Understanding Attenuators - Know What You're Hearing
By Michael Farnsworth
A properly designed attenuator can be a minor miracle of sorts. For instance, they can keep you from being booted out of your apartment, encourage your pets to stay home more often and have probably saved plenty of marriages. They also prevent hearing loss, decrease complaints from sound engineers and eliminate death threats from fellow musicians. As a guitar player, I can confirm they're versatile and useful devices. They can be used not only for bedroom practice but also for rehearsals (with reasonable drummers) and most especially in recording sessions. One often overlooked reason to use an amplifier attenuator is simply because you want to hear what you want to hear from your amp at the volume you're most comfortable with. After all, our hearing generally doesn't get better with age!
Okay, I'm going to get technical for a sentence or two. What are attenuators? A device that "attenuates" in the audio world is a device that turns the sound down. It does this by diverting some of the electrical energy present in a circuit from the signal path to either "ground" or heat. (The Weber MASS attenuator I bought incorporates an ingenious combination of both methods thus preserving the accuracy of tone.) Although many people call these things "speaker attenuators" they don't really attenuate the speakers - that's what baffles are for. These devices are amplifier attenuators; they attenuate the energy coming out of the amplifier before it gets to the speakers.
Before I bought my attenuator I spent hours and hours doing research on the web. As a writer, I do research almost every day of my life and, in the case of looking for something I want/need like an attenuator for a guitar amp, I have a couple added advantages: I've played since 1961 and have owned many models of classic Fender and Marshall amps as well as Gibson and Fender guitars right back to tweeds and PAF's. I also served over thirty years hard time as an audio engineer and sound designer, so I'm very, very picky. I concluded the only way to judge if the MASS was right for me was to hear it for myself. I ordered the Weber Mass model that can handle high and low-powered amps up to 50 watts continuous and put it to work on an Epiphone Valve Junior, a couple of old blackface Fender Deluxe Reverbs and a '63 Fender Blonde Bassman.
After firing up the MASS, I was immediately delighted and satisfied. I remembered some things that were mentioned in a few online reviews regarding sonic weaknesses or undesirable artifacts. One thing I read at the Weber site was that an attenuated amp won't sound exactly like a "loud" amp, but no detailed explanation was given. I also read some reviews where folks with attenuators complained about this sort of phenomena. So, why wasn't I disappointed? Simply put, I knew what to expect and why. It's all got to do with perception and the physics of the way we hear, plus some basic acoustic principles.
Ever notice the "loudness" control on a home stereo amplifier? Most folks don't know what this thing really does even though they often use it. The loudness control is designed to compensate for the way we hear at low volumes. Human hearing spans a remarkable volume range and the amount of treble and bass (high frequencies and low frequencies) we hear depends on the volume at which we are listening. Turn the loudness function on your stereo off (if you have one) then take a CD with a full and well-balanced audio spectrum and you can demonstrate this for yourself. Turn it down as low as possible, just barely loud enough to hear anything at all and listen. If there are vocals you will likely be able to hear that mid-range content but little in the way of cymbals or bass. If it's an instrumental CD you may hear whatever instrument is in the mid-range. The full frequency spectrum is virtually inaudible at low volume. It doesn't matter if you're the most golden-eared wunderkind in the universe, that's just how we hear. Our hearing is most sensitive right in the middle at around 1,000 hz. The further away we get from that point in either direction, higher or lower, the less we hear of the extremes relative to the midrange content when the volume is low.
Now, slowly turn the volume up and listen closely. You will start to hear more bass come up in the balance first, then you'll hear some high frequencies start to come in until you reach a fairly loud (but still listenable) volume of between 82 and 89 decibels. That's the volume level that is most spectrum-balanced for human beings. If you live in an apartment, you're probably starting to push your luck with the landlord at this volume, but doesn't the music sound full? You can hear everything in the mix nicely balanced! Now, slowly turn the volume back down and observe what you hear ... the cymbals and other very high frequency content will start to go away along with our much beloved bass frequencies. When the volume is pretty far down, but still audible, push in the loudness button (if you have one) and suddenly you will hear a fuller sound, much closer to the full spectrum frequency balance you heard when the volume was up much higher.
Why is this? Human hearing "attenuates" higher and lower frequencies at low volumes, probably due to some inherent survival instinct. When we feel very low frequencies it that alerts us to large, moving objects that might eat or crush us. The loudness button typically boosts the bass frequency content in the sound mix by eight or ten decibels centered at 100 hz, it leaves the midrange area "flat" (unaffected) and boosts the high frequencies four decibels usually at around 10,000 hz. This compensates for the way we hear the frequency spectrum in relation to the volume we are listening at and the end result is that we hear a more enjoyable and fuller mix at lower volumes.
A guitar amplifier attenuator doesn't necessarily have that sort of compensation circuit built into it. Therefore you need to know that when you turn the volume down, it won't change the tone of the signal. It's your own hearing that creates the perceived tonal change. That's why some attenuators have high-frequency boost capabilities which compensate for the perceived high frequency loss. Most of the bass that we perceive as disappearing is actually felt almost as much as heard when played at loud volumes. Boosting the bass in an attenuator would not restore the "felt" content of the sound and would generally only make the sound mushy, so that's why most attenuators don't have them. Your pant legs won't flap from the air being pushed by the speakers at low volume and you won't feel the thump from a muted power stroke on the lower strings for the same reason: the volume of air being moved by the speaker is greatly reduced. It isn't the attenuator's fault, it's simply doing what you're asking it to do!
Sympathetic resonance, or feedback, is another way the physics of sound affects the attenuated tone. If you're using your amp cranked to let the speaker sound drive the strings for additional sustain and/or controlled feedback (think early Slowhand i.e. "Fresh Cream" or Jimi) that won't necessarily be possible when using an attenuator. This is because feedback (desirable or not) is the result of the amplified sound from the speaker traveling as sound waves through the air and vibrating anything they can. When the guitar string producing a sustained note is hit by waves which are at the same frequency as it is, this causes the string to vibrate and sustain even more. In excess, the upper harmonics, or even the pickups themselves, can go wild and produce unwanted feedback frequencies but, when controlled in a musical manner, the sound can be a thing to behold. If you're attenuating the sound volume then the audible acoustic energy sent from the speakers is diminished by the time it hits your strings, the chances of achieving additional sustain and the extra harmonic overtones is also diminished proportionally.
One other thing I read on some review sites - by a few players who didn't know about the difference between a potentiometer-style standard volume and a rheostat - is that they thought that the volume pot on the attenuator felt cheap and inaccurate. A regular volume pot will be smooth feeling because it's a wiper contact point being moved over a very smooth and usually lubricated surface called a carbon track. An audio taper pot is designed so that the volume one hears is proportionate to the way the human ear hears, not mathematically logarithmic. If you've ever used a "log" pot on an old amp where all the volume seems to be in the the 0-4 range and little changes between 4 and 10, you'll understand what I mean. The Fender Deville amp's master volume works like that and this has produced a whack of folks developing little kits to put an audio taper pot in the effects loop to give finer grain control as a second master volume. A rheostat, which is needed on the MASS to handle the heavy current of an amplifier output transformer, is a loosely-spaced coil of wire wrapped around a 'former' which holds it in place. The wiper feels rough compared to a regular pot when turned because it isn't traveling over a smooth surface, it's traveling over winds of copper wire. This also accounts for why the volume level isn't infinitely variable, meaning that it doesn't have the fine grain control a pot does. It may look like a regular volume control but it isn't and it can't be designed or built that way due to simple physics.
The speaker load also affects the tone of the amplifier because the load the amp sees is constantly variable as the speaker reacts to it and vice versa. This is referred to as "reactance". Amplified music creates a signal that is constantly changing in frequency and volume therefore the magnetic field of the speaker coil changes, too. The amplifier and speakers together create one big circuit that have interactive tonal characteristics and having the attenuator in the circuit alters the characteristics of this interaction to a degree. It's similar to (but not exactly the same thing) as the difference between speakers with AlNiCo magnets and ceramic magnets. The AlNiCo magnets create their own natural compression when pushed at higher levels whereas the ceramic magnets don't in the same audible way. Since the speakers are being driven with less juice after the attenuator they may not reach the sweet spot where that natural AlNiCo compression is achieved. One very nice thing about the MASS (and a major reason for me choosing that model and type of attenuator) is that is has an actual speaker motor in it which is designed to create the interaction described above. The speaker motor doesn't make any sound but it does look the same as a speaker to the amplifier, electrically speaking. This emulates the reactance effect that a speaker would provide. You can't have everything with an attenuator but I think the MASS does its very best to give you everything possible.
Now, in the real world, what does all this mean? Let me give you an excellent example. The weekend before the MASS arrived, I decided to use the Epiphone Valve Junior as the primary testing amplifier. I spent a couple hours going through all the tubes in the storage container and there were maybe twenty various old RCA's, GE's, Sylvania's, Mullard's, some new Tung Sol's, JAN Philips, EH, TAD's, Ruby's, and, well, you know the routine. In order to thoroughly test the MASS, I wanted to be able to dime the amp while using a preamp tube that would provide infinite sustain yet be balanced at all frequencies. My wife (for some unfathomable reason) found it completely beyond comprehension that I would play the same licks over and over at full volume while wearing industrial hearing protectors. (She remained outside gardening for the rest of the preparation procedure looking miffed and, when it was over, came back in to complain about the volume she could hear outside.) The MASS arrived and I dimed the amp but turned the MASS down as low as possible and played for a while. She didn't interrupt to let me know she was being disturbed, so I sought her out and asked her what she thought of the tone. (She has amazing ears and I rely on her opinion for getting a quick objective opinion on how something sounds. This is one very good reason she is a 'keeper'.) She said it sounded really loud but not as loud as during the preparation procedure. Happily, I didn't appear to be in hot water.
Later that evening I was playing at exactly the same volume through the MASS and she was in the adjoining room making dinner with the TV on across the house. After ten minutes she poked her head in the door and said, "That volume is fine, you can play it like that as much as you want and it's okay." Great, but I noticed she looked a bit confused and later I asked her why. She said it had the sound of an amp that was turned up way too loud, but she realized that, in reality, it wasn't loud at all. It just had the sound of an amp that's being played loud.
Bingo, blam, direct hit! The sound I wanted had finally arrived.
Now, here's a wonderful revelation. When you use an amplifier/speaker attenuator in the extreme and then mic it for recording, you'll notice that all of the amplifier noise will be completely gone. At least it was in my case. If you have three hundred noisy stomp boxes in your path going into the preamp your mileage may vary ... but in my case it was silent and that's a word I don't use lightly. So, it sounds very loud, but physically isn't, which makes it easier to record huge meaty tone in a smaller room and the random noise is either gone or so severely attenuated it doesn't cloud your mix. What's not to love?
As an added bonus, my wife has heard overdriven compressors, tube screamers, cranked tube preamps, etc., etc., etc. before, but this was the only time she ever said it sounded loud at low volume. Coupled with the fact she listens to hours of music every day and has great ears, this tells me that using the MASS has brought me much closer to the end of my personal rig Tone Quest. If it fooled her, it can fool any microphone. And for playing live, I can keep the stage volume at humane levels while letting the PA system provide any needed horsepower.
Now I have two keepers!
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