OF CINEMA LOUDSPEAKERS
The history of motion picture sound has been well documented
over the years, covering all aspects of film and disc systems, microphones,
recorders and amplifiers. But there is a scarcity of information on the
development of the cinema loudspeaker which has always been the weakest
link in the chain. Most of the early inventors such as Hepworth, Lauste,
Poulsen, Engl and Vogt, were concerned mainly with various methods of sound
recording and synchronisation, and the absence of audio amplifiers (not
yet invented) meant that nobody was working on loudspeakers. Here are the
to the "Wee Small Voice"
Thomas Edison was probably the first person to show talking
pictures commercially, connecting his cylinder phonograph to his new Kinetoscope,
by giving 'peep-show' demonstrations in a New York parlour. Sound was heard
by means of ear tubes similar to a stethoscope, connected to a diaphragm
and stylus assembly which traced the hill and dale recording on the cylinder.
But the sound was very weak with words barely distinguishable, and his
equipment became known as the machine with the "Wee Small Voice". Connecting
a metal horn was no great improvement.
studio for making talking pictures (1896)
engineer Leon Gaumont was granted patents for loudspeaker systems to go
with his sound on disc talking films, which used one of Berliner's Gramophones.
He was the first to suggest placing loudspeakers behind the screen, and
carrying them about to follow the images on the screen! In the same year
in Germany Oscar Messter patented his new Auxtephone system which used
compressed air amplifiers to feed special loudspeakers.
Lee de Forest, a graduate from Yale University, was granted a patent for
a revolutionary electronic tube. Using Ambrose Fleming's recent invention
of a thermionic diode, de Forest added a third electrode as a control grid
which allowed an audio signal to modulate the electron flow. He called
his new tube the Audion, and found it could provide amplification. This
basic US patent, more than anything, paved the way for the development
of audio amplifiers.
Leon Gaumont demonstrated his Chronophone system at the
Gaumont Palace in Paris with enough sound volume for 4000 people! He used
his own design of loudspeaker which he called the Eglephone. This device
released compressed air through a double distributor, which ensured a constant
flow of air into the loudspeaker without affecting the distributing disc.
Due to the limitation on the size of the Gramophone record, the longest
picture length which could be recorded was only 200ft. Projection was at
16 frames per second.
American Technical Society published a Cyclopedia of Motion Picture Work
which said, "The Gramophone has not yet been perfected for the minor
sounds of nature, and the human voice is about the limit for the sound
record. In producing talking pictures the method is to make the talking
record first, and then fit; a motion picture to it. To do this actors are
well drilled in their parts so that they will be able to produce the performance
companies were now making audio amplifiers, such as the Bell Telephone
Laboratories and Western Electric in America, and Seimens and Halske in
Germany. This generated a demand for loudspeakers to go with them. The
first units were designed by telephone engineers, and were basically modified
telephone earpieces called loudspeaking receivers. But they had a poor
frequency response and lacked sensitivity. Theodore Case, another Yale
man, realised more than anyone that if sound pictures were going to survive
it would be necessary to perfect a system so that the illusion of Cinema
is good enough to forget the mechanics. He bought a three stage audio amplifier
from de Forest, and together they developed a sound on film system called
Phonofilm, which used a glow tube for recording called the Aeolight.
loudspeaker as used by de Forest (1923)
Messrs Case and de Forest gave the first public demonstration
of their system as part of a programme at the Rivoli Theatre on Broadway.
Unfortunately the reproduced sound was not good enough to impress the public.
The acoustic consultant who designed the loudspeaker system was Professor
Stewart, who had developed horns for the detection of enemy aircraft in
World War l. He was the inventor of acoustic wave-guides and well versed
in directional characteristics. He used a cluster of horns above the screen
directed at various parts of the auditorium. The drive units were of the
diaphragm type with a balanced armature mechanism which was the latest
idea at the time. Unfortunately these units were not very sensitive and
had a limited frequency response on either side of the armature resonance.
Consequently it was often difficult to distinguish between male and female
and de Forest parted company, and Case continued to try and solve outstanding
technical problems. He designed a sound head which was fitted below the
picture head, thus establishing the 20 frame separation on the combined
print. At the same time Edward Kellogg and Chester Rice, who were engineers
at the General Electric Company, invented the first moving coil cone loudspeaker
with an electrically energised magnet system. This loudspeaker so surpassed
its predecessors in sound quality that its use soon became universal. Kellogg
concerned himself with every aspect of high quality sound for motion pictures,
and worked on mechanisms, optical systems, and electrical pick-ups as well
as loudspeakers. In all Kellogg was granted 107 patents for his inventions
Case joined with Fox Studios for the exploitation of his sound system,
which was to be called Movietone. Edward Sponable, who had recently arrived
at the studios to help with the introduction of sound, suggested a perforated
screen suitable for picture projection and relatively transparent to sound.
This enabled loudspeakers to be placed behind the screen as forecast by
Leon Gaumont, and was immediately adopted by the industry. In the same
year Messrs Wente and Thuras of the Bell telephone Laboratories produced
a horn loudspeaker with a moving coil mechanism driving a diaphragm and
a powerful magnet system with a battery energised field coil. This gave
a power efficiency as high as 25% which enabled sound to be reproduced
at much higher levels and with improved quality. This was important since
the amplifiers at that time only had an output of 2 watts. The driver unit
was attached to an exponential horn, curved so as to conserve space behind
the screen, and multiple throats which allowed 2 or more units to be attached
for increased output. Prototypes of these loudspeakers were used for the
premiere of The Jazz Singer at the Warner Theater in October, as part of
the Western Electric sound on disc system.
The Jazz Singer gave the industry the boost it needed,
and events moved swiftly with everybody clamouring for conversion to sound.
Western Electric were offering either disc or film systems with projectors
suitable for both, whilst RCA in America, Petersen in Denmark, Gaumont
in France, Kalee in England, Klangfilm in Germany, all concentrated with
sound on film. To improve sound reproduction the projection was also changed
at this time from 16 to 24 frames per second.
efficiency folded horn with moving coil driver (1928)
Telephone Laboratories were working on a new high efficiency cone-type
loudspeaker coupled to a large throat horn, which considerably extended
the low frequency range. An entirely new design of electrostatic loudspeaker
was manufactured by a Chicago firm, and actually installed in several cinemas.
It gave good fidelity with low distortion, but was lacking in bass frequencies.
Messrs Vogt and Engl also tried electrostatic loudspeakers in Germany,
using three types differing in size in an attempt to get a wide frequency
short horn with large cone driver unit (1929)
the frequency response of recordings improved, further attempts were made
to improve loudspeaker performance. Since most units were lacking in both
high and low frequencies, equalisation was introduced into the theatre
amplifier to compensate for these deficiencies. New power tubes also became
available which were capable of giving as much as 8 watts.
It was found that high frequencies demanded
a different design of loudspeaker unit, which led to the use of a small
horn type unit for the efficient radiation of frequencies above 3000Hz.
These were used in addition to the standard sized horn unit, and the result
was a marked improvement. The next requirement was a loudspeaker specifically
for frequencies below 300Hz, and the answer was a Rice Kellogg moving coil
cone unit mounted on a large baffle of rigid construction. This three-way
configuration formed the basis of the Western Electric Wide Range System
and the RCA High Fidelity System, and was used for studio monitoring as
well as in cinemas.
As theatre sound systems improved, the high
background noise of photographic sound tracks became objectionable, especially
in low level passages. As long ago as November 1927, Edward Sponable was
aware of this problem and had passed his thoughts over to Western Electric
in a telephone call, together with a possible solution. But it was only
in 1931 that noise reduction systems were introduced by the equipment manufacturers.
Telephone Laboratories were giving a series of demonstrations of their
new loudspeakers, and Douglas Shearer of MGM's sound department was so
impressed that he initiated a theatre loudspeaker development programme.
Photographic test films were also becoming available, mostly direct positive
recordings made by a Mr RO Strock of Eastern Service Studios. Over the
years he had recorded well over 1,000,000 feet, and each film was carefully
calibrated by Bell Telephone Laboratories against their original. But although
readings were taken with these films there was no standard of performance
to interpret the results.
The result of MGM's research was the Shearer Two-Way
Horn System in which the low frequency range was covered by a large folded
throat wooden horn driven by a large cone moving coil unit, and the high
frequency range was covered by a small throat horn with a cellular opening
driven by a diaphragm type dynamic unit. A passive frequency crossover
unit comprising high and low pass filters allocated inputs to both loudspeaker
units, the crossover frequency being 500Hz. This loudspeaker combination
provided a uniform frequency response from 40 to 10,000Hz, controlled directivity,
high efficiency and high power. It was notable for reproducing amplifier
hum, which had previously been ignored and now had to be eliminated. Flutter
in projection systems remained a problem and also became more noticeable
as a result of the new loudspeakers. New sprockets had to be designed to
take account of film shrinkage, and these were a great improvement. The
need for a small portable flutter bridge became acute, and a suitable one
was developed for use in the field.
Shearer two way horn system.
received a Technical Academy Award for the Shearer Horn System, and Western
Electric incorporated it into their new 'Mirrophonic' Sound System which
provided a new standard of sound quality in those theatres which could
afford to re-equip.
However, thousands of
exhibitors had equipment still performing well and they settled instead
for equalisation which was less expensive. An engineer with 'golden ears'
sat in the auditorium with a set of variable tone controls rather like
a graphic equaliser. When he was satisfied that the best results were being
obtained from a sample product, the frequency correction was measured and
built into the amplifier system. This method was not entirely satisfactory,
since many cinemas contained acoustic defects which could not be corrected
The Academy of Motion
Picture Arts and Sciences set up a committee to study the standardisation
of theatre sound equipment. A test reel was prepared containing samples
of dialogue and music from the major studios, and after repeated running
in several theatres a final characteristic was agreed. This limited the
high frequency response to 7kHz, and was effective in reducing high frequency
background noise. It became known as the Academy Curve, and was to last
almost 50 years. But it was only an electrical curve measured at the amplifier
output, and did not take into account loudspeaker performance or theatre
Academy's Committee now began a programme to study the variation in acoustics
of a number of Cinemas both large and small, and in particular the reverberation
time of each auditorium. It was difficult at this time to measure reverberation
without very sophisticated equipment, but compact portable equipment was
developed for this purpose. The signal source used was the Academy Warble
Film consisting of single tones varying in frequency by plus or minus 25Hz
at a 10Hz rate. This was to overcome the problem of standing waves. Considerable
differences were found between the various auditoria, which explained why
a particular film did not sound the same in all cinemas even though they
had all been aligned to the Academy Curve. The Committee again found that
auditoria with pronounced acoustical faults could not be corrected electrically,
and not a great deal could be done economically to existing structures.
New high flux magnetic
alloys, such as 'Alneco', were now becoming available which meant that
cinema loudspeaker units could be manufactured with permanent magnets.
The cumbersome field supply units used with existing loudspeakers (which
were a source of hum) could be dispensed with. But this did not happen
overnight; for instance the large amount of equipment used for the road
show of "Fantasia" still had energised loudspeakers, as did thousands of
cinemas. Other good news was that due to the manufacture of new output
tubes, such as the KT66 and the 6L6 (and PX25 in the UK), amplifier power
was no longer a problem. By using 4 tubes in parallel push-pull as much
as 60 to 80 watts was available for large auditoria.
Although unknown at the
time, Messrs Braunmühl and Weber who worked at the Berlin Radio Station
were recording on a 6.35mm paper tape covered with a fine oxide powder.
Their breakthrough came when they replaced their DC bias with RP bias,
and the high background noise virtually disappeared. The rest is history.
manufactured new equipment for recording on 35mm magnetic film, but studios
and manufacturers in Europe and America were slow to embrace this new technology.
The high frequency response was greatly improved in relation to photographic
sound, and the Academy Curve had to be adjusted accordingly.
This caused a demand for
better loudspeakers, and a relatively new firm called Altec Lansing developed
a directional HF horn called the 'Mantoray' It was designed to aim high
frequencies where they were wanted without spill-over into other areas,
which was important for the new stereo sound systems making their appearance.
Western Electric produced what they called an Acoustic Lens for their HF
horn consisting of a series of perforated discs, equally spaced in front
of the diaphragm, and a series of slanting vanes. These ensured a vertical
distribution into the balcony as well as the stalls. (How many cinemas
have balconies today?) Other improvements included plastic diaphragms and
suspensions which gave a reduction in failure compared with the aluminium
alloy diaphragms previously used, and aluminium voice coils appeared in
Altec also produced a
new bass enclosure driven by two or more units which were coupled at the
back. An air enclosure was ported at the front which extended the bass
frequency. However, most cinemas tended to hang on to their existing equipment
for as long as they could.
The last decade saw the introduction of multi-track magnetic
sound from Cinemascope, Cinerama and 70mm release prints. Cinema loudspeakers
had become standardised by now with two 15in diameter bass units in a reflex
cabinet (available in several sizes), and high frequency horns (also available
in several sizes) with multi-cellular flared openings. Large auditoria
would have two bass cabinets bolted together for each channel, with side
wings to increase the baffle area, and two or more high frequency horns.
Passive frequency dividing networks were still used, the crossover frequency
being typically 500Hz.This caused the overall loudspeaker response to dip
at this frequency, which could only be corrected by equalisation. Not a
lot of loudspeaker research was being carried out at this time, mainly
because the market for new equipment was relatively small.
Loudspeaker System with Acoustic Lens (1958)
Laboratories decided to try and improve photographic sound by using their
Type A noise reduction system which had been so successful with magnetic
recording. Commencing in London with a team of four headed by Ioan Allen,
the first thing they found was that most items of equipment such as microphones,
amplifiers and film recorders (especially with the introduction of new
sound negative emulsions), were capable of achieving a frequency response
in excess of 10kHz. Only a small 'slit loss' equaliser was required in
the projector to match the replay response. The limitations were the loudspeakers
and the ubiquitous Academy Curve. The latter could be removed, and the
loudspeakers equalised effectively or exchanged for more modern units.
Dolby used a modern spectrum analyser which gave a visible display of the
response curve, together with a pink noise generator. (Pink Noise is a
continuous spectrum noise having constant energy over a given bandwidth).
An adjustable equaliser with 27 one third-octave filters was built into
every cinema installation, together with a signal processor which provided
all the electronics from the photocell to power amplifier. The first Dolby
encoded film was Callan with a mono track, shown at the Cannes Film Festival
in 1974, followed later in the same year by a demonstration of Dolby Stereo
(SVA) at the SMPTE Conference in Toronto. New standards of sound reproduction
were being achieved, and a new International Standard was prepared for
the alignment of cinemas world-wide. This Standard, known as ISO 2969,
took into account the reproduction of Dolby SVA prints, older films recorded
to the Academy curve, films with magnetic sound and the newer digital systems.
It was also designed to be used for re-recording theatres and monitoring
spite of the tremendous achievement of Dolby Laboratories and the introduction
of Spectral Recording (Dolby Stereo SR), there were still some problems
remaining with the reproduction of 70mm films with magnetic stripes. Some
studios were using bass pre-emphasis whilst others were not, and there
were still discrepancies between their various monitoring systems. So cinemas
had to be aligned for each film, using a multi-frequency test film supplied
by the studio concerned.
When George Lucas embarked
on his Star Wars films he found that they sounded different in different
cinemas, and asked his chief engineer Tomlinson Holman to do something
about it. Holman found that all the cinema loudspeakers built since 1940
had been designed for high efficiency at a time when amplifier power was
expensive, whereas amplifier power was now virtually unlimited. Loudspeaker
units could be designed to give an improved bass response, less distortion
at high sound pressure levels, an improved uniform coverage at high frequencies,
as well as removing some of undesirable characteristics of crossover networks.
Quite a tall order. The result was the THX Sound System which takes into
account everything between the output of the Dolby signal processor and
the ears of the listener.
The THX system specified
loudspeakers units manufactured by JBL who have had many years of experience
in cinema sound. Two 15in bass units are flush mounted side by side in
a vented cabinet with two ports, the cabinet being attached to a large
baffle board 10 inches behind the screen. The tweeter sits on top of the
bass cabinet with the front of the horn protruding through the baffle.
New high frequency drivers were developed using titanium instead of aluminium
for increased reliability, and new horns provided a uniform cover varying
from 125o x 40o to as narrow as 50o x
One of the most successful
items of the system is the electronic crossover situated ahead of the power
amplifiers. Lucasfilm consider this crossover so important that it is delivered
as a sealed unit with no external adjustments. Inside are the usual high
and low pass filters and an HF sensitivity control, together with a unique
filter giving a 6dB lift at the crossover frequency to eliminate the "camel
hump' response associated with passive crossover units. A further refinement
is a delay line for the feed to the bass unit to compensate for the fact
that the tweeter voice coil lies further behind the screen than the bass
voice coil. The delay line brings them into phase. Stereo power amplifiers
are used for each channel, one half for HF and one half for LF, which permits
higher sound levels without distortion.
To round off the THX system
an optional sub-woofer is available to enhance low frequencies, installed
in a cut-out at the bottom of the loudspeaker baffle. The system also includes
a complete review of auditorium acoustics, and is compatible with the ISO
Standard 2969 over a frequency range of 40Hz to 16kHz. The first THX system
installed in the UK was at the Warner Theatre Leicester Square in 1986,
and the Empire Theatre was not far behind.
proliferation of multiplex cinemas provided a greatly increased market
for loudspeaker system, and JBL speakers can now be found in many cinemas.
Dolby introduced their Digital Sound System, Dolby SR.D, in 1991 and 43
films using this system were released in the following two years. Sony
demonstrated their Digital System (SDDS) at the Odeon Leicester Square
in 1993, and although units were not immediately available for installation
it was obvious that the reader heads and sound processor were incompatible
with any other system currently available. September 1993 saw the reopening
of the Warner Westend Theatre with nine screens on five floors, all with
JBL loudspeakers and two THX installations.
Pro-Logic System with THX loudspeakers (1996)
There is now a domestic version of THX, and although it cannot dictate
the acoustic conditions it does lay down stringent specifications for all
the hardware including loudspeakers. So it's into the next century with
Dolby Pro-Logic and THX in the living room. Who was it who said that there
was no commercial future for talking pictures? Answer: Thomas A. Edison
in 1926! We've come a long way since the days of the "Wee Small Voice".
Original URL: http://www.amps.net/newsletters/Issue21/21_cine.htm