_2tpc-lt, the 2 telephone pickup coils pick up (“sniff”) the electromagnetic waves produced by a laptop.

examples of the sounds that can be produced by sniffing a MacBook Pro (13-inch, 2012) are given below, accompanied by images specifying the locations on the surface of the laptop at which the telephone pickup coil was placed. they were recorded while the laptop was on and in sleep mode, as well as while it was turned on and off, while it was put to sleep and woken up, and while a program (Adobe Illustrator) was opened and closed.

(of course, this is only an overview; different laptops sound differently, and even the same laptop does not always sound the same.)
 

the circuit board of a Sony SRF-18

the figure above is a map of the solder side of the circuit board of a Sony SRF-18. the original package was removed and the board was mounted on a wooden support built by scenographer and technician Jonas Vogel. because of the conductivity of human flesh, touching the circuit board with bare fingertips bridges between different components and adds to the existing circuit free-range resistors and capacitors, whose values depend on the amount of pressure that is being applied and the dampness of the fingertips (a small glass of water and a towel should therefore be part of the setup as well). the letters on the map indicate specific locations on the circuit board to be touched. touching the locations indicated with the letter X (in red) with wet fingertips should be avoided, as it could mute the signal for a long, indefinable duration of time (it is recommended to cover these locations with pieces of electrical tape).

below is a compilation of video examples demonstrating the sounds the circuit board of a Sony SRF-18 can produce. it is followed by a list of the examples, including time stamps and additional comments.

1. AUDIO IN (no input)

[dry fingertips]
1-1 0:09 electric hum
1-2 0:18 noisy rustles
1-3 0:22 electric hum (very soft)
1-4 0:27 electric hum (filtered)
[wet fingertips]
1-5 0:54 electric hum
1-6 1:00 a few drops of water flowing around the board
1-5b 1:11 electric hum (continued)
1-7 1:20 noisy rustles
1-8 1:24 electric hum (filtered)
1-9 1:45 low feedback
1-10 1:52 “birdsong”
1-11 2:00 + harsh feedback
1-12 2:10 + high feedback

feedback in AUDIO IN mode is not always very responsive and controllable. more predictable feedback (but not as varied) is possible in RADIO mode (see below).

2. AUDIO IN (hard drive chord)

[dry fingertips]
2-1 2:32 hard drive chord
[wet fingertips]
2-2 2:36 + electric hum
2-3 2:46 + noisy rustles
2-4 2:50 + electric hum (filtered)
2-5 3:15 + feedback (distorted)

different inputs respond differently. electric hum and noisy rustles can also be produced with dry fingertips (as they were in the previous section).

3. RADIO (FM)

[dry fingertips]
3-1 3:45 radio stations
[wet fingertips]
3-2 4:07 feedback (high, low, medium)
3-3 4:29 feedback + radio stations
3-4 4:44 feedback glissandi
3-5 4:56 feedback glissandi (+ “birdsong”)

electric hum and noisy rustles can also be produced in RADIO mode by touching locations E and H. feedback is more responsive in RADIO mode than in AUDIO IN mode and can be produced also by touching E + A, B, or C.

4. RADIO (AM)

[dry fingertips]
4-1 5:17 radio stations
4-2 5:33 + iPhone 4
4-3 6:04 + electric toothbrush (Oral-B BRAUN)
4-4 6:28 + electric frothing wand 1 (GEFU)
4-5 6:37 + electric frothing wand 2 (GEFU, slightly bent)
[wet fingertips]
4-6 6:45 feedback (high*, low, medium**) (*sounds more like filtered noise **filtered noise with medium to high resonance)
4-7 7:05 feedback glissandi
4-8 7:20 feedback glissandi (+ “birdsong”)

when set to AM, also the radio — like the telephone pickup coils — translates electromagnetic waves into sound (this is why AM generally sounds rougher than FM, as in addition to the desired station, the radio also picks up the electromagnetic waves produced by the electric devices around it). the translation of the electromagnetic waves produced by an iPhone 4, an electric toothbrush, and 2 electric frothing wands is demonstrated in examples 4-2 to 4-5.
 

an *IMPORTANT* safety issue

in order to make the sounds produced by the circuit board of a Sony SRF-18 audible, it has to be connected to an amplifier and a loudspeaker. this raises an important safety issue, as in the case of a ground fault it could be dangerous to touch the circuit board while it is connected to any device powered by the mains. this issue is also relevant to the 2 telephone pickup coils sniffing the laptop, as to match the radio’s loudness, they must also be connected to an amplifier before they are connected to a Sony SRF-18).

the scheme below suggests a safer version of SRF18-cb_2tpc-lt. the 2 telephone pickup coils sniffing the laptop are connected to a battery-powered headphones amplifier, which is connected to the audio input (so the telephone pickup coils match the radio’s loudness). the headphones output is connected to a volume pedal (so it is easy to control the volume while playing), and the volume pedal is connected to a battery-powered stereo loudspeaker. the electromagnetic waves produced by the loudspeaker’s 2 speaker drivers are picked up by 2 additional telephone pickup coils, and these can be safely connected to any mains-powered mixer/PA system. note, however, that this solution has a major drawback: it significantly softens the electric hum and the noisy rustles.

 
SRF18-cb_2tpc-lt was used in 24d24iS_esO4bsPSpbVRssS2-EPB, MUSIC FOR ONESELF v2, and ccloudlab1.

SRF18-cb_2tpc-lt was inspired by chapters 3 and 11 from the book Handmade Electronic Music by Nicolas Collins.

a Sony SRF-18 is listed as an object “capable of producing noise sounds” in the collection of objects objects4JamesSaunders.