Choosing Components, submitted by Jonathan
Part 1 of a tutorial on how to choose components for a sound system. All will stay in this thread to keep from clogging up others. This is by request :D
So what's important when matching speakers together? How do you choose a speaker, and how do you determine their limits? This one covers midbass/midrange drivers, design theories, tradeoffs. The midrange region is the heart of the frequency spectrum. The critical portion of audio spectrum is considered to be 300hz-3khz, so it's important to get this one right.
Soft parts:
There are three things you have to balance with speaker cones and their surrounding soft parts.
1: Stiffness. Stiffness is critical for quality audio reproduction, as it resists torsion and flexing. In the real world and it's limitations, the rigidity of a driver determines where resonances occur.
2: Damping. Anyone who has sound deadened a vehicle can tell you the importance of damping. It's purpose is to prevent fatiguing resonances and breakups.
3: Sensitivity. Not really critical to performance, just the fact that you don't want a midrange driver requiring 1kw to move it.
These three properties, unfortunately, are exclusive. Striking a balance means that as we add to one, we're taking from another.
Philosophies:
Rigid drivers: Examples are metal cone drivers (aluminum, magnesium, beryllium), Kevlar, Carbon Fiber, Ceramic, Nomex, and other proprietary alloys and composites developed by many different manufacturers. These drivers are capable of near pistonic motion throughout their bandwidth, resulting in the lowest distortion and the lowest energy storage within their intended range. Why aren't they all over the place then? The resonance has to occur somewhere. In the case of rigid drivers (especially metal drivers), this occurs in upper midrange or lower treble, severely limiting their bandwidth and requiring a clever crossover designer.
Damped (soft cone) drivers: The most prominent are paper and polyprophylene. Past that there are plenty of variations and tons of proprietary composites from given manufacturers. The resonances of these drivers are well damped, showing no nasty peaks anywhere. They don't typically require a lot of crossover magic to get them to integrate well with a tweeter, and aren't unbearable if you play the driver where resonances occur. These typically have a wider bandwidth than a rigid driver, making them much more flexible with potential combinations. The bad? They don't perform as well throughout the passband. Higher distortion, higher energy storage.
How do I decide a suitable range for a driver
Where do resonances occur? How bad are they? What will I have to do to compensate? These are questions you will (hopefully) be asking yourself when choosing a driver. We, the audiophiles, must depend on both manufacturer specifications as well as independant testing data.
Let's look at a rigid cone driver:
http://www.acoustictechnology.sg/t_reference.html
This is a technical overview of the SEAS Lotus driver. I chose it to keep you from having to download PDF spec sheets. Load the FR chart and the harmonic distortion graph. See that breakup at 4.5k, and the resulting distortion spike? We'd have to take that into account. More specifically, we'd deliver a precision strike to ~4.5k down with a notch filter ;)
"Not too bad" some may say.
Not so fast. Look at the distortion chart again. What are those peaks in distortion BELOW 4.5khz? Harmonics. 2nd and 3rd order harmonics, to be more specific. Why? Harmonic distortion can still excite this resonance peak, which requires us to cross the driver lower. A general rule of thumb is to cross a driver over roughly 2 octaves below any breakup peaks. Assuming that the breakup is at 4.5khz, abiding by the 2 octave rule means we will be crossing the driver over at about 1.1khz. Yikes, I pity the poor tweeter that would have to take over duty there. So what they end up doing is crossing the driver so that acoustic rolloff occurs at roughly 1.7khz, which is roughly 1.5 octaves down from 4.5khz. 3rd order harmonics affect a breakup 1.5 octaves above that, so it works pretty well. Desireable? No. Tolerable? Sure. But what tweeter to mate with it? It takes a beast of a tweeter to deliver with that low a x-over frequency. More on that later.
It takes a good motor to make a good driver (this applies to any driver). Repeat that to yourself as many times as needed :). With rigid drivers this is moreso. The lower distortion, the less their peak is excited. Ever wondered why a lot of metal dome tweeters can be so harsh? Especially the cutesy palm in your hand neo compact car audio domes? Turn the volume down. Chances are they aren't too bad assuming all is set up correctly. But the more and more the volume turns up, they get harsher, don't they? And by the time you're at their limits, they're screaming at you, aren't they? Your typical 1" metal tweeter has a breakup around 25khz. We can't hear 25khz. But if we have a breakup at 25khz, that means that 2nd order distortion can excite this peak, which means 12.5khz would be able to excite this peak, and this breakup can bleed into the audible region. And 3rd order distortion means that ~8.3khz is capable of exciting this breakup. This means you need a motor with extremely low distortion.
So why not do a damped driver? It doesn't have these limitations of crossover right?
Sort of. Let's look at a "soft cone" driver.
The Adire Audio Extremis:
http://www.adireaudio.com/Home/ExtremisMidwoofers.htm
Distortion is low, no crazy breakups in FR. Should be easy to mate with a tweeter. But look at the waterfall:
http://www.adireaudio.com/Home/Images/Extremis%20Waterfall.gif
See that energy storage below 2khz? In case you've never seen a waterfall, it's that big peak around 2khz-it's the one that's sticking out in the open. So we have pinpointed where resonance is at it's worst with this driver. The bad news is that it's a lot of energy storage, and it's in a very audible frequency range. It's also in a lower frequency range than the above driver. The good news is that it doesn't cause any crazy spikes in distortion or FR. Because it is damped, we don't have to take the same measures that we did with the above driver. But the breakups of both drivers are very much audible, we just treat them differently. Remember what I just said above:
Jonathan wrote: Desireable? No. Tolerable? Sure
So that SEAS Lotus midwoofer shouldn't be crossed past 1.7khz at the most, but the Extremis? It's damped enough that you could run it above 3khz if you wanted to. Maybe even higher, your choice, but it will partially be determined by my next point:
Dispersion]
Drivers are only so good off axis. What is this relative to? The size and shape of the cone. If the short high frequency waves radiated from one side of the cone meet the same sound waves from the other side of
the driver, those sound waves will interfere to cancel and reinforce each
other. The result: Uneven frequency response and poor off-axis performance.
Sometimes this is countered with phase plugs, but they are only good to an extent. What's important to remember? Once the wavelength becomes shorter than the diameter of the speaker cone, "beaming" will occur. In other words, off axis dispersion suffers. If pushed high enough, dispersion literally narrows to a beam.
For a 7" midbass driver (or "oversized 6.5"), this typically occurs around 2khz. Meaning your off axis dispersion will suffer past that. Just take a look at a FR graph and notice where the off axis plots fall off at. Cone shape can aid to an extent, but like phase plugs they are not a cure.
I found this chart a while back:
Speaker Diameter (inches)
Theoretical Maximum Frequency Before Beaming (Hz)
.75 18,240
1 13,680
2 6,840
3 5,472
5 3,316
6.5 2,672
8 2,105
10 1,658
12 1,335
15 1,052
18 903
Just keep the word theoretical in mind.
Motors
So we've covered theory of soft parts. Now the most important part: the motor. It doesn't matter what kind of speaker you're making, the motor is the backbone of a good speaker driver.
Is it about crazy x-max? Not always. With a midbass/midrange, what's most important is the consistency. What consistency? A flat BL curve and a flat inductance curve Low inductance is important, too, but it must also be consistent over the range of a driver's excursion. The good news is that motor design has improved substantially with the aid of computers-primarily the ability to analyze motors via FEA, Klippel and other tricks. What are some good examples of speaker motors? Some of the best I've seen and dealt with come from Peerless, Scan-Speak, SEAS, and the XBL^2 units. A couple of examples:
http://www.diymobileaudio.com/forum/showthread.php?t=2698
http://www.diymobileaudio.com/forum/showthread.php?t=617
http://www.diymobileaudio.com/forum/showthread.php?t=1025
http://www.diymobileaudio.com/forum/showthread.php?t=832
http://www.diymobileaudio.com/forum/showthread.php?t=842
I will further delve into motor design later. This is my first stopping point. If it seems incomplete, it is. There is more on the way, it's that I've posted a lot of graphs, charts, said a lot of "lingo", and made a lot of points in a really long post.