A suspension bicycle seat post can save your back.

Your bicycle seat post information was right on.  My back and I can't thank you enough"

The seat post simply attaches the saddle to the bicycle. That said, this is a very complex topic for what you would think would be a very simple item. 

Through your bicycle seat post, you can adjust both the height and the fore/aft position of the saddle on the bicycle.  You can also choose different materials to change the quality and experience of your ride.

Here is some general information to help you understand the importance of this piece of equipment.

Construction

A bicycle seat post consists of two parts: the pillar and the clamp.  Some types of posts integrate these parts into one unit.  

This is called a one-piece seat post.  

The two-part seat post is made up of a tube with a clamp around the upper end.  This clamp holds the rails of the saddle in the position chosen and fastens the saddle to the top of the seat post.  The clamp is generally made of steel and is serrated to allow set adjustments tilting the nose up or down.  Generally the serrations are very coarse to allow enough friction to hold the weight of a rider.  You can't do micro adjustments on the tilt angle very easily on this type of clamp setup. It just gets too hard.

Micro adjustments of the tilt angle are possible with seat posts that have a bolt running vertically through the clamp.  Even better are the posts that have two bolts. This is a pull/pull type setup allowing you to make very fine adjustments to the tilt angle of the seat.  Thomson makes a fine example of this seat post.

Can the material make a difference?

The combination of a good, comfortable saddle and a quality bicycle seat post that meets the needs of the rider can nearly equal the comfort given by a suspension on the frame of a tandem.  Proper adjustment of the seat post can alleviate much of the discomfort that you will feel when riding.

Generally speaking, the only person on a tandem to get a suspension on the bicycle seat post is the stoker. This is for a few reasons:

1. Inability to see upcoming bumps
2. generally women are stokers and the issue of comfort is more prevalent with women. Men don't want to admit it isn't comfortable
3. Reduce stoker fatigue that comes from dealing with the unseen bumps

Rigid Seat post

Even a bicycle seat post without a mechanical means to reduce the bumps, known as a rigid post, is going to provide some dampening of the bumps a ride produces. The type of material, steel, aluminum, titanium or carbon fiber will determine how much dampening will be achieved. Each of these materials offers specific characteristics of dampening.

• Carbon Fiber - A great material for combining strength and lightness along with vibration dampening. Care has to be taken when clamping down as there is a specific point where the post can be crushed. These posts are known for their ability to take the buzz out of a ride. Not really prone to breaking unless extreme force is applied a carbon fiber seat post will break into a very nasty end you don't want to contact.
• Steel seat post - Known in the biking community to give a nice smooth ride in frames, the same holds true for a steel seat post. A little flex in the post provides some shock absorption for the stoker. Will not break very easily, rather when one fails, and it is rare, it will bend over. Steel can be flexed and flexed to a certain point without getting any weaker, unlike aluminum.
• Aluminum seat post- Known to be very stiff, not much suspension given by aluminum. Many riders feel an aluminum seat post will transmit a lot of road vibration to the rider. Not a good thing for your stoker!
• Titanium seat post -also known as "Ti" for the name on the periodic table, this is a magnificent material. Super durable and elastic this seat post will flex a little, weigh almost nothing and never rust. Very good choice but also very expensive.

Suspension Seat Post

The mechanical means these posts use make a major difference in the amount of bumps felt by the stoker on a tandem. Not seen very often on single road bikes, the suspension seat post is pretty much something you will see on mountain bikes and tandem bicycles. It can really add to the comfort of the ride if set up properly and the stoker knows the limits of what a suspension seat post can deliver.

The thudbuster seat post is what I have on the back of my tandem for my wife. It offers a very nice ride that is adjustable for the weight of the rider.
One big advantage this has over other types of suspension bicycle seat posts is that the suspension components are merely elastomer rings that won't ever rust or seize up. A disadvantage is that as the seat travels downwards it also travels backwards changing the stokers relative position to the pedals and the handlebars. My wife really hasn't complained but some stokers might.

A telescoping style suspension seat post originally came on our tandem and we rode it that way for a year. It didn't offer too much travel, at least the model we had. The ride was pretty good. I was the stoker once when a friend piloted. It had a bit of "stiction" at the beginning of the travel so the bumps initially felt hard.