The Rule of 10
If we cannot hear above 20kHz why do engineers insist on building amplifiers with ever higher bandwidths?
While I can’t tell you why designers other than our own like to extend amplifier bandwidth into the ultrasonic regions I can explain why we do. It’s called the Rule of 10.
The rule of 10 is a lofty engineering goal that simply states we should strive to build products with 10 times the required average use case. So, for example, if we want to make sure we can deliver flat frequency response out to 20kHz then we should try and extend the amp’s bandwidth by a factor of 10, or 200kHz. In the same vein if we want to be flat to 20Hz we’d make certain to extend that by a factor of 10, to 2Hz.
We don’t always get what we want nor do we use the Rule of 10 as a strict formula, only a guide. The reasons for this are simple. By stretching the parameters beyond requirements we gain headroom, staying within the comfortable bounds of an amp’s abilities. Taken too far in any one direction this goal can actually make things worse. For instance, demanding too much ultrasonic performance can require design changes that have negative impacts elsewhere.
The Rule of 10 is valuable advice if it is kept in balance and harmony with the rest of the design.
Without design compromise
Origin stories of Infinity’s flagship loudspeaker, the IRS, have as many variations as M&Ms but this is the one I was told.
It all took place over a dinner attended by Infinity partners Cary Christie and Arnie Nudell along with their international sales manager Leon Kuby. It was Kuby who challenged Arnie to consider building a line source loudspeaker without design compromise. Nudell is reported to have scoffed at the idea saying such a speaker would be absurd, taking up most of the room and costing a king’s ransom. Over time the challenge moved from the absurd to the possible and finally to the practical.
From TAS’ Jim Hannon:
“Like Infinity’s previous flagship loudspeakers, the goal of the formidable seven-feet, six-inches-tall, four-tower, Infinity Reference Standard (IRS), introduced in 1980, was to reduce “the musical distance between the live performance and its reproduced illusion.” Its sole design objective was to “achieve the world’s highest level of musical accuracy, and to develop the new technology needed to attain that objective.” Originally conceived as a statement of what a large line-source dipole without any design compromises could achieve, the IRS attained surprising commercial success, and served as HP’s long-time reference. That alone should be enough for the IRS to reach iconic status!”
In fact, Stan and my first meeting with HP of the Absolute Sound was at a time when this very speaker was his reference. We had come to visit Harry and show off our new little phono stage, a $120 silver box about the size of a pack of English muffins that was our sole product. Harry kept promising to give the phono preamplifier an audition while we sat transfixed by the sound of the IRS—but the audition never happened. When 2 a.m. rolled around we were all tired. Stan and I went back to California the next day changed by those speakers. Our horizons had been forever extended as we witnessed what few people ever get the chance to do: be in the same room when the musical distance between a live performance and its reproduced illusion had been reduced to near nothingness.
The IRS loudspeaker system changed my life and the lives of others. It was a seminal work that deserves its place in history. It will live in permanency at our new building with its own dedicated room. Sometimes history has to be preserved so we can understand our roots.