Pickups and Saddles
How sound is generated
As noted in the section on tonewoods, the sound from a uke is generated by plucking a string. Strings themselves generate almost no audible sound, but their vibration is translated through the saddle, to the bridge and from there to the top, which creates the audible sound.
The top vibrates forward, creating a sound wave that rushes forward to the listener. How well the top vibrates creates the quality of the note.
The top also vibrates inward. This sound bounces around inside the uke and is shaped by the inside space as well as how it reflects off or is absorbed by the back and side woods. This second wave is released through the soundhole as a changed and more complex pattern. The listener hears both, very close together, as a rich, complex tone full of harmonics. See the section on saddles, below for more information.
When you are using a pickup, the sound you get comes from the saddle vibrations, and the complexities of the tonewood top, the inside sound wave and the harmonics are less noticeable. The tone can be rather simple and lacking in depth. That's why some players prefer to use a microphone to capture the sound coming from in front of the instrument, rather than from a pickup. The mic'ed sound is more representative of the actual acoustics of the uke than the electronically reproduced sound.
And that opens another debate on dynamic versus condenser mics. Usually you'll want a condenser mic for recording at home (plus it will need a pre-amp or power source). Dynamic is the better choice for vocals, however. But omni-, uni- or bi-directional? Unidirectional is best, unless you have multiple instruments being played simultaneously and only one mic to record with.
Acoustic ukulele pickups come in two flavours: active and passive. Each has its advantages, as well as disadvantages. Both are based on a piezoelectric element located either under the saddle or in the bridge.
Electric steel-string instruments use magnetic pickups, a different technology. Their pickups are essentially antenna with small magnets that vibrate within a coil of wire. More details on these are also here.
Active pickups have an onboard pre-amp and often various controls such as tone or volume. Some, like the Applause have more features, such as an three-band equalizer, mid-shift and wave shape. The Riptide pre-amp offers separate bass and treble controls, plus a tuner. The Boat Paddle, however, has no controls whatsoever and depends on the amp to shape the output, although it does have a pre-amp.
On ukes the active pickups I've seen are all piezoelectic. Steel string pickups use different technology, including magnetic coils.
Active pickups boost the signal, so are often louder than passive pickups and the controls give the user an opportunity to balance the sound (sometimes including tone) before it gets to the amp. This is often important when more than one player is sharing an amplifier.
Active pickups are described as having a "hotter (louder) output than passive pickups. There is less signal loss on the way to the bass amp."
Active pickups have another drawback: they depend on an external power source to work. This requires a battery, often a small one near the side and easily changed, or, in the case of the Applause, a larger one very inconveniently hidden inside the body. The Boat Paddle has a rechargeable battery that doesn't need changing but does need recharging. Batteries can last months to years, but it's wise to have a spare one on hand if you plan to play for an audience. When the battery dies, the pickup stops working.
Be aware that batteries should be checked at least annually, since they can corrode in place and damage the components if left unchanged.
Active electronics also require the uke body be altered to accommodate the circuitry, usually involving a section of the side cut out. This may subtly alter the tone of the sound coming from the soundhole, since it will affect how it bounces around inside the body.
Passive pickups are also piezoelectric, and send a raw signal to the amplifier, and are often softer in volume because the signal is weaker. They often have no controls to moderate the signal, so all processing must be done at the amp. When they do have controls, they are designed to merely limit (lower) the output, rather than alter (boost) it.
Passive pickups are described as having a "warm, full, round, dynamic tone. Their fat, punchy tone is their appeal."
Stick-on or clip-on pickups are also passive piezo-electric devices. They take the sound from the vibrations of the top (or bridge if stuck there), not from the energy of the strings.
One area I've never seen discussed is the effect of an under-saddle pickup on the acoustics of an instrument. Under-saddle pickups are made of a piezoelectric ceramic. The string passes its energies to the saddle which then passes them on to pickup and then to the bridge. Obviously a pickup also acts as a filter, just like the bridge and saddle do. But just what impedance does the pickup provide? How does it affect the overall tone of the sound heard unamplified? I have yet to discover that, but I suspect it is important to the tone.
There are two kinds of piezo under-saddle pickups. One is a long bar that stretches the entire width of the saddle, and is therefore connected to all the strings. This means that if you pluck a G string, the entire pickup responds, the level of excitement falling gradually away from the primary source of the energy. This is the kind most commonly used on ukuleles. It can be passive or active.
The other kind places individual piezos under each string, with some material (possible saddle material) between them to reduce cross-pollination effects. So when you pluck the G string, the G-string pickup responds most. The other pickups respond less so (although minimally, they do respond because the bridge and top board vibrate, which can excite the piezos to discharge). This type tends to have better definition for individual strings - which is why it's used on the Kala U-bass.
And the other option is to put a small microphone into the ukulele body. This doesn't capture all of the uke's sound, because it doesn't "hear" what the top is sending forward. As Ralph Shaw writes, "This gives an accurate sound of what you'd hear if you were inside a ukulele."
The saddle is the first contact for the vibration of the string and is thus a critical component in sound reproduction. The material of the saddle determines how the sound is transmitted to the pickup and to the bridge, which then spreads the vibrations to the top. It's a complex, but fascinating science.
The saddle does several things, depending on its composition. It can filter frequencies, it can enhance frequencies, and it can alter frequencies. The most common materials for saddles are dense woods, bone or a plastic.
Some materials, like brass and other metals, are excellent at transmitting higher frequencies. Woods may lower overall frequencies, thus provide more lower ("warmer") tones, but at reduced amplitude (lower volume). Glass is apparently excellent for transmitting the fidelity of tones, but I have yet to see glass saddles, at least not for a ukulele (tempered glass would be best, but plate glass would probably work for ukes).
Very dense woods like ebony are often used for saddles, but softer woods are entirely inappropriate. Dense woods should also be used for the bridge, which also acts as a filter/transmitter for the vibration, spreading it to the topboard.
One of the easiest and most dramatic changes you can make in the sound is to swap saddles. A lot of saddles are held in place by the friction of the slot, as well as the pressure of the strings, so they are easily removed. If your uke has an under-saddle pickup, however, you have to be very careful when removing the saddle because the pickups often have fragile foil contacts that are easily damaged.
A lot of inexpensive ukuleles have mediocre plastic saddles. Changing these to a better material can really improve the sound. Saddles are often easily obtained from your local music store an are not an expensive upgrade - and one even a klutz like me can do easily.
Ivory was once a popular choice for saddles because it is even denser than most bone, but is now unavailable because so many animals were slaughtered simply for their ivory that entire species were at risk. You can still find some antique ivory saddles. Enterprising luthiers could construct an ivory saddle from an antique item, or even laminating the ivory tops of old piano keys.
While bone is often prized by luthiers and musicians as a material, it can be inconsistent and have pockets of lower density within. These pockets can deteriorate, not improve, sound transmission. If you really want to try bone, you can try baked (white) cow bone available at most pet stores as a dog treat. Use a small, fine-toothed saw to slice a piece lengthwise, then sand it down to fit. The best bone is the very dense, fossilized walrus or mammoth tusk, but it is also very expensive.
There are several artificial materials such as Tusq and Nubone that simulate bone, but offer a broader frequency range, as well as consistency across the whole saddle. I have successfully used Tusq saddles to brighten the tone of some mellow instruments like my mango wood tenor and cedar cigar-box uke. Other synthetic materials used for saddles include Corian (a countertop material) and Micarta (a synthetic bone substitute).
Unfortunately for ukulele players, few manufacturers provide sell ukulele-specific saddles. You may have to buy a guitar saddle and shave/sand it down. This is somewhat painstaking and usually involves many tiny adjustments in height and width until the saddle fits correctly.
If the action of the strings (the height above the fretboard) is too high, especially at the 12th fret and higher, reducing the height of the saddle can alleviate this, but be careful not to sand away too much so the strings buzz.
Electric/steel stringed ukes (and guitars) may not have a saddle, since they transit the sound differently. The pickups essentially act as antennae, causing small magnets to move within wire coils, generating a signal. They don't depend on sound being transmitted through a saddle, although on a semi-hollow instrument some resonance is also produced by the body.