Monday, August 20, 2007

Acoustics: Ultrasonic stethoscope

The US Army Aeromedical Research Laboratory (USAARL) has sponsored the development of a new, noise-immune stethoscope technology based on an ultrasonic transducer (article link).  The envisioned operational environments include not only battlefields, but also noisy civilian emergency situations such as helicopter air-evacuations, traffic accidents, sporting events, and the like.

The technology works on doppler shift, which is a frequency shift caused by movement - things moving away from you tend to stretch out their sound waves and things moving toward you tend to compress them, causing respective lowering and raising of the sound frequencies.  A practical example is when you hear an ambulance go by with its siren blaring.  At first the pitch (frequency) seems to go up (get higher) and then as it passes, it goes down.

Anyway, the article I linked to above (from the EMSResponder website) has a well written report. It is light on technical details, but it does a good job of pointing out advantages as well as potential problems

Friday, August 17, 2007

Indoor Acoustics: Restaurants and noise

I've recently begun following a blog called Cross-Spectrum/Acoustics - Sound, Vibration, and a Little Noise.  It has quite an interesting collection of links to articles on various topics on acoustics and noise, as the title implies, as well as good original content by the author.

On to the topic of this post.  One of the perennial challenges for forensic audio cleanup is to reduce the amount of interference on recordings made in bars, pubs, and restaurants.  In recent years, say the past ten, the amount of noise found in restaurants in the USA has risen dramatically thanks to a trend in restaurant design, which has resulted in an increase in the number of cases requiring forensic restoration and enhancement. Cross-Spectrum has a post with links to recent articles on the restaurant noise phenomenon.

"Restaurant noise" is a little too broad of a term in itself to describe the problem from a forensic standpoint, however.  Consider the different types of noises that may be found there:
  • music (instrumental and/or vocal; live or pre-recorded)
  • talking (speech; the cocktail party problem of too many voices blending together into babble)
  • dropped flatware (knives, forks)
  • clinking glassware
  • shouts (e.g. to get attention of wait staff)
  • resonance and reverberation
  • hum (fluorescent lighting)
Forensic audio practitioners will recognize lots of things about this list, for instance, impulsive noises (flatware, glassware, and shouts), broadband noise (talking), and harmonic noises (instrumental noise, hum and resonance).  Each of these types of noises is different and may require different treatment; however, one of these noises rises above the others in difficulty and that is talking (speech).  

Filtering out noise generally requires recognizing the noise signal as being somehow different from the desired signal, which is usually speech, in some domain we can measure it in (i.e. time, frequency, amplitude, or space) and then removing the noise using a filter implemented in the corresponding domain.  That process falls apart when the noise is speech itself - how do you tell the difference between interfering speech and desired speech on a mono, omni-directional recording made in an uncontrolled environment?  Therein lies the problem.  There are practical solutions, as well as impractical ones for that matter, but the best ones require prior-preparation.  Unfortunately, I'll have to leave this filtering discussion at this point as going in much deeper could lead to a book-length article!  (If you are interested in going in deeper, feel free to ask away in a comment or contact me directly using the email link found near the top-right of the page.)

As you can hopefully now appreciate, our job would be so much easier without this recent trend in designing restaurants, bars, and pubs to strengthen the cocktail party problem, hence my interest in the Cross-Spectrum posting.

Saturday, August 11, 2007

Human Auditory System: Using hearing test to predict SIDS

Dr. Daniel Rubens of Children's Hospital and Regional Medical Center in Seattle has announced a breakthrough in predicting and possibly even understanding SIDS (Sudden Infant Death Syndrome, also known as cot or crib death).  Using data from infant hearing tests, Dr. Rubens found a pattern where infants who later died of SIDS tested differently than normal infants.  

ScienceDaily has an article.

Acoustics: Detecting landmines with sound - cheaply

The trick to detecting landmines using sound is to do it before it goes "BANG", of course.  All attempts at humor aside, this is not an easy problem to do and gets harder (and less reliable) the cheaper you try to make the detection system.

The engineers at GIT (Georgia Institute of Technology) have been leaders in this research for a while.  They now claim to be able to reduce the cost of a landmine detection system that uses a low frequency sound wave to vibrate the landmines and a cheap microphone to detect the sound from the resulting movement of the mines.  New Scientist has a write-up.  Their paper is in the Journal of the Acoustical Society of America (JASA, July 2007)

Wednesday, August 01, 2007

Admin - Posting will be light

Due to many pressing work commitments, posting will be light during the next few weeks.  I'm saving up lots of material, so as soon as things slow down a bit (or I end up in an airport with a decent free or T-Mobile WiFi connection and some time to kill), I'll be back to posting in earnest.
Kind Regards,