Sound We Can't Hear and GeekNews
Stanford instructor and regular Geek Ben Jaffe informs us about ultrasound and infra-sound. And we all cover the Week in GeekNews.
Sphingomonas bacteria was identified by 11th grade student Daniel Burd that digests common plastic grocery bags made from polyethylene.
!!Infrasound and Ultrasound Notes
The Range of human hearing is 20hz-20khz
We can feel sound below 20hz (infrasound), and ultrasonic pitches can affect us, though we can’t hear them.
!!!Infrasound (below 20 hertz)
- Some film soundtracks and music use infrasound to produce unease or disorientation in the audience and listeners
- A 17 Hz tone was integrated into random samples of music, and the study participants were not told which pieces included the tone. About 1/4 of respondents reporting anxiety, uneasiness, sorrow, nervousness, chills down the spine, or physical reactions.
- The Department of Defense – phased arrays of infrasonic emitters — i.e. HUGE custom subwoofer (normal ones stop at 20 Hz lowest), and output of this sub goes via pipes to several openings which are usually the distance of 1 wavelength apart. Outputs at about 7 Hz, and each of these openings output in phase with each other, so you get a bit more directionality.
- The so-called “Brown Note” is a sub-20Hz tone that allegedly causes people to involuntarily vacate their bowles. This has been debunked several times. Rick says that if this were true, it should be used as colonoscopy prep.
- Research has shown that the frequencies of 19 Hz can be responsible for optical hallucinations in one’s peripheral vision. Nasa found the resonant frequency of the eye to be approximately 18 Hz, and the resonation of one’s eyeball can cause these visuals.
- Dog Whistle is around 18-22 kHz, which is why children can often hear the whistles
30-40kHz Animals and Chemistry
- Bats use echolocation to navigate caves and find prey. Nocuid moths have a reflex that those frequencies trigger, which causes them to drop a few inches in flight
- 250kHz – 2mHz Medical Diagnostic Ultrasound
- 1-10MHz Chemotherapy (Acoustic Targeted Drug Delivery), also used to find flaws in dense materials, measure thickness
!!Producing Directional Sound Using Ultrasound
Correlation between the directionality of sound, and its frequency, so ultrasonic frequencies are far more focused than audible sound
Technology was first developed 30 years ago, but could only reproduce simple tones. The audio quality was poor, but now, the quality is usable for commercial applications.
High intensity ultrasonic frequencies are required to produce the effect. The SPL (sound pressure level) involved was typically greater than 100dB of ultrasound at a nominal distance of 1m from the face of the ultrasonic transducer. (The threshold of pain for audio within our hearing range is 120-130dB. 100 dB is about the audio level of
Exposure to more intense ultrasound over 140dB near the audible range (20-40kHz) can lead to a syndrome involving manifestations of nausea, headache, tinnitus, pain, dizziness and fatigue, but the technology does not use levels this high, and the frequencies it uses are often higher.
Here are some interesting pages about these topics:
- [Hearing Range|http://en.wikipedia.org/wiki/Hearing_range]
- [Brown Note|http://en.wikipedia.org/wiki/Brown_note]
- [Sound from Ultrasound|http://en.wikipedia.org/wiki/Sound_from_ultrasound]