Look at the specifications for loudspeakers. You’ll get dimensions and weight. Often, you’ll get nominal impedance and a frequency response range (e.g. 40 – 20k Hz). If you’re lucky that range will include a +/- decibel tolerance (40 – 20k Hz +/-3dB). And if you’re really, really lucky they might even show you a frequency response graph. If you want to see distortion measurements, fat chance, go measure it yourself. And then there are other important considerations that you’ll never see. You want to know about off-axis response? Or intermodulation? Or resonance? Fahgeddaboudit. These three factors, off-axis response, intermodulation, and resonance, are consistently ignored. It’s not for lack of consequence to the sound quality. No, together these factors separate the good from the outstanding. Rather, it’s for convenience. These issues get complicated. And that’s not the only reason they’re not talked about. The number one reason is that manufacturers think you’re too stupid to understand, and what’s more, if you did understand you’d cross their speakers off your short list in a heartbeat.
However, you can understand. Let’s start with the off-axis response. If you sit directly in front of a speaker, you are on-axis. That direct sound coming straight from a speaker is what’s measured in the frequency response figures. Most speakers have a reasonably flat on-axis response. What that means is the speaker produces sound from low bass frequencies through the audible spectrum up to the high treble frequencies at relatively the same volume. For music to sound real, this is square one. The off-axis response is how a speaker sounds as you walk around it. Now, you might argue, why does that matter, “I’m not going to listen from the side or back.” Neither am I, but that off-axis sound is pouring into your room, and unless you live in an anechoic chamber (a room that reflects no sound, as if the walls were acoustically black), that poor off-axis response is adding to the total sound you hear. In fact, the reverberant sound field, all the sound waves bouncing off the walls, ceiling and floor of your room, usually makes up more than half of the sound you hear. Repeat, more than half of what you hear. This fact alone is the startling reality of listening to speakers in a typical room. Because a significant portion of the sound is reflected, when that reflected sound is out of balance, the music and voices you hear won’t sound real. It will be lacking in large parts of the spectrum. The high frequencies are lost first, and depending on how poor the off-axis response is, even much of the midrange gets diminished. That stuffy reverberant sound field mixes with the direct on-axis sound and adds up to off-color the entire presentation. Acoustic room treatment and controlled dispersion speakers, both of which are trying to reduce the room’s contribution, actually end up exacerbating the problem by making the reverberant field even more skewed towards the bass.
Imagine for a moment we’re trying to judge color. To see color, we need light. As you know, judging color by daylight is easy, by fluorescent light it’s more difficult, and by yellow sodium vapor street lights, impossible. Daylight is white-light. It has about the same intensity of all the colors of visible light, in other words, the full spectrum. You need full spectrum light in equal amounts to accurately judge color. Those other sources do not have equally balanced intensity, and in the case of the sodium light, there’s little else but yellow frequencies. Now, let’s pretend we have a light bulb that shines nicely balanced white-light from a small area on one side, but as we go around the bulb the light gets gradually yellow, then orange, and finally on backside it’s a deep red. If we put an object in front of the white part, very close, we’ll see accurate color. Now, what happens when we put that light bulb in a room? Can we judge color even if we were standing directly in front of the white spot? Not very well. Why? Because of all the off-color light radiating around the bulb is also filling the room, reflecting off surfaces, and skewing the color balance everywhere around the room. The same thing happens with sound waves.
The importance of a linear, full spectrum (no color shift) off-axis response is equally important for loudspeakers playing in a room. Unless you eliminate the walls, floor, and ceiling, or build yourself an anechoic chamber, the room’s reverberant sound field is unavoidable. The speaker’s off-axis response is critical for hearing the full spectrum of music, and hearing it balanced on and off-axis, including the reverberant sound field.
The Parallel Audio Project is an omnidirectional loudspeaker which lights up the room with full spectrum, flat frequency response sound in all directions. The reverberant sound field has the same frequency content as the direct sound. You hear the open, natural sound of music all around. This is only one of many important distinguishing features that makes the Project stand out from the crowd.
Next we’ll take on intermodulation in part 2.