 Drivers and the Myth of Tweeter Dome Materials
by Alan Lofft
One recent morning, I put on a new CD I’d received from Amazon, an orchestral work—California Suite – plus several piano concertos, by an obscure and overlooked 20th Century American composer, Hubert Headley, and relaxed with my second mug of coffee.
As I listened to a percussion and timpani passage, there was the metallic “ping” of the tiny triangle deep at the rear of the soundstage, and I thought, “Isn’t it amazing I’m hearing a triangle reproduced so lifelike that it reminds me of how the triangle sounded at the rear of the Carnegie Hall stage at a recent concert.” (The recording, quite nicely done, was actually made in Moscow in 2006 for the Naxos American Classics series, with the Russian Philharmonic conducted by Dmitry Yablonsky and Anna Bogolyubova, piano.)
Modern Marvel
All of us take for granted lots of modern marvels, but it nevertheless still impressed me in that instant that a tiny reciprocating electro-mechanical motor— the voice coil of the Axiom M22 tweeter—moves a titanium dome back and forth 10,000 times per second to precisely reproduce that convincing triangle sound. (The triangle is an instrument of indefinite pitch but the bulk of its output is centered around 10,000 Hz.)
Think about it for a moment: the audio voltage, an AC signal representing the triangle’s complex acoustic spectrum zips out of the output transistors along the speaker cables reaching the tweeter’s voice-coil and magnet structure. In the second or so that the triangle sound resonates and decays, the voice-coil moves a few millimeters back and forth 10,000 times, and so does the tweeter’s titanium dome, compressing and rarefying the air molecules at the same rate (10 kHz). These rapid pressure waves travel across the room and reach my ears 10 milliseconds later (sound travels about 1 foot per millisecond—my couch is about 10 feet from my speakers).
Eyes closed, the better to imagine the orchestra spread across the front of my living room, I react by thinking, gee, now that sounds real. It’s just as amazing that my eardrum also vibrates at the same rate and my brain interprets that signal as “triangle”! But that is a subject for the future; for now we’ll stay with drivers.
Silk vs. Polypropylene vs. Titanium/Aluminum
Why titanium and not silk or cloth for the tweeter dome, you might wonder? Isn’t cloth or silk inherently softer than metal, hence better suited to reproduce music? This touches on a common misunderstanding of loudspeaker design, especially of tweeters, by some professional reviewers and lots of enthusiasts. They believe that a hard dome material imparts a “hard” or harsh quality to the sound, or conversely, a “soft-dome” tweeter made of cloth or silk will yield “soft” or “silken” traits to strings and other sounds. This is nonsense, based on lack of knowledge of how a tweeter dome operates.
It is not the material the dome is made of that imparts so-called musical qualities to loudspeaker sound reproduction; it is the linearity or accuracy at which the dome precisely vibrates, without distortion, to recreate the waveform of the triangle that convinces us of a triangle’s sound. The lighter the dome is, the less energy it takes for the voice coil to easily move it back and forth, stopping and starting its travel as the voltage moves between positive and negative cycles. Ideally, a tweeter dome should exhibit perfect pistonic motion during its rapid back-and-forth excursions. The dome must be rigid and not change shape during these rapid fluctuations. Titanium is ideal – light yet rigid—and functions as a heat sink to drain heat away from the tweeter voice coil during loud dynamic swings in the music. If cloth or silk doped with a stiffener were used to shape it into a dome (still used on many speakers) or polypropylene (plastic) material, it might be less rigid and could change shape, especially from overheating, which would alter the accuracy of the reproduced waveform just a little, perhaps enough that the triangle sound might not have that lovely ring. During a recent double blind listening test of a competitor’s speaker, I described the treble and midrange as “shrill” and “screechy.” The tweeter was a cloth-dome design, so if the material made any difference, it should have sounded “soft,” correct?
In fact, it was the jagged and unbalanced frequency response of the competitor’s speaker that made it sound shrill, not the cone or dome materials. Over many years of double-blind listening tests both at Axiom and at the National Research Council in Ottawa, I’ve heard speakers that used a variety of different materials for the tweeter dome. Some cloth or silk dome tweeters exhibited edgy aggressive highs, but when the tests were over, you could quickly spot the jagged, unbalanced treble frequency response in the measured curves that was responsible. Similarly, I’ve praised some speakers that used aluminum and metal-dome tweeters very highly and downgraded others. It’s all about smooth balanced frequency response free of jagged nasty resonances, not about the driver dome or cone materials.
Getting Loud
What happens inside a tweeter when the music gets loud? When a big cymbal crash comes along, a bigger voltage hits the voice coil and it momentarily heats up, but the Axiom titanium dome helps conduct heat away from the voice coil. Moreover, the voice coil is immersed in a bath of magnetic ferrofluid to help move heat away from the vibrating voice coil (ferrofluid has the ability to increase heat transfer from seven to ten times that of air). This is one reason why Axiom tweeters can cleanly reproduce very high-volume sound and cleanly handle dynamic peaks.
For its woofers and midrange cone drivers Axiom uses aluminum because it, like titanium, is lightweight and rigid, so the voice coils can easily move the cone using fewer watts, hence gaining sensitivity and efficiency. Like the tweeter, the midrange and woofer voice coils on Axiom speakers are connected to the aluminum cone, so again the cone works as a heat sink, cooling the voice coil and enabling it to accept higher power levels and play cleanly and loud without problems. The rigid nature of aluminum also helps keep the cone moving as a perfect piston without changing shape, ensuring that the acoustic waveforms of sounds are accurately reproduced.
How It Works
You may be curious about why the voice coil/magnet assembly is termed the “motor” of a loudspeaker. It’s called that because a finely wound coil of wire (the voice coil) surrounds a permanent magnet pole piece but it is separated by a tiny gap. It becomes an electro-magnetic motor when an electrical signal is applied to the voice coil. As the varying audio voltage surges through it moving between positive and negative swings of the waveform, the magnetic properties of the voice coil change and interact with the surrounding permanent magnetic field, causing the voice coil to move back and forth with the varying audio voltage. The voice coil drives the cone or dome to which it’s connected (hence the term “driver”) which moves along with the voice coil, compressing or rarefying the air molecules in front of the dome or cone. And when those pressure waves hit our eardrums, presto, sound!
Woofers
Midrange cone drivers and especially woofers must use much larger magnets, voice coils and cones because they have to move a lot more air to reproduce lower frequencies than tweeters. Bass instruments have long wavelengths; hence the reciprocating woofer cone must generate lots of big pressure waves. That’s also why the woofers consume much of the power (watts) because the large motor assembly needs lots of watts to move the big cone back and forth. This introduces problems of excursion—the distance the voice coil and cone must move to produce loud deep bass. If the voice coil moves too far, it may move out of the magnetic field of the speaker magnet. When that happens, the cone movement becomes non-linear, so it distorts or doesn’t accurately reproduce the audio waveform. One investigator found that a 15% movement of the voice coil outside the magnetic field is equivalent to about 3% distortion.
To ensure accurate translation of the audio electrical signal into deep bass sound waves at higher volume levels, the voice coil can sometimes be made longer so the cone travel (excursion) can be increased, hence the cone can move a greater distance without moving the voice coil outside of the magnetic field. Long excursion woofers may be equipped with dual voice coils, as they are in all of Axiom’s subwoofers. Wired in parallel, these dual voice coils present a lower impedance to the amplifier, letting the amplifier deliver more current and power to the woofer.
There’s much more to good loudspeaker design than I’ve covered here, but this brief tutorial should help you appreciate the amazing accuracy and operation of Axiom loudspeaker drivers. In future newsletters, we’ll look at other aspects of loudspeaker design. |
Audio Oddities: Solving Subwoofer Troubles
It’s not uncommon for a new subwoofer owner to have problems getting the subwoofer to produce audible output. More often than not, it’s usually a matter of choosing the right menu items in the AV receiver, but there are some simple tests you can make to ensure that the subwoofer itself is activating properly and producing output.
Don’t get frustrated if you initially don’t hear anything from the subwoofer. You can easily test it. Disconnect the AV receiver subwoofer cable from the sub input jack and make sure the sub is connected to AC power and set to “On” or Auto-ON. You will need a portable iPod or music player of some kind with a headphone output-to-male-RCA jack cable. Set the sub’s own volume control midway in its rotation and connect the RCA cable from the iPod or music player (a CD player will also work fine) to the sub input jack and play some music. The sub should activate and you’ll hear muffled sounding deep bass. If you don’t have a CD player or iPod handy, just slowly insert the center pin of the male RCA jack into the input. You’ll hear a sudden loud buzz. That means the sub is working. Remove the RCA pin and re-connect the sub cable from the AV receiver.
While it would be wonderful to be able to totally rely on the auto-setup modes built into new AV receivers, the new subwoofer owner must still do some initial steps.
First, you have to tell the AV receiver that you have a subwoofer and that you want it activated. While menu systems vary with each brand of AV receiver, in most cases, you’ll need to answer a prompt: Subwoofer? Yes/No. Sub On/Off? Once you answer “Yes” and “On” the AV receiver will activate the Low-Frequency Effects output (usually labeled “Sub Out”) on the receiver’s back panel. You may also have to do this not only for a movie soundtrack mode but also for the stereo mode, if you only listen to music in stereo using the two main speakers.
Most subwoofers need a significant voltage sent from the sub output on the receiver in order to “turn on” or activate the sub (be sure the subwoofer AC switch is set to “On” or “Auto On/Off”). You can regulate the size of that signal by accessing the Sub Output level control in the AV receiver menu. Set it to about +3 dB or more to get things working; you can trim the setting later on. The auto setup mode may set it too low with insufficient voltage to turn on the sub.
As to “hearing” your sub, it isn’t like a regular loudspeaker. Our ears are not nearly as sensitive to deep bass as they are to higher frequencies, so the output from a sub won’t be as immediately obvious. All you may hear is a low rumbling sound and that’s good. Once you play music or movies, however, it will fill out the bottom octaves and deliver the power of special effects and the foundation of music.
There are no absolute rules about what overall level you want to run the subwoofer at, as deep bass is partly a matter of personal preference. First-timers often run them way too loud—they expect to “hear” the sub like a conventional speaker. It will sound unnatural or boomy if turned up too high. A subwoofer should never call attention to itself as a separate loudspeaker. It should just fill out the lowest octaves of music and reproduce the “oomph” of special effects in movie soundtracks. The actual volume setting on the rear of the subwoofer may vary from one model and brand to another. Likewise, the menu setting for subwoofer level in the AV receiver will vary from model to model but typically you should have the sub output level on the receiver set to “0 dB” or +1 to +3 dB. If you set it too high, you may introduce distortion either at the sub output jack or you may overload the input at the subwoofer. Choose a setting that is neither too high nor too low, and that applies to the subwoofer volume control as well. Following these steps will ensure that you get the performance you paid for and want from your subwoofer. – A.L. |
Question of the Month: Connecting Two Amps to the Same Speakers
Q. So here may be the dumbest idea you have heard all week. I am a confirmed 2-channel guy. I recently bought an Axiom subwoofer that I plan to use only when watching movies. I have been toying with the idea of going “all the way” and setting up a 5.1 system to use separately from my 2-channel system when watching movies. I want to continue to use my existing 2-channel preamp and amp when listening to music. It seems silly to buy new main speakers when I have my existing speakers. I am trying to figure out a way to share my main speakers between a 2-channel setup and a 5.1-channel setup. It’s not possible (is it?) to run 2 amps (2-channel and 5.1- channel) into one speaker as long as only one amp is powered up at a time (two sets of speaker cables to each speaker’s binding posts)? Would this cause a short? I am trying to avoid having to switch speaker wires back and forth every time I want to switch from movies to music. If this is a really silly idea, can you think of any other workarounds? – Mike
A. It’s not all that dumb an idea, Mike, but don’t try it. And you are not the first person to ask this question. It’s not so much the possibility of a “short” but rather that if your amp/preamp were switched off when you decided to use the 5.1-channel receiver, it would be pushing the amplified audio output not only into your left and right main speakers but also into the output circuitry of your turned-off amplifier. The voltages would likely damage the circuitry of your 2-channel amplifier and possibly the AV receiver. Or the reverse would happen if you used the 2-channel amp while the AV receiver was turned off but remained connected to your main left and right speakers.
The electrons from your 2-channel amp don’t know that you just want them to drive your main speakers and not travel up the other speaker wires to your AV receiver. They just go wherever there is a copper wire connection. It’s also possible to set up a switching system that would remove the 2-channel amplifier connections to your speakers when you wanted to run the 5.1 system. There is a manual switch but I won’t recommend it because you might forget about the switch and end up turning on either the 5.1 receiver or the 2-channel amp/preamp when one or the other wasn’t connected to your speakers. Amplifiers don’t like operating without a load on the speaker outputs; they may just shut down. However, Niles makes an audio-triggered automatic amplifier switch that should do exactly what you want. The Niles ABS-1 costs about $100 and automatically switches the speakers to whichever amplifier you turn on. If both amps are turned on, it defaults to the "A" amplifier to avoid damage to the other amp.
Alternatively, consider using the 5.1-channel AV receiver in the 2-channel mode, with or without the subwoofer, and in the surround sound mode for watching movies. All AV receivers offer you that option. Or you could sell the AV receiver and the 2-channel preamp and upgrade to a separate AV preamp/processor with impeccable specs and use your existing 2-channel power amp to power the main speakers for 2-channel listening with or without the subwoofer. You’d need to add a 3-channel power amp for the surrounds and center channel that would function when you want to use the 5.1-channel mode. The pre-outs from the AV preamp/processor would remain connected to all the amplifiers, so it’s as simple as you choosing the operating mode of your AV preamp—either 2-channel or 5.1 channel.
And though you say you are a confirmed 2-channel guy, you just might discover that you actually like some stereo recordings processed through Dolby Pro LogicIIx (or dts Neo:6 or H/K’s Logic7) in 5.1 channels. Sometimes the difference in realism can be a revelation, especially with good jazz or classical recordings and most live rock/pop recordings. If you don’t like the effect, you can simply switch it off and go back to 2-channel listening! – A.L. |
