Image Stability
In a sonically quiet room there can be found a number of objects, each of which causes a different sounding early reflections. The tonal differences are due to the variation in shape, size, composition and location. Shape and size determine the low frequency cutoff of the reflections. Composition tends to reduce the higher frequency reflectivity and location varies the sound of a reflection because the tweeter and mid-range speakers do not have a uniform beam pattern, different frequencies will be louder or quieter at any given location in the room.
With the speakers set up to produce a fairly strong mono stereo center image, instead of playing music use a signal generator to run the system through a “sine sweep”. Ignore the variations in loudness, particularly in the bass range but concentrate on the location of the mono stereo center image as the sweep moves into the upper treble range, above 500 Hz (octave above middle C).
The tight center image will not stay in one place, it will move left and right, up and down along a path as if it was flying around between the speakers.
Clearly, early reflections cause problems with the imaging of the stereo system. By adding larger sized sound panels close to the speakers, the spread of the off-axis sound, the beam pattern of the speaker, is narrowed which reduces the number of off-axis early reflections. The stereo imaging clears up and its position is stabilized.
Imaging Omni Speakers
Omni speakers play sound equally in all directions. About 10 % of the hifi speakers are omni speakers. The original popular omni speaker was Bose 901, in the 60’s and continues to evolve omni speakers. Walsh Ohm speaker came in the 70’s and continues through today with their call to action “hear the music not the speakers”. Omni speakers have evolved, MBL, Linkwitz, Duevel to name a few.
With Omni speakers the same sound expands equally in all directions. One of the more curious aspects of omni speaker owners is their insistence on having no acoustic treatment. The more reflections the better which leads us to explore what the imaging of omni speakers in hard surfaced rooms is like. We are not talking about omni speakers being used in large venues, where they seem to deliver fantastic sound, but when used in regular sized home listening rooms.
Omnis, like most all other speakers, deliver a direct signal before any of the reflections arrive. This creates a first arrival effect which in this case retains a semblance of a left and right speaker playing stereo. Outside of this, the imaging as we “know it” seems to disappear. When an audiophile, who regularly listens to a precision imaging system, visits an omni room what the “sees” is a giant “fog bank” of sound devoid of any detail. It’s not like sound beams blasting through a foggy sound stage, there are no image beams cutting through the fog bank of sound, no sound stage, just great music.
With omnis, there is formally speaking, no image to be seen. But dedicated omni-philes talk about the sound stage of their system even though no L-R ear information is being received. Their image is not a perceived image, it is their imagined image. Concerts, night clubs, choirs all have traditional footprint. It’s just that in audio we are wary about “hearing” what we wanted, believed or hoped to hear rather than the truth of what we did hear. With omnis, it’s all good, we get to relax and hear what we want to hear and don’t give it a second thought. Stressless playback. |
