Physics of the Turntable


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Have you ever wondered how a record player works? Probably not. After all, who still listens to records? Surprisingly enough, turntables are making a come back. With the recent surge of interest in hip hop music, popular attention has been turned towards the turntable, used by DJs to provide beats, loops and scratching for virtually all of today's hip hop groups.

The inner workings of the turntable may seem complex at first but after reading this paper it should become clear that, like all things, the record player works on basic principals of physics. In fact, the turntable is remarkable in that the basic physical principles behind it are quite simple. Some of these will be explored here. Please enjoy your visit.

How a record player works is quite simple. A motor is somehow connected to a solid disc so that the disc is rotated at a constant speed. On top of the rotating disc (platter), The record is placed on top, with a slip mat in between. The slip mat can serve two functions. In the past to hold the record in place so that it would not rotate independently of the platter. Now, however, the slip mat serves a much different function. Instead of holding the record in place, the slip mat is now used to reduce the friction between the spinning platter and the record. This way a DJ can scratch (manually move the record, usually at high speeds) the record while the platter continues to spin underneath. Once the record is rotating, a stylus glides along the grooves and picks up the vibrations, these are then converted into audible sound.

There are many different models of turntables still being manufactured. Of those being sold, it is possible to divide them into two separate categories based upon their motor system. Virtually all record players being manufactured today have either belt drive or direct drive motors. For the reasons discussed below, direct drives are accepted as the industry standard for professional DJs and turntabilists.

Belt Drive
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There are two advantages to the belt drive design. The motor in a belt driven turntable is set away from the platter by means of a continuous belt loop. This minimizes vibration to the platter and thus needle skipping. Also, belt drive models tend to be much cheaper than their direct drive counter parts.

These advantages, however, do not balance the many short falls of the belt drive design.

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Over time the belt can become loose and begin to slip, causing variations in the platter speed. Belt drive turntables have much lower torque. This leads record to be much more susceptible to outside forces. The contact between the stylus and record causes a frictional force in the form of torque, as the stylus angular path gets closer to the center of rotation the force decreases. Thus, at the start of a record, the stylus contact could actually decrease the speed of the record by up to 3%, while at the end of the recording, the distance from the center is much less. The torque is much smaller and will have little or no noticeable affect on the speed of the record.

Direct Drive

The sole disadvantage to direct drive turntables is the vibration from the motor, which is located directly under the center of the platter. However, in recent years, shock-absorbing (less dense) material, placed between the motor and platter, has been used to cut back on vibrations. Again the deciding factor is torque. Since the motor is directly connected to the platter through a system of gears, the torque is usually much higher than in the belt drive models. Higher torque means the platter speed is less susceptible to outside forces (stylus, hand). A higher torque also means that the platter will accelerate faster, therefore, there will be less distortion heard when the record starts to play.

As the record spins around with the platter, the stylus glides through the grooves. The stylus is held in place by the cartridge, which is attached to the tone arm. A counter weight attached to the opposite end keeps the pressure on the needle very low. As the record spins the contact between the stylus and groove walls causes vibrations in the stylus. These vibrations move up the needle to the top, where a magnet, surrounded by four wire coils, is attached. The vibrations create a mechanical force, which is proportional to the groove modulation on a phonograph record, which is used to change the position of the magnetic fields of the magnet. This induces a voltage in the coil which is equal to the number of turns in the coil multiplied by the time rate of change of the magnetic flux linkage through the coil.

For most people the word "turntable" brings to mind the scratchy sound of the ancient record players popular in the 1950's. If you think the turntable is nothing but an out-dated and obsolete sound reproduction device, then you are mistaken. Believe it or not, the best quality sound does not come from a CD or even a DVD but from good old fashioned vinyl. Surprised? You shouldn't be. A vinyl record is recorded in analog format. Sound waves through the air, recorded in analog format, are transcribed directly into identical waves on a different medium (vinyl, oxide tape, steel). Because the entire wave is copied, the recorded sound is nearly identical to the sound in the air. CDs and DVDs record in digital format; they take samples of the wave at different times (44,100 times/second for a tape) and convert the general shape of the wave into numbers. As you can see in the graph below, neither CDs of DVDs are able to match the sound wave perfectly.

Even those most experienced with the record player can sometimes have trouble. DJs and turntablists around the world are constantly trying to find ways to improve their abilities. Hard work, dedication and practice can only get you so far if your equipment is not functioning correctly. Certain difficulties are virtually impossible to remedy, such as spilling an adult beverage on your turntable during a show or putting out a pile of burning records, these types of problems should be written off as acts of God. However, other problems can be solved using simple physics. Thank God I took physics 211, where, oh where would I be without it?

Problem: When ever I touch the spinning record, to perform a scratch, it slows down.

Solution- The friction between the slip mat and the record is too high. The force of your hand on the record is being transferred into fictional force acting on the platter. Possible ways to fix this are:
A) You can reduce the friction by reducing the kinetic coefficient of friction between the slip mat and the platter. To do this you can place a layer of wax paper between the slip mat and the platter.
B) If you decrease the area of contact between the slip mat and the platter you will reduce the force of friction. To do this you simply cut away an inch or two from the edge of the slip mat.
C) If there is more inertia, it will take more frictional force to slow the platter. To increase the inertia you need to add more weight to the platter. The best place to add weight is the outer rim because the greater the distance the greater the increase in inertia. To add the weight, tape a row of quarters to the rim of the platter.

Problem: My needle skips when I try to perform a scratch.

Solution- The force holding the stylus to the record is not great enough. To correct this you can:

A) Add more downward force to the stylus. Just move the counter weight towards the center of the tone arm. If still more force is needed place a coin on top of the cartridge. DANGER! This will cause your records to wear out very quickly.
B) If you angle the stylus so that it hits the record at less then 90 degrees, the friction between the stylus and the record will increase making it more difficult for the stylus to jump out of the groove.



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