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    The Knob With the Misleading Name

    Issue 151

    Component audio systems were still comparatively rare when my college roommates and I put one together (see my article in Issue 149), but there was at least one other in our dormitory. Two flights below us, my friend Skip had a simple mono system consisting of a big Klipschorn, a 10-watt integrated amp, and a turntable (possibly a 45-only changer like my first turntable).

    I was still new to audio components, so I took a close look at the amp’s control panel. it had a source selector switch of course, plus bass and treble controls, a volume knob, and another labeled “Loudness.” Volume and loudness? Weren’t those both the same thing?

    Skip’s explanation, if he gave one, went right over my audio-newbie head. But I do recall him saying that it added bass.

    But why, if there was already a bass control? “Well,” said Skip, “I like a lot of bass. That’s why I got the Klipschorn, for starters. But to get the bass I like, I start with 45rpm records that have heavy jukebox bass. And though the turntable I play them on has a ceramic cartridge, I feed it into the magnetic phono input, because the RIAA equalization boosts the bass,” (that much, I understood), “and I turn the bass control all the way up, and then I set the loudness control for maximum bass boost.”

    I still wondered why his amp had two controls that boosted bass, and why one was labeled “Loudness.” But then he turned the system on, and I just listened. Even with just 10 watts, Klipschorns can play very loud. And there sure was a lot of bass! I could no longer hear Skip’s system when I went back to my room, two flights above, but I could swear that, standing in my stocking feet, I could just about discern the beat. And I began wondering about that knob again…

    I’d have wondered less if the knob had been labeled properly – not “loudness,” but “loudness compensation.” It compensated for the way our hearing system’s frequency response changes with sound level. Make a sound softer and it seems to lose some bass and a smidgen of treble. Those frequencies are still there, but we no longer hear them as well.

    We don’t notice this when the music itself gets softer, because we’re used to the difference in sound between a forte and a pianissimo. But if we’re listening to loud music at soft levels, as we often do at home, the sound is noticeably thinner and may lose some of its sparkle.

    This is often called the Fletcher-Munson Effect, after the two Bell Labs researchers who first measured it. In 1933, they published a set of “equal-loudness contour” curves showing, for sound pressure levels (SPL) from 0 to 120 dB, what amount of boost (and, sometimes, cut) would be needed to make tones at various frequencies sound as loud as a 1-kHz tone. These curves have been revised a few times since [1], by others, but they all show basically the same thing.

     

    Fletcher-Munson curves. Note that before frequency was referred to as "Hertz," it was given in cycles per second.

    Fletcher-Munson curves. Note that before frequency was referred to as “Hertz,” it was given in cycles per second.

     

    To counteract this effect, a loudness compensation circuit modifies the signal as you turn your volume control down, lowering the volume less in the bass and upper treble than at mid-frequencies. Today, that’s usually controlled by a simple switch rather than a knob, though I’ve seen fairly recent gear from Yamaha, Marantz, and others with loudness knobs.

    Loudness switches are less confusing and expensive than variable loudness-compensation knobs and they do help restore a natural fullness to the sound, adding compensation as the volume is turned down. But variable loudness controls offer greater precision and accuracy, because the position of the volume knob is far from the only factor governing how much compensation is needed.

    Proper compensation varies with both the sound level of the original performance and the difference between that and the sound level reaching your ears at home. According to the Fletcher-Munson curve [2], reducing the volume 30 dB would require about 8 dB of boost at 100 Hz for sonic accuracy if the performance’s sound level was 100 dB SPL – but would require about 25 dB of boost if the performers had been playing at 70 dB SPL.

     

    ISO 2003 loudness curves, compared to the Fletcher-Munson curves.

    ISO 2003 loudness curves, compared to the Fletcher-Munson curves.

     

    So, the first factor that loudness compensation must take into account is the recording or broadcast you’re listening to. How loudly were the performers playing? And how much headroom did the recording and mastering engineers leave between the loudest recorded note and the point at which the signal would become distorted? And how are these affected by the output levels from your phono cartridge, CD player, or streaming device? The amplifier’s designer can only guess.

    That designer will at least know your amp’s input sensitivity, its gain for every volume setting, and its output power – but not the sensitivity of your loudspeakers. Skip’s 10-watt amp could deliver a lot of sound from Klipschorns, which were and are among the most efficient speakers ever [3], but the results using acoustic-suspension speakers like the then-new AR-3 [4] would have been much quieter.

    The room’s acoustics are another variable. Sound levels in a large room with a lot of carpets and upholstered furniture will be lower than in a small room with fewer soft surfaces.

    For an amplifier with a simple loudness switch, a designer will calibrate the compensation circuit for average recordings, played through a speaker of average sensitivity, in an average room. That will satisfy most listeners, because few pay close attention to the sound in background listening. But if the designer wants his amplifier’s loudness compensation to be really accurate, he’ll give it a knob that lets you adjust it for this recording, over your system, in your room.

    How do you set that knob? According to both a Yamaha manual I found online, and an article I wrote in 1965, you adjust such controls by raising the volume to the highest level you expect to listen at, then you use the loudness knob like a volume control. That fits with my recollections of Skip’s amp, whose volume and loudness knobs were both the same size.

    But I’ve also heard that you should play music at normal volume, then turn the volume down and adjust the loudness compensation until the frequency balance sounds the same as it did before you changed the volume setting. After that, leave the loudness knob alone, because you’ve now set loudness compensation for the variables in your room and system (at least for one of your signal sources) – the next time you’ll need to touch that knob is when you change your room’s furnishings or your system’s components. That fits with the fact that loudness knobs on modern amps I’ve seen are usually much smaller than volume controls.

    I don’t really know which version is correct, but I’ll bet someone who does will soon tell us in the Comments.

    Even if Skip’s still listening, I doubt he’ll care. Though his amp’s loudness control was meant to preserve realism, he joyfully perverted it to give him the unreality he wanted – fun over fidelity. And nowadays, he’d have a better way to make his walls rock: subwoofers.

     

    [1] In 1957, D.W. Robinson and R.S. Dadson, of Britain’s National Physics Laboratory, produced a new set of curves, which became an ISO standard 30 years later. That standard was revised in 2003

    [2] Chosen here because it’s the simplest curve, and easiest to read.

    [3] Rated 105 dB SPL for 1 watt at 1 meter, though 1959 models might have been a bit different.

    [4] I don’t think AR published a sensitivity spec, but I’ve seen it quoted as 83 dB, which means it would have taken about 100x as much power as the Klipsch to achieve the same output.

    Header image: the variable loudness knob on a Yamaha receiver.

    11 comments on “The Knob With the Misleading Name”

    1. Great article
      I studied with JD HARRIS a well known
      Psychoacoudtician who published many articles on loudness growth etc.
      Also George Von Bekesy was one of my profs.
      Just wondered if the test parameters for the FM studies were similar to the 2003 study.
      What is interesting is what happens in the cochlear with increased volume.
      Enjoyed the article

    2. The loudness switch attempts to primarily compensate for frequency response in the context of volume. As it was pointed out other variables include average listening room, your equipment, mastering of the tapes, etc…… everything in the recording path, but mostly importantly personal taste was not mentioned. Recently I listened to Bob Dylan’s Biograph, the 60’s NY recordings were sonically dismal, “Slow Train Coming” was better, however when I listened to Dire Straits “Making Movies” I was delighted. Where in the sonic path was the failure, recording equipment, playback equipment, mastering failure, personal preference etc. At this point I am considering an EQ device to manage past preferences made by recording engineers, recording and mastering equipment etc.. Why is this solution sometimes pooh-poohed in the audiophile world.

      1. Every circuit element–especially active ones–in the signal path has the potential to degrade the signal. Hence audiophile avoidance of EQ, tone controls, etc. (I like the middle path of including circuit modifiers but letting them be switched out of the signal path when not in use.) That said, it is not necessarily true that this potential is always realized, or always audible when it is.

        1. Hooked up a mid 80’s low end EQ, inserted it into a tape loop path. I am able to switch this EQ in and out of the signal path as well as switch all tone controls out of the path via source and the direct path tone control.
          Since I do not rattle the walls with volume and drive power hungry speakers I rely on the loudness and tone controls to boost the lower frequencies. I listened to some Texas swing, Asleep at the Wheel, and found no EQ necessary. I then played some Bob Wills and the Texas Playboys and it needed EQ. What was a lack luster recording was transformed into a sound not quite rivaling the Asleep at the Wheel recording, however, it was a very enjoyable recording. I believe EQ is an absolute necessity in managing less than acceptable mastering, older recording technologies, or whatever, providing enjoyable listening experiences.

        2. Ivan, in my forays through audiodom it took me about 20 years to fully resign myself to the detrimental effects of accumulated circuitry between the source & my ears. Even with “no effect” activated, the signal was clearly degraded when I cycled the Tape Loop into which any external processor had been inserted. OTOH, the subject traditional loudness circuit was an extremely minimal R-C tack-on to the volume pot – introducing no further circuitry, and with a very broad phase shift (I’d claim to be inaudible). It can’t be implemented on volume controls since mid-Nineties because most either no longer carry the audio signal or are circuit-based only – they regulate volume on a voltage bias through a tang on a chip. Or are digital, to which the loudness concept is entirely inapplicable. I still have a bucket of tapped pots, but they’re of no use in most of today’s “affordable” gear.

          I suspect you’ve heard enough of me, but I couldn’t resist chiming in on a beloved audio tweak.

    3. I miss the loudness comp mode more than about anything else in modern audio head eqmpt [except Tape monitoring loops]. I greatly enjoyed this feature since my first apartment in 1970 – boosting highs, as well as lows, was indispensable for maintaining body in music without bothering neighbors; there was an added benefit in my early apartments by suppressing mid-range glare off the extremely live walls & ceilings. The Yamaha solution was very effective, as the varying levels of different sources could make simple “loudness” comp a bust on some material or with very high/very low efficiency speakers (noted by Mr. Mihalka above).

      Ivan: the kissin’ cousin to loudness comp is dynamic range compression (COMPRESSION ??!!) to permit enjoyment of a great Firebird Suite or much of well-recorded Debussy work. Conversely, I hope I don’t get blacklisted for dare mentioning dynamic range expanders for some very flat cuts. And, yes, these devices were quickly switched in & out of the audio chain by the good ol’ Tape loop.

      1. I had one of those. It was a dbx box with a knob and some blinkie lights, one red and another yellow. It expanded the dynamic range of really flat tracks. Back in those days, many of us listened to music in really “dead” rooms. Remember the couches with such great “suck” you had a hard time getting out of once you sat down, green shag carpets, swag lamps, and for those in the southwest, Mexican blankets hung on the walls. It sucked the life out of bookshelf speakers and the dbx was the way to get it back. With the rise of vinyl again, I’m surprised the dynamic range expanders haven’t emerged again for the vintage vinyl enthusiasts.

        1. Musta been channeling you, Jack – same room down to the green shag, but my couch was a wonderful full-body bean bag that easily formed to whatever position you best aligned with for music – including a guest, if I so indulged. I had an RG Dynamics, DBX, Pioneer RG-2, but the smoothest & least detectable was the Phase 1000. I was in the audio biz in the Seventies, so I wrung ’em all out. I might still have a Phase, but each one I acquired soon suffered from poor quality switching/pots, and I tired of swearing & chasing intermittent opens all the time. Besides, expansion was as sensitive to varying input levels & sources as a fixed loudness was, so I had to keep notes on settings per album. I was such a twisted audiophool in those times that I actually banked my favorite listens with settings on my Teac 6010. Old, old, bygone times, indeed.

    4. I didn’t give Skip’s full name in the article because I wanted his permission first, and couldn’t remember his real first name. But I thought he might get a kick out of his 15 microseconds of additional fame, so I tracked him down this evening. Alas, Gerald L. “Skip” Koff died in 2014, shortly after retiring as a professor of psychology at Walsh College, in North Canton, Ohio.

      But according to his obit (https://www.legacy.com/us/obituaries/indeonline/name/gerald-koff-obituary?n=gerald-koff&pid=173360718 ) he remained an audiophile for life. In his youth, he worked for a high-end audio dealer in Philadelphia; after moving to Ohio, he “took a sledge hammer to the interior walls to create his ideal listening room, then carefully assembled a system with speakers from Greece and French vacuum tube amplifiers. His preference in sound was analog, not digital. It was purer and truer—like himself.”

      But what particularly impressed me in his obit was this: “His teaching emphasized critical analysis based on facts which the media often overlooked.”

      Hear, hear.

    5. I have a 1986 Yamaha R-9 receiver that featured a “New Continuously Variable Loudness Control” that remains in service to this day. The brochure and the manual discuss its operation in great detail. The design was such that the compensation control was intended to be used to adjust the volume once the source material was selected. As mentioned in the article, you set the max volume with the inner volume control, and then used the outer ring compensation control as the “volume” control. This provided as much as -40db of compensated attenuation. It was wonderful and I used it with wonderful effect for almost 30 years until I purchased a new AVR receiver and relegated the wonderful amp in this receiver to driving zoned auxiliary speakers in my system. As an aside, this was a wonderful receiver that featured a 125 RMS amp that could also provide pure class A power. It’s built like a tank with massive heat sinks and lots of head room. It’s phono input, both MM and MC were terrific, as was the FM section.

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