Hi-Rez Highway: Re: The reality of figure of 8 microphones: by Jean-Marie Geijsen

Dear Mr. Greene,

1)â€œSpaced omni recordings present confusing and internally contradictory locational cues to the listener to the point that things are not exactly anywhere.â€ Yes you are right, the problem is that as long as we use two speakers, the same applies to our playback system. In other words, any recording done with spaced omnis, ORTF or crossed pairs, played back on two speakers located 10 feet apart, will suffer from these â€œcontradictory locational cuesâ€ due to our spaced loudspeaker set up and the fact that we can not prevent people moving around when they listen to music. See also point 3.

2)â€œthe least colored(or should I say coloured) vocal sound was that of the Coles ribbon(a figure 8).â€ I do not now how this microphone was set up in relation to the voice, but for voice recordings, the distance between microphone and source is rather small, let say around 30 cm. Compare this to a orchestral recording where the microphone distance to the source is often 5 meter or more. When manufacturers give specifications of the frequency response of their directional microphone, they always mention the measurement distance between sound source and microphone. Simply because the output of their microphone, for the low end, can vary up to 20 dB or more when the source to microphone distance varies from extremely close to 5 meters or more.

3)â€œplay a mono pink noise signal through two speakersâ€, I am happy that you mention this test. This is exactly one of the tests we use for setting up our loudspeaker set up. This mono signal tells you a lot about your loudspeaker set up and the ability of our stereo loudspeakers, spread 8 - 10 feet apart, to produce a non coloured phantom image. To hear this pink noise signal uncoloured you have to listen with one ear closed, and the other ear positioned exactly on the central axis. It will be very difficult to locate the origin of the sound but at least it is uncoloured. If you listen with both ears, you have to sit again exactly on the central axis and now you will hear a nice phantom image in the middle but it is already coloured due to the delayed arrival of the left speaker signal at you right ear, interfering with the non delayed signal from the right speaker, causing a comb filter. Needless to say that the same happens on the other ear. When you move side ways as you suggest, these comb filters will change frequency, and therefore the pink noise will be different coloured depending on your position. Now attenuate the signal on your right channel about 3 dB. As a result the image will shift to the left. Now move again and notice that you will hear the same comb filters as before, less extreme, but they are still there. What we also can hear is a sharp shift of the phantom image position to the left when you move left. This test tells us that our 2 loudspeaker set up is not capable of producing a non coloured, stable phantom image ANYWHERE between the loudspeakers when the signal is a mono signal. As long as we use two speakers for stereo, we will suffer from this unwanted artefact.
Ergo: we have to work with a stereo loudspeaker set up that is far from perfect.

Lets see what happens when we use different main microphone set ups in combination with our spaced loudspeaker set up.

We are listening to a recording made using a coincident microphone set up. The spatial image is completely build upon level differences so the left an right signals are identical (mono) signals. The only thing that varies from left to right is level. Now we have a sound source halfway to the right on stage, which is audible half way to the right on the speaker set up. Now move a little bit off centre to the left. Immediately the phantom image will move to the left. Furthermore, we will have a comb filter as we have seen in our test before. The fact that the comb filter frequency on the playback side is independent of the angle of incident of the sound on the main microphone system is a crucial observation. This comb filter is very strong with deep notches from only one (actually two, since we have two ears,) pronounced comb filter frequency and is the same for direct sound and diffuse sound on the recording. So for an orchestra, spread from left to right between the loudspeakers, the tonal balance of the orchestra and the acoustics of the hall will suffer equally from this single pronounced comb filter when not sitting exactly in the middle since a single comb filter is extremely audible.

Now we look at the spaced microphone set up. Imaging is mainly based on time differences. We start with the same source on stage, halfway to the right, and our phantom image half way to the right on the speaker set up. Sitting exactly in the middle, the direct sound will suffers from a comb filter due to the fact that there is a time difference between left and right. When we move a little bit off centre, the phantom image will also move, but less compared to the coincident set up. The comb filter will change frequency like before. But the comb filter frequency for the direct sound and the diffuse sound will change independent from each other. The reason for this is the fact that at the playback side, the comb filter, which is the result of moving position, is super imposed on the infinite number of diffuse sound comb filters created by the pick up of the spaced omni set up.
The time difference between the left and right signal varies with the angle of incident on the main system and therefore, on the playback side, there is not one single pronounced comb filter but a infinite number of different comb filters. So for the reverb of the hall, the infinite amount of different comb filters will effectively compensate each other and as a result we have a smooth flat frequency response for the diffuse sound field almost independent of the listening position. The comb filter on the playback side is â€œabsorbedâ€ by the infinite comb filters on the recording side. If we look at the orchestra positioned from extreme left to extreme right, the comb filters will also vary, dependant on the angle of incident. For the strings, which are large groups of people, this comb filter will almost not exist. For solo instruments like flute or trumpet, the comb filters will be more pronounced, but at the same time the comb filter notches will be filled by reflections, so at the end, the resulting comb filter is not as extreme as one could expect from theory.
So my conclusion is that although a spaced omni set up is maybe not perfect, it coops much better with the problems we encounter on the playback side. I prefer the less deep and less pronounced comb filters combined with a more stable (not sharp) image compared to the more instable and sensitivity to single frequency comb filters of the coincident techniques. I have tried many different main systems. 15 years ago I have done at least 25 recording with crossed fig of eight and/or ORTF systems before I decided to continue with spaced omniâ€™s.
My personal view is that since we can not prevent listeners moving their head when listening to music, we have to take the shortcomings of our loudspeaker set up in account when discussing microphone set ups.

4)â€œOmnis pick up diffuse field sound in abundance â€œ, yes, omniâ€™s do pick up a lot of diffuse sound compared to crossed fig. of eight when mounted at the same distance to a sound source. But the crossed fig. of eight are mostly positioned twice (1.7 to be exact) as far away from the sound source compared to omnis, so the direct to indirect (diffuse) sound ratio will be equal for both systems. The last time we did a recording in the Philharmonia in St. Peterburg we had our omniâ€™s 1.5 meter in front and 3.5 meters above the orchestra. During the recording Mr. Alexander did, the main system was 15 feet (5meter) in front of and 10 feet up, more then 3 times as far. So probably there was at least as much diffuse sound ,if not more, on this recording then we had.

I do appreciate your response very much, and I find this a very stimulating discussion. So please, if you find my writing offensive, please remember that I am not English.

Best regards,

Jean-Marie Geijsen