Abstract: An object is to provide a hydraulic oil composition which is excellent in oxidative stability, lubricating properties and biodegradability. A hydraulic oil composition comprising vegetable oil with a total degree of unsaturation of 0.3 or less as base oil, and comprising at least one antioxidant selected from the group consisting 6f a phenol antioxidant, an amine antioxidant and a zinc dithiophosphate antioxidant in an amount of 0.01 to 5% by mass based on the total amount of the composition.

Abstract: An object is to provide a hydraulic oil composition which is excellent in oxidative stability, lubricating properties and biodegradability. A hydraulic oil composition comprising vegetable oil with a total degree of unsaturation of 0.3 or less as base oil, and comprising at least one antioxidant selected from the group consisting of a phenol antioxidant, an amine antioxidant and a zinc dithiophosphate antioxidant in an amount of 0.01 to 5% by mass based on the total amount of the composition.

Abstract: A headphone device comprising a pair of headphones for receiving output signals from various acoustic apparatus, wherein a direct sound signal or an output signal of an acoustic apparatus is applied to a reverberation supply circuit to produce an indirect sound signal, said indirect sound signal being mixed with said direct sound signal with a ratio of more than 1 : 10 to control the feeling of distance to a sound image existing outside the head, and further the direct sound signal is added to another channel through an attenuator to control the feeling of the direction of the sound image, whereby a natural listening condition similar to the case of loudspeaker listening is obtained.

Abstract: The present invention relates to a headphone apparatus which is adapted to be connected, for headphone listening, to a reproduction apparatus for reproducing a program source such as a record, a magnetic tape or an acoustic apparatus such as a radio receiving set. In accordance with the present invention, there is provided a headphone apparatus in which a direct audio frequency signal as an output signal from the electroacoustic apparatus is applied to reverberation circuits to generate indirect audio frequency signals and these direct and indirect audio frequency signals are mixed and applied to both the left and right electroacoustic transducers of the headphone apparatus, whereby an acoustic image can be established outside the listener's head just as in the case of listening to speakers.