A History of ProAc and Celef
As a young man, Stewart Tyler’s main objective was to build a compact loudspeaker with good bass extension, smooth and extended high frequencies with a clear and uncoloured midrange. Developing the bass extension and diminishing cabinet colourations would be his biggest challenge.
He decided to design a two-way system since most threeway models of the time suffered from phasing and integration problems. The drive units he chose were an 8-inch plastic cone bass/midrange driver and a 1 1/4-inch melinex dome tweeter with phase correction.
With the aid of an anechoic chamber for amplitude measurements and nothing more than his own ears, Stewart evolved a nine-element crossover with a tuned circuit that dramatically reduced the break-up mode, which was characteristic of the otherwise reliable 8-inch plastic cone.
To further alleviate any colouration, he experimented with cabinet bracing, but he found that it merely changed their frequency. However, by attaching bituminous impregnated fibreboard to the inner sides of the enclosure, colourations were minimised by being consigned to the low frequency area.
The hardest part was yet to come, which was to achieve good bass extension free of overhang whilst providing the necessary efficiency and power handling. He found that a totally enclosed box gave a well-controlled bass with a slightly artificial sound quality, whilst normal port reflex loading produced a natural character to the bass and greater efficiency, but however, did not control bass unit excursions at higher volumes. Loading the port with various fibrous materials had the desired effect at high volumes but was both inconsistent and unnatural at low volumes. The problem was to add enough friction to the air in the port tube to be effective at high volume levels but impose minimal damping at low levels.
Finally, Stewart had the idea of loading the reflex port with small tubes, each of which would add its own friction at high levels without any significant effect at low volume.
The end result was a revelation – the speakers sounded both natural and free of colourations, with improved power handling and impressive bass response. After extended listening sesslons at home, Stewart eventually decided to replace the tweeter with a 1-inch soft fabric dome unit which provided greater high frequency response. At last he was satisfied.
He decided to demonstrate his loudspeakers at the hi-fi store where he had purchased all of his own audio equipment. So impressed were the staff at the performance of these small units, even in comparison with that of many larger and more expensive systems, that the manager offered without hesitation to sell the loudspeaker if Stewart would care to produce more.
Soon another three pairs were delivered to the shop. They were sold in one day, and Celef Audio was born. The name Celef was derived from the drive units; a Celestion tweeter and a KEF bass driver.
Stewart’s next step was to send a pair of these first models, now christened the Celef Monitor, to a popular Hi-Fi magazine for review. The review was ecstatic and over the next five years, through an expanded dealer network, the Celef Monitor became a byword for exceptional sound quality.
Soon, a new model followed with similar success, the Celef Mini, based on the same design but using 5-inch plastic cone bass units in an even smaller enclosure.
The name Celef became synonymous with high quality, accuracy and a spacious soundstage not generally found in loudspeakers of that period.
As Celef began to expand, Stewart developed the family business which was to provide the cornerstone of his reputation for quality, consistency and reliability. His parents joined him at the Borehamwood headquarters, his father taking charge of production and his mother overseeing quality control. Their unstinting attention to every detail became vital as demand and production increased.
Celef continued to prosper and many more new models were introduced. But Celef loudspeakers had generally been positioned as a medium-cost range and Stewart felt the need to break new ground.
The catalyst for change was his discovery in 1979 of the new ATC range of bass and midrange drive units which offered sensational performance – but at sensational prices. Enclosures utilising such exotic drive units could not sensibly be marketed under the Celef banner and in 1979 Stewart formed a new company – ProAc, an abbreviation of Professional Acoustics.
ProAc models were designed for professional and studio use as well as the discerning domestic listener. The first model launched was the ProAc Studio Three, a three-way design which was quickly acclaimed as the finest box loudspeaker in the world. Using a refined version of the Celef resistive port, it exhibited extremely low levels of colouration with immense power handling and bass extension.
Encouraged by this success, Stewart decided to undertake his most difficult challenge to date – a miniature loudspeaker with real bass extension and good power handling. The dilemma over totally enclosed cabinets and reflex port designs was once again resolved by using a special version of the resistive reflex port.
Using a 4-inch bass driver, specially coated to Stewart’s own specification, a miniature loudspeaker evolved which produced exceptional power handling and remarkable bass extension combined with minimal colouration. This unit became the ProAc Tablette, which in modified form is still selling strongly today.
The product range evolved steadily. The Studio Three was transformed into the EBS Monitor (Extended Bass Studio) which itself was developed into a floor-standing model as the Studio Tower. Further models were added – the Studio One, a compact unit, and the Super Tower and Mini Tower, both elegant, slimline floor-standing enclosures.
Throughout the development of these models, Stewart’s policy was one of constant experimentation and improvement. Most of the drive units used in his designs are now produced to his own specification by such companies as Scanspeak and SEAS.
In 1987 Stewart suffered the shock of his mother’s death. Tribute must be paid to her vital contribution to the success of Celef and ProAc alike – her insistence on perfection in the production of each enclosure cannot be overvalued. She was a great loss to Stewart, his family and to the Industry. Fortunately, her expertise was passed on to her “right-hand-man” who had worked alongside her for many years. Her rigid standards of quality control still prevail at ProAc today.
By this time most of ProAc’s production was carried out at a new main factory at Milton Keynes, with only the EBS and Studio Tower being manufactured at Borehamwood. Stewart divided his time between the two centres, ensuring that production and quality control were at their peak.
Following the launch of the Response Range in 1989 to wide critical acclaim, things went from strength to strength.
In 1990 a new factory in Brackley was established and all production moved from both Borehamwood and Milton Keynes. The ProAc factory remains there to this day although now a lot bigger.
The 1990’s saw the Tablette range progress and the production of a ProAc home cinema set up, resulting in the Hexa 5.1 surround sound system.
Stewart continued experimenting with different materials and designs and in the early 2000’s gave us the Future Series, a speaker well before it’s time in both its appearance and design approach of using a chamber with no parallels. The Future Series was also the first ProAc speaker to utilise the ribbon tweeter, thinner than a human hair, giving excellent response and detail.
As the year’s went on, the use of carbon fibre was introduced with the Carbon Pro 8’s, ribbon tweeters were introduced to the Response Range, the Tablette range progressed and the high-end K Series (using Kevlar cones) were launched.
The Response Range and award winning K Series have now established themselves at the very highest end of the market and continue to impress audiophiles and music lovers around the globe today.
After pushing the boundaries of speaker design and manufacturing for well over 40 years, Stewart Tyler sadly passed away in July 2021.
ProAc continues to flourish as a tightly-knit family operation keeping his legacy alive and producing loudspeakers which will be revered by hi-fi enthusiasts, sound engineers, music producers and music lovers worldwide, for many years to come.
"I imagine designers carry designs around in their heads that maybe never come into fruition, and I am no exception. I like to sit outside under the stars and often get many good ideas from this peace and tranquility."