AES Section Meeting Reports

Pacific Northwest - March 20, 2017

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The PNW Section's March 2017 meeting explored the concept of impedance in its audio forms. 57 persons (25 AES members), including the AES Western Region VP Leslie Gaston-Bird, met at Dr. Mike Matesky's beautiful Opus 4 Studios in Bothell, WA. A panel of local experts spoke, including Steve Macatee (retired from Rane), Colin Isler (still working for the Rane Division of InMusic Brands Inc.), Dana Olson (retired from Boeing, Physio Control & Cypress Semiconductors) and Mark Rogers (semi-retired from The Greenbusch Group consulting engineers). Unfortunately, acoustician Dennis Noson took ill and could not present on acoustic impedance issues as scheduled.

Steve Macatee started the ball rolling by reviewing basic electricity concepts to define resistance and reactance; comparing them to the familiar water analogy. Then he went over several dictionary definitions of impedance, and finally the nature of alternating current (AC), audio waves, and acoustic waves.

Colin Isler then focused on the typical audio interface and what those impedance specs all mean. He spoke about the historical development of modern audio circuits from the earlier telegraph and telephone circuitry through to more familiar solid-state devices. Originally, most audio devices used tubes, and were designed to maximize power transfer, thus impedances of input and output stages needed to be as close to identical as possible. Colin explained the need to reconsider the once sacred notion of "matching" impedance between stages, since solid state devices, hence most modern equipment, are voltage driven rather than power driven. Ideal voltage transfer calls for impedance bridging - the practice of ensuring the impedance of the input stage of a circuit is several orders of magnitude higher than the output impedance of the previous stage. This concept of the difference between impedance matching and bridging remains a confusing one for some in the industry. He went on to describe balanced and unbalanced circuit topology, and to talk in more depth about the physical construction of devices, and the unexpected sources of impedance in real world circuits, including capacitive and inductive reactance resulting from the physical construction of cables and connectors.

After a break including a delicious cake, cookies and some time to chat, the group reassembled in the studio for the awarding of door prizes.

Returning to the substance of the meeting, Dana Olson took the discussion into the realm of loudspeaker impedance issues. This introduces more layers to the onion, because one must consider acoustic impedance imposed by the air around the speaker, and the air inside, mechanical impedance introduced by multiple materials in the speaker (surround, spider or suspension, mass of the cone itself) electrical impedance resulting from the back EMF created by the movement of the coil through the magnetic lines of force, and in a ported speaker, the impedance offered by the port itself, or by the addition of a passive radiator. Dana described and demonstrated extremely cost effective measurement techniques, and showed how one can derive the impedance in the circuit by measuring the current flowing through a known resistor in series with the speaker voice coil. Considerable discussion of the anomalies evident in measured impedance curves in real devices ensued.

Finally, Mark Rogers put a point on the discussion by describing several real-world situations where systems designers got into trouble by failing to take into account the impedance and resistance offered by the interconnection wire itself - and reminded us that most tables listing the loss in electrical cables fail to communicate that they list only the impedance of ONE conductor, when in reality we must always remember that it's the ROUND-TRIP we need to consider - a 300' run means a 600' round trip path to be calculated.

More details, audio and slide decks are on the PNW Section site:

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AES - Audio Engineering Society