Saturday, November 25, 2006

Sexual Rhythms

The swaying of breasts, the menstrual cycle, and hip thrusts are just a few of the important oscillations in our sex lives. Physics provides a simple and powerful description of rhythmic motion and cycles, and can help you get the most out of oscillations, from choosing the best bed for your sexual pleasure, to enjoying and exploiting the natural rhythms of your body parts during sex.

Listen to the podcast with roboreaders Audrey and Paul.

Seasons shift through the course of a year, the ocean tides ebb and flow every day, and your mood may swing with the periodic changes in the chemicals in your brain, but the most common types of oscillators are mechanical - a tree bending back and forth in a breeze, a string bowed on a violin, or a couple making love.

Mechanical oscillators work by transferring energy between two forms - kinetic and potential. Anything that moves has kinetic energy. Stored energy is potential, as in the case of a ball poised to roll down a mountain. The energy in oscillators is sometimes kinetic, sometimes potential, and usually a bit of each.

When a playground swing moves back and forth it's briefly stationary at the highest point in its arc. For an instant it has no kinetic energy. Because it is higher above the ground than at other times in its motion, it has a maximum amount of potential energy, just like a ball on the verge of rolling down a hill. When the swing descends, it speeds up as its potential energy is converted to kinetic. The swing is moving fastest, with the most kinetic and least potential energy, at the lowest point of the arc. As it swings up in the other direction, the kinetic energy is converted back into potential.

Similarly, when you make love on your bed - at least when one partner is bouncing up and down on top - you rhythmically compress the mattress springs. The springs have lots of potential energy when they are compressed, but as the springs extend and push you upward, the potential energy is converted into the kinetic energy of your moving bodies.

The rate that the energy flows back and forth in an oscillator is its resonance frequency. The frequency of an oscillator is measured in hertz, which is the number of oscillations in a second. A clock ticks at one hertz , or once per second; your heart can beat at two hertz or more during heavy exercise and sex; and middle C on a piano is a 440 262 hertz vibration in air, which we hear as a musical note.

All oscillators have at least one resonance frequency. Many, such as violin strings, have several resonances. The distinctive sound of an instrument has a lot to do with the many resonances that are produced along with every note; it's the combination of resonances that ensures that a violin and a piano produce rich and distinct sounds even when they are playing the same note.

One resonance, however, is usually more important than the rest - it's the fundamental resonance. In the case of a musical instrument, the fundamental is the note a musician is playing. When a violinist chooses to play middle C, for example, he or she presses on a string to ensure that its fundamental resonance occurs at the 440 hertz frequency we identify with the note. Other, lesser resonances are called harmonics.

Lovers on a bed form an oscillator with a fundamental resonance and a spectrum of harmonics. Just as a gifted musician can make sweet music by adroitly manipulating an instrument's resonances, lovers can add to their resonant bliss in the bedroom by understanding and controlling their bed's oscillations.

When you sit on a bed, you'll sink into the mattress until you reach an equilibrium position. At that point, the force of gravity pulling you down is balanced by the force of the bed springs pushing you up. If you start to bounce up and down, you'll find that there's a certain frequency that allows you to get a big steady bounce. That's the bed's fundamental resonance. Most beds resonate at frequencies of a few hertz.

By rhythmically bouncing on the bed, you're doing what physicists call driving the oscillator. It's easiest to drive an oscillator at its resonance frequency. At frequencies just below or above resonance, it takes much more force to get a big bounce. If you start out very slowly, you'll probably end up oscillating well below your bed's fundamental resonance and won't bounce on the bed much at all.

Increasing your speed can bring you into resonance, allowing you to achieve large bounces with seemingly little effort. Once you're moving too quickly , you may pass the resonance. As a result, you'll end up working against the bed's rhythmic sweet spot. If that happens you will have to exert much more force to get a good bounce at the same time that you're trying to move quickly. The chances are, you'll be rapidly exhausted. If you stick close to the resonance, on the other hand, you get maximum motion for minimal energy input, which helps you keep going longer before you wear out.

Many things can affect a bed's dynamics, but it's the mattress' firmness that plays the greatest part in determining the resonance frequency. Firmer mattresses have higher resonances than soft mattresses. If you visit a mattress store, you can check this for yourself. Sit on several beds with different firmness and bounce on each. You'll find that the frequency varies from a very firm to very soft mattress.

Beds get their bounce from springs, and springs have resonance frequencies that depend both on their firmness, which physicists call the spring constant, and the mass on top of the bed. That means the resonance frequency will be different for you than it will be if you have someone in bed with you. In fact, if you and your partner are about the same weight, the resonance frequency will be roughly half two-thirds as fast with the two of you close together on the bed as it will be with just one of you. (To be more precise, it will be 1/(the square root of 2) or about 0.7 times slower.)

Determining a bed's resonant frequency is only part of the issue. After all, you may not be content within the confines of one rhythm. Fortunately, there's another factor that affects bed motions. Harmonic oscillators, and beds in particular, often include a certain amount of damping, which gives you a little more leeway in choosing your own rhythms.

Shock absorbers in cars are a good example of damping. Car suspensions consist primarily of simple springs. When a car hits a bump, the springs allow the wheels to travel up or down relative to the car, maintaining contact with the road. If it weren't for shock absorbers, a car would continue to oscillate on its springs after hitting a speed bump or a pothole, leading to a nauseatingly bouncy ride. Shocks settle a car down quickly by dissipating the energy of the bounce. As a result, they reduce the resonance frequency and make the resonance less pronounced. If you push down on the bumper of a car with bad or missing shocks, you can easily get it to resonate and bounce dramatically. It is much more difficult to find the resonance frequency of a car with good shocks.

Damping has the same effect on a bed that shock absorbers have on a car - it will be harder to drive a resonance on a very damped bed, but at least you won't suffer the frustration that comes from trying to move at rates higher than resonance.

Although no commercial beds currently come with automotive-type shocks, padding in the mattress adds damping. Alternatively, you can add your own damping by spreading a thick comforter on the bed and making love on top of it. A few well-placed pillows under you can increase damping too.

The bottom line is this: if you want to use the resonance to your advantage and you like it fast, choose a firm bed; if you like it slow, go with a soft bed; and for maximum flexibility, buy a firm bed but keep a few soft comforters and pillows around to dampen the resonance to suit your mood.

If you have the soul of an experimental physicist, try making love on a trampoline, which has almost no damping and a powerful resonance. Then try it on a water bed, which also has little damping and strong resonance, as the water sloshes from one place to another, but at a much lower frequency than a trampoline. To round things out, make love on a squishy foam bed, like the Tempurpedic mattress, to experience lots of damping with very little resonance. It can be an exhausting and frustrating challenge.

Some people find that their favorite lover is a machine - specifically, their vibrator. Vibrators get their buzz from an electrically powered oscillator.

In battery-powered models, vibrations generally come from an electric motor attached to a rotating disk, with its weight placed off-center. The principle is the same thing that causes unbalanced washing machines - which are some, other, people's favorite lovers - > to buck violently when more of the laundry is on one side of the washer drum than the other. The faster the motor turns, the higher the vibrator frequency.

Many of the vibrators that plug into the wall generate oscillations with a different type of electric motor called a solenoid. Instead of spinning an unbalanced weight, electricity passing through a coil of wire forces a metal slug to vibrate rapidly back and forth. The speed of vibration in a plug-in vibrator is related to the 60 hertz oscillations of the electricity in wall sockets. They are usually more powerful than battery vibes. Unfortunately, their power comes at a price - they can only vibrate at the frequency of the electricity in the wall, at a multiple of the electrical frequency, or certain fractions of the wall frequency. Unlike battery powered vibes, which can run at a spectrum of speeds, most plug in vibrators have only one, two or three speed settings.

Ideally, vibrators would also come with adjustments to increase the strength of the vibrations independently of the speed, but that is not the case with any vibrators currently on the market. This is likely due to the fact that adding a power setting would complicate vibrator design, but you can always adjust the power you feel by changing how hard you press the vibrator against your body.

Vibrators have resonances just as beds and cars do. That means that increasing the speed of a continuously variable model can either increase the power of the vibrations or decrease them, depending on whether you are approaching or passing the resonance. As a rule, the power of the vibrations will slowly increase as you turn up the speed, then reach a maximum at the resonance frequency, and slowly fall as you continue to turn it up.

Fortunately, you can gain a bit more control over your vibe with damping, just as you can use damping to adjust a bed's dynamics. Changing how tightly you grip a vibrator and where you hold it will change the amount of damping, allowing you to modulate the speed and intensity. Inserting it into your vagina or anus will also change the speed as the soft tissue touching the vibrator absorbs energy. If you listen to the pitch as the vibrator moves in and out, you can hear the speed change. Alternatively, pressing the vibrator against a soft rubber or gel can also slow it down.

Some dildos have a cavity that allows you to insert vibrators into them. The softer and heavier the dildo, the more it will dampen the vibrations and slow the resonance. Many manufacturers of plug-in vibrators offer soft sleeves and attachments to allow you to dampen oscillations in the same way.

Damping is the reason that vibrators are more comfortable when used in the anus or vagina than on a hard penis or clitoris. A rigid clitoris or penis has much less damping than softer and more enveloping anal and vaginal tissue. The vibrational motion, and resulting energy, is transmitted at full intensity to sensitive nerves in a small area where the vibrator makes contact with the rigid tissue. In the vagina and anus, the energy is distributed to more tissue and nerves, which feels less intense.

Springs, like those in a mattress, are just one classic type of oscillator. Pendulums are another. A pendulum consists of a weight at the end of a rod or string. It will swing at a rate determined by its length, regardless of the amount of weight on the end. A long pendulum swings slowly, and a short pendulum swings quickly. It's easy to adjust a pendulum's frequency by changing its length. Old fashioned clock pendulums included adjustments for fine tuning the length of a pendulum depending on whether the clock ran fast or slow; if it ran too fast you could turn a screw to lengthen the pendulum, or turn it the other way to shorten it if the clock ran slow.

Much of sex involves motions that have the characteristics of a little of both pendulums and springs. From a physics point of view, a woman's breasts are a complicated combination of springs and pendulums. Depending on whether she is lying on her back, standing, or on her hands and knees, her breasts are more like one or the other, which can have a huge effect on how they move.

Take a woman on her hands and knees, for example. Her breasts will hang down and sway as a result of her motions. Because hanging breasts are similar to pendulums, they have resonances determined primarily by the length they extend from a woman's chest. The resilience of her skin and breast tissue will also have an effect, but for gentle motion, the pendulum-like aspects are most important. Breasts will naturally swing slower if they hang farther from the chest, and swing faster if they are more compact.

It's easy to tell when hanging breasts have reached their resonance frequency, because they will swing dramatically back and forth. Increasing the frequency of the forces driving her breasts will reduce the amplitude of their motion, as they pass the resonance frequency, until they stop moving altogether.

When a woman is on her back, her breasts will tend to resemble springs more than pendulums, and will resonate at a frequency that has more to do with their resilience and mass. Heavier breasts have lower resonance frequencies, and tauter breasts (with higher spring constants) have higher resonances.

Resonant frequencies of breasts vary dramatically from one woman to another, and will vary even in a specific woman as her breast size and tissue resilience changes over time, or as she changes position from standing to lying on her back to getting up on her hands and knees.

Resonances are also the reason some women need sports bras when they exercise. It can be painful if a woman moves with rhythms close to her breasts' resonance frequency because that is when they are moving the most. Sports bras solve the problem by compressing breasts and making them, in effect, more taut. This raises the resonance frequency, hopefully beyond the frequency of jogging and other repetitive motions. Depending on a woman's cup size and the design of the bra, it may just move the resonance frequency up enough to make one exercise comfortable while making another painful.

For instance, if a woman with large breasts finds that her resonance frequency comes at a slow jog, it's possible that she will experience less motion if she sprints at a rate that drives the breasts at frequencies above resonance. Essentially, speeding up changes the bounce to a jiggle. Potentially, if she were to wear a sports bra that makes jogging comfortable it could move the frequency up to the point that things get bouncing and painful when she sprints.

Penises too have natural frequencies, which can change depending on arousal. Although the first sports bra (according to one story of their origin) was made from a pair of modified jock straps, men do not usually need extra support to prevent the sorts of resonances that plague women's breasts during exercise. A hanging, flaccid penis and testicles form short pendulums with resonance frequencies well above the frequencies of nearly any athletic activity. All jock straps do is lift the testicles up and forward to keep them from being squashed between the thighs - resonances aren't usually an issue during exercise.

An erect penis, however is like a large mass on a spring attached at the pelvis - the larger and heavier the penis, the lower the resonance frequency - and it may well resonate at rates that would interfere with many sports, but a man with an erection probably isn't in the right state of mind for jogging, basketball or soccer anyway.

Some types of penis enlargement involve snipping the tendon that supports the penis, allowing it to extend farther from the pelvis. This can radically reduce the spring constant of the penis attachment, and lower the resonance frequency of the erect penis a great deal. It's hardly iron-clad evidence, but if you watch a porn movie, you may notice that two men with similar sized erections seem to have very different penile resonance frequencies. (Look for the motion when their penises are not being touched directly, but are being driven by some indirect motion, say shifting position on the bed or walking across a room in the nude.) It's possible that a fellow with a low frequency oscillation has had his penile tendon snipped.


Bouncing beds, humming vibrators, and oscillating body parts are only a few of the ways that simple harmonic motion is important in the boudoir. In fact, your entire body is a kind of a simple harmonic oscillator. Moving from one position to another changes the portions of your body that come into play and affects the resulting resonances. The rhythm that feels natural with one partner on top is likely to be different from the rhythm when the other partner is on top, particularly if they are significantly different sizes.

Clearly, we've only touched on a fraction of the ways rhythms are important in sex. We hope it's enough to convince you to keep oscillations and resonance frequencies in mind, in order to help you enhance your sexual experience - whether you're choosing a bed, buying a vibrator, or searching for a new position.


coturnix said...

Buzz - e-mail me ASAP. It is very important (hint - publication....)

Dai Conrad said...

Middle C isn't 440 Hz. Middle C is about 262 Hz, if I recall correctly. 440 Hz is "concert A", the A above Middle C which an orchestra tunes to.

Buzz Skyline said...

Oops, you're right Dai. I fixed it now. Thanks for catching it.


elephant8 said...

I noticed the 440Hz mistake as well. Unfortunately the audio podcast was not corrected.