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What is a 'fast' or 'slow' speaker?

Posted by SteveJewels (A) on November 13, 2021 at 05:08:44

In Reply to: Is fast bass...... overly "tight" and "dry" bass? posted by GreatSound on November 11, 2021 at 14:46:11:

The ability or inability of the transducer to respond accurately to the input voltage from the amplifier is what is referred to when describing a speaker as 'fast or 'slow'.

Audio voltages have a 'rise time' and fall time'. Rise time is the amount of time it takes for the voltage to increase from zero to its maximum although people working with these measurements usually measure and refer to the 10 to 90% rise or fall time.

Cone woofers have significant mass and take more time (if only milliseconds or fractions of a millisecond more) to respond to the input voltage so the movement of the transducer cannot faithfully follow the input voltage.

Additionally, the inertia of the additional mass causes the cone to be more difficult to stop at the peaks (positive and negative) therefore the 'slowness' is exacerbated at the peaks. Servo speakers are an attempt to ameliorate this effect and why, I believe, they are regarded as a better match to planars. They have better control of change of direction of the transducer and of overshoot.

Because cone speakers have mass and woofers have more mass they are less able to respond instantaneously. If you make them lighter they are usually less stiff and physically deform (15" and 18" Altecs and Klipsch's) from the shape in which they are made. If you make them stiffer to hold their shape better they are usually heavier. If they are heavier you need a stronger motor as in a heavier voice coil and pole piece and stronger magnets although I don't think magnets contribute to a speaker being 'slow'.

To reproduce lower frequencies larger cones are needed. The larger the cone, 8", 10", 12", 15", 18" the more serious the detrimental effects of the mass. Some speaker manufacturers use advanced materials and/or methods to increase stiffness, sometimes without increasing mass but this has it's own problems. Stiffer materials such as metals, tend to ring or have their characteristic voicing, think titanium vs aluminum, vs silk vs beryllium vs paper, carbon fiber or perfectium unobtanium but voicing is a different topic. JBL uses aquaplas on paper cone speaker for damping and rigidity. "Aquaplas is a sound damping elastic used to provide greater bass control and more accurate woofer movement."

Another phenomenon that affects transducer performance is that a it can apply more force to the air when it pushes than when it pulls. When the transducer pushes the air in front of it becomes (however slightly) compressed and more force can be imparted into it. When the transducer pulls, the air becomes less dense and less force can be imparted to it. This includes the mass of the entire column of air through which the sound is conducted from the transducer to your ears. This is why the speed of sound is faster (I knew I could get that word in somehow. :)) through liquids and ever faster through solids than in air, the molecules are closer together.

I'm not sure I can identify the characteristics that result in perceiving planars as 'faster' than cone speakers but I can hear it.

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