How driver materials affect sound quality

September 6, 2017
 by Paul McGowan

4 comments on “How driver materials affect sound quality”

  1. Enjoyed the talk, Paul, but once again the link to this page works fine from Ask Paul, but is broken from Paul’s Posts (there is a spurious ” %u200E” appended).

  2. Thanks for the oil can memory Paul! My first “real” job was pumping gas on the graveyard shift at a Baltimore Sunoco station during high school summer in the mid 70’s. I can still hear that distinctive sound that you described in my head to this day. It was a full service station and we were required to clean everyone’s windshield and ask if they would like their oil checked, no matter how busy we were. For those that need a visual, google The Jerk Oil Can Scene.

    On topic, unlike some posters I really like my Wharfedale Jade 3 aluminum tweeters. DMP/DSD combo really makes them sing.

  3. I hadn’t given much thought to this question until maybe 10 or 15 years ago. The material makes a big difference. I first heard of the term oil canning back probably in the 1980s when someone, I think Julian Hirsch did a review of a speaker with a metal cone. However, later discussions of lossy woofer cones got me thinking. Metal cones are without a doubt the worst possible choice. Metal is a good conductor of sound. resonances in the cone structure itself will not dissipate quickly. After WWII some experimenters made cones out of aluminum pie plates. Ping one with your finger and you’ll see what the ringing effect is. At the opposite end the surprise is that one of the best materials is paper. The pulp fibers in the paper lose energy to each other very quickly making them fairly dead. Henry Kloss said Edgar Villchur founder of AR was obsessed with paper quality. One nice thing about acoustic suspension woofers is that since the restoring force is mostly due to air pressure, the force is applied uniformly over the entire surface of the cone. Unlike mechanical suspension speakers there is no difference in the force around the circumference of the outer suspension and the inner suspension, the spider and there is no difference between the restoring force from the innermost part of the cone to the outermost part. The restoring force is applied by uniform pressure over the surface of the cone reducing its tendency to break up into harmonic modes. Acoustic suspension woofers need to have a relatively high mass to get their Fs as low as possible. An AR 12″ woofer has a free air resonance frequency of around 16 to 19 hz. This means the cone will be thicker and stronger. Plastic cones are good for subwoofers but not for woofers that will extend into the upper bass. The Tonegen 1259 Ken Kantor helped Fostex develop based on the AR woofer has a plastic cone but is used as a subwoofer in a 4 way system, the NHT 3.3. One drawback of the 1259 is that it requires an enclosure with twice as much volume as an AR 12″ woofer. The suspension material is critical also. The tight accordion pleated stiff suspensions that were common many years ago and still used in professional speakers led to a high free air resonance and were not uniform with frequency. Villchur first used a cloth suspension cut from a bed sheet. Early variants of his speakers used cloth suspensions that didn’t wear out. However over the years the dopant which is some sort of nasty butyl rubber disolved in toluene I think dries out and becomes porous. Foam suspensions and other rubber suspensions deteriorated over time and had to be replaced. Perhaps better suspension materials are in use today. Efforts to reseal cloth surrounds using various mixtures of glues are invariably a failure as they increase Fs as they cure. This compromises bass performance by raising F3 and Q.

    Dome tweeters also invented by Villchur are strong for the weight of their cones. Paper, silk, and even mylar seem to work out fairly well. Metal domes have a poor reputation for their bright shrill sound probably due to internal cone resonances that don’t dissipate energy. How well does metal transmit sound without loss? They say you can put your ear to a railroad track and hear a train 100 miles away. I never tried it.

    Looking at the FR of Kevlar drivers, they have a nasty high end peak and must be crossed over below that peak or they will not sound good at all. I’m not familiar with Nomex but I suspect it’s probably a good material for woofer cones.

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