Ponemos SW por encima de nuestra capacidad auditiva, ¿por que no supertweeters ?

Os pongo un texto interesante que he encontrado al respecto:

El estudio íntegro, aquí: http://jn.physiology.org/content/83/6/3548

Inaudible High-Frequency Sounds Affect Brain Activity: Hypersonic Effect

Tsutomu Oohashi, Emi Nishina, Manabu Honda, Yoshiharu Yonekura, Yoshitaka Fuwamoto, Norie Kawai, Tadao Maekawa, Satoshi Nakamura, Hidenao Fukuyama, Hiroshi Shibasaki
Journal of Neurophysiology Published 1 June 2000 Vol. 83 no. 6, 3548-3558 DOI:


Abstract

Although it is generally accepted that humans cannot perceive sounds in the frequency range above 20 kHz, the question of whether the existence of such “inaudible” high-frequency components may affect the acoustic perception of audible sounds remains unanswered. In this study, we used noninvasive physiological measurements of brain responses to provide evidence that sounds containing high-frequency components (HFCs) above the audible range significantly affect the brain activity of listeners. We used the gamelan music of Bali, which is extremely rich in HFCs with a nonstationary structure, as a natural sound source, dividing it into two components: an audible low-frequency component (LFC) below 22 kHz and an HFC above 22 kHz. Brain electrical activity and regional cerebral blood flow (rCBF) were measured as markers of neuronal activity while subjects were exposed to sounds with various combinations of LFCs and HFCs. None of the subjects recognized the HFC as sound when it was presented alone. Nevertheless, the power spectra of the alpha frequency range of the spontaneous electroencephalogram (alpha-EEG) recorded from the occipital region increased with statistical significance when the subjects were exposed to sound containing both an HFC and an LFC, compared with an otherwise identical sound from which the HFC was removed (i.e., LFC alone). In contrast, compared with the baseline, no enhancement of alpha-EEG was evident when either an HFC or an LFC was presented separately. Positron emission tomography measurements revealed that, when an HFC and an LFC were presented together, the rCBF in the brain stem and the left thalamus increased significantly compared with a sound lacking the HFC above 22 kHz but that was otherwise identical. Simultaneous EEG measurements showed that the power of occipital alpha-EEGs correlated significantly with the rCBF in the left thalamus. Psychological evaluation indicated that the subjects felt the sound containing an HFC to be more pleasant than the same sound lacking an HFC. These results suggest the existence of a previously unrecognized response to complex sound containing particular types of high frequencies above the audible range. We term this phenomenon the “hypersonic effect.”


Aparte de lo que atañe a este estudio y el objeto de este hilo, es muy interesante lo mencionado en el este párrafo introductorio acerca del estudio de Muraoka en 1978 acerca de la calidad percibida a partir de 15 kHz...
[QUOTE]INTRODUCTION

It is generally accepted that audio frequencies above 20 kHz do not affect human sensory perception since they are beyond the audible range (Durrant and Lovrinc 1977; Snow 1931; Wegel 1922). Thus for example, most of the conventional commercial digital audio formats [e.g., compact disks (CDs), digital audio tapes (DATs), and digital audio broadcasting] have been standardized to a frequency range that does not allow such high-frequency components (HFCs) of sounds to be included. As a premise for determining these formats, several psychological experiments were performed to evaluate sound quality subjectively by means of questionnaires, according to the recommendation of the ComitéConsultatif International Radiophonique (CCIR 1978) or its modified versions. Studies by Muraoka et al. (1978)and Plenge et al. (1979), as well as other studies, concluded that listeners did not consciously recognize the inclusion of sounds with a frequency range above 15 kHz as making a difference in sound quality. Nevertheless, and interestingly enough, artists and engineers working to produce acoustically perfect music for commercial purposes are convinced that the intentional manipulation of HFC above the audible range can positively affect the perception of sound quality (Neve 1992). Indeed, the Advanced Audio Conference organized by the Japan Audio Society (1999) proposed two next-generation advanced digital audio formats: super audio compact disk (SACD) and digital versatile disk audio (DVD-audio). These formats have a frequency response of up to 100 kHz and 96kHz, respectively. However, the proposal was not based on scientific data about the biological effects of the HFCs that would become available with these advanced formats. Although recently there have been several attempts to explore the psychological effect of inaudible HFCs on sound perception using a digital audio format with a higher sampling rate of 96 kHz (Theiss and Hawksford 1997; Yamamoto 1996; Yoshikawa et al. 1995,1997), none of these studies has convincingly explained the biological mechanism of the phenomenon. This may reflect in part the limitations of the conventional audio engineering approach for determining sound quality, which is solely based on a subjective evaluation obtained via questionnaires.


There are two factors that may have some bearing on this issue. First, it has been suggested that infrasonic exposure may possibly have an adverse effect on human health (Danielsson and Landstrom 1985), suggesting that the biological sensitivity of human beings may not be parallel with the “conscious” audibility of air vibration. Second, the natural environment, such as tropical rain forests, usually contains sounds that are extremely rich in HFCs over 100 kHz. From an anthropogenetic point of view, the sensory system of human beings exposed to a natural environment would stand a good chance of developing some physiological sensitivity to HFCs. It is premature to conclude that consciously inaudible high-frequency sounds have no effect on the physiological state of listeners.



In the present study, therefore, we addressed this issue by using quantifiable and reproducible measurements of brain activity. To measure human physiological responses to HFCs, we selected two noninvasive techniques: analysis of electroencephalogram (EEG) and positron emission tomography (PET) measurements of the regional cerebral blood flow (rCBF). These methods have complementary characteristics. EEG has excellent time resolution, is sensitive to the state of human brain functioning, and places fewer physical and mental constraints on subjects than do other techniques such as functional magnetic resonance imaging (fMRI). This is of special importance because some responses might be distorted by a stressful measurement environment itself. On the other hand, PET provides us with detailed spatial information on the neuroanatomical substrates of brain activity. Combining these two techniques with psychological assessments, we provide evidence herein that inaudible high-frequency sounds have a significant effect on humans.
[/QUOTE

METHODS

Subjects

Twenty-eight Japanese volunteers (15 males and 13 females, 19–43 years old) participated in the EEG experiments; 12 Japanese volunteers (8 males and 4 females, 19–34 years old) participated in the PET experiment; and 26 Japanese volunteers (15 males and 11 females, 18–31 years old) participated in the psychological experiment. None of the subjects had any history of neurological or psychiatric disorders. Written informed consent was obtained from all subjects before the experiments. The PET and EEG experiments were performed in accordance with the approval of the Committee of Medical Ethics, Graduate School of Medicine, Kyoto University. All subjects were familiar with the actual sounds of the instruments used as a sound source.

Sound materials and presentation systems

Traditional gamelan music of Bali Island, Indonesia, a natural sound source containing the richest amount of high frequencies with a conspicuously fluctuating structure, was chosen as the sound source for all experiments. A traditional gamelan composition, “Gambang Kuta,” played by “Gunung Jati,” an internationally recognized gamelan ensemble from Bali, was recorded using a B&K 4135 microphone, a B&K 2633 microphone preamplifier, and a B&K 2804 power supplier, all manufactured by Brüel and Kjær (Nærum, Denmark). The signals were digitally coded by Y. Yamasaki's high-speed one-bit coding signal processor (United States Patent No. 5351048) (Yamasaki 1991) with an A/D sampling frequency of 1.92 MHz and stored in a DRU-8 digital data recorder (Yamaha, Hamamatsu, Japan). This system has a generally flat frequency response of over 100 kHz.

Most of the conventional audio systems that have been used to present sound for determining sound quality were found to be unsuitable for this particular study. In the conventional systems, sounds containing HFCs are presented as unfiltered source signals through an all-pass circuit and sounds without HFCs are produced by passing the source signals through a low-pass filter (Muraoka et al. 1978;Plenge et al. 1979). Thus the audible low-frequency components (LFCs) are presented through different pathways that may have different transmission characteristics, including frequency response and group delay. In addition, inter-modulation distortion may differentially affect LFCs. Therefore it is difficult to exclude the possibility that any observed differences between the two different sounds, those with and those without HFCs, may result from differences in the audible LFCs rather than from the existence of HFCs. To overcome this problem, we developed a bi-channel sound presentation system that enabled us to present the audible LFCs and the nonaudible HFCs either separately or simultaneously. First, the source signals from the D/A converter of Y. Yamasaki's high-speed, one-bit coding signal processor were divided in two. Then, LFCs and HFCs were produced by passing these signals through programmable low-pass and high-pass filters (FV-661, NF Electronic Instruments, Tokyo, Japan), respectively, with a crossover frequency of 26 or 22 kHz and a cutoff attenuation of 170 or 80 dB/octave, depending on the type of test. Then, LFCs and HFCs were separately amplified with P-800 and P-300L power amplifiers (Accuphase, Yokohama, Japan), respectively, and presented through a speaker system consisting of twin cone-type woofers and a horn-type tweeter for the LFCs and a dome-type super tweeter with a diamond diaphragm for the HFCs. The speaker system was designed by one of the authors (T. Oohashi) and manufactured by Pioneer Co., Ltd. (Tokyo, Japan). This sound reproduction system had a flat frequency response of over 100 kHz. The level of the presented sound pressure was individually adjusted so that each subject felt comfortable; thus the maximum level was approximately 80–90 dB sound pressure level (SPL) at the listening position.

Using the bi-channel sound presentation system, four different sound combinations were prepared as follows: 1) full-range sound (FRS) = HFC + LFC; 2) high-cut sound (HCS) = LFC only; 3) low-cut sound (LCS) = HFC only; and,4) baseline = no sound except for ambient noise. All experiments were performed in an acoustically shielded room. In the PET experiment, there was a very low-level fan noise from the PET scanner, which did not annoy the subjects. Figure1 A shows the averaged power spectrum of the source signal obtained from the music with a CF-5220 fast Fourier transform (FFT) analyzer (Ono Sokki, Tokyo, Japan) over an analysis period of 200 s. It contained a significant amount of HFCs above the audible range, often exceeding 50 kHz and, at certain times, 100 kHz. Figure 1 B shows the averaged power spectra of the actual sounds reproduced with a 22 kHz cutoff frequency for the filter and recorded at the subject's head position. The spectrum of FRS was essentially the same as that of the source and contained both LFCs below and HFCs above 22 kHz. None of the blindfolded subjects could distinguish LCS (i.e., HFC only) from silence when it was presented alone. Therefore we concluded that the HFC employed in the present experimental setting was, at least, a consciously unrecognizable air vibration.

Fig. 1. Power spectra of the sound used in this study. A: the averaged power spectrum calculated from the entire 200-s period of the recorded sound source signal using a CF-5220 fast Fourier transform (FFT) analyzer (Ono Sokki, Tokyo, Japan). It contains a significant amount of high-frequency components above the audible range. B: the averaged power spectra of the sounds reproduced by the bi-channel sound presentation system (see text) in different conditions. The power was calculated from the signal actually recorded at the subject's head position using a B&K 4135 microphone (Brüel and Kjær, Nærum, Denmark). The top, middle, and bottom panels represent full-range sound (FRS), high-cut sound (HCS), and low-cut sound (LCS), respectively. The power spectrum of FRS is essentially identical to the spectrum of the source and contains both a low-frequency component (LFC) (i.e., the one used in the HCS condition) and a high-frequency component (HFC) (in the LCS condition).


EEG recordings and analysis

The EEG experiments were performed in the EEG laboratory of the National Institute of Multimedia Education. Subjects were asked to sit on a chair in a relaxed position. The distance from the speakers to the subjects' ears was approximately 2.5 m. Special attention was paid to the subjects' immediate environment to avoid discomfort. For example, the room was decorated with plants, lacquered masks, and landscape paintings. The equipment for the EEG recordings was hidden from the subjects' view and all cables for the experimental equipment were in a pit below the floor. The subjects were instructed to enjoy the music without any cognitive tasks during the sound presentation. The subjects were able to see outdoors through a wide, double-glass window that acoustically shielded the experimental room from outside sounds. Two different EEG experiments were performed. In the first experiment, to explore the physiological effect of sounds with a nonaudible frequency range, we employed a strictly controlled experimental setting of sound presentation combined with conventional EEG measurements. In the second experiment, the same effect was examined under more ordinary listening conditions.
EXPERIMENT 1.

To examine the physiological effect of sounds with an inaudible frequency range, 11 subjects were presented with the FRS, HCS, and baseline conditions. In this experiment, a cutoff frequency of 26 kHz with a steeper cutoff attenuation of 170 dB/octave was employed to separate HFCs from LFCs. This relatively high cutoff frequency was chosen because when a cutoff frequency lower than 26 kHz is used the skirts of the power spectrum of the filtered HFCs extend below 20 kHz and generate sounds containing components below 20 kHz. It is widely known that the upper limit of the audible range of humans varies considerably. It usually corresponds to around 15 or 16 kHz in young adults and sometimes below 13 kHz in the elderly, and some people can recognize air vibrations of 20 kHz as sound. When a cutoff frequency of 26 kHz is employed with the steeper cutoff attenuation, the power spectrum of the filtered HFCs under 20 kHz falls below the system noise level. Therefore we selected a cutoff frequency of 26 kHz, which is sufficiently high to completely exclude contamination by audible sound components in all of the subjects. In accordance with conventional recordings of background EEG activity, subjects were asked to keep their eyes naturally closed during the experiment to eliminate any effects of visual input. The presentation of the sounds in both FRS and HCS conditions lasted 200 s, which included the entire piece of music. The baseline condition also lasted 200 s without sound presentation. The inter-session intervals were 10 s. Two recording sessions were repeated for each condition in the following order: baseline–FRS–HCS–FRS–HCS–baseline.

EXPERIMENT 2.

The validity of the digital audio format internationally employed for CDs was evaluated under more ordinary listening conditions. Seventeen subjects were presented with sounds using a cutoff frequency of 22 kHz, which corresponds to the upper range of sounds recorded by a CD. Subjects were then asked to keep their eyes naturally open as they usually do when they listen to music. The open-eye condition was also appropriate to control the subjects' vigilance. Each subject was presented with four types of conditions: FRS, HCS, and baseline, as in Experiment 1, plus LCS to elucidate the effect of an HFC when it is presented alone. As in Experiment 1, each condition lasted 200 s. Before the actual recording sessions, HCS was presented once to familiarize the subjects with the experimental environment. To avoid any influence by the order of presentation, the four different conditions were performed in random order across the subjects. After a 10-min rest, the same four conditions were repeated in reverse order. Neither the subjects nor the experimenters knew which conditions were being performed.


The EEGs, recorded using the WEE-6112 telemetric system (Nihon-Koden, Tokyo, Japan) to minimize constraint on the subjects, were stored on magnetic tape for off-line analysis. The EEGs were recorded continuously, including the intervals between the sessions. Data were recorded from 12 scalp sites (Fp1, Fp2, F7, Fz, F8, C3, C4, T5, Pz, T6, O1, and O2 according to the International 10-20 System) using linked earlobe electrodes as the reference with a filter setting of 1–60 Hz (−3 dB). The impedance of all electrodes was kept below 5 kΩ. The EEGs obtained were subjected to power spectra analysis. The power spectrum of the EEG at each electrode was calculated by fast Fourier transform (FFT) analysis for every 2-s epoch, with an overlap of 1 s, at a frequency resolution of 0.5 Hz with a sampling frequency of 256 Hz. Then the averaged power spectrum within a 10-s time window was calculated. Each analysis window was designated by the time at its middle point measured from the beginning of the sound presentation. For example, the time window labeled as 100-s contains data from 95 to 105 s from the beginning. Then the square root of the averaged power level in a frequency range of 8.0–13.0 Hz at each electrode position was calculated as the equivalent potential of EEGs in an alpha band (alpha-EEG). To eliminate a possible effect of inter-subject variability, the alpha-EEG at each electrode position was normalized with respect to the mean value across all time epochs, conditions, and electrode positions for each subject. To obtain an overview of the data, to check for contamination by artifacts, and to characterize the spatial distribution of the alpha-EEG, we constructed colored contour line maps using 2,565 scalp grid points with linear interpolation and extrapolation. This type of map is called a brain electrical activity map (BEAM) (Duffy et al. 1979). To avoid contamination by artifacts arising from eye movement, we calculated occipital alpha-EEGs by averaging the alpha-EEGs at the electrodes on the posterior one-third of the scalp. The BEAMs and occipital alpha-EEGs were averaged over multiple time epochs and subjected to a statistical evaluation of condition effects. Since the time course of the alpha-EEG change revealed a considerable time lag with respect to the sound presentation (see results and Fig.2C), we made a statistical evaluation of the data obtained from all time epochs as well as of the data from only the latter half of the session (from the 100-s to 200-s class marks). We used analysis of variance (ANOVA) followed by Fishers' protected least significant difference (PLSD) post hoc test to assess statistical significance for the different conditions.




Fig. 2.
Normalized potentials from the alpha frequency range of the spontaneous electroencephalogram (alpha-EEG) under each experimental condition (FRS, HCS, and baseline) and time course in the successive FRS and HCS conditions in EEG Experiment 1. A: brain electrical activity maps (BEAMs) averaged across the 11 subjects over the entire time epoch of sound presentation. Darker red indicates higher alpha-EEG potential. Note that the alpha-EEG is enhanced in the parieto-occipital region exclusively in the FRS condition. B: mean and standard error of the occipital alpha-EEG for all 11 subjects. FRS significantly enhanced the occipital alpha-EEG relative to HCS.C: time course of grand average BEAMs across all 11 subjects. Two sessions for each condition were averaged in this figure. The occipital alpha-EEG shows a gradual increase during the FRS presentation and a gradual decrease while HCS was successively presented



Sigue......