Koichi Yoshihisa

Exciting demonstration in acoustics by high‐school teachers’ group: ‘‘Stray Cats.’’

In Japan, to get students interested in a subject, high‐school teachers often form a group to share their ideas and inventions on education. ‘‘Stray Cats’’ is one of the most active groups in physics. The group has been proposing many exciting demonstrations, which were contrived to support students’ learning process with intuitive understanding of physics. Here, instead of using commercial equipment, they developed simple teaching tools that show physical phenomena in an exciting and attractive way, using quite common materials and daily goods. For example, the velocity of sound is measured by a pipe filled with a gas (air, CO2, helium, etc), setting a loudspeaker and two microphones in the pipe. Interference of sounds is demonstrated by two pipes with attached cone‐shaped horns at one end, which collect a source sound in different phase, and merging them into one at the other end, which produce louder or quieter sound as a result of interference. Hitting or rubbing different length of aluminum rods aids students’ understanding of longitudinal waves and transverse wave with the relationship between rod’s length and pitch, as well as a pleasant experience with beautiful tones. These educational tools will be presented with videos taken with the Stray Cats group.

Observations of vibration velocity on wind turbine with propagated sound to surroundings

The authors newly tried measurements of the excited vibration on many parts of wind turbine such as outside surface of nacelle storing power generation system and tall tower by using a laser Doppler vibration meter. Observation and analysis of the natural vibration frequencies of blade in the stopping were done. Observations on sound in surroundings of wind turbine were also done. FFT analyses on them with high resolution to obtain detailed frequency characteristics and to know the relations between vibration velocity and propagated sound to surroundings were done in addition to ordinal spectral analyses. Sharp spectra at near 1 Hz as slightly lower or higher were appeared in the analysed results on blade and tower. In both the analyzed results of vibration velocity on wind turbine and propagated sound, a lot of sharp spectral peaks were recognized in wide frequency range from very low frequency to several hundreds of hertz with high coherencies between vibration velocity and propagated sound at their pea...

Experimental study on noise radiation from wind turbines

Noise emitted from wind turbines is composed of aerodynamic and mechanical sound and has directional radiation characteristics. To investigate the horizontal sound directivity around a wind turbine under various wind conditions, field measurements of noise generated from two different wind turbines have been performed over long periods. Wind turbine operational data such as the nacelle direction and rotor rotational speed were collected at 1 s intervals along with corresponding acoustic data. An empirical formula for the directivity correction was derived from the A-weighted sound pressure levels measured at the several receiving points around the wind turbine. We also focused on the amplitude modulation components of wind turbine noise in emission areas and compared the estimated rotor speeds by using measured sound pressure levels with actual values. The results showed that the directivity pattern of the A-weighted sound pressure level for two different wind turbines is almost the same, whereas the freq...

Acoustical diagnoses of porous drainage pavements

Road traffic noise reduction of drainage pavement with small air gaps among grains is well known. However, traffic flow over many years fills those gaps with mud and sand, thereby degrading the noise reduction effect. To regain that effect, cleansing or reconstructing the pavement is necessary after precise assessment of the air gaps. For that purpose, acoustical diagnoses of drainage pavement are proposed and test results are introduced. One method is to check sound absorption of the pavement surface, which normally has high absorption near frequencies of 1 kHz, which is related to the reduction of road traffic noise. The use of a particle velocity sensor together with a microphone greatly simplifies measurements and judgments. Another analysis method is based on sound propagation through air gaps in the pavement. Transmission characteristics clearly indicate pavement conditions. Low-attenuation sound transmissions are shown in a wide frequency range below 1 kHz for sound air gaps, and weak transmissions in a narrow frequency range are shown when air gaps are filled. These acoustic diagnostic methods are effective in investigating porous drainage pavement.

Field experiments on the influences of wind speed and direction on outdoor sound propagation over flat ground

The influences of wind on outdoor sound propagation are investigated both by field experiments and numerical simulations. At first, measurements of sound propagation at a distance of 160 m were carried out under various wind speed and direction. The relationship between vector wind speed and the variation of sound pressure level has been examined not only for the up/down wind conditions but also for the cross wind conditions. Secondly, the short term sound fluctuations were investigated by the use of a high energy impulsive sound generator and a loud speaker as sound sources. Sound propagation up to 300 m was examined with frequency range from 16 Hz to 4 kHz octave band. Finally, the sound speed profiles in the measurement field were estimated by two methods; one was based on the time interval during the sound propagation from the source to the receivers, the other was based on the wind speed at height of 0.6 m, 1.2 m, 2.4 m and 4.8m, and the temperature. By using those sound speed profiles, the excess at...

Effect of atmospheric absorption on aircraft noise propagation in various world regions during the year

The attenuation of sound due to atmospheric absorption depends strongly on environmental temperature and humidity. For that reason, it varies in response to changing meteorological conditions on a variety of time scales. To perform long‐term prediction of outdoor sound propagation more accurately, it is necessary to examine the fluctuation of the attenuation coefficients for atmospheric absorption during the year. Large variations occur according to the time and place in actual meteorological conditions, on which the attenuation coefficients depend strongly. The attenuation coefficients for atmospheric absorption were calculated from hourly meteorological data obtained at ten international airports throughout the world during a year using the calculation method described in ISO 9613‐1. Calculated results show that the variability characteristics of the attenuation coefficients during the year depend strongly upon both the sonic frequency and the place. The effects of atmospheric absorption on aircraft noi...

Measurements of acoustic impedance and their application to sound propagation calculations and audible simulations

Acoustic impedance is an important value that determines the boundary condition of each sound field, but collections of actual values are not sufficient for evaluation of many sound fields. First, measurements using a particle velocity sensor for acoustic impedance were tested on various fields. Such measurement results were applied to calculations of sound propagation. Frequency characteristics of sound propagation were obtained along such surfaces as fields of lawns and snow, and areas paved by porous asphalt for drainage. Those characteristics showed fair correspondence with inspected field measurement results. Then, fine calculations in the frequency domain were converted to the impulse response for each sound field model. Convolution operations based on the impulse response and on voice, music, and some noise sources readily produced an ideal sound field to the audible sound file. Furthermore, simulations of the noise, including noise reduction effects from a car running through a drainage pavement a...