Hi-Rez Highway: Re: The reality of figure of 8 microphones: by Jean-Marie Geijsen New high resolution SACD releases, players and technology.

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Posted by Jean-Marie Geijsen (P ) on July 27, 2005 at 13:17:25

In Reply to: WARNING! Sad to say I am very disappointed in the new Water Lily Acoustic SACD posted by Teresa on July 19, 2005 at 20:47:06:

Dear music lovers,

Just some additional info (or fuel) for this very interesting discussion.

The reality of figure of 8 microphones:

Figure of 8 microphones are pressure gradient microphones. Meaning, they are only sensitive to the pressure difference between the front and back of the membrane. For long wavelength, i.e. low frequencies, this pressure difference will be small. For higher frequencies, shorter wavelength, the pressure difference will be bigger. As a result you have a transducer with HP characteristics, a 6dB/oct filter. To compensate this HP behaviour, somewhere between 200 to 300 hz, there is a LP filter to achieve a “flat” response from that frequency upwards and to achieve a reasonable sensitivity of the mic. (the lower frequency of this LP filter, the lower the sensitivity will be). This microphone is a very nice EQ, about -15 dB @ 50Hz.

The frequency response varies with sound source distance. If the source is close, about 6 inches, you get reasonable bass. When the source is far away, 15 to 20 feet, there is very little bass. This is called the “proximity effect”.

The rear of the microphone will pick up the reflections and reverb of the hall in antiphase, not necessarily a problem.

Distortion of this type of microphone is in average 20dB higher compared to omnidirectional microphones.

Off axis response not ideal.

Extremely sensitive for contact noise and airflow.

This is why recordings done solely with fig of 8 mic’s also have their own “sound”.

This is microphone theory (and practice), free available on internet.

Jean-Marie Geijsen

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