100 YEARS 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 known facts.

Listening 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.

Edison's studio for making talking pictures (1896)

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.

Early cinema 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 voices!

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.

High efficiency folded horn with moving coil driver (1928)

Experimental electrostatic loudspeaker (1929)

Large throat short horn with large cone driver unit (1929)

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.  

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.

The Shearer two way horn system.

    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 by equalisation.  

    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 acoustics.  

    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.  

    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 tweeters.  

    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.

Westrex Loudspeaker System with Acoustic Lens (1958)

    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 125^o x 40^o to as narrow as 50^o x 30^o.  

    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.  

Dolby 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".