With open baffle speakers (dipoles) I find that although the same direction colorations are audible to some extent (the subjective quality of which is very dependent on distance from the front wall), I also find, and this is perhaps the main reason I decided to build OB; the embedded reverbs in the program content took on a 3-D nature. The sense of being in a different place became significantly more believable. Since the horizontal off axis response of a dipole drops pretty far at their sides, there would also be less lateral wall reflections (fake spaciousness), which would limit spaciousness to what's in the recording, which can be great if the recording is good.
I also use my own brew of the Carver Holographic Generator to reduce the interaural crosstalk, and now the spaciousness is such that I've been fooled into thinking my side speakers were turned on when they weren't. My side speakers are at the ceiling, aimed directly across at each other, about half way back from the front wall of my living room. I usually only turn them on for DVD's or TV.
A couple of ETC (energy time curves) from a Peter D'Antonio paper. He pushes the RPG diffusors.
In both cases the top curve is for the untreated room and the bottom curve is with a considerable number of diffusors added at the locations of the primary bounces.
Each diffusor will replace a single strong specular reflection with a wider and lower spread of multiple reflections. You can use the technique to try and get your -20 dB reflection levels without killing off all of the loudspeaker's energy.
I'm not sure why the dipole shows a strong sidewall reflection as I would have thought that the null would be in that direction (these were big Apogee ribbons). The large rear wall bounce is as expected. It is essentially the same strength as the direct sound.
In the end, any radiation pattern can probably be modified to acceptable performance with enough room treatment, but how many enthusiasts do so??
David S.
In both cases the top curve is for the untreated room and the bottom curve is with a considerable number of diffusors added at the locations of the primary bounces.
Each diffusor will replace a single strong specular reflection with a wider and lower spread of multiple reflections. You can use the technique to try and get your -20 dB reflection levels without killing off all of the loudspeaker's energy.
I'm not sure why the dipole shows a strong sidewall reflection as I would have thought that the null would be in that direction (these were big Apogee ribbons). The large rear wall bounce is as expected. It is essentially the same strength as the direct sound.
In the end, any radiation pattern can probably be modified to acceptable performance with enough room treatment, but how many enthusiasts do so??
David S.
It could be argued that once you leave the anechoic chamber, more room reflections are better than just a few. It seemed counter-intuitive to me at first. Each reflection creates cancellations at your ear when it adds to the direct signal. Since most other reflections cause cancellations at different frequencies, the energies tend to fill in each others cancellations. The more different reflections, the more any cancellations are likely to get filled in, giving you a more flat frequency response at the listening position. I think this is also why tweeter arrays (3 and above closely spaced), even woofer arrays, and wideband vertical line arrays have an advantage. The pink noise variations and boomyness in the bass should be noticeably less.
Many AV receivers have the option to decode 2 channel stereo into L+C+R channels, do you think this would work well for a wide L-R separation + centre channel on music, or would the source material need to be discrete 3 channels ?
Although I've had AV receivers that could do this before I've never bothered to set up a proper centre channel (that accurately matches L and R) to see whether it would be effective.
The decoding is certainly able to accurately extract dead centre in phase signals (removing them from L & R completely) and leave full left and right signals unaltered, but I wonder how well it would do with sounds that are partially panned or only partially correlated.
Mind you even if you had 3 channel recordings on 3 channel media you still have the same problem, just pushed back into the hands of the mixing engineer...
Hi Oliver,
Let me take an advantage of your provoking question, and give my provoking(?) answer
In the treble range the signal phase has no meaning. At high freqs perception does not happen in the signal domain, but in the modulation domain. There is the valid concept of envelope ITD that is present but the extracted information follows the modulation. Natural sounds, like speech and music, are relatively slow processes. Here for example the AM modulation analysis based on a psychoacoustic model of the most important sound for a man: female speech
The modulation bandwidth in this case is less than 20Hz. One could say only in every 50ms there will be new information present.
The carrier signal at 2kHz has 100 cycles within 50ms. According to my knowledge perception system cannot follow the phase of each carrier cycles at 2kHz (and above) because of the low pass character of the modulation transfer function of the acoustic nerve fibers cut off around at 500Hz (for 2kHz carrier) already. The modulation transfer function cut off seems to follow constant Q characteristics (from physiology studies).
- Elias
Let me take an advantage of your provoking question, and give my provoking(?) answer
In the treble range the signal phase has no meaning. At high freqs perception does not happen in the signal domain, but in the modulation domain. There is the valid concept of envelope ITD that is present but the extracted information follows the modulation. Natural sounds, like speech and music, are relatively slow processes. Here for example the AM modulation analysis based on a psychoacoustic model of the most important sound for a man: female speech
An externally hosted image should be here but it was not working when we last tested it.
The modulation bandwidth in this case is less than 20Hz. One could say only in every 50ms there will be new information present.
The carrier signal at 2kHz has 100 cycles within 50ms. According to my knowledge perception system cannot follow the phase of each carrier cycles at 2kHz (and above) because of the low pass character of the modulation transfer function of the acoustic nerve fibers cut off around at 500Hz (for 2kHz carrier) already. The modulation transfer function cut off seems to follow constant Q characteristics (from physiology studies).
- Elias
In the discussion about in-head localization in anechoic chambers, Graaf put forth this link:
Sound reproduction: loudspeakers and ... - Google Books
, which is basically an excerpt from Toole's book. I quote, "Perceptions of sounds originating in the head, which routinely occur in headphone listening, can also occur in loudspeaker listening, when the direct sound is not supported by the right amount and kind of reflected sound." Toole goes on to describe his experience of listening to stereo in an anechoic chamber, where the image did form in the head.
This just illustrates my earlier point that not all reflections are bad. Some are necessary for proper localization of the sound source. Loudspeakers with narrow dispersion have the central phantom image form way in front of the speakers and that is mainly due to the brain not receiving enough reflected cues to determine the proper location of the phantom image. Here is what Toole has to say, "... it (in-head localization) can happen in a normal room with loudspeakers that have high directivity or in any situation where the direct sound is heard without appropriate reflections."
Sound reproduction: loudspeakers and ... - Google Books
, which is basically an excerpt from Toole's book. I quote, "Perceptions of sounds originating in the head, which routinely occur in headphone listening, can also occur in loudspeaker listening, when the direct sound is not supported by the right amount and kind of reflected sound." Toole goes on to describe his experience of listening to stereo in an anechoic chamber, where the image did form in the head.
This just illustrates my earlier point that not all reflections are bad. Some are necessary for proper localization of the sound source. Loudspeakers with narrow dispersion have the central phantom image form way in front of the speakers and that is mainly due to the brain not receiving enough reflected cues to determine the proper location of the phantom image. Here is what Toole has to say, "... it (in-head localization) can happen in a normal room with loudspeakers that have high directivity or in any situation where the direct sound is heard without appropriate reflections."
Hi,
A good one to start on human sound perception in modulation domain:
http://physrev.physiology.org/content/84/2/541.full.pdf
- Elias
A good one to start on human sound perception in modulation domain:
http://physrev.physiology.org/content/84/2/541.full.pdf
- Elias
To get back to the original point of "What is the ideal directivity pattern for stereo speakers?" and where Toole has already given his insight (quoted) in it's "naked ugliness" about stereophonic cross talk, which I consider one reason besides pinna localisation for the cause of another stereophonic artefact namely pin point imaging, or pin point imaging of the two tweeters. Just now once again I disappointedly listen to a stereo triangle in my living room, and instead of perceiving a high freq phantom image between I hear two tweeters making sound at +/-30 lateral degrees. Even I block the half frontal hemisphere with thick absorbing pillow i.e. only one ear sees one speaker, now I perceive one tweeter playing but image is less pin point.
There are two cures to this stereophonic artefact that I have observd succesfull:
1) block the direct sound (see my experiments on stereolith thread)
or
2) use a very wide directivity tweeter to spatially homogenise high freq cues.
So to answer the question:
What is the ideal directivity pattern for stereo speakers?
My answer is two folded:
Ideal directivity pattern above 1kHz is such that left speaker illuminates the left wall only, and vise versa for the right speaker. Direct sound should not be present. To maximise the illumination side firing center speaker could be used.
Below 1kHz, where most of the music signal information is located in terms of modulation, very high directivity is beneficial to maximise information transfer through the room. Dipole line array maybe.
- Elias
There are two cures to this stereophonic artefact that I have observd succesfull:
1) block the direct sound (see my experiments on stereolith thread)
or
2) use a very wide directivity tweeter to spatially homogenise high freq cues.
So to answer the question:
What is the ideal directivity pattern for stereo speakers?
My answer is two folded:
Ideal directivity pattern above 1kHz is such that left speaker illuminates the left wall only, and vise versa for the right speaker. Direct sound should not be present. To maximise the illumination side firing center speaker could be used.
Below 1kHz, where most of the music signal information is located in terms of modulation, very high directivity is beneficial to maximise information transfer through the room. Dipole line array maybe.
- Elias
I don't have this problem, and there is nothing exotic with my setup.
OTOH it seems to be normal for high frequencies to be mixed left and right. Maybe the idea is to create space in the middle for the lead instruments or vocals?
Suggest you try carefully panning a high frequency signal from left to right channels. Does it 'pop' from 100% left to 100% right? Mine doesn't and I doubt if yours will either, unless there is something wrong.
This is not an implementation thread, as far as i understood ...
Typical bending wave planars tend to exhibit "chaotic bipole" behaviour, gross
radiation pattern is not far from omni.
I would opt for restricion of the gross pattern from kardoid- to dipole like.
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We are drifting well off from the topic.
In the end the first question is "what reflection pattern do you want your home system to create?" This means what secondary arrivals do we want, from what direction, at what strength and what delay? If you can answer that then we can speculate about typical rooms and see if a realistic polar pattern can achieve this reflection pattern. The other view of the objective is "what balance between direct and reflected?" That is, are we in the direct field, or at a distance where direct and reflected are equal or perhaps well into the reverberent field?
These are the micro and macro views of the subject. Even if we speculate that a certain reflection pattern is ideal we may not be able to achieve it within the dimensions of our room. If it turns out we can achieve it then likely a small variety of speaker room combinations might achieve it equally (deadish room and wide dispersion, versus lively room and directional speaker).
Viewing the matter in any way other than this is hard to justify. How can you discuss a speaker dispersion pattern without considering the total response it will create in the typical room? This is the only path that gets you back to that which we can test in psychoacoutics: our perception of various reflections that arrive from different directions and different delays and different strengths.
So what ETC (energy time curve) are we looking for?
David S.
In the end the first question is "what reflection pattern do you want your home system to create?" This means what secondary arrivals do we want, from what direction, at what strength and what delay? If you can answer that then we can speculate about typical rooms and see if a realistic polar pattern can achieve this reflection pattern. The other view of the objective is "what balance between direct and reflected?" That is, are we in the direct field, or at a distance where direct and reflected are equal or perhaps well into the reverberent field?
These are the micro and macro views of the subject. Even if we speculate that a certain reflection pattern is ideal we may not be able to achieve it within the dimensions of our room. If it turns out we can achieve it then likely a small variety of speaker room combinations might achieve it equally (deadish room and wide dispersion, versus lively room and directional speaker).
Viewing the matter in any way other than this is hard to justify. How can you discuss a speaker dispersion pattern without considering the total response it will create in the typical room? This is the only path that gets you back to that which we can test in psychoacoutics: our perception of various reflections that arrive from different directions and different delays and different strengths.
So what ETC (energy time curve) are we looking for?
David S.
Listening in the direct field i'd like to have
an initial time delay gap af say >10ms after
the direct sound.
Floor, ceiling, near side wall reflections of low
intensity are tolerated.
The time delay gap is ended by a smooth (non sparse)
onset of the reverb continuously rising to a max intensity
and a monotone decay follows.
Increasing the speaker/listener distance the initial time
delay shrinks continuously, but without the gross shape
of the ETC changing too much.
Somewhat verbose, but that is how i imagine a prototypical
ETC should be in most likely living rooms that particular
"well behaved" speaker will be placed in.
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Definitely synergy, or even they need to be considered as a combined system. Dead room with wider dispersion and live room with narrow dispersion can certainly have comparable direct to reflected ratios. In fact of you look at the range of d.i. that you typically see in available speakers you can easily have the same direct to reflected ratio from various system if you allow modest variation in listening distance.
For example a wide dispersion system might have an average d.i. of 6dB. A fairly narrow system (say a 90 x 40 CD horn) might have a d.i. of 12 dB. These numbers would be for the upper range only, from 500 or 1000Hz up. Below that range they would tend to have the same d.i. In this case, doubling the distance from listener to the CD horn system would give the same direct to reflected ratio as the wide dispersion system.
Who can proclaim that one is universally more desirable than the other, at least without talking about the room and listening distance?
David S.
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