Avoid a large sweet spot

Why you should be sour on a wide "Sweet Spot" for two channel playback by Jim Smith.

A wide sweet spot is almost like having your own harmonic distortion generator! There's simply no way a serious listener should be satisfied to sit more than a foot away from the 'equal path length intersection' (center point) of sound from a pair of loudspeakers. Inter-channel phase and timing information has just been badly compromised, destroying instrumental timbres.

How is it that Audiophiles will accept only phase and time-aligned loudspeakers and then expect to sit off the acoustic center-point, totally destroying the inter-channel phase/time information? Look at it this way.....First, since you probably know this stuff, please forgive the simplified averaged wavelengths, but for purposes of illustration, let's assume that a 1100 Hz tone (or harmonic) has a length of about 12 inches. Then 550 Hz is almost 2 feet in length (from the top of the sound-wave crest to the top of the next). And 2 kHz is almost 6 inches in length, 4 kHz is 3 inches, etc.

Now imagine that a female vocalist is recorded with her image centrally located in the stereo stage. If you sit two feet off center, that means that any fundamental notes and their harmonics from at least 500 Hz and above have been altered, some dramatically, some slightly.

This is audible, and it's depressingly measurable! Before we examine the disastrous effects, let's look at what's happened to cause the problem...

But wait a minute! What about imaging?
OK, let's say that now you're about a foot closer to the left speaker than you are to the right one. Imagine a centrally recorded image that is reproduced at equal volume (amplitude) from both speakers in order to give the illusion of a precise center image.

Without going too far into recording techniques or speaker dispersion patterns, a panned mono center image (such as is produced in a studio) may appear to have shifted left somewhat. While a center image recorded from a stereo pair of mics seems to 'stay put' a little better. But these are phantom images at best, lacking in the ultimate richness of tone and body. Here's why...

It's not the potential 'image wander' that's troublesome. It's the harmonic distortion! (Technically, it's not distortion, but the alteration of harmonic relationships.) The positive cycle (top of the wave crest) of a 1000 Hz overtone arrives at your ear from the (closer) left speaker before it does from the right one. There'll be an audible - and very measurable - change at that frequency (or harmonic overtone).

Should the distance be equivalent to a half-wavelength further (6 inches), then that particular overtone (harmonic) will arrive exactly out of phase. And you know how your stereo plays less bass when the speakers are out of phase? Well, the effect is exactly the same - a reduction in level at that particular frequency.

Avoid a large sweet spot

Why is this important? You've heard of voiceprints. That's where a recording of your voice can be used to positively identify you, no matter how hard you try to shift the sound of your voice.

How does it work? The unique relationships of vocal overtones are different for each voice. For example, the first harmonic may be 87.3% of the fundamental, the second just 48.1%, the third 54.7%, etc.

The exact relationship of these overtones (their relative strength, compared to the fundamental) is the identifying 'genetic code' of your voice. Well it turns out that all instruments and voices have their own particular set of harmonic ratios.

That's how we know to differentiate two different instruments that are playing exactly the same note - say A (440 Hz). And that's how an original Guarneri will be chosen over a 'replica' - it's all in the 'tone'- which is actually the harmonic - or overtone structure.

So, if you're sitting where the path lengths are significantly unequal from the left and right speakers, you are absolutely guaranteed to hear wild shifts in the harmonics, meaning that an instrument or voice will not sound exactly as it should. This is not just some subjective acoustic theory. It's not only audible; it's also measurable in your room at your listening seat!

The sad fact is, you've just altered your system's harmonic relationships. So why did you buy all that stuff with 'vanishingly low distortion' if you're going to introduce a far worse version by not sitting in the center point where the path lengths are equal? Incidentally, this is an incontrovertible law of physics that is part of the good - and the bad - of stereophony.

From a perfectionist's standpoint, it doesn't matter if your loudspeakers produce a smooth response off-axis. The varying wavelengths at a listening position off the acoustic center will always produce uneven response on centrally recorded images (actually all images, but it's easier to think about the centrally recorded image for the purposes of illustration).

Here's a simple test for you. Put on a Sheffield or other disc that contains pink noise in both channels (pink noise is best, because it contains equal energy per octave, just like music). If you can, put your preamp in mono.

Stereo or mono, what you want is equal amplitude in each channel. Now, from the center position, slowly move your head to the left or right. That huge change in mid/treble tonal balance is exactly what happens if you sit off axis.

And because the wavelengths vary according to frequency, the varying time arrivals of harmonics also produce an unpredictable cancellation effect (well, it is predictable in that it's never a good thing). And a 'wide sweet spot' isn't really so sweet...

Now that we've told it like it is, let's also admit to having absolutely wonderful experiences listening to music while others have occupied the best seat. If a system has dynamics, if it's effortless, if it at least starts out being pretty accurate timbrally, then it can be quite listenable off-axis. Just remember that the phantom image produced off-axis in stereo is only an approximation.

Sweet, it ain't!

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