by Richard Honeycutt
In this article, Richard addresses four common myths that has grown up in the audio field.
Through the years, a number of urban legends have grown up in the audio field. These are promulgated by two types of people, those who don’t know any better, and those who should. Let’s look at a few of these myths.
You have to talk really close to a [insert brand/model] mic for it to pick you up.
I once heard this one in the form of a lady’s request for me to recommend a mic that “would pick up better at a distance.” This pretty much comes down to an issue of proximity effect: almost any directional mic boosts bass when used at a short working distance. The exact amount of bass boost depends upon D, a design parameter of the mic. Also, many mics are designed with a built-in low-cut filter to keep the mic from sounding too bassy when used up-close. But the filter just cuts bass, not specifically bass caused by proximity effect. The more aggressive this filter is, the thinner the mic sounds if it is used at a greater working distance. Mics that are equalized for tonsil position will sound really thin if used at 2 feet. This thin sound causes some folks to think that the mic is not picking up. One type of mic – the Variable-D mic – is built with internal acoustical circuitry that avoids proximity effect. The old E-V 660, 664, 665, 666, RE10, RE11, and RE15 were variable-D mics, as are the current RE16 and RE20. The lady who requested a mic to pick up at a distance was very pleased with the variable-D mic I recommended. Of course it did not really pick up any better at a distance than did her other mics, but the lack of proximity effect made the timbre of the sound pretty much the same at 6” as it was at 4’, so she thought it picked up better at a distance.
A sound system needs to have a damping factor of 100 or better to sound good.
This came from the old days when power amplifier output impedances were not vanishingly low as they are now. It has since been revived in discussions of speaker lead wire resistance. Damping factor is sometimes given as the ratio of the stated speaker impedance to the sum of the amplifier output impedance and the speaker lead wire resistance. Low damping factor improves the amplifier’s control of speaker cone motion. When the cone is displaced from equilibrium position and then released, it oscillates briefly before coming to rest. This effect stretches transients, hurting definition. It can also exacerbate resonant peaks in the frequency response, depending upon the enclosure design. When the voice coil oscillates in the speaker’s magnetic field, it generates electrical energy. The resistances in series with the voice coil dissipate this energy as heat, providing a braking motion.
For an 8 Ω speaker, a damping factor of 100 would imply a combined amplifier output impedance and wire resistance of 0.08 Ω or lower. In fact, since we have a series circuit, the resistance of the voice coil is every bit as important in affecting damping as are the amplifier output impedance and the lead wire resistance. With a typical 8 Ω speaker having a voice coil whose electrical resistance is about 6 Ω, one can easily see that fractional-ohm changes in amplifier output impedance or lead wire resistance will not make much difference in actual damping. In the 1969 edition of the Audio Cyclopedia, Howard Tremaine clearly demonstrated that trying to get combined amplifier output resistance and speaker lead wire resistance below 0.5 Ω is of little value, regardless of what the purveyors of “Beastie” cables may claim. (As discussed in my article in Radio Electronics in February 1991, skin effect is also negligible at audio frequencies – but that’s a whole ‘nother set of myths.) A total 0.5 Ω round-trip lead wire resistance for a 100’ run calls for #14 AWG or heavier wire, although heavier will not necessarily be significantly better. And BTW, if you keep total lead wire resistance under 0.5 Ω, you’re pretty much covered in terms of current capacity, wire heating, etc. Of course, for 4 Ω speakers, this becomes 0.25 Ω, etc.
The sound system in this room is so loud that it makes its own acoustics.
The only way this one can be true is if the sound is so loud that is exceeds the range in which air behavior can reasonably be approximated as linear, in which case, your hearing apparatus has become a bloody mess and what the room sounds like is no longer important to you. It is true that very loud sounds do cause our ear/brain system to go into compression, both long-term and short-term; and that above the high-nineties of dBSPL, harmonic distortion in the hearing mechanism becomes significant. But this does not affect the reverberation time, clarity, definition, or other acoustical properties of the room.
Horns always cause added distortion.
This myth came about because the distortion caused by air nonlinearity in the throat of a horn is easily predicted, so many textbooks include charts showing it. Distortion mechanisms of direct-radiator speakers are more complicated to predict, and so are often ignored in textbooks. In fact, as Alexander Voishvillo pointed out in an AES presentation back in 2003 or so, the sound field in front of a trombone section of a symphony orchestra can produce distortion due to air nonlinearity. Driver distortion is almost always dependent upon excursion: the more excursion, the greater the distortion. Since horn drivers require less excursion to produce a given sound output, they also produce less distortion. It is also true that a part of the distortion calculation for a horn driver involves the ratio of high to low cutoff frequencies, so that a midrange horn that covers less than a decade will produce less distortion in its upper range than would a full-range horn, if any such thing were available.
There are many more myths ripe for discussion, but the others will have to wait for a later blog. rh
Richard A. Honeycutt developed an interest in acoustics and electronics while in elementary school. He assisted with film projection, PA system operation, and audio recording throughout middle and high school. He has been an active holder of the First Class Commercial FCC Radiotelephone license since 1969, and graduated with a BS in Physics from Wake Forest University in 1970, after serving as Student Engineer and Student Station Manager at 50-kW WFDD-FM. His career includes writing engineering and maintenance documents for the Bell Telephone System, operating a loudspeaker manufacture company, teaching Electronics Engineering Technology at the college level, designing and installing audio and video systems, and consulting in acoustics and audio/video design. He earned his Ph.D. in Electroacoustics from the Union Institute in 2004. He is known worldwide as a writer on electronics, acoustics, and philosophy. His two most recent books are Acoustics in Performance and The State of Hollow-State Audio, both published by Elektor.