Psychoacoustic considerations for my OB project - diyAudio
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Old 29th August 2010, 02:35 PM   #1
wowo101 is offline wowo101  Germany
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Default Psychoacoustic considerations for my OB project

After having settled for the drivers and the basic construction (H-baffle with 2 Peerless SLS-10, naked TB-1320 and Neo3), I am now considering what should into go into the equalization and positioning of my first OB speakers.

Especially two points got my attention:

First, the approach of Dr. Geddes (p. 7 and 9) et al. to broaden the stereo sweet spot by compensating the distance induced SPL loss from the far speaker by an angle induced SPL loss from the near speaker; second, SL´s recent Orion voicing modifications that supposedly improve imaging and perceived tonal balance and that he traces back to our hearing sensibility varying with frequency and listening angle.

As to the first point:

It is usually said that SPL drops with 6dB per double distance. This drop could be accomodated by positioning the speakers so that the distance from the far speaker is twice that from the near speaker, while the listening angle of the near speaker is 60°, which also produces a 6dB drop in SPL due to dipole directivity. This should roughly give equal SPL from both speakers across a wide sweet spot (while, of course, it doesn´t account for timing differences).

In Floyd Toole´s "Sound reproduction", though, I found a compilation of in-room-measurements of omnidirectional sources as well as of speakers with different directivity characteristics that strongly suggest an SPL drop of 3dB per double distance (fig. 4.13, p. 60). I guess this is due to diffuse or reverberant sound contributing to the measured SPL.

So my questions here are: Do you think my outlined approach is gerenally valid? And if so, which of the numbers for SPL loss should be the starting point?

As to the second point: SL argues that in higher frequency ranges we are more sensitive to sound coming from angles around 30° than to sound coming from the front, making harmonics and overtones sound too bright compared to their fundamentals, which in turn not only leads to a perceivedly unbalanced sound but also to compromised imaging because our brain gets incoherent clues for soundstage reconstruction. (Or at least this is what I understood so far.)

My question here ist this: I found plenty of research on the topic of hearing sensitivity varying with listening angle, still SL seems to be the first to draw these conclusions concerning stereo imaging. So I´m asking myself if his reasoning here ist correct, or if the perceived imbalance of the "old" Orion is really due to dipole blooming in the relevant frequency regions that leads to imbalances in total radiated sound power which in turn could be responsible for the noted effects. What do you think?

Thanks in advance for your feedback & input!
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Old 29th August 2010, 03:40 PM   #2
Rudolf is offline Rudolf  Germany
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wowo,

one directivity is not equal to all others. Note that with Earl's waveguide SP levels fall quite constantly with increasing off-axis angles. This is not the case with dipoles, where SPL tends to fall off mostly above 45°. While your considerations may be true at 60°, they don't equally apply for 30°. With a true dipole you simply won't get a sweet spot as wide as with a OS WG.

Just toe in the baffles so that the driver axes meet well in front of you and hope for the best.

About Linkwitz: I don't buy his argument about the 30° sensitivity. I rather believe that it is a blooming of his dipole pattern, caused by the overly wide baffle, which makes his reflected sound top-heavy. We would need to see some polar response measurement of the Orion++ to verify this, but I don't know of any.
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Old 29th August 2010, 04:58 PM   #3
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Quote:
Originally Posted by Rudolf View Post

About Linkwitz: I don't buy his argument about the 30° sensitivity. I rather believe that it is a blooming of his dipole pattern, caused by the overly wide baffle, which makes his reflected sound top-heavy. We would need to see some polar response measurement of the Orion++ to verify this, but I don't know of any.
I think this may be a good time for me to chime in on this. I tend to agree with you about the blooming. As you may be aware I am working on my new speaker, the NaO Note, which maintains fairly good dipole directivity to about 8 kHz before the pattern stars to narrow at higher frequency. When I listened to the Note in comparison to my older design, the NaO II, I noticed that it sounded different to a greater extent than I imagined. The NaO II has an ESL quality to it. Like the Orion, the radiation pattern blooms to the sides above the crossover point due to the wider dispersion of the conventional dome tweeter. The Note sounds more natural to me. I found myself wanting to understand what the source of the difference was. The Note has exceptionally flat on axis response. The NaO II has a little more irregularity above 4k Hz. I thought that perhaps this was the cause.

What I did was to use some DSP software from SoundEasy and corrected the response of both speakers to ruler flat on axis with matched levels and listened. The difference was still present even though the direct sound was identical. The conclusion can only be that the difference is due to the difference in radiation patterns of the NaO II and the NaO Note above 2k Hz or so, and particularly in the 2+ to 8 k Hz range. If fact I can make the Note sound like the NaO II by boosting the high frequency response of the Note, or I can make the NaO II sound more like the Note by cutting the NaO II tweeter level. Thus the difference in sound seems to be more associated with the total radiated power above the mid to tweeter crossover than with anything related to the direct sound. The NaO II is similar to the Orion, dipole mid with conventional tweeter, but the crossover point is higher in the NaO II, which was initially intended to help counter the high frequency blooming.

Having the NaO Note alongside the NaO II and being able to able to digitally correct the axial response of both systems to be identical has show how significant variation is the polar response is on the sound.

The polar plots of the Note can be found here. The polar response of the NaO II here.
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Old 29th August 2010, 05:36 PM   #4
wowo101 is offline wowo101  Germany
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Thanks, Rudolf und John, for the quick responses!

Of course Rudolf is right on the first point - I didn´t take into account that dipole SPL doesn´t fall off constantly with increasing angle...

Concerning the Orion issue: I´m more than glad that my reasoning is in line with that of people far more knowledgeable than me. It seems my learning efforts in things dipole and OB start to pay off after all. (Thanks to you both, I have to say!)

I remember seeing polar response plots of the Orion somewhere around here, although their validity was heavily questioned...
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Old 29th August 2010, 06:20 PM   #5
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Originally Posted by wowo101 View Post
Thanks, Rudolf und John, for the quick responses!

Of course Rudolf is right on the first point - I didn´t take into account that dipole SPL doesn´t fall off constantly with increasing angle...

Concerning the Orion issue: I´m more than glad that my reasoning is in line with that of people far more knowledgeable than me. It seems my learning efforts in things dipole and OB start to pay off after all. (Thanks to you both, I have to say!)

I remember seeing polar response plots of the Orion somewhere around here, although their validity was heavily questioned...
Earl Geddes had presented an application with some plots indicative of the polar response. There was some question about the on axis response which he corrected. However, it would be safe to say that the Orion polar plots woudl look much like the NaO II when normalized by the on axis response. Since the Orion and the NaO II use the same tweeter the dispersion is going to be very similar. On thing different about the NaO II and the Orion is the NaO II uses a narrower baffle.

I would also point out that the NaO II has always had a tweeter cut level control. I tend to like it with the tweeter cut a little. Additionally in the last update I made the speaker, (several years ago now) the cap, C3 in the tweeter network was split into a cap in parallel with "a cap in series with a resistor". This was done based on listening and truns out to cut the tweeter response in the 2 to 8k range.

Click the image to open in full size.
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Old 30th August 2010, 09:41 PM   #6
cuibono is offline cuibono  United States
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Quote:
Originally Posted by Rudolf View Post
wowo,

I rather believe that it is a blooming of his dipole pattern, caused by the overly wide baffle, which makes his reflected sound top-heavy.
I agree here too - I don't know why SL offered the explanation he did, because it should have been very obvious to him (and other with dipole measurement experience) that between 1-4kHz, the Orions have irregular polar patterns. No wonder they need to be tweaked by ear in this region. I was surprised right from when he posted his update, because he has been one of the biggest proponents of the importance of the off-axis response.



Quote:
Originally Posted by john k... View Post
Thus the difference in sound seems to be more associated with the total radiated power
Brilliant work John. For a while now, I've suspected that the total radiate power may actually have a bigger impact on the sound than the direct ("flat") response.

Now, back to the OP! Wowo, first I don't have any experience with WG based speakers, so I can't comment on the 'size' of the sweet spot compared to other types of speakers, for instance dipoles. But I can tell you that my current dipole speaker, which is similar to a number of other designs on this website, sounds as good, or better, outside the 'sweet spot'. Like Rudolf suggested, I actually prefer a seat several feet behind the traditional sweet spot location - but I don't toe my speakers in either. That spot seems to have the most natural balance between direct and reverberant sound. I find that the tonality of the music does not change no matter where I am in the room, and that is due to having a total linear response from the speaker itself.

Something to consider though - compensating for SPL losses looks difficult in a reverberant environment (with different things coming from each speaker), but timing dominates localization, in my book. No stereo speaker system can compensate for the time differences due to being closer to one speaker than another. So not matter what, if you are out of the sweet spot, the soundstage will become distorted. My impression, from what I've heard, is that if you want a rock solid soundstage, go with a 5.1 or 7.1 system. The one time I heard a nice surround system, it really kicked stereo's *** in terms of soundstage stability.

Somewhere on the internet, there is an interesting experiment with the localization of sound (well known in psychoacoustics, I think). The test signal is a stereo recording, with one channel much lounder than the other - but the quieter signal starts a few mS before the louder. When you listen to the recording, you always localize the sound as coming from the quieter speaker. Hmmm....
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