Choosing midbass drivers

[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

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/E ... erfall.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:

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/sho ... php?t=2698
http://www.diymobileaudio.com/forum/sho ... .php?t=617
http://www.diymobileaudio.com/forum/sho ... php?t=1025
http://www.diymobileaudio.com/forum/sho ... .php?t=832
http://www.diymobileaudio.com/forum/sho ... .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. I'm going to sit back, rest my fingers a bit, and wait to answer the first round of questions :)

[High Society]

Why isn't this in the FAQ section?? lol

[Jonathan]

Maybe once it's all finished

[JeremyC]

Holly crap man. Get this dude a book deal. I think I just felt my head get a little bigger, and I have a big noggen.

[Ryan from Ohio]

Will be in the FAQ when Jon requests. Quite a bit of info here!

[Team AI]

Wow I think I may begin to understand this once I have read it about 10 more times. I'm glad you posted this one as it has a lot of good information an I soon hope to start my first SQ project in my wifes car and the more I learn before I start them more I know once I begin. Thanks I can't wait till this is all completed Jonathan and will be adding it to my private information library with your permission so I have it on hand for review purposes.

[joeyc]

I just installed the new bravox carbon fiber's in my truck jonathan, and as we all know carbon fiber is a VERY rigid material. HOWEVER, the carbon fiber in these bravox carbon has NOT been baked. It is still in its material form....sounds great though. weird

Edit: by the way, on the post jonathan, wow i'm not sure if i have gained intelligence, or just feel stupid

[Jonathan]

I just installed the new bravox carbon fiber's in my truck jonathan, and as we all know carbon fiber is a VERY rigid material. HOWEVER, the carbon fiber in these bravox carbon has NOT been baked. It is still in its material form....sounds great though. weird

Edit: by the way, on the post jonathan, wow i'm not sure if i have gained intelligence, or just feel stupid

I'd like to see how those perform. Carbon fiber is still gaining popularity and research, and has very desirable characteristics if done right. Most I've seen have a characteristic double peak in upper midrange, but there have been some examples I've been very fond of. Illusion Audio's older drivers come to mind, though the quality went down once they went to India

[solacedagony]

What exactly is "energy storage"? I can't find a definition of it anywhere.

[Jonathan]

Part 2:

Motors (continued....)

There are many motor topologies and many different ways to make them perform well. Motors tend to have certain characteristics, but it's important not to stick to generalization. If you looked at the drivers linked above, you'd swear they were underhung or XBL^2 by looking at the BL curve. And all except that CSS mid use overhung motors. Bottom line is that there are excellent and poor implementations of any motor topology, and you won't be able to determine this unless you look at testing of a specific driver. 'Nuff said.

What's #1 when referring to motors? IMO-quality control. Engineering gives you an idea of what a driver is supposed to do. QC determines what will arrive on your doorstep. It's too easy to screw up a speaker. Peerless is an example of a company that has some of the best QC in the industry. Every Peerless driver I've seen tested has performed VERY close to the specifications.

How much excursion do you REALLY need? Good question. For every octave lower, you have to increase excursion 4 times. Repeat that, 4 times. Chances are if you think that extra millimeter of x-max in a midwoofer is doing something-not really. If you have a 3mm x-max midwoofer, you'll need a 12mm x-max woofer to go an octave lower at the same volume level. But do you want your driver producing delicate midrange and 50hz simultaneously? This brings my next point:

Intermodulation

We've all heard it, typically considered doppler distortion. Muddiness. Cluttering of the frequency spectrum. Loss of resolution. Fatigue. Harsh, screechy midrange.

Intermodulation is the interference within speaker components. As you can imagine, this increases as more power is applied. Excursion at low frequencies tends to modulate middle and higher frequencies. And at the top end of the spectrum, the chaotic resonances and vibrations that occur within a speaker cone can cause lower frequencies to sound harsh.

How to prevent this? There is a commonly accepted theory that a speaker should produce no more than 3 octaves. If you have a midbass driver covering 80hz on up, this would mean that it would hand off to a dedicated midrange driver at 640hz. So on and so forth.

As you're probably thinking, this is difficult to do in a car. 2 ways are generally the best thing for most vehicles. My recommendation: If you can't do a dedicated midbass driver, then don't try to get a midwoofer doing 50hz. Cross it as high as possible without destroying the imaging and upfront bass. With some tweaking you can make this work. Remember, 100hz is an octave higher than 50, meaning it requires 1/4 the excursion that 50hz does. This cuts down distortion dramatically.

So, to summarize.

When choosing midbass drivers you want to make sure that:

1: You know where resonances lie, if they need specific attention, and if a driver performs well enough to make your list.

2: Consider the dispersion capabilities of the driver, especially if you are going to be installing in a location that is far off axis.

3: Choose a driver of good motor design. Also choose a driver of good quality control.

4: Don't make a driver do too much. Using a single driver over a wide range is rarely as desirable as using a driver intended for a specific bandwidth. The most attention should be put into not trying to make a midbass play too low for your intended listening level.

Part 3 will discuss choosing drivers based on the limitations of the car environment.