01/11/11 All Things Audio (Part 1)
3 points of perception with electronic engineering
Hearing is one of our five primary senses yet it does not always gain the attention it may deserve. Sight is often given prime position but hearing is often posted a poor second place, especially in the consumer oriented world. This is not an ode to poetic music but more of a challenge to the perception of audio.
The reality is that sound plays a significant part in many actions – a hammer hitting a nail (incidental), or the siren on an emergency vehicle (deliberate). In the evolving technical world, sound is becoming as relevant as “sight” in many realms.
Audio assault
Our busy blustered world bombards our bodies on a daily basis. You might think that that “bodies” should have been “ears” but the subject of hearing through the skull and vibration on the torso comes later. Our interests lie in our intended actions without the disruption of external stimuli. To this extent ear defenders may be employed to simply reduce the amount of unwanted or damaging sound reaching our ears. Beyond this there is a more pervasive action of sound in our busy world. It ranges from diminished concentration to “stress”.
Noise cancelling headphones are a joy to those who bought adequately and frequent trains, planes and road transport. These devices use microphones to present the “unwanted” sound back to some electronics. The theory is that you turn the sound upside down and play it back with the “wanted” sound. In many instances the wanted sound is music or a sound track to a film. There is no reason why the wanted sound should be nothing – silence. Unfortunately there is likely to be a very low residual noise called hiss.
Now we reach the first point of “all things audio” – humans of the genus “consumer” are perceptive, hard to please and may have little reference to the natural world. Firstly, the consumer ambition desires complete silence as a goal. It seems simple but when was the last time you heard complete silence? Silence is not within miles of any large city… perhaps by a lakeside on a very quiet and still night? You might have been able to go into an anechoic room where there are no echoes and with the door shut there is certainly a hush. Many people feel that this is stifled and unnatural. Silence in the absolute sense is almost non-existent and very natural ambience is an experience.
So we return to hiss, where we find that the user experience ranges from “feeling that your ears are being sucked out” to “pleasant”. What is the difference between the two? The answer is all about the audio engineering. Pleasant hiss is of a low level and more importantly of a particular spectrum. This all starts to make the audio business more complicated than just dealing with the microphone response, time delays and all the other techno stuff; the residual hiss must be of a very low level and the right type!
Acoustic devices
“Audio” taken in isolation is often considered as analogous to Hi-Fi/music systems or devices like noise cancelling headphones. In the everyday world there is a wider spread of audio than might first be realised.
The sirens on emergency vehicles in many countries have changed over the decades. In dense traffic and built up locations, the multi-tone and “whooshing” type noises have prevailed. Human perception can determine direction and speed of travel far better with a wide, “whoosh”, spectrum of noise – the classic siren sound is just the recognised warning.
Some sounds are intended or disruptive but what use are the incidental sounds? On the whole they go nowhere and add to our noisy existence. Otherwise sound can be a useful means of measurement and detection.
There are security systems for “listening” to sound on fences, walls and other perimeters. Prisons, airports and international borders are all instances where this type of security may be deployed. Electronics do the listening which requires anything from simple filters to complex algorithms to decide what type of event is occurring. At all times the goal is detection but there is no point in a system that is going to trigger an alarm at every occurrence of sound.
Real security breaches occur rarely but wind, rain and traffic noise occur frequently. In this case there is a great deal of discrimination required. It becomes necessary to separate the random, peaked and unwanted from the target events that signify an alarm condition. The system will spend most of its time processing “nothing”, whilst discriminating most. As back up to these systems, many have one other feature – a facility to play-back the event sound as an audio feed for humans. Why?
The simple answer is point two. As humans, we have a very fast ability to interpret sounds and their associations or meaning, which we often take for granted. It also implies considerable processing power and significant pattern recognition.
Skull sound
Telephone answer machines used to be voice recorders using a tape cassette in the days when you “left a message”. These days we have “voicemail” and the recording is in 1’s and 0’s on some piece of solid state hardware. When you hear your own voice recorded on either system it probably does not sound like you. Everyone else sounds about right on the recording, just not you.
It is quite likely that the majority of people are completely unaware of what they perceive through their skull. The primary reason for “you” not sounding like you on the voicemail is because your perception lies inside your head. We hear with our ears but listen to all perceptions. There are devices called bone conductors that play sound directly onto your head using the skull as transport medium, bypassing the ears completely. The listening experience is remarkable and you use it unaware most of the time.
The voicemail allows you only to hear your friends and colleagues. You do not sense them as you do your own voice, where bone conduction is also in play. This difference in hearing between ears and bone can aid certain types of deafness and is promoted as a therapeutic means of treatment for ADHD and autism.
Moving traffic, bustling pavements and perhaps trains and planes form a part of most day experiences. We perceive it and hear it but if we just recorded it would it feel like the real thing? Our perception is, again, greater than just the sound. The massive sounds of large vehicles and great crowds of people extend below simple hearing. The low frequency pressure waves are “felt” across the body where the cavities in the torso are most effective. The visceral nature of low frequency energy awareness is another naturally given facility.
The world of audio cannot be summarised in 3 perceptions. The final point is that human recognition, experience and meaning of sounds are complex. Some primary mechanisms for listening are rarely experienced in isolation, which leads to common misconception about the part that audio plays in our world.
We perceive sound with more than our ears but unless there is a market, the consumer world will not hear of it.
Posted by: Peter Hawkins on 01/11/11.