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LOUDSPEAKERS

Fri 26 May 06 @ 12:50 pm

Finally i did it! After long searching on net and putting articles together :)
Here is everything you should know about loudspeakers if you are a real DJ ;)
Enjoy reading!

Dynamic Loudspeaker Principle

A current-carrying wire in a magnetic field experiences a magnetic force perpendicular to the wire.

Loudspeaker Basics

The loudspeakers are almost always the limiting element on the fidelity of a reproduced sound in either home or theater. The other stages in sound reproduction are mostly electronic, and the electronic components are highly developed. The loudspeaker involves electromechanical processes where the amplified audio signal must move a cone or other mechanical device to produce sound like the original sound wave. This process involves many difficulties, and usually is the most imperfect of the steps in sound reproduction. Choose your speakers carefully. Some basic ideas about speaker enclosures might help with perspective.
Once you have chosen a good loudspeaker from a reputable manufacturer and paid a good price for it, you might presume that you would get good sound reproduction from it. But you won't --- not without a good enclosure. The enclosure is an essential part of sound production because of the following problems with a direct radiating loudspeaker.

Loudspeaker Details

An enormous amount of engineering work has gone into the design of today's dynamic loudspeaker. A light voice coil is mounted so that it can move freely inside the magnetic field of a strong permanent magnet. The speaker cone is attached to the voice coil and attached with a flexible mounting to the outer ring of the speaker support. Because there is a definite "home" or equilibrium position for the speaker cone and there is elasticity of the mounting structure, there is inevitably a free cone resonant frequency like that of a mass on a spring. The frequency can be determined by adjusting the mass and stiffness of the cone and voice coil, and it can be damped and broadened by the nature of the construction, but that natural mechanical frequency of vibration is always there and enhances the frequencies in the frequency range near resonance. Part of the role of a good enclosure is to minimize the impact of this resonant frequency.

Types of Enclosures

The production of a good high-fidelity loudspeaker requires that the speakers be enclosed because of a number of basic properties of loudspeakers. Just putting a single dynamic loudspeaker in a closed box will improve its sound quality dramatically. Modern loudspeaker enclosures typically involve multiple loudspeakers with a crossover network to provide a more nearly uniform frequency response across the audio frequency range. Other techniques such as those used in bass reflex enclosures may be used to extend the useful bass range of the loudspeakers.

Use of Multiple Drivers in Loudspeakers

Even with a good enclosure, a single loudspeaker cannot be expected to deliver optimally balanced sound over the full audible sound spectrum. For the production of high frequencies, the driving element should be small and light to be able to respond rapidly to the applied signal. Such high frequency speakers are called "tweeters". On the other hand, a bass speaker should be large to efficiently impedance match to the air. Such speakers, called "woofers", must also be supplied with more power since the signal must drive a larger mass. Another factor is that the ear's response curves discriminate against bass, so that more acoustic power must be supplied in the bass range. It is usually desirable to have a third, mid-range, speaker to achieve a smooth frequency response. The appropriate frequency signals are routed to the speakers by a crossover network.

Crossover Networks for Loudspeakers

Most loudspeakers use multiple drivers and employ crossover networks to route the appropriate frequency ranges to the different drivers.

Two-Way Crossover

Combinations of capacitors, inductors, and resistors can direct high frequencies to the tweeter and low frequencies to the woofer. This amounts to filter action. A two-way crossover network divides the frequency range between two speakers.

Three-Way Crossover

Combinations of capacitors, inductors, and resistors can direct high frequencies to the tweeter and low frequencies to the woofer. This amounts to filter action. A three-way crossover network divides the frequency range between three speakers.
A capacitor has lower impedance for high frequencies. In series with the high frequency speaker (tweeter), it acts to block low frequencies and let high frequencies through.

The inductor has a lower impedance for low frequencies. In series with the low-frequency speaker (woofer), it acts to block high frequencies and let low frequencies through.

Inductors

Inductance is typified by the behavior of a coil of wire in resisting any change of electric current through the coil.

Arising from Faraday's law, the inductance L may be defined in terms of the emf generated to oppose a given change in current:

Ported Bass-Reflex Enclosure

The bass-reflex enclosure makes use of a tuned port which projects some of the sound energy from the back of the loudspeaker, energy which is lost in a sealed enclosure. But care must be taken to avoid the back-to-front cancelation of low frequencies which characterizes unenclosed loudspeakers. This is avoided by tuning the cavity resonant frequency of the enclosure to the free-cone resonant frequency of the loudspeaker. This has the effect of projecting bass frequencies from the port in phase with the sound from the front of the cone, at least at the resonant frequency. The overall effect is the increasing of bass efficiency and the extension of the bass response to lower frequencies.

Enclosure Effects on Resonance

Putting a loudspeaker in a closed box will eliminate the back-to-front cancelation effect, but will shift the ouput curve upward in frequency compared to the infinite baffle. A bass reflex enclosure can extend the bass response significantly below the loudspeaker resonance.

Back-to-Front Cancelation

While the front surface of the cone of a loudspeaker is pushing forward to create a sound wave by increasing air pressure, the back surface of the cone is lowering the air pressure. Since the wavelengths of low frequency sound are large compared to the size of the speaker, and since those low frequencies readily diffract around the speaker cone, the sound wave from the back of the cone will tend to cancel that from the front of the cone. For most bass frequencies, the wavelength is so much longer than the speaker diameter that the phase difference approaches 180°, so there is severe loss of bass from this back-to-front cancelation.

This is one of the reasons why even the best cone-type loudspeaker must have an enclosure to produce good sound.

Coupling Loudspeaker to Air

How hard can you punch a handkerchief? Not very hard, because it offers so little resistance. A loudspeaker has a similar problem when it tries to punch sound energy into the air. The usual language is that the speaker has a poor "impedance match" to the air.

A loudspeaker without an enclosure does a very poor job of producing sounds whose wavelengths are longer than the diameter of the loudspeaker. For an 8-inch speaker, diameter of speaker equals wavelength at about 1700 Hz. Even for a 16-inch speaker, the diameter equals the wavelength at 850 Hz.

This is one of the reasons why even the best cone-type loudspeaker must have an enclosure to produce good sound. The enclosure increases the effective size of the loudspeaker.

Loudspeaker Resonance

Direct-radiating cone-type loudspeakers must be mounted so that they are free to vibrate. This mounting is elastic, so there is an inherent resonant frequency of the speaker cone assembly -- like a mass on a spring. This free cone resonant frequency distorts the sound by responding more strongly to signals near its natural vibration frequency. This non-uniform response changes the frequency content in terms of the relative intensities of the harmonics and thus changes the timbre of the sound. Since the cone is undamped, it tends to produce "ringing" or "hangover" with frequencies near resonance. If the resonance is in the bass range, the bass will be "boomy".

Audible Sound

Usually "sound" is used to mean sound which can be perceived by the human ear, i.e., "sound" refers to audible sound unless otherwise classified. A reasonably standard definition of audible sound is that it is a pressure wave with frequency between 20 Hz and 20,000 Hz and with an intensity above the standard threshold of hearing. Since the ear is surrounded by air, or perhaps under water, the sound waves are constrained to be longitudinal waves. Normal ranges of sound pressure and sound intensity may also be specified.

Frequency: 20 Hz - 20,000 Hz (corresponds with pitch)

Intensity: 10-12 - 10 watts/m2 (0 to 130 decibels)

Pressure: 2 x 10-5 - 60 Newtons/m2 2 x 10-10 - .0006 atmospheres

For an air temperature of 20°C where the sound speed is 344 m/s, the audible sound waves have wavelengths from 0.0172 m (0.68 inches) to 17.2 meters (56.4 feet).

Sound Intensity

Sound intensity is defined as the sound power per unit area. The usual context is the measurement of sound intensity in the air at a listener's location. The basic units are watts/m2 or watts/cm2 . Many sound intensity measurements are made relative to a standard threshold of hearing intensity I0 :

The most common approach to sound intensity measurement is to use the decibel scale:

Decibels measure the ratio of a given intensity I to the threshold of hearing intensity , so that this threshold takes the value 0 decibels (0 dB). To assess sound loudness, as distinct from an objective intensity measurement, the sensitivity of the ear must be factored in.

Sound Pressure

Since audible sound consists of pressure waves, one of the ways to quantify the sound is to state the amount of pressure variation relative to atmospheric pressure caused by the sound. Because of the great sensitivity of human hearing, the threshold of hearing corresponds to a pressure variation less than a billionth of atmospheric pressure.

The standard threshold of hearing can be stated in terms of pressure and the sound intensity in decibels can be expressed in terms of the sound pressure:

The pressure P here is to be understood as the amplitude of the pressure wave. The power carried by a traveling wave is proportional to the square of the amplitude. The factor of 20 comes from the fact that the logarithm of the square of a quantity is equal to 2 x the logarithm of the quantity. Since common microphones such as dynamic microphones produce a voltage which is proportional to the sound pressure, then changes in sound intensity incident on the microphone can be calculated from

where V1 and V2 are the measured voltage amplitudes .

Threshold of Hearing

Sound level measurements in decibels are generally referenced to a standard threshold of hearing at 1000 Hz for the human ear which can be stated in terms of sound intensity:

or in terms of sound pressure:

This value has wide acceptance as a nominal standard threshold and corresponds to 0 decibels. It represents a pressure change of less than one billionth of standard atmospheric pressure. This is indicative of the incredible sensitivity of human hearing. The actual average threshold of hearing at 1000 Hz is more like 2.5 x 10-12 watts/m2 or about 4 decibels, but zero decibels is a convenient reference. The threshold of hearing varies with frequency, as illustrated by the measured hearing curves.

Annotated Equal Loudness Curves

Loudness

Loudness is not simply sound intensity!

Sound loudness is a subjective term describing the strength of the ear's perception of a sound. It is intimately related to sound intensity but can by no means be considered identical to intensity. The sound intensity must be factored by the ear's sensitivity to the particular frequencies contained in the sound. This is the kind of information contained in equal loudness curves for the human ear. It must also be considered that the ear's response to increasing sound intensity is a "power of ten" or logarithmic relationship. This is one of the motivations for using the decibel scale to measure sound intensity. A general "rule of thumb" for loudness is that the power must be increased by about a factor of ten to sound twice as loud. To more realistically assess sound loudness, the ear's sensitivity curves are factored in to produce a phon scale for loudness. The factor of ten rule of thumb can then be used to produce the sone scale of loudness. In practical sound level measurement, filter contours such as the A, B, and C contours are used to make the measuring instrument more nearly approximate the ear.

"Rule of Thumb" for Loudness

A widely used "rule of thumb" for the loudness of a particular sound is that the sound must be increased in intensity by a factor of ten for the sound to be perceived as twice as loud. A common way of stating it is that it takes 10 violins to sound twice as loud as one violin. Another way to state the rule is to say that the loudness doubles for every 10 phon increase in the sound loudness level. Although this rule is widely used, it must be emphasized that it is an approximate general statement based upon a great deal of investigation of average human hearing but it is not to be taken as a hard and fast rule.

Why is it that doubling the sound intensity to the ear does not produce a dramatic increase in loudness? We cannot give answers with complete confidence, but it appears that there are saturation effects. Nerve cells have maximum rates at which they can fire, and it appears that doubling the sound energy to the sensitive inner ear does not double the strength of the nerve signal to the brain. This is just a model, but it seems to correlate with the general observations which suggest that something like ten times the intensity is required to double the signal from the innner ear.

One difficulty with this "rule of thumb" for loudness is that it is applicable only to adding loudness for identical sounds. If a second sound is widely enough separated in frequency to be outside the critical band of the first, then this rule does not apply at all.

While not a precise rule even for the increase of the same sound, the rule has considerable utility along with the just noticeable difference in sound intensity when judging the significance of changes in sound level.

Threshold of Pain

The nominal dynamic range of human hearing is from the standard threshold of hearing to the threshold of pain. A nominal figure for the threshold of pain is 130 decibels, but that which may be considered painful for one may be welcomed as entertainment by others. Generally, younger persons are more tolerant of loud sounds than older persons because their protective mechanisms are more effective. This tolerance does not make them immune to the damage that loud sounds can produce.

Some sources quote 120 dB as the pain threshold and define the audible sound frequency range as ending at about 20,000 Hz where the threshold of hearing and the threshold of pain meet.

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