by Richard S. Burwen
Have you ever listened to a trumpet outdoors, with its bell pointed at your ears?  That is flat response with no sound reflections.  It
sounds awful.

To make the trumpet enjoyable, listen to it in a reasonably live acoustic space, at an angle of 45 degrees or so off its axis.  
Multiple sound reflections from the surfaces of the room (preferably non-parallel) add and subtract at various frequencies via
 filtering to produce hundreds or thousands of resonant ripples in the frequency response.  That is what makes instruments
sound musical.

The multiple resonances make it harder to hear noises and other imperfections in the trumpet sound.

What is missing in a real room is and artificial reverberation systems is significant sound reflections higher than 5 kHz.  That is
where Burwen patent pending high frequency reverberation comes in.  It does for high frequency imperfections in recorded sound
what lower frequency reflections do for the trumpet.

The trick is to get thousands of small ripples in the frequency response without getting a few whoppers that bother you.

Automatic equalizers use microphone measurements to reduce the biggest resonances, generally below 500 Hz, so the average
bass response is flatter.  What you do not want is a system so sensitive that it attempts to remove reverberation.  That cannot be
done anywhere other than at the microphone location and is likely to degrade the sound where you listen.  While this type of
equalizer is of some help, it can balance the overall tone quality of a speaker and room only to within 4 dB or so of optimum.

What you ear thinks of as tonal balance is the general trend of the frequency response curve, amongst all the ripples.  For poorly
balanced sound, 1 or 2 dB change in the frequency response curve is difficult to hear.  I used to think 0.5 dB change at middle
frequencies was all I could hear.  Not so.  Once the sound is nearly perfectly balanced, as little 0.1 dB tilt in the frequency
response becomes significant.

For example, I remastered one of my own most favorite live classical concert recordings 7 times using
time making some improvement in the sound.  The final frequency response curve has 15 dB boost at 15 Hz and 3 dB attenuation
at 5 kHz.  As I reached "perfection" I spent the last 15 minutes deciding on a 0.1 dB increase in the 3 kHz slider.  Without it the
orchestra was more natural.  With it the cymbals were more natural.

All the tone sliders in the BURWEN BOBCAT TONE BALANCER  produce gradual changes in frequency response in fine steps of
0.1 dB.  The LOW, MID HIGH, and HIGH sliders, which each have a +/- 30 dB range are 601 position switches.  BASS, MIDDLE,
and TREBLE are 401 position switches covering +/- 20 dB.   

How did you pick your speaker system?  Probably you thought it had very well balanced sound on program material you
considered to be a reference.  In assessing tonal balance your ear averaged out ripples in frequency response caused by room
reflections and speaker imperfections.

How close can the average tonal balance of your speaker system be to what you would consider to be best tonal balance on your
reference program?  If you paid $100,000, maybe +/-2 dB in the frequency range from 100 Hz to 5,000 Hz.  You need high
resolution tone controls to bring it within tenths of a dB.  

In rare instances it is possible make it seem as though one or a few players are in your own listening room.  For most recordings
the best that can be done is to try to transport you to the original recording environment or a different hall.  Two parts of the audio
range have a big effect on your perception of closeness - middles around 800 Hz and extreme high frequencies above 5 kHz.

Pushing both MIDDLE and TREBLE sliders down can help vocals by humping up the 1 kHz region and placing a small dip at 3300
Hz where screech occurs and your ear is most sensitive.  A peak of only 1 to 2 dB may be all you need, produced by setting both
MIDDLE and TREBLE at -3 to -4 dB.

Pushing the MID HIGH and/or HIGH controls up a few dB will clarify words and increase sibilance.  Often I like pushing MID HIGH
down and HIGH up to keep the sound from getting too thin.

The choice of high frequency reverberation also affects presence.  For vocals I use either BASIC which is most natural or
RECITAL which provides more ambiance, hinting of a small hall, and seems more exciting.  Clicking RVRB OFF highlights the
artificial reverberation in the recording itself and it may be annoying.

When you move any slider, the BURWEN BOBCAT TONE BALANCER automatically adjusts the listening volume in attempt to
make it psychologically the same, but the compensation is not perfect.  So in judging presence you may want to readjust the
volume slightly.  Louder sounds closer.  Often the louder of two sounds seems superior even when it is inferior.

Pushing LOW up can boost 15 Hz as much as 30 dB and may be able to extend the bass response of your speaker an octave
lower.  Beyond a certain point the bass becomes boomy, but this can be alleviated by pushing the BASS slider down.  How much
boost you can use depends upon the low frequency content of the recording, the power capability of your amplifier-speaker
combination, and your listening volume.  A resonant rise at the low corner frequency of your speaker cannot be perfectly
compensated the way an automatic equalizer can reduce a resonance, but the effect can be pleasing.  An efficient speaker fed by
a high damping factor semiconductor amplifier will usually have a smaller resonant peak at its low corner frequency and will be
more accurately compensated by the BURWEN BOBCAT TONE BALANCER.

All 3 reverberation selections are designed to spread the stereo image a little wider than the speakers.  Whether this actually
occurs depends very much on the tone slider settings.  Boosting high frequency content via the MID HIGH and/or the HIGH slider
widens the stereo image beyond the speakers.   Attenuating the high frequencies or boosting the middle frequencies will narrow
the stereo image.  Boosting middle frequencies and extreme high frequencies at the same time can increase presence while
maintaining a normal or very slightly widened stereo image.

The stereo image width can also be controlled by mixing.  Use the INPUT CH - SPKRS 43 position mix selector to deliberately
widen or narrow the stereo image.  Widening is accomplished by slightly blending the left and right channels out of phase.  
Normally this is done when a front center speaker is used because it tends to narrow the stereo image.  Use 2 - ALL, 2 WIDER -
ALL, or 2 WIDE - ALL for 3 progressively higher degrees of widening, accomplished by varying the level of blending.

To narrow the stereo image and bring a vocalist closer, the PHONE + ALL mix does the opposite.  It slightly blends the left and
right channels in-phase.  Stereo recordings heard via headphones are too wide.  This mix restores normal width.  FL + FR - ALL
provides extreme in-phase blending to monophonic sound.

Some extremely bad recordings can be surprisingly improved by drastic measures.  That is why the tone sliders have such huge
dB ranges that can add up to more than 70 dB boost or cut at 15 Hz and 20 kHz.  The greatest amount of bass boost I ever used
was 68 dB on a laser disc containing the movie La Traviata.  The low bass had been cut off but there was a trace of it left that
could retrieved via the huge bass boost.  Some very rough, raspy voice recordings can become tolerable by attenuating the high
frequencies as much as 40 dB.