Noise muffler for magnetic tape vacuum column

Abstract

Noise in the frequency range of 60 to 3,000 Hz generated by movement of magnetic tape in a vacuum column is muffled by use of Helmholtz cavities. The connecting ports to the cavities are placed near the top of the vacuum column close to the source of the noise and also where they provide an impedance mismatch barrier to sound waves trying to exit from the top of the vacuum column. The size of the ports and location of the ports in the vacuum columns are predetermined so that the ports will not inhibit the loading of the tape loop into the vacuum column.

What is claimed is: 
     
       1. A noise muffler for a vacuum column in a magnetic tape transport for preventing the emission of noise from the vacuum column, said muffler comprising: means for mismatching the acoustical impedance near the top of said column so that sound waves in the column will tend to be reflected at the top of the column back into the column;   means for generating interference between sound waves within the vacuum column so as to dissipate the noise within the vacuum column.   
     
     
       2. The apparatus of claim 1 wherein said means for generating comprises: a plurality of resonant cavities connected to the vacuum column by ports near the top of the column, the natural frequency of each resonant cavity being such as to generate a cancelling sound wave in the vacuum column when the resonant cavity is energized by a sound wave in the vacuum column that is near the natural frequency of the resonant cavity.   
     
     
       3. The apparatus of claim 1 wherein said means for mismatching comprises: a plurality of acoustical side branches connected to the vacuum column near the top of the vacuum column, each side branch forming an acoustical discontinuity so that sound waves in the vacuum column will tend to be reflected back within the vacuum column rather than passed out the top of the vacuum column.   
     
     
       4. The apparatus of claim 3 wherein said acoustical branches are small enough in size and located so that during tape loading in the vacuum column, the vacuum of the column will not be dumped around the tape via one or more of said side branches. 
     
     
       5. Method for muffling noise due to tape movement in a vacuum column of a magnetic tape transport comprising the steps of: reflecting the sound waves generates by the tape in the vacuum column back into the vacuum column;   dissipating the energy of sound waves in the vacuum column so that the noise generated by the tape movement in the vacuum column is contained in and dissipated in the vacuum column.   
     
     
       6. The method of claim 5 wherein said step of reflecting comprises the steps of: forming an acoustical discontinuity;   positioning the acoustical discontinuity at the top of the vacuum column so that there is an acoustical impedance mismatch at the top of the vacuum column forming a barrier to the transmission of sound waves out the top of the vacuum column.   
     
     
       7. The method of claim 5 wherein said dissipating step comprises the steps of: dissipating the energy of sound waves from the vacuum column in resonant cavities attached to the vacuum column;   generating interference sound waves in resonant cavities attached to the vacuum column;   passing these interference sound waves from the resonant cavity into the vacuum column to interfere with the sound waves of the same frequency inside the vacuum column.   
     
     
       8. In a magnetic tape transport having a vacuum column for buffering high speed movement of the magnetic tape, apparatus for muffling noise generated by movement of tape in the vacuum column so that the noise will not be emitted from the mouth of the vacuum column, said apparatus comprising: a plurality of Helmholtz cavities attached to the vacuum column near the mouth of the vacuum column, the Helmholtz cavities being of various sizes and having various size ports connecting said cavities to the vacuum column so that a band of frequencies making up the noise generated by the magnetic tape movement will be muffled;   said ports connecting the vacuum column to said cavities being positioned relative to a wall in the vacuum column such that the distance between the port and the wall is nλ/4 where λ is wavelength of the natural frequency of the cavity and n=1, 3, 5, 7, . . . .   
     
     
       9. The apparatus of claim 8 wherein said connecting ports are of such size and position in the vacuum column that during loading of the magnetic tape into the column, the vacuum of the vacuum column will not be dumped around the magnetic tape as it passes across one or more connecting ports. .Iadd. 10. A magnetic tape unit comprising: a file reel and a machine reel;   a magnetic head mounted intermediate said file reel and said machine reel;   a capstan for driving magnetic tape in both directions past said head and between said file reel and said machine reel;   a vacuum buffer zone on each side of said magnetic head for receiving tape; and   muffler means communicating with each said buffer zone for reducing acoustical noise generated within said buffer zone. .Iaddend..Iadd. 11. The magnetic tape unit of claim 10 wherein each said buffer zone comprises a vertical buffer column and a tapered buffer chamber, said muffler means communicating with said buffer zone at the junction between said vertical buffer column and said tapered buffer chamber. .Iaddend. .Iadd. 12. The magnetic tape unit of claim 11 further comprising hinged door means closing said buffer zone, said muffler means being located in said door means. .Iaddend.