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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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In Reply to: Re: Rear-firing tweeters : gimmick or real improvement ? posted by David Yost on November 11, 2006 at 08:05:22:
In some ways my method is similar but there are some important differences too. It starts with the same basic conclusions and assumptions but its method is different. For one thing, the impulse method of measuring concert halls IMO is not adequate. My method starts with an array of unidirectional micorphones. The test sound source is a loudspeaker array which is excited at one single steady state sine wave tone at a time. When the microphones in the test array reach steady state, the output to the speaker array is suddenly shut off. If the microphone output is A(t) then -d[A(t)]/dt is the contribution each echo makes to the total reverberant field at that frequency. By repeating the test over the full audio spectrum one frequency at a time, you build up the spectral transfer function profile of each echo. Because the microphones are unidirectional, each one gives only the one vector component. It is important to know that the reverberant field is composed of vector components as is the direct field. The lack of taking this into account is why binaural recording and reproduction doesn't work and the full appreciation of it is why conventional surround sound systems can't work also. The total transfer function is the triple integral SSS B(t)dsdfdt for a specific directed coordinate pair of source and listener where s is a surface element around each microphone through which sound flux passes. It is only necessary to reproduce two such transfer functions, one for each stereophonic channel to reconstruct the total field as each channel is a loci of sound. The difference between the left and right channels at the source being recorded on the opposite channel is not a problem because sounds from stage left will arrive at stage right (and visa versa) a short time later and slightly softer and can and should be processed in exactly the same manner as sounds originating from stage right. Therefore microphone placement during the recording is not an issue as long as they are reasonably close to the musicians. The minimum criteria for the loudspeakers reproducing the reverberant field is that a) they must reproduce a vector field and b) their location cannot be detected by ear. to achieve this a minimum of 4 reverberant channels are necessary and a sufficient number of loudspeakers required so that at any point suitable for listening, the vector component from any one loudspeaker must not be as loud as the aggregate of the components from all of the others combined. One technique to achieve this is to have many loudspeakers (my current arrangement has 16 around the perimeter of the room), another is to aim the speakers indirectly at the listener so that the field from each one is more diffuse when it arrives at the listener. The use of many panel type arrays will work but is totally unfeasible in most cases due to obvious practical limitations. This simulates the very low vector gradient as a function of angle in an acceptable concert hall. The ambient speakers must also generate a field whose output reaching the listener with an unprocessed signal has the same or very similar FR as the main speakers.In over 30 years of experience with this type of sound system, a number of things have become apparant. All of the channels must be carefully equalized. Due to differences in the spectral balance of recordings, it is necessary to be able to equalize each one separately. There is a correct loudness for each simulation and a correct relationship between the direct and reverberant fields. There are optimal simulation settings for each particular type of music and individual recordings must also be compensated for within each type. This is due to the observation that there are actually three transfer functions operative. First there is the one programmed into electronics for the optimal simulation program for the type of music selected. Secondly there is the one imposed by the listening room itself. This is both a hinderance (it gives clues to the size of the listening rooom by the reverberant field it creates from the main speakers) but a factor which can be exploited at the same time. Third, there is the transfer function on each recording itself. Some are more reverberant and some less, the degree varies considerably but there are none which are compltely devoid of early reflections. This has several advantages including eliminating the necessity of resynthesizing every last echo and the fact that longer initial delays than 40 ms can be applied to the signal due to the inherent echoes reproduced by the main speakers which fills the time gap. Due to the number of variables to be adjusted for and the fact that there are no standards for making recordings, at this time the only way to satisfactorily adjust such a system is from memory of actual listening experience. It is therefore well beyond the capability of virtually all except a tiny percentage of audiophiles. The only hope to apply this principle on a large scale would be to design and install completely engineered sound systems which are field calibrated by the installer and to encode recording in a way which either controls the parameters of the circuitry or which had all of the necessary processed signals encoded at the recording site. That would mean a minimum of 6 discrete channels.
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- Re: Rear-firing tweeters : gimmick or real improvement ? - Soundmind 06:07:31 11/14/06 (0)
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