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Old 19th October 2012, 10:43 PM   #551
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Originally Posted by gedlee View Post
As far as the Greisinger comments go, I have trouble with the fact that a 100 Hz tone has a ten foot wavelength and we are supposed to somehow believe that a reasonable "gradient" can be achived between the ears? At 20 Hz the wavelength is 50 feet. Come on, the signal at the two ears at these frequencies has to be extremely similar if not identical. The correlation has to be nearly 1.0.
Don't be so quick to dismiss gradients between both ears at bass frequencies. At 20Hz ? No. At 100Hz ? Yes definitely, I've seen the effect as low as 60-70Hz in some rooms.

Although wavelengths are long, if there are strong lateral standing waves present notches at certain frequencies due to destructive interference can be very deep and very localised spatially.

In one of my previous rooms there was a spot almost exactly at the listening position where 65Hz was almost perfectly notched out, at least for one ear at a time. Moving my head sideways would put one ear or the other directly in the notch, allowing the opposite ear to hear something, a very disconcerting effect much like listening to bass in one ear with headphones, with a phase reversal between the ears as the centre of the head crosses the cancellation point.

When phase between the direct path and reflection are almost exactly 180 degrees and the levels are very close to equal the change in amplitude with small phase shifts (physical displacement) is very rapid, many orders of magnitude more rapid than if they were summing in phase. It's easy to do the calculations based on summing sine waves at different phase angles to see this.

With normal in phase mono bass this effect will generally only happen if there are serious standing wave problems in the room, so bass quality overall is going to be poor anyway... but there is some music that puts the bass either in only one channel or in deliberate reverse phase between left and right (often with some levels mismatch on purpose to prevent total cancellation) and in these circumstances a bass gradient can happen between the ears especially if you're near equidistant from the speakers.

Again, the effect is quite disconcerting and I don't like it, but it does have an interesting effect on localisation.

Now is this effect true "localisation" or is it just an artefact of standing waves and destructive interference fooling the ears by causing a localised imbalance of bass ? The latter I think, as the exact perceived effect is highly dependant on the room standing wave structure, the precise listener position and so on, so while an interesting gimmick its not a reliable or reproducible way of introducing a sense of localisation at low frequencies. Certainly it is not reproducing any localisation captured during the recording process, and is therefore just a reproduction artefact.

A good multi-sub setup is going to almost completely eliminate any type of low frequency localisation by minimising standing wave issues and therefore steep gradients, and if its a mono bass setup it will eliminate reverse phase bass effects as well. (I'm not convinced however that a multi-sub setup should necessarily be configured for mono bass reproduction)
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Old 19th October 2012, 11:07 PM   #552
KSTR is offline KSTR  Germany
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Simon,

that's basically what I've been saying in #518.

Especially when using close-field woofers/subs where 1/r level differences are still effective**) it is possible to get almost headphone-like L/R-seperation down low, with the appropriate signals coded in the source material.

**) and dipoles are best for this because there are two 1/r mechanisms

Last edited by KSTR; 19th October 2012 at 11:10 PM.
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Old 19th October 2012, 11:20 PM   #553
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Simon,

that's basically what I've been saying in #518.

Especially when using close-field woofers/subs where 1/r level differences are still effective**) it is possible to get almost headphone-like L/R-seperation down low, with the appropriate signals coded in the source material.

**) and dipoles are best for this because there are two 1/r mechanisms
Sorry, I've only been following the thread with one eye, so I missed post #518.

There's no question that imbalance in bass between left and right ears will cause a sense of left-right localisation (one need only try it with headphones to prove that) the question has only ever been whether a sufficient gradient between the ears can occur without headphones so the effect can manifest in non-headphone listening.

In free space with a single source - no. In rooms with complex standing wave patterns and boundary cancellation effects, yes, at certain frequencies and in certain circumstances it can occur, but usually not in any predictable way that could be exploited by a recording engineer.

I had the same debate with a certain well known member of rec.audio.tech about 10 years ago, he also didn't believe that a sufficient gradient was possible between the ears at bass frequencies even when listening near points of cancellation. (Simple calculations and empirical testing prove otherwise however)
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Old 20th October 2012, 01:48 AM   #554
gedlee is offline gedlee  United States
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DBMandrake

You are talking about psychoacoustic "tricks", not about good LF design for sound reproduction. Unless what you are talking about works broadband, I would not consider it a viable technique for sound reproduction. Everything that I am saying is broadband.
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Old 20th October 2012, 02:01 PM   #555
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Thanks to Markus and John for their good work and posting the graphs

These graphs provides ample objective evidence to the subjective perceptions of many favoring dipoles and the comments I had posted in #458 :
"Even with the same diameter drivers, equalised to same LF extension, listened at moderate SPL, to even nearfield (2-3 Meters) there is that slight overhang in sealed units that makes it sound like a "speaker" which is not apparent in a dipole(H or Uframe) making it sound that last bit different ... or "natural'."

It is great to know that 3 independent experimenters (including Elias) using 3 different measuring methods have shown readings which show that a dipole as opposed to a monopole, has a better transient response and smoother frequency response both in nearfield and farfield listening.

It might be that multiple equalised monopoles as proposed by Dr. Geddes is the best "cost no object" way to get good bass but for an average room for stereo speakers, dipoles are as good if not better and I would agree with Markus in post #539

"I believe it's THE solution for everybody that wants really good bass and doesn't have a dedicated listening room with proper acoustic treatments. It's also limited to a few seats within one row but is stereo/multichannel anything more than a one seat solution?"
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Old 20th October 2012, 02:18 PM   #556
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nearfield (2-3 Meters)
2-3m is NOT near field. Near field subs have to be placed within about 2ft (60cm).
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Old 20th October 2012, 02:30 PM   #557
Rudolf is offline Rudolf  Germany
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Originally Posted by soundaatma View Post
These graphs provides ample objective evidence to the subjective perceptions of many favoring dipoles and the comments I had posted in #458
Ample? Objective?
I think that these measurements are just some building blocks, but by no means a "house of wisdom". If pictures look to ones eyes like what he hears with his ears, we are talking about religion, but not about science. We need to know the exact physical reasons, why and from what those perceptional differences arise. Before that is achieved we are just guessing imho.

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Old 20th October 2012, 02:50 PM   #558
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I beginning to think that the "exact physical reason" lies between the ear lobes.
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Old 20th October 2012, 02:58 PM   #559
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#551

This is way off topic; both monopole and dipole speakers are capable of setting up LF standing waves in appropriate sized rooms.

But perhaps some clarification:

"....gradients between both ears...." and "minimizing standing wave issues and therefore steep gradients....." is confusing language when describing ears as straddling a standing wave nodal point. Particle velocity is zero, and smoothly increases to maximum at antinode 1/4 wavelength away. For 65Hz this is roughly 132cm.

Hair cells appear to have a zero crossing function (and as I recall it is for negative going pressure at eardrum), and neighboring hair cells thus tend to phase lock. This behavior is exhibited below about 600Hz.

With ears straddling the node, particle velocities are 180 degrees out of phase, and so are hair cell firings of left and right ears.

With one ear effectively placed at node, it receives no signal while the other ear does.

In both cases a localization cue is generated.

For typical stereo speaker placement in typical room, generation of the requisite lateral standing wave formation seems highly unlikely.

#555:

Quote:
It is great to know that 3 independent experimenters (including Elias) using 3 different measuring methods have shown readings which show that a dipole as opposed to a monopole, has a better transient response and smoother frequency response both in nearfield and farfield listening.
What are you missing from my post #444? Both frequency response and transient responses may be equalized to be the same for monopole, dipole, and cardioid radiators.

If "there is that slight overhang in sealed units" it means they haven't been correctly equalized.

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Old 20th October 2012, 04:47 PM   #560
gedlee is offline gedlee  United States
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BarleyWater

You make very good points, unfortunately the tight coupling between the frequency response and the impulse response is not always appreciated by many.

I had some doubts about your #444 post at first, but then after I figured out what you had actually done, it became clear. I think that what got missed by many (even myself at first) is that you EQ'd for a linear phase band-pass system and not a minimum phase system. You could have done either correct? Or better said, either is possible. Had you done the minimum phase we would have seen the kinds of impulse responses that we are used to seeing.

You are completly correct about the zero crossing firings below 600 Hz, which is precisely why the ear resolution falls below this frequency - fewer and fewer firings occur in a given time resulting is less and less information to the brain for it to process. This is also why we see our hearing sensitivity falls below 600 Hz.

I completely agree that "For typical stereo speaker placement in typical room, generation of the requisite lateral standing wave formation seems highly unlikely."
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