Sunday, December 30, 2007

Human Auditory System: Mosquito test your hearing

I've blogged before about the Mosquito ringtone - the teenagers' ringtone that adults mostly cannot hear.  Brian Dipert's excellent blog over at EDN (a technical magazine for the electronics industry) calls our attention to an in-depth article from last year in Sound and Vision Magazine on the Mosquito ringtone.  The article includes links to various MP3 recordings. Brian, who still has some high frequency hearing left even though, as I recall, he enjoys attending, as well as recording, live music performances that use sound reinforcement (i.e. loud, loudspeakers), is running a poll to see how many of us can hear the higher frequencies.

I missed the Sound and Vision article when it came out, but read it with interest today.  I do have some nits to pick with it though.  For starters, exposure to loud sounds is understood to be less harmful to young people than to us older folks.  The human ear has some automatic protection built in and although its effectiveness degrades as we age, it does enable younger people to be exposed to louder sounds without permanent damage.  Of course, there are limits to this ability, so this should not be taken by anyone as a license to go and blow their eardrums or cilia out(!).

My second nit is the author (David Ranada) slams higher sample rate audio formats.  His comments are certainly valid from the standpoint of continuous tones, but I believe that there are a good number of audiophiles and audio engineers who would argue that higher sample rates provide better reproduction of transient sounds.  This technical argument is based around the limitations of Fourier Transforms, which, very simply put, try to model all sounds as continuous sine waves.  There are also technical arguments for higher sampling rates based around the design of the anti-alias filters, which are easier and cleaner to implement at the higher rates.  Finally, I've heard that there has been some scientific research related to endorphin release in the brain - simply put, at low sample rates, we don't emotionally respond to digitally sampled music nearly as well as we do at high ones.

Now, I know that the whole topic of sampling rates is almost guaranteed to get most audiophiles riled, so let me close by saying that my opinion is not fixed on this point.  I was merely pointing out the arguments for the other side from a technical point of view. There are, of course, application, environmental, cost, convenience, and other factors that are involved in deciding what audio format to use and I have addressed none of those in this post.  That being said, I'll be happy to discuss them in the comments if anyone would care to.  I also have the best of intentions to write something on the use of high sampling rates in forensic audio and video processing in the near future.

Keith McElveen

P.S. For the record, Brian could hear the highest frequencies on his machine, but I could not, at least on my MacBook's speaker.

Thursday, December 27, 2007

Noise Cancellation: Quieting an aircraft

Aviation Week has a short but fascinating article on the design of passive and active noise cancellation for the King Air airplane.  The design seems very sensible for an aircraft application - use passive cancellation (absorption) for the high frequencies and active cancellation for the low frequencies.  

Aside:  For those unfamiliar with active cancellation, this is the same technology that is used in the Bose line of headphones where an out-of-phase signal is induced under the ear cup that cancels out the sounds that leak in from outside, thereby letting you hear the audio being cabled in from the media player/device (e.g. MP3 player, radio, etc.) even in moderately noisy environments.

The design trade-off is governed by the constraints that active noise cancellation does not work very well at high frequencies but does work well at low frequencies, while passive cancellation at the low frequencies requires massive (i.e. heavy) sound absorbing materials, which would weigh the airplane down.  At high frequencies, the materials can be much lighter. So, as you can see, the design trade-off works out very neatly - active at low frequencies and passive at the high frequencies.

One design detail that stood out to me is that they use 24 microphones to provide feedback to the cancellation algorithm so it can continuously adapt to the changing noise environment in the cabin.  Those 24 microphones are coupled to 12 loudspeakers to produce the anti-phase cancellation signal.  The numbers of microphones and loudspeakers tell me that this is a serious system that is designed to reduce noise through-out the cabin, not just in a limited area.  My hat is off to them as this was a seriously challenging design problem.

Human Auditory System: Apple patent application for hearing protection

The iPodObserver, among others, is reporting on an Apple (US consumer electronics and computer company) patent application on a way to automatically protect a listener's hearing based not only on a maximum level constraint but also adding the total listening time variable.  To put it another way, the longer one listens to elevated sound levels, the quieter they need to be as the time progresses to avoid hearing damage.  I haven't read the patent application yet, so I am uncertain as to exactly what the novel aspect of the invention is, but taken by itself, I see it as a good move an industry leader to make their product safer.

Off Topic: Interesting end of year stories

I'll admit it up front - I load up my browser with lots of tabs during the normal work week with stories that I have the best of intentions to read and report (the best of) and thanks to my surfing over the holidays, I've got way too many to blog in my normal way about. SO, I'll post links below of a bunch of them that are interesting from a forensics point of view or simply just interesting on their own. I hope you enjoy them.

New Scientist Round-up of 2007 Biology and Medicine stories

Acoustic Recognition: Toward teaching a machine to label music

The New Scientist Technology Blog posts about Major Minor, an on-line music labeling game by Dan Ellis and Michael Mandell at Columbia University (USA).  The goal is to build up a database of sounds that have been tagged by users (all humans, presumably) with descriptive words.  The scoring system seems very well thought out to generate appropriate tags, weed out inappropriate ones, and to motivate participation.  

Why does the world need such a database?  Well, just like with humans, in order to teach a machine (e.g. a computer) to recognize some characteristic of a sound, you have to feed it many, many examples of sounds with that characteristic for it to learn from.  In this case, the researchers use the Internet to provide the means to efficiently and effectively get people to the "test site" and their well-constructed, slightly addictive game for the incentive to get the test subjects to willing participate and give up their neural processing time to help out.  Good luck to the team at Columbia U!

Tuesday, December 18, 2007

Military: Long Range Acoustic Device (LRAD)

The Strategy Page has been on a roll lately with interesting audio and video articles. This week they discuss the Long Range Acoustic Device (LRAD), which allows long range broadcast of intelligible speech (up to 300 meters or yards).

The device was originally developed for the US Navy for spoken communications at sea (to warn off vessels approaching without permission), but has also found use as a less-than-lethal weapon in Iraq.  It also is being used for what may be impromptu PSYOPS, referring to the "Voice of God" field applications mentioned in the Strategy Page article.  The device has been around for several years, so news articles on it are easy to find - I think I have blogged on it at least once.  For an overview, you can refer to the Wikipedia entry.  From published information, it appears to be a high-volume loudspeaker array, i.e. a collection of speakers driven in-phase with each other, thereby creating a directional beam pattern.

Monday, December 17, 2007

Computer Audio: Audacity 1.3.4 Released for Windows and Linux

Audacity 1.3.4 has been released for Windows and Linux - sorry, but we'll have to wait for Mac OS-X!  To download, click here (

Wednesday, December 12, 2007

Military: Digital camouflage- now on warplanes

I've posted before about digital camouflage being used on military uniforms.  Now, StrategyPage tells us about how it is being used on warplanes, at least on the tops of the airplanes. On the bottoms, which would be seen by people looking up at the sky, they are still using a sky-colored monochrome.  Interestingly, there are multiple patterns for the top sides to match different terrains, such as arctic and desert.

Sunday, December 09, 2007

Biometrics: Battlefield forensic identification

The Strategy Page has a brief article on how the United States' military has adopted tactics used by law enforcement, such as the collection of fingerprints, photographs, retinal scans, and DNA samples for use in identification by biometrics.  

One thing to notice in the article is the taking of photographs from multiple angles in order obtain more information than is possible from single one.  You can immediately demonstrate to yourself why it is useful to have shots from at least two angles by closing one eye while looking at someone - you immediately lose the depth information provided by your own stereoscopic vision, assuming you are normally-sighted.  This same depth information is useful in calculating biometrics for face recognition.

Friday, December 07, 2007

Human Visual System: Chemical identified that improves visual sensitivity

ScienceDaily reports on research findings by a team of neuroscientists at New York University (USA) that identified the chemical acetylcholine (ACh) and its effect on the human visual system - namely to enhance its ability to detect weak signals.

Speech Recognition: Application to university lectures

Technology Review (an MIT publication) has a comprehensive article on an up-to-date application of speech recognition, including both the recognition engine and the user interface layers - namely, MIT's Lecture Browser website, a search application for video lectures.  The article is very well written and is approachable by anyone with a bit of science background.

Equipment: In defense of audiophiles

Slate Magazine has an article by Fred Kaplin on why high-end audio equipment is indeed worth the money if one is interested in experiencing the nuances of well performed and recorded music. 

N.B. I normally would not link to something that was not G-rated, but in this case there is one occurrence of sexually-oriented language that I found in bad taste.  After some consideration, I decided to post this after all with this warning to those of sensitive natures.