ACK Attack is BACK!!

ACK Attack is BACK!!

Written by Jim Smith

ACK Attack for serious listeners, part one –

[You may want to refer to Jim’s original piece on the ACK Attack, in issue #12 of Copper—Ed.]

 Why you should be sour on a wide “sweetspot”

Hey, we might as well get this topic kicked off with a bang…A “wide sweet spot” is almost like having your own harmonic distortion generator! With stereo sound, 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, especially obvious with 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 an 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 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 of stereo comb filtering (what I’m euphemistically calling harmonic distortion), let’s look at what’s happened to our stereo imaging…

OK, let’s say that, by sitting to the left of center, you’re now about two feet 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 microphones seems to “stay put” a little better. Bu 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 an 1100 Hz overtone arrives at your ear from the (closer) left speaker before it does from the right one. Due to phase cancellation, 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.

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 disguise your voice.

How does it work?

The unique relationships of vocal overtones are different for each voice. For example, the first overtone (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. 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—let’s say A (440 Hz). It’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 unwanted 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 shockingly 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. It has no bearing whatsoever upon sitting off -center in the concert hall, because the sound is not being reproduced from a pair of widely spaced loudspeakers which are subject to severe comb-filtering due to varying time arrivals at your head.

From a perfectionist’s standpoint, it doesn’t matter if your loudspeakers produce a smooth response off -axis. Even in an acoustically dead room, the varying wavelengths from two loudspeakers received 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 disk 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.

Whether stereo or mono, what you want is equal amplitude in each channel. Now, as the cut of pink noise is playing, while sitting in the center (equidistant to the speakers) position, slowly move your head to the left or right. That huge change in mid/treble tonal balance is exactly what happens when 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 I’ve told it like it is, I’ll also admit to having absolutely wonderful experiences listening to music while others have occupied the best seat. If a system has dynamics, presence & tone – if it’s effortless, if it at least starts out being pretty accurate tonally, then it can be quite listenable off -axis. I have thought that this technically incorrect listening experience can be so musically involving because – like live music – it’s a shared experience.

Just remember that the phantom image produced off-axis in stereo is only an approximation.

Sweet, it isn’t!

How to create a wider listening area – NOT a wide sweet spot…

When a wider listening area is desired, a trick the British introduced years ago may be useful. It’s called Intensity Stereophony (a big name for such a simple idea).

Basically, the idea is to crossfire the speakers in front of your seat, as opposed to a “normal installation,” where you might normally toe them in behind you. In other words, if you expected to have the speakers crossfire two feet behind your head, in this case, it might be two feet in front.

This results in a left side listener hearing increasingly more of the further right speaker’s direct radiation and less of the nearer left speaker’s. And vice versa for the right side listener. The practical effect is that images tend to be a bit more stationary for off-axis listeners. In general this technique works most effectively when the loudspeakers’ off-axis response is smoother than average.

Additionally, you may need to introduce more loudspeaker separation, to avoid the image “bunching up in the center.”

However, sitting off-center still exhibits the same time/phase frequency response roughness we discussed above. The “phantom center” image is simply wider and more forgiving for off -center listeners.

Finally, it must be noted that line source loudspeakers get progressively less loud as you get closer to them, so sometimes a line source speaker will help to provide a larger listening area, especially with Intensity Stereophony.

Just don’t think of it as a wide sweet spot…Above copy excerpted and edited from Get Better Sound.
You can read Jim’s work at his website.   

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