A Sound wave can be described by two basic characteristics, the speed of the vibration (Frequency), and the extent of the vibration (Amplitude).

When a speaker moves forward, backward and back to its original position, this is called a cycle. Frequency is determined as to the number of cycles per second. It is labeled Hertz (Hz) after an early scientist in the field of acoustics. Diagram 1 is a graphical representation of a sound wave. The top half of the curve is where pressure increases, the bottom half is the decreasing of the pressure.

It has been determined that sound travels through air at sea level at a speed of 1128 feet per second. The wavelength is the distance a sound travels in one cycle. This distance can be determined by dividing 1128 by frequency. With this formula we determine that a 20 Hz note is 56.4 feet long. On the other hand a 20kHz (20,000 Hz) is .056 feet which is about 5/8 of an inch. Understanding this relationship between distance and soundwaves will be important in understanding how we hear, and therefore how to properly set up a sound system.

Yet another description of the soundwave is phase. Phase refers to the pressure change of the soundwave at a certain time and place. Once again, refer to diagram 1, and see how the first part of the curve is above the zero line. This is known as positive phase (increase of pressure). When the curve is below the zero line, it is known as negative phase (decrease of pressure). When Two or more waves combine the combination can be referred to as being in phase or out of phase. In phase means that the soundwaves are combining to form a wave that is larger than either of the two. When the waves are out of phase, meaning the phase of one wave is positive and the other is negative, they will subtract from each other, therefore reducing the pressure level. Soundwaves are constantly interacting with one another and creating what is known as a complex wave. All musical instruments create complex sound waves. The instruments combine waves to create the sound that makes them unique.

Our ears have a tremendous ability to evaluate sounds that range from very soft to very loud. The decibel (dB), named after Alexander Graham Bell, is a method of describing acoustic pressure, without having to deal with the billion-fold range of sound pressures to which our ears are sensitive. A Bel is the difference in loudness produced by a ten fold increase in power.

A decibel is 1/10th of a Bel. A doubling of sound pressure level (SPL) is a 10 dB increase, where a halving of SPL is a 10dB decrease. The human ear can detect differences as small as 1 dB, however 3dB is commonly referred to as a level where a change is readily apparent.

A doubling of power input into a speaker will produce a 3dB change in output, a doubling of cone area will result in a 3dB increase over that of a single driver.

A doubling of distance from a sound source reduces SPL by 6dB

By comparing arrival times at both ears, our brain can determine the direction from which a sound came from. A sound arriving at both ears simultaneously tells our brain that the sound is centered either in front, above, below, or behind us. The brain then looks at the frequencies received, and compares these frequencies with sounds that we have heard before. When the brain remembers a sound like the sound it just heard, it looks at the late soundwaves that arrived at the eardrum that were reflected off the outer ear. This process allows us to perceive height. When a soundwave comes from behind, the soundwave must pass through the pinna. This causes a filtering of high frequencies in comparison with reflections off of walls, ceilings, and floors. This is what allows us to localize sound sources behind us.

It becomes increasingly difficult to localize sound sources as the frequency decreases. This is due to the wavelength being so much larger than our heads, that it becomes difficult to differentiate between the arrival times at our ears. This effect keeps us from localizing pure tones below around 100 Hz. If the sound source is not pure, however, and there are harmonics associated with the source, we can localize these sounds and therefore localize the source.

For current to flow, there must be a potential difference between two objects. This potential difference is created from an excess of electrons at the source, and a lack of electrons, or positive charge, at the destination. Using our hose analogy, the potential difference, measured in volts, would be the pressure of the water within the hose. The higher the pressure, the more potential difference, and the higher the voltage.

It also can be shown that a larger hose can move a greater volume of water (more current) than a smaller hose. This would correspond to the size of the conductor. A wire conductor’s size is called its gauge. The smaller the number, the heavier the gauge, and the larger the wire. Heavier gauge wire can conduct more current than a smaller gauge.

A capacitor is a device that resists change in voltage. It stores a charge. For our uses it will pass AC but not DC (while it is charging it will pass DC). Their value is expressed in Farads (or more commonly in microfarads). They have a working voltage (called DCWV) that must never be exceeded. They can be found in both polarized and nonpolarized versions. The most commonly used types are the electrolytic, mylar and poly (polypropylene, polystyrene, polyester). They are used primarily for filtration (in power supplies), noise suppression, and speaker crossovers.

For noise suppression, a capacitor is placed between the alternator, or component power lead, and ground. This will present an open circuit to the 12 volts DC once the cap is charged, but will provide a short circuit for the small AC “ripple”. Polarized electrolytics are most commonly used. They are much cheaper than the other types, especially in larger values. A minimum of 25 DCWV is needed for safe operation in a 12 volt system.
Another use for the capacitor is in speaker crossovers. Although they pass AC, their impedance is inversely proportional with frequency. Placed in series with a speaker, it forms a 6dB / octave high-pass filter. A nonpolarized capacitor must be used.

Electrolytic caps are the cheapest for their size. If using electrolytics for crossovers be sure that the DCWV is at least 100 volts for amplified systems. Using a cap with too small of a working voltage will usually cause the cap to explode. Mylar caps are a bit more expensive than electrolytics of the same value, but offer higher reliability and sonic improvement. Metalized film poly caps are very expensive, but offer the highest sonic quality in critical listening applications.

An inductor is a device which resists change in current flow. Usually it has a low DC resistance but a high AC impedance. This means that it passes DC well, but does not pass AC easily. It is electrically opposite to the capacitor. Its value is expressed in Henries (more commonly milliHenries). There are two major types of inductors, air core and iron core. The other important factor is the gauge of wire that is used to make the inductor. The heavier the gauge, the less signal loss and the higher the current capability. For a given value and wire gauge, air core inductors are more expensive, but they do offer some benefits. The greater the current flowing through an inductor, the larger the magnetic field produced. With iron core inductors, there is a point where the core saturates and no greater field may be produced. At this point, severe distortion sets in. Air core inductors for the most part do not exhibit saturation.

An inductor’s impedance is proportional to frequency. They are used in noise suppression and speaker crossovers. Placed in series with a speaker, it forms a 6 dB / octave low pass filter. Placed in series with a components B+ lead, they will suppress noise.

A diode (or rectifier) is a semiconductor which allows current to flow in only one direction. It can be used to change (rectify) AC into DC. The two leads of a diode are labeled anode and cathode. A diode is said to be forward biased when its cathode is at least .7 volt more negative than its anode. A diode is reverse biased when its cathode is not at least .7 volt more negative than its anode. Forward biasing a diode causes current to flow, and reverse biasing causes no current to flow. Although there are numerous types of diodes, the ones of interest to the mobile electronics installer are power diodes, or rectifiers. A rectifier’s value is expressed in amps, and is the maximum forward current which the device can safely handle.

There are other parameters which are of importance when selecting a rectifier. The peak inverse voltage (PIV) is the highest voltage which can be present while the rectifier is reverse biased.

Transistors are semiconductor devices which either act as current amplifiers, voltage amplifiers, or switches. The two most often used in mobile electronics are the bipolar and the metal oxide semiconductor field effect transistor (Mosfet). They act as “valves” where a small change in control current or voltage on the input yields a great change in current or voltage at the output.

The chief disadvantage to using bipolar transistors is “thermal runaway”. Thermal runaway is characteristic of bipolar transistors which causes more current to flow when the transistor gets hot. As more current flows, more heat is dissipated allowing even more current to flow. This cycle continues until the transistor destroys itself.

Mosfets are more similar to vacuum tubes in operation than bipolar transistors. They are transconductance devices, which means that the input works on voltage only without the need for current. They have extremely high input impedances. They act primarily as voltage amplifiers. Mosfets do not exhibit thermal runaway like bipolars do, as when Mosfets get hot, they actually allow less current to flow.

How loud a speaker can play depends on how much air it can move without overheating. How much air can be moved is determined by the surface area of the cone and the excursion capability of the motor system.

Xmax is defined as the width of the voice coil that extends beyond the front plate (See Diagram 2). This relates to how far the speaker can move in either direction without appreciable distortion.The amount of power required to move a speaker to its maximum excursion is referred to as the displacement limited power handling. Please note that this number varies with enclosure size and frequency.

Loudspeaker power handling ratings are one of the most commonly quoted, but most poorly understood of specifications given by loudspeaker manufacturers. It seems that every company has its own way of measuring and specifying power handling. That’s because Marketing departments are always looking for ways to be able to list higher numbers for power handling in order to impress their customers with the apparent ruggedness of their products. It is sometimes difficult for product users to understand how these specifications relate to real world amplifiers or how they relate to the way they listen to their favorite kinds of music on loudspeaker systems. Loudspeakers fail in one of two ways - mechanically or thermally. Mechanical failures occur when one of the moving parts of the speaker such as the surround, spider, or cone become fatigued, tear, or break from the effects of continued long excursions. Thermal failures occur when the electrical power dissipated in the voice coil as heat causes the adhesives holding the turns of voice coil wire together to break down, or the insulation on the wire to fail, resulting in shorted turns. Also, the wire itself can melt, which means an open voice coil, or the coil support can melt or burn, again meaning failure of the loudspeaker.

For many years at MTX, we have been power testing loudspeakers using pink noise band limited from 20 Hz to 20 kHz and crest factor limited to six decibels (four to one peak to average power ratio). Loudspeakers are tested in free air, without an enclosure, for a period of eight hours, and must pass this test without a permanent change in performance characteristics. This is a very conservative test, resulting in realistic power handling ratings, and allows in-house direct comparisons of the power handling characteristics of different models in our various product lines. However, because the speaker is tested in free air without the benefit of an enclosure to add stiffness to the suspension, and the test signal contains full power signals down to 20 Hz, this is very severe mechanically for the driver. Since voice coil excursions must quadruple for every halving of frequency to produce the same acoustic output, surrounds and spiders are quickly stretched to a maximum. As a result, a majority of speaker failures are mechanical, without having stressed the voice coil thermally.

On the other hand, many audio companies ascribe to the Electronic Industries Association (EIA) standard for loudspeaker power testing called RS-426A. This test calls for a white noise signal to be applied to a band limited filter from 40 Hz to 318 Hz with a six decibel crest factor. However, the resulting signal, when displayed on an RTA or any constant percentage bandwidth analyzer, has a bandwidth from approximately 500 Hz to 8 kHz with a peak near 2 kHz. This means that it has relatively little low frequency content. The other test conditions include operation in free air without an enclosure, and a test period of eight hours. While useful for testing general purpose loudspeakers, it has limited application with large woofers. In addition, because its test signal is centered around 2 kHz, where the impedance of most woofers has risen to two or three times its nominal value, the actual power delivered to the speaker is much less than is indicated by the calculation of power using the nominal impedance. The result is that most failures using this test are thermal and the applied voltages required to cause the failures are excessive and difficult to generate with normal power amplifiers. More importantly, the power handling ratings that come from this type of testing are unrealistically high and do not represent actual usage conditions.

Some companies, including MTX, have recognized the need for a better method of testing and rating the power handling of loudspeakers. A committee of members of the EIA have developed a new test signal based on a cross section of the spectral content of recorded music. For several months, power testing at MTX has been done using this new test signal which has been proposed as a “B” revision for the RS-426 standard. The test signal has a flat response from 40 Hz to 1 kHz, with a three decibel per octave rolloff from 1 kHz to 10 kHz, and with a rapid rolloff above 10 kHz and below 40 Hz. In order to encourage its widespread use by loudspeaker manufacturers, the EIA has made the proposed “B” version test signal available on compact disc. The other conditions of the test remain the same as before, the signal being crest factor limited at six decibels, with the driver in free air, for a duration of eight hours.

The results of testing loudspeakers in this way has been to stress drivers both mechanically and thermally to nearly the same extent, with a test signal that is a close representation of real music. The power handling ratings that would result give the customer a very good idea of the size of power amplifiers that could be safely used with a given loudspeaker.

Because of the excellent testing results which have been obtained using this proposed revision of EIA RS-426, MTX has decided to adopt it as a standard within our company. All power handling ratings published on new loudspeaker products will be based on this internal standard.

Since the use of the term “RMS power” is so widespread in our industry, even though its meaning is ambiguous, MTX has decided to continue its use while assigning it a real definition. “RMS power” listed on MTX loudspeaker data sheets will be the power the speaker withstands under the test conditions of the proposed RS-426B standard. In the same way, the term “peak power” has been misused with an even wider variation of so called definitions (or sometimes no definition at all). MTX data sheets will drop the term “peak power” in favor of the phrase “total power” which will be a maximum of four times the “RMS power” rating in order to reflect the amplifier power required to test the speaker under the conditions of the new standard. This is based on the dynamic range of the test signal being limited to a crest factor of six decibels, which corresponds to a power ratio of four times.

It is hoped that the adoption of these terms as defined will end some of the confusion about loudspeaker power handling and give our customers useful information to help them in selecting power amplifiers to be used with the speakers in their audio systems.

Loudspeaker power handling ratings that have been discussed here are based on standard tests intended to simulate actual use conditions as closely as possible, while still being reasonably practical for the manufacturer to perform. However, customers will typically use drivers in some kind of enclosure, whether it be sealed, vented, or bandpass. The size and type of enclosure will help to determine the mechanical power handling limit of the driver based on its maximum linear displacement. The customer should always determine the power and frequency limitations for his particular driver/enclosure combination before using them with large power amplifiers. Many of the popular computer enclosure design programs can provide this information, or MTX customer service personnel will be glad to help with recommendations for specific drivers and enclosures.

Research done in the study of loudspeakers has been conducted by a number of individuals. The works of Neville Thiele and Richard Small are considered to have the most impact on the loudspeaker design field.

A method was found, so that one could predict the frequency response performance of a loudspeaker system, based on its physical characteristics.

The Physical Characteristics of a speaker are:
Re:  The D.C. resistance of the voice coil measured in Ohms.
Sd:  The surface area of the speaker’s cone.
BL:  The magnetic strength of the motor structure.
Mms:  The total moving mass of the speaker including the small amount of air in front of and behind the cone.
Cms:  The stiffness of the driver’s suspension.
Rms:  The losses due to the suspension.

By understanding the relationship of these physical parameters and how to change them, we may alter the response parameters to fit the desired goal.

The Thiele/Small Response parameters are:
Re:  The D.C. resistance of the voice coil measured in Ohms.
Sd:  The surface area of the speaker.
Fs:  The resonant frequency of the speaker.
Qes:  The electrical “Q” of the speaker.
Qms:  The mechanical “Q” of the speaker.
Qts:  The total "Q" of the speaker.
Vas:  The volume of air having the same acoustic compliance as the speaker’s suspension.

Understanding the response parameters allows us to calculate the predicted frequency response of a given speaker system. The formulas that accomplish this are rather lengthy and complex, and are best left to a computer. There are a number of high quality computer programs on the market that automate the design process of building an enclosure.