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.

Plastic bag eating bacteria

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)


!!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: