A very interesting thread this, so here are the facts for your
consideration:
Many years ago I asked Dudley Harwood, my predecessor about the 'BBC dip' or
to be fair 'The Gundry Dip'.
(Gundry senior worked at the BBC but passed away some years ago: Gundry
'junior' works or worked for Dolby Labs)
First take a look at the 'How we hear' pages on the Harbeth web site. Yes,
incomplete but note the huge boost in the ear's *sensitivity* centred around
3.5kHz.
Next, consider that all Harbeth (and probably the vast majority of other
speakers crossover at 3kHz-ish ...... that's a most unfortunate choice of
frequency unless your are *extremely* careful in blending the midrange and
tweeter .... seems that we are, hence the 'Harbeth sound'.
Then, as Harwood stated, there was significant colouration in the 'presence
region' with early conventional plastics ... so reducing the energy in this
high-sensitivity region partly masked the colouration ... and finally ...
BBC monitors were designed for virtually near field use i.e. you could
(probably) reach out and touch them from the studio managers position at the
desk. A little depression in the 1-4kHz region pushed the stereo image
backwards, subjectively, (i.e. the performers seemed to recede a little into
the cabinets) and made the speaker less oppressive to work with. (As a side
issue this image positioning is why a speaker designer should *never* design
a new model as a single, mono speaker and then clone into stereo. I know,
because it is one of the first traps I fell into as a novice some 20 years
ago ... mercifully the product never went into production. Do all the
listening tests in stereo right from the very start of the design coming
together.)
That's the whole story. Anything else you may hear is, I'm sorry to say, not
factual.
Alan Shaw
Harbeth