Psychology, Psychoacoustics and Neuropsychology of Sound
Any sound, wanted or unwanted, creates an envelope that does more than intrude into your acoustic space. But, what is more important, it intrudes into your mind.
Humans interact with the acoustic environment in complex ways. The human hearing system consists of the ear and extremely sophisticated transduction system digitally connected with the cortex. Moreover, it also encompasses conduction and mediation via bones, flesh, and body cavities. Acoustic energy profoundly influences an individual's brain waves, respiratory cycles, nervous system, muscle function, heart rate and glandular function. Be careful, an acoustic space can readily be turned to influence against the individual and profoundly change cortex activity. The brain is a dual-processing system. The left hemisphere is involved in more analytical tasks like setting levels and various functions, whereas the right hemisphere handles creative tasks and emotional responses; it feels rather than thinks. As we listen it will be easier to stay in right-brain mode but low frequency resonances can change regional activity of the brain.
Acoustic intrusions reduce your freedom of thought. There is no escaping sound. When it meets your body (no matter are you consciousness or not, are you sleeping or aware) it forcibly enters your mind. Not through your ears only but via your bones, flesh, and the body cavities. The low frequency sound (110 Hz) resonances have extreme power because it can change the cortex activity and state of your mind. This was known by our ancestors before the pyramids were built. The archaeoacoustics proofs are in subterranean paleolithic temple Hypogeum Hal-Saflieni Malta, Maeshow Orkney, Newgrange Ireland, Nekromanteion at Acheront etc...
Psychology and psychoacoustics: Human perception = current sensory inputs + past experience
Neuropsychology: Past experience = emotional systems + rational cognitive systems
What does it means? If different people listen the same audio signal their perception will be function, not only of (the same) current sensory input, but it will depend on their past experience which means it will be function of their emotions and rational cognitive systems. But, this is not the end; we can go further: the integration neurons in auditory cortex are affected by the simultaneously presentation of the audio-visual stimuli.
Faced with the stimuli in the nature, brain needs to coordinate information from multiple sensory systems. The brain task is to create subjective representations of the human environment in outside world. We don't know a lot about the neuronal mechanisms for this cross-modal processing during online sensory perception but the analysis technique can capture how the phase of cortical responses is coupled to aspects of sound and vision stimulus dynamics. Surprisingly; auditory cortex not only tracks auditory stimulus but also reflects dynamic aspects of the visual signal. Similarly, visual cortex mainly follows the visual properties of a stimulus, but it shows sensitivity to the auditory aspects of a scene. What does it mean? Can we listen by eyes and see by ears? No exactly; but picture can change perceptible sound and sound can change perceptible picture. One of the proofs is McGurk effect.
We can go further still: Although humans cannot perceive sounds in the frequency range above 20 kHz, there is a question of whether the existence of inaudible high-frequency components may affect the acoustic perception of audible sounds. Recent experiments with the positron emission tomography (PET) measurements revealed that, when a frequency components above and below 20 kHz were presented together, the regional cerebral blood flow (rCBF) in the brain stem and the left thalamus increased significantly compared with a sound lacking the components above 20 kHz. Simultaneous electroencephalogram measurements showed that the power of occipital alpha-EEGs correlated significantly with the regional cerebral blood flow (rCBF) in the left thalamus. Psychological evaluation indicated that the subjects felt the sound containing a high frequency components above 20 kHz to be more pleasant than the same sound without it. The results suggest the existence of a human response to sound containing high frequencies above 20 kHz. The name of this phenomenon is the Hypersonic effect.
On the other side of the audio spectrum it has been suggested that infrasonic exposure have an adverse effect on human health, suggesting that the biological sensitivity of human beings may not be parallel with the conscious audibility. The World Health Organization stated that health effects due to low-frequency components in noise are estimated to be more severe than for community noises in general. The subsonic frequencies lower than about 20 Hz cannot be heard by the average person, but they can be sensed as vibrations, as most people have experienced near a subwoofer. A minority of people are able to hear sounds below 20 Hz with pressure levels more than 20 dB less than the hearing threshold for normal-hearing persons. Several hydromechanical and neural mechanisms in the cochlea may be responsible for this increased sensitivity. Long term exposure to excessive levels of high intensity low frequency sound, produced by highly amplified bass music, wind turbines, airplanes, cars,... can not only be harmful, but can cause death. The World Health Organization with European Comission's Joint Research Centre published paper claim that noise pollution causes 3000 deaths per year in Europe. Many scientific articles confirm the symptoms associated with low frequency noise exposure include annoyance, stress, sleep disturbance, headaches, difficulty concentrating, irritability, fatigue, heart ailments anxiety, stitch and beating palpitation but also include a number of otologic symptoms including dizziness or vertigo, tinnitus and the sensation of aural pain or pressure. The most severe are wind turbine noise which causes greater annoyance than any other sounds of similar level. Beside them the effects of extra-low-frequency atmospheric pressure oscillations affects human mental activity. Slight atmospheric pressure oscillations in the extra-low-frequency range below 0.1 Hz, which occur naturally, influence human mental activity. Experiments with a volunteers exposed to oscilations similar to the natural quasichaotic atmospheric perturbations in the frequency range 0.011–0.17 Hz caused significant changes in attention, short-term memory functions, performance rate, and mental processing flexibility disrupting mental activity.
We can easily understand the fact that very high levels of sound affect human perception and physiology but it is less known the fact that the absence of sound in very quiet places affects human with constant distress and deprivation. The environment of sleeping room with 15 - 20 dB background noise is quiet place but in anechoic chamber we have 0-5 dB background noise levels or even less. The experience of being in an anechoic chamber after 10 minutes produces the phenomena of an unpleasant sensation and symptoms of stress. After about 30 minutes the ear has adjusted to very low level sounds, and the heartbeat is constantly heard with increasingly louder intensity. After 60 minutes, the rushing of blood in veins becomes audible and noises so small such as the ones produced by hair. Finally, after 120 minutes, the adjusting mechanism of the ear is reaching its maximum and all the previous noises are now heard as loud sounds. Moreover, one begins to hear a constant hiss produced by random air molecules crashing on the eardrum. Another aspect of psychoacoustics called the concept of spatial sound display is based on the brain subconscious use of sound to map out the spaces. Based on the reverberation time and early reflections, the brain immediately sonically maps the surroundings and gives a feeling of the size and volume of the space. This becomes more observable in a very dark space. In an anechoic room, the sound mapping function of the brain is in constant distress since it is not able to function under such unnatural conditions so this situation produces intense deprivation effects on human physiology.
Remember: Since auditory neurons are influenced by the visual stimulus and past experience it is extremely unlikely that two persons will have the same perception while listening the same audio system or the same sound. But for the individual the sound pressure at the two ear drums is a sufficient stimulus. Producing the same sound pressure will produce the same auditory perception. Exact reproduction of the sound pressure is not necessary for producing the same auditory perception. The limitations of neural responses allow different and simpler stimuli to produce the same response.
If your browser supports the audio element you are listening now Gnossienne 1 by Erik Satie (in lossy encoding of only 38 kb/s) but the picture above shows a FFT spectrogram of high resolution recording by audiophile label (lossless 4608 kb/s). It is a recording of the double bass in L channel, the electric guitar in the R channel and female vocal in the center. Spectrogram shows a sum of L+R. We can see that there is no components, in the human audible dynamic range, below 30Hz or above 8 kHz. Well, how it sounds? In short: very life-like realistic and spectacular. If we analyze the recording with studio class headphones we can hear subtle DSP effects, which are otherwise masked by room acoustics.
For the end a curiosity about the speed of sound: If a temperature is 3.0 degrees Celsius, height above sea level 3.0 meters and relative humidity (very dry) at 3.0%, the speed of sound will be (with 99.9949% accuracy) 333.3 m/sec.
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