Hammond Model A, Manual, Page: 140 / 298

OPERATIONAL ADJUSTMENTS OF FLUID TYPE
It is a well known acoustical phenomenon that audibility of some frequencies is
emphasized over others in any given enclosure. Range of frequencies affected
depends upon the size and type of reflecting surfaces such as walls and ceilings.
Thus if a musical instrument such as an organ is played in an enclosure of almost
any size, some frequencies will sound louder in one portion of the listener area
than in another, and conversely some frequencies will sound weak. This can be
effectively demonstrated by playing the organ in a small room with a microphone,
then listening to the signal picked up by the microphone in another room. Variations
in loudness will be startling especially when single frequencies are sounded.
The reverberation unit similarly produces a response pattern which tends to
emphasize some frequencies over others to a slight degree. This is an operating
phenomenon of the equipment and cannot be eliminated. This room pattern effect has
not proved seriously objectionable, because as described above it stimulates an
acoustical effect which is present in some degree whenever any musical instrument
producing a wide range of frequencies is played in an enclosure.
If some notes on the organ sound excessively loud while others sound weak it may be
traceable to the reverberation control system. In investigating this, disconnect the
reverberation system by turning the switch on the reverberation preamplifier or
amplifier to the off position. 1£ notes then sound at equal loudness, turn
reverberation system on again and make the following adjustments:
1. The two-pole plug, which is connected to wire carrying signal
to the driving unit at the top of the reverberation unit, may be
inserted in two positions. Reversing this plug by turning it at 90 degrees
will reverse the input signal phase, thus changing the response
pattern of the reverberation system. Reversing this plug will
often improve evenness of overall frequency response for a
given installation.
2. Sometimes evenness of frequency response can be improved by
cutting down amplitude of the reverberated signal. This is accomplished
by changing the position of the reverberation switch.
If switch is on HI move it to Med, and if switch is on Med
move it to Lo.
Exact recommendation on adjustment of this switch is somewhat difficult as the
purpose of the reverberation control is to compensate for lack of natural
reverberation. Adjustment of it therefore should be made in cooperation with the
organist, who must understand its intent. In large installations the use of two
reverberation units will reduce room pattern to the point where it is negligible.
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