AMAZON multi-meters discounts AMAZON oscilloscope discounts
On this page: 1. How a Speaker Works---Parts of a Speaker-Frequency
Response-Transient Response-Dispersion-Cone Resonance-Compliance Damping-Impedance-Efficiency-Power
Rating--Speaker Polarity-Kinds of Drivers
The speaker is the part of a sound system that has the fewest specifications, but the greatest effect on quality. Most listeners can't hear the difference with a change of amplifiers, but can immediately sense a switch in speakers. If you want to upgrade your stereo set, the speaker system is a good place to start.
PARTS OF A SPEAKER
The working parts of a dynamic speaker are the cone with its suspension, the voice coil, and the magnet (Fig. 1-1). When an electric current flows through a wire, it sets up a magnetic field around that wire, and for a coiled wire the field is increased. If the coil of wire is located in an external magnetic field, provided by a magnet, the field of the coil interacts with that of the magnet to apply force to the coil. If the current is an alternating current, the field around the coil builds up and collapses in response to the frequencies of the current. In a speaker this changing field interacts with the constant field of the magnet, causing the coil to move in response to the current. As the voice coil moves, it moves the cone, which makes pressure waves in the air near the cone. These pressure waves are heard as sound.
The cone suspension allows the cone to move without shifting sideways far enough for the voice coil to rub the center pole of the magnet. Modern speakers have an outer cone suspension, the surround, and a coil centering device called the spider.
If you connect the leads of a sensitive AC voltmeter to the terminals of a dynamic speaker and gently push the cone with your ...
Fig. 1-1. Parts of a loudspeaker.
... hand, the meter pointer will move, indicating that you have produced a small voltage across the terminals. When a speaker is connected to the input of a high gain amplifier, it can act as a microphone. Many intercom sets use this principle, with a small speaker in each station that works in a dual role of speaker during the receiving mode and microphone during the sending mode.
When a speaker is in use, each voice coil movement produces a voltage in the same way. The direction of the electromotive force (emf) produced is always that which will oppose the change of current direction in the current that drives the speaker. Because of its direction, this force is called the back emf. The stronger the magnetic field, the greater the back emf and the higher the electrical damping on the voice coil and cone movement.
FREQUENCY RESPONSE
In selecting a speaker for music reproduction, smoothness of response is more important than extended range alone. A speaker with a smooth response from 100 to 10,000 Hz can produce music more faithfully than one with a 50 to 15,000 Hz range but with significant peaks within that range.
The requirements for a wide frequency range from a single cone are contradictory. For good performance above 10,000 Hz the cone must be light, with a mass no greater than about 5 grams. But light cones bend at low frequencies, producing distortion. For good bass response the cone should be large to "grab" enough air to have good radiation resistance, but for good dispersion at high frequencies it must be small. A heavy bass cone will have a lower frequency of resonance and a smoother response than a light one, but the heavy cone will have poorer transient response as well as a limited high range.
One way that speaker designers solve these problems in full range speakers is to add a secondary cone, called a whizzer. A whizzer cone is less expensive than a separate tweeter because it is driven by the same magnet and voice coil as the main cone. It can improve the radiation pattern of the highs as well as extend the frequency range.
Even those large cones that have no whizzer can produce some high frequency sound. They do this by cone reduction. At low frequencies the entire cone vibrates as a single piston, but as the frequency of the signal is increased, the central part of the cone vibrates independently. Having lower mass, a small section of the cone can emit sound at relatively high frequencies.
TRANSIENT RESPONSE
A speaker's ability to handle short pulses without altering their duration is called its transient response. For good transient performance a speaker must start to move almost immediately after receiving the amplifier's signal to do so, then stop promptly when the signal ends. The trait of oscillating after the signal has ended is called hangover.
The first requirement for good transient response is a smooth frequency response. A peaky response curve indicates multiple cone resonances, and each resonance can be kicked off by any signal which contains the resonance frequency in either the fundamental tones or in its natural overtones. Each resonance adds its share of hangover, and the aural effect of hangover is muddy sound.
Even if a speaker has a smooth frequency response without higher resonances, it will have at least one resonance, the fundamental cone resonance. The prominence of this resonance varies with system Q; a high Q speaker has low magnetic damping and is prone to peaking at its frequency of resonance.
DISPERSION
All speakers produce sound that is more directional at the upper end of their frequency range. As a rule of thumb a speaker is omnidirectional only up to the frequency where the effective cone diameter is equal to the wavelength of the sound. Following this rule, a 12" speaker is fully omnidirectional to about 1300 Hz, an 8" speaker to 2000, or a 4" speaker to 4000. Speakers can be used to perform at higher frequencies than these because of cone reduction and the use of whizzer cones.
A speaker that has poor dispersion at high frequencies will sound harsh when you are in the beam but dull when you move aside. If such speakers are used in a stereo set, the stereo image may move unnaturally as you turn your head. A speaker that spreads the highs around the room will sound more expansive and the highs will have an airy quality like those of live music.
For the ultimate in good dispersion, small dome tweeters are hard to beat. The dome shape permits the necessary strength for a small vibrating surface, and the small size provides the superior dispersion.
CONE RESONANCE
If you suspend a mass on a spring (Fig. 1-2) and set the mass in motion, it will always vibrate at a certain frequency which is its natural resonance. To change the frequency of resonance you can alter the mass or the stiffness of the spring. If you add a blob of modeling clay to the mass, it will vibrate more slowly. Or a more compliant spring will have the same effect. To increase the frequency you would either reduce the mass or get a stiffer spring.
Each speaker has a fundamental frequency of resonance which is determined by the mass of the cone and the compliance of its suspension. Large cones, having greater mass, usually have a lower frequency of resonance than small cones. When the frequency of resonance is measured on a bare speaker, it is called the free air resonance.
If you sprinkle some talcum powder on a speaker cone and watch the powder as you vary the frequency of the drive signal from an audio generator, you will see that the cone's vibration increases as you approach the speaker's resonance. At resonance it vibrates wildly. At this frequency the speaker is extremely efficient at converting electrical energy into sound, but, by its greater voice coil movement, the speaker produces more back emf at this frequency than at any other. The stronger the magnet, the higher the opposition, or impedance, to the flow of current through the coil. This is why a strong magnet controls the cone movement at resonance, damping it.
COMPLIANCE
The traditional suspension, which was no more than a single wrinkle around the circumference of the cone, has been largely replaced by a roll edge in high fidelity speakers. High compliance woofers, with roll edge suspensions, can perform well in compact boxes.
Another change in speaker compliance is the way in which the compliance is reported. Instead of the distance the cone moves per unit of applied force, it is reported as that cubic volume of air which has the same compliance for the cone as the speaker's suspension.
When reported as a cubic volume, the term is called the VAS Because any volume of air will offer more resistance to the movement of a large piston than that of a small one, large woofers almost invariably have a high VAS. Before one can say whether a certain box size is large or small for a given speaker, the speaker's VAC AS must be considered.
--------
Fig. 1-2. How changing mass or compliance affects resonance. STIFF SPRING MASS STIFF SPRING LARGER MASS REDUCED FREQUENCY OF VIBRATION; WEAK SPRING MASS
----------------------
DAMPING
The degree of damping for any speaker depends on several factors, the size of the magnet, the suspension, the mass of the moving parts, and the internal resistance of the amplifier. If other factors are equal, an increase in magnet size produces more damping. But the rule "the bigger the better" doesn't go; too much magnet can overdamp a speaker, reducing the level of bass response. Magnet size isn't everything, but you can get an indication of speaker quality by the size of the magnet simply because the magnet is the most expensive part of a speaker.
Manufacturers aren't likely to squander the cost of a big magnet on a poor speaker. For optimum bass performance the magnet must be fitted to the size of speaker and the kind of enclosure to be used.
As a measure of the degree of damping on a speaker a numerical term, called speaker Q is used. Q stands for resonance magnification, the tendency of the speaker to peak in response at the frequency of resonance. Values for Q can range from about 0.2 up to 2 or 3 or more. The greater the damping on a speaker, the lower is its output at resonance and the lower is its Q. When a speaker is put into a closed box, its Q, as well as its frequency of resonance, will be raised. It is often accepted that a Q of 1 is a useful design goal for closed box speakers. This is a good compromise between a system that is under-damped and one that is over-damped.
IMPEDANCE
The opposition of the voice coil to current flow at any frequency is called its impedance. Most speakers have a nominal impedance rating of 8 ohms, which suggests that a speaker is like a resistor. The units of resistance and impedance (ohms) are the same, but there are important differences between the impedance of a speaker and pure resistance.
If you measure the resistance of your speaker's voice coil with an ohmmeter, you will find that it is about 75 % of the rated impedance. An 8-ohm speaker, for example, will usually have a resistance of about 6 ohms. This tells you that impedance is something more than simple resistance. The ohmmeter measures resistance by putting a direct current through the voice coil, but the speaker must operate on alternating current. When an alternating current passes through the coil, the constantly swinging flow sets up its own magnetic field which grows and collapses with the frequency of the current. This moves lines of force through the coil, causing a reactance to the alternating current. The more rapidly the current reverses itself, the greater the reactance. This tendency of a coil to resist the flow of high frequency current is called inductive reactance. The more turns of wire in the coil, the greater its inductance.
A coil can also have capacitive reactance, which produces the opposite effect of inductive reactance. At frequencies where the capacitive reactance is equal to the inductive reactance, the two reactances cancel, and the speaker's impedance is equal to the DC resistance of the voice coil. At all other frequencies the total reactance to current flow will be greater.
Impedance varies with frequency (Fig. 1-3). Note the hump at the frequency of resonance where the voice coil's back emf is greatest. The rise at high frequencies is caused by voice coil inductance. While the uneven impedance curve may look bad, remember that this is not a response curve. The high peak at resonance is a sign of a strong magnetic field.
When connecting speakers in your stereo system, the important thing to remember about impedance is to avoid a connection that puts a low impedance load on your receiver or amplifier. For many components the danger line is approached if the impedance drops below 4 ohms. Just make sure you don't wire more than two 8-ohm speakers in parallel. And never wire 4-ohm speakers in parallel with any other speakers. Many receivers and amplifiers have speaker switches that put the main speaker system and a secondary set of speakers in parallel when both sets are on, so you should be careful about using 4-ohm speakers in such combinations.
Fig. 1-3. How a speaker's impedance varies with frequency.
EFFICIENCY
The measure of any speaker's ability to convert electrical energy into sound energy is its efficiency. Mathematically, efficiency is sound power output (in acoustical watts) divided by electrical power input (in electrical watts). There are two ways to increase efficiency: reduce the cone's resistance to motion, or increase the force of the voice coil for a given flow of current.
The easiest way to reduce the cone's resistance to motion is to make it lighter. If the cone mass is halved, the speaker will be 4 times more efficient in the mid-frequency band. But reducing cone mass increases the frequency of resonance and often produces a rougher response curve. Another way to reduce motion resistance is to increase the compliance of the suspension.
To increase the force on the voice coil, the manufacturer can use a stronger magnet or put more turns of wire in the coil. Careful design must be used with these methods to prevent overkill. Too much magnet will over-damp the cone, and if too much wire is added to the coil, the increase in mass will begin to cut efficiency more than the field of the added turns can increase it.
Efficiency is one speaker characteristic that is rarely quoted.
For many purposes it is not especially important because of the available power from modern receivers and amplifiers. It may be of importance to listeners who like extremely high sound levels.
Compact, closed box speakers are the most inefficient kind, with efficiency ratings of from 0.25 to 0.5 %. Large floor models often have much higher efficiency, sometimes as high as 20 times those of the most compact speakers.
Another way of rating efficiency is to show the sound pressure level (SPL) in decibels (dB) when the speaker is fed 1 watt of electrical energy. This measurement is made with the microphone at 1 meter from the speaker. If you check such ratings, you will see that small speakers and tweeters almost invariably show higher dB ratings than large woofers. This is why tweeters and mid-range speakers often need either specially designed crossover networks or variable controls for proper balance. Such dB ratings should be used as a rough guide and have almost no value in rating speaker quality. A heavy cone woofer, suitable for use in a compact enclosure, will usually seem inefficient when compared to full range speakers of the same size.
POWER RATING
The power rating that most manufacturers assign to a speaker is the amount of power the speaker can absorb without damage.
Some of the electrical energy that goes into a speaker's voice coil is converted to heat by the coil's resistance. The larger the coil, the better it can dissipate heat, so you can estimate the power handling ability of a speaker by checking the diameter of its voice coil. Speakers with the smallest voice coils, from 1/2 to 9/16", are usually rated at no more than 5 watts. The power ratings rise with increased coil diameter, so that a speaker with a 1" voice coil can handle from 15 to 30 watts. Speakers with coils larger than 1" can usually handle much more power. A 2" voice coil speaker, for example, can be rated at 100 watts or higher.
These ratings are rms values, which means that you can use an amplifier with a much higher power rating than the speaker. Music is full of transient sounds rather than sustained tones. When music power ratings are quoted, the figure is always considerably higher than the rms figure.
In some cases an amplifier or receiver with a high power rating is safer for your speakers. Low powered amplifiers can produce distortion that adds upper harmonics, placing an undue load on small tweeters. Ordinary music contains little power in the high frequency range, but a poor 10-watt amplifier can blow a tweeter which would normally be perfectly safe in a 20-watt or even more powerful system. By going into harmonic distortion at transient peaks, the low powered amplifier tremendously in creases the power to the tweeter from the normal milliwatt range into several watts.
SPEAKER POLARITY
If you hook up a single speaker to a mono amplifier or receiver, it makes no difference which speaker lead goes to each terminal. But when two or more speakers cover the same frequency range in the same room, they must push and pull together or the sound from one will cancel that of the other (Fig. 1-4).
Stereo speakers have coded speaker terminals. The positive terminal is usually marked with a red dot, sometimes with a plus mark. Make sure that the lead from the positive terminal of each stereo speaker goes to the receiver terminal of the same polarity in each channel.
If you think your speakers may be out of phase, try reversing the leads to one speaker. Note that you must reverse the wires to just one speaker; if you reverse the wires to both stereo speakers, their polarity with relation to each other will be the same as in the first wiring. The easiest way to note proper polarity is to place the two speakers face to face and feed a mono signal with prominent bass tones to them. Switch the connections on one speaker only.
Choose the connection that gives the greatest bass response.
Fig. 1-4. Why stereo speakers must be in phase.
It is also important to observe polarity in wiring together woofers and tweeters in 2-way systems or the three drivers in 3-way systems. Failure to do this can produce holes in the response curve. Unless you are instructed to do otherwise, the various drivers should be wired in phase. Some crossover networks, such as second-order 12 dB per octave networks, produce a phase difference in adjacent drivers of 180° and require reversed phase wiring to prevent a hole in the response curve. If reversed polarity is necessary, it will be indicated in either the instructions or the schematic diagram.
KINDS OF DRIVERS
Any direct radiator speaker is called a driver. Because of the conflicting demands placed on a single cone driver for full-range duty, nearly all systems with a driver larger than 8" in diameter are multiple speaker systems. These systems have a large driver for the low frequencies, often called a woofer, and at least one other driver for the highs, a tweeter. A 2-way system consists of a woofer and tweeter; a 3-way system of a woofer, mid-range driver, and tweeter.
Most woofers have an advertised diameter of at least 8 inches.
If a driver is designed to be a woofer, it will have limited high frequency response. This limitation is desirable because it makes the crossover network's job easier. A woofer will also have a low frequency of resonance, low enough that its bass range will be adequate when the speaker is put into a suitable box.
Mid-range drivers are often considered unimportant on the theory that any speaker can reproduce mid-range. This attitude is a mistake because the ear is most sensitive to response variations in the mid-range. For best mid-range performance, choose a driver that is designed for the purpose. And if your woofer is larger than 8" or 10" in diameter, you will probably need a mid-range driver for good mid-range performance.
The best tweeters are usually small in diameter, giving better dispersion than large tweeters. This is no problem in 3-way systems; the tweeter can be a 1" dome. But small tweeters must be used with adequate crossover networks. If the crossover frequency is placed too low, or if the tweeter is incorrectly wired, it won't last long. Remember that the power ratings assigned to tweeters are based on the assumption that an adequate crossover network will be used.
Piezoelectric tweeters can solve many problems of high frequency reproduction. These tweeters have such a high impedance at low frequencies that they are effectively out of the circuit there. This means that they can be used without a crossover network, wired directly to the amplifier output line. Such tweeters are unusually rugged, able to take driving voltages of up to 35 volts rms without failing. For continuous high power level operation a current-limiting resistor is suggested because these tweeters have low impedance at frequencies above 50,000 Hz.