In recent years, an increasing number of patients is suffering from respiratory diseases such as asthma, pulmonary emphysema, chronic bronchitis, etc. One of the most effective therapies for these diseases is oxygen inhalation therapy. Such oxygen inhalation therapy makes the patient inhale oxygen gas or oxygen-enriched air. An oxygen concentration device, liquid oxygen, an oxygen gas cylinder, etc. are known as the oxygen source, among which the oxygen concentration device is mainly used for home oxygen therapy due to its convenience in using and easiness in maintenance.
The oxygen concentration device concentrates oxygen present in the air at about 21% and supplies an oxygen-enriched gas. The oxygen concentration device includes a membrane-type oxygen concentration device utilizing a membrane which selectively permeates oxygen and a pressure-swing adsorption type oxygen concentration device utilizing an adsorbent which is capable of preferentially adsorb nitrogen or oxygen. The latter is mainly used because higher concentration of oxygen is obtained.
In the pressure-swing adsorption type oxygen concentration device, oxygen is generated by supplying air compressed using a compressor to an adsorbent cylinder filled with molecular sieve zeolite such as 5A type, 13X type, Li—X type, etc. as the adsorbent which preferentially adsorbs nitrogen over oxygen. Highly concentrated oxygen-enriched gas is continuously generated by alternately repeating a pressurization/adsorption step in which nitrogen is adsorbed on the adsorbent in the adsorbent cylinder under a pressurized condition and unadsorbed oxygen-enriched gas is obtained and a depressurization/desorption step in which the adsorbent is regenerated by reducing the pressure in the adsorbent cylinder to atmospheric pressure or lower and purging nitrogen adsorbed on the adsorbent.
Such an oxygen concentration device is intended to be placed relatively close to the patient and is essentially used continuously throughout the day by the patient regardless of dining or bedtime. Therefore, a noise generated from the oxygen concentration device is directly heard by the patient or patient's family, etc. and may give unpleasant feeling to them. In particular, the noise generated by the device has a significant influence on the patient or the family during sleep, and there is concern that the noise generated by the oxygen concentration device negatively affects their mental health by disturbing their sleep.
Sources of the noise generated by a pressure-swing adsorption type oxygen concentration device include a structure-borne sound from a compressor which supplies pressurized air to an adsorption cylinder and used for pressure variation, a suction sound and an exhaust sound from the compressor, an operating noise from a motor for driving the compressor, a purge gas flow noise generated when purging the adsorbent cylinder, and an operating noise from a cooling fan for cooling the interior of the oxygen concentration device housing. Among them, noises originated from the compressor such as the structure-borne sound radiation generated by the compressor itself, suction sound and exhaust sound of the compressor account for a large proportion of noises generated by the entire device.
As described above, reduction of the noise from noise sources such as the compressor is a vital issue in the oxygen concentration device. A sound muffler called a cavity type or an expansion type has been used to reduce the suction sound and the exhaust sound of the compressor, or gas flow sounds such as various purge sounds in a conventional oxygen concentration device. Furthermore, there are strong demands by patients for downsizing and weight reduction of the oxygen concentration device. As a method for satisfying both of the requests, in a sound muffler described in Japanese Patent Laid-Open Publication No. H10-245203, for example, it has been proposed that space saving can be achieved by eliminating dead space by shaping the sound muffler into a cuboid and by integrating the sound muffler into an attachment part of the air filter.
However, the expansion-chamber muffler described above has such a feature that, as the ratio of the spatial cross-sectional area of an expansion chamber to that of a tube increases, the attenuation rate of the emitted sound increases, and the length of the cavity portion of the expansion chamber is related to the frequency of the sound desired to be reduced. Therefore, the physical size of the cavity portion of the expansion chamber is determined by the frequency band and the attenuation rate of the noise desired to be reduced, which has been one of the obstacles for achieving downsizing and weight reduction of the body of a low-noise oxygen concentration device.
Thus, in order to perform downsizing and weight reduction of the device mounting a sound muffler, while increasing the sound reducing effect, Japanese Patent Laid-Open Publication No. 2003-235982, for example, discloses a technique in which a small form of the device is maintained while serving sound attenuation by bringing an expansion-chamber muffler and a resonance muffler together and placing each muffler into the dead space inside the housing of the device.
As a sound muffler for attenuating a noise in high frequency band and low frequency band based on different principles, such technique as described in Japanese Patent Laid-Open Publication No. 2005-6731 has been disclosed. In this patented technology, two sound mufflers are provided to reduce noises in high frequency band and low frequency band, where a long flow path is constituted using a sound-absorbing material for reducing the former, and this sound muffler is in turn incorporated in an expansion-chamber muffler for reducing the latter noise in the low frequency band, thereby achieving sound attenuation as well as downsizing and weight reduction. However, the expansion-chamber muffler requires the same size as before to reduce the noise in the low frequency band, and the fundamental technical problem remains unsolved.