Advanced Psychoacoustics 2: Silent Subliminals

Do you want to influence a humans behavior ?
Or maybe induce some thoughts in the brain of all people listening to a specific radio channel ?
Then you found the right blog entry, which will present you a Pure Data program to change the attitude of your desired audience ... Wink

Some weeks ago I was searching for mind control techniques and stumbled about an interesting United States Patent of a psychoacoustic phenomenon I never heard during my audio engineering studies. The Patent Nr. 5,159,703 is from 1992 and called Silent Subliminal Presentation System, its abstract reads:

A silent communications system in which nonaural carriers, in the very low or very high audio frequency range or in the adjacent ultrasonic frequency spectrum, are amplitude or frequency modulated with the desired intelligence and propagated acoustically or vibrationally, for inducement into the brain, typically through the use of loudspeakers, earphones or piezoelectric transducers. The modulated carriers may be transmitted directly in real time or may be conveniently recorded and stored on mechanical, magnetic or optical media for delayed or repeated transmission to the listener.

Hm, so I thought that might be interesting - let's try it!

Silent Subliminals

Silent Subliminals are messages, which are inaudible to the listener, yet they are perceived by the ear for use by the subconscious mind. To be effective, such signals are transmitted to a target person at a constant, high level of signal strength and should be "decoded" by its brain/ear.

The system presented here works as follows: An audio signal including the desired information is modulated with a carrier in the upper frequency region of the audio spectrum (for instance 14500 Hz). This signal is now strong (60 to 90 decibel), but essentially unnoticed by humans and can be optionally mixed with any kind of foreground music.

On the receiving end, the upper slope of the ear's response curve demodulates the original modulated signal, which is finally passed subliminally to the inner ear. The listener can expect his subconscious mind to accept the suggestions with repeated listening.
This demodulation process is similar to a technique called slope detection, which was used in early FM radio receivers (see for instance Simpler radios for FM reception). In those receivers, the response curve is formed by the properties of the electronic components, whereas here it is formed by the natural response curve of the human ear. The same technique could be used at the low frequency end of the ear response curve, however, this region has a much smaller available bandwidth and is therefore more restricted.

Information Encoding: FM and AM Method

To encode silent information, the mentioned patent proposes two analog techniques: a frequency modulation (FM) and a single sideband amplitude modulation (AM) system with carrier suppression. I transfered these techniques into a digital design, so that they can be used on any computer.

The digital FM system can be found in the attached Pure Data patch silent_subliminal_fm.pd. Here the target signal is first band-pass filtered to contain only frequencies in a small band (100 to 3000 Hz, for speech). Then it is upsampled by a factor 4 to avoid aliasing, and frequency modulated with a carrier of 14500 Hz. For more details on FM see Miller Puckette's book Theory and Techniques of Electronic Music, Chapter 5.4. Finally the signal is filtered by a sharp anti-aliasing filter at 20 kHz and downsampled again to the original sampling rate. An important parameter in this system is the volume of the subliminal signal, which controls the modulation index of the frequency modulation. The higher this index, the broader the FM spectrum and more artifacts can be heard.

Furthermore I implemented the single sideband AM system, see the Pure Data patch silent_subliminal_ssb.pd. Again the signal is first filtered and upsampled by a factor 4. Then the single sideband modulation is implemented using a Hilbert Transform, as explained in Miller's book Theory and Techniques of Electronic Music, Chapter 8.4.3 or shown in the Pd example patch H09.ssb.modulation.pd. Afterwards the signal is filtered to avoid aliasing and downsampled to its original sampling rate.

Pure Data Patches and Practical Tips

If you are not interested in all this theory, forget what you have read so far and just use the Pure Data patches.
To execute these programs, the free and open-source Pure Data system (including the iemlib external) is needed, e.g. install the Pd-extended distribution, which includes all necessary libraries. Furthermore some loudspeakers or headphones working in the high frequency band (maybe up to 17-19 kHz) are necessary, but that shouldn't be such a problem if you don't have really crappy equipment.

The following screenshot shows the single sideband modulation patch, called silent_subliminal_ssb.pd: It is possible to load one sample which contains the subliminal information (left), and another sample can be used as foreground music (right). The volume of the subliminal sound should be adjusted, so that no more annoying artifacts can be perceived. If your loudspeakers work correct, this high frequency signal will still be very strong (equivalent to the audio power of a loud conversation) and can be demodulated by the ear.

Now a screenshot of the frequency modulation patch, called silent_subliminal_fm.pd: Here you will find one additional parameter: the FM modulation index (which is the volume of the subliminal sound). Higher values of this modulation index cause a broader FM spectrum, which means that there are more audible artifacts. That's why I prefer the other patch, because it's not really clear to me, which values of the modulation index should give good results. I guess one has to try Smile.


At the end I want to mention one more paper: Synthetic Telepathy and the Early Mind Wars, by Richard Alan Miller, 2001. It presents the history and techniques of mind control and contains a really comprehensive reference list of books and papers. In the following just some random quotes about silent subliminals from that paper:

  • "All schematics, however, have been classified by the US Government and we are not allowed to reveal the exact details... ... we make tapes and CDs for the German Government, even the former Soviet Union countries! All with the permission of the US State Department, of course... The system was used throughout Operation Desert Storm (Iraq) quite successfully."
  • "However, the fact that the sounds are subliminal makes them virtually undetectable and possibly dangerous to the general public."
  • "Free-market uses for this technology are the common self-help tapes, positive affirmation, relaxation and meditation tapes, as well as methods to increase learning capabilities. But there is strong evidence that this technology is being developed toward global mind control."

Wow, these statements sound heavy ... so maybe the here presented Pure Data patches will be a very powerful tool Wink !
I did not make any systematic listening tests so far, therefore everyone is invited to try these patches and see how they can influence people. Good luck and please let me know if you get some results ...

Silent Subliminal Pure Data Patches (zip)5.52 KB