Q. What are the advantages of omnidirectional speaker systems, if any?
A. The advantages are a diffused sound, a wide stereo image, and a certain smoothness in the treble, as peaks in the response are "ironed out." Compared with more directional systems, the disadvantages are an imprecise, confused stereo image, inferior transient response, and greater dependence on room acoustics. Some systems like the Bose 901 attempt to combine the virtues of both methods by having a front speaker for direct sound in addition to the rear speakers which use the walls as a reflector.
above: IMF Studio uses the transmission line principle.
Q. Are electrostatic speakers that much better than dynamics?
A. ESL's have a smooth transparent sound which is only equaled by the very best dynamic systems. This is partly due to inherent virtues and partly because there are no cabinet resonances. No cabinet-no resonances! On the other hand, full range ESL's have a restricted low frequency because of cancellation effects (all those presently available are doublets, that is they radiate in two directions) and to the electrode spacing limitations. The diaphragm has to be close to the plate or plates to obtain reasonable sensitivity and spacing must be large enough to permit the large movements necessary at low frequencies. Thus, a compromise has to be accepted.
Q. I bought a pair of loudspeakers after hearing them at a dealer's, but when I connected them up at home, the bass response seems much less. I am using an amplifier similar to the one at the dealer's. What could be wrong? I have checked the phasing according to the instruction book.
A. The listening room must be considered acoustically as an extension of the loudspeaker, and as room sizes and characteristics vary, so will the actual sound. Best position for maximum bass is usually in the corners but this will not necessarily give the best overall balance.
So do not be afraid to experiment (domestic conditions permitting). Assuming the speakers check out all right and the bass loss is due to the room, it is perfectly permissible to use a little bass lift on the amplifier-that's what tone controls are for. However, too much lift may cause distortion and give a "chesty sound" to speech. An equalizer, such as the Amplidyne SE 111 or the Soundcraftsmen 20-12, might be worth considering. These units can lift the extreme bass without affecting frequencies in the 200 Hz region.
Fig. 1--Typical labyrinth system.
Q. What is a labyrinth system?
A. It basically consists of a long tunnel or pipe at the rear of the loudspeaker. Its purpose is to provide an effective termination for the cone and increase the path length of the rear wave so that any radiation will not cancel the output from the front. Output from the tunnel can be positioned at the front or the back of the enclosure--or even at the sides. Figure 1 shows a typical arrangement. The cross-section of the tunnel should be at least equal to the cone area and the length is usually equal to the quarter wavelength of the speaker cone resonance. Thus, a loaded speaker resonance of 40 Hz would need a path of about nine feet. Since the loudspeaker sees a high impedance when the tunnel length is quarter wave, the cone excursions and distortion are reduced to a minimum. At frequencies below this mode, the port and diaphragm radiation is out of phase so radiation falls off steeply. At higher frequencies, the port radiation would be in or out of phase with the diaphragm output so the tuner has to be heavily damped with acoustic material.
Q. What is a transmission line system?
A. A labyrinth is really a transmission line, but recently the term has been applied to a variation developed by Dr. Bailey. Instead of lining the tunnel, it is completely filled with absorbent material (usually long-hair wool). This obviously reduces output above 150 Hz or so, provided a higher resistance loading for the cone and increases the effective length of the tube! Speakers using this system include the IMF, Trans-Static, Electrostatic Sound, Radford, and Infinity. Some of these use 1/8th wave length tunnels instead of 1/4.
(Audio magazine, Mar. 1972)
Also see:
Q's and A's on Loudspeakers (Mar. 1970)
How to Choose a Loudspeaker (Mar. 1970)
What Price Loudspeaker Response Curves (Mar. 1972)
The Acoustic Feedback Loudspeaker System (Jan. 1972)
Speaker Tests: Room Test by Richard C. Heyser (Jan. 1975)
Speaker Tests--Phase Response (by Richard C. Heyser) (Dec. 1974)
The Listening Room by E.T. Canby (Apr. 1970)
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