THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

580 19. Sound Reproductiontuned aperture of a specified cross-sectional area and port length in which theenclosed air mass resonates with the enclosure’s air spring. This causes a woofer’sback wave to communicate with the external acoustic space with the enclosureeffectively acting as a Helmholtz resonator.Another version of the bass reflex system is the port/passive radiator (PR). Thevent is replaced with an acoustically driven diaphragm (aptly called a drone cone).Another type of enclosure is the transmission line (TL) that effectively functionsas a low-pass filter with a 90 ◦ phase shift, absorbing all of the rear wave energyof the woofer except for frequencies below 75 Hz. The TL enclosure provides afolded path or a labyrinth equal in length to the 25% of wavelength at or just abovethe resonance of the woofer. Damping material such as Dacron r○ , fiber wool,or fiberglass fills much of the labyrinth. The TL enclosures generally provideexcellent, clean bass response from relatively compact floor-standing cabinets.The principal disadvantage of TL is the complexity of the cabinet structure.Other types of speaker systems are available in the market. Among these are theelectrostatic speakers, the ribbon-type speaker system, and there are others thatconsist of hybrids, e.g., a system that combines the electrodynamic drivers withan electrostatic or ribbon tweeter. A ribbon type of drive essentially consists of avoice coil patterned (like the conductive pattern on a printed circuit board) onto amagnetizable diaphragm (i.e., the “ribbon”) made of strong, flexible materials suchas Mylar. The diaphragm is suspended in the front of a flat magnet that matchesthe area of the diaphragm. A varying signal sent into the coil causes a change inthe magnetic field that causes the ribbon to move with respect to the magnet, thuscausing sound to be radiated. If the magnet is perforated in one way or the other,the ribbon acts as a dipole sound generator.In the electrostatic speaker (which is similar operationally to the condensermicrophone but functioning in reverse), a voltage is maintained between a thindiaphragm and another surface. In effect the two thinly separated surfaces areacting as a large condenser, with the varying voltages fed to one of the elements,causing the diaphragm to move. Because of the small excursions of the diaphragm,the electrostatic driver does not function well in the low-frequency range butit is capable of providing excellent quality high-frequency signals. The highlycapacitive loading of the electrostatic speaker may be problematic for some poweramplifiers that are usually designed to handle primarily resistive impedances.Headphones are essentially miniaturized speakers mounted in earcups, whichare in turn attached to headbands that serve the purpose of holding these driversagainst the ears. There are also a number of headphone models that use electrostaticelements either by themselves or as supplemental components to electrodynamicelements.19.4 The Iosono SystemOn the basis of research conducted for more than 15 years at the Technical Universityin Delft, the Netherlands, Karlheinz Brandenburg, the director of Fraunhofer