All electronics today have frequency response capabilities that extends well beyond the audio range. The measured sound spectrum is 20Hz-20KHz. A speaker and most musical instruments have a limited frequency response, and only accurate within 3 octaves (octave is ratio 1:2) or 1 decade (decade is ratio 1:10). 3.2 octaves being 1 decade. The standard way of measuring a speaker or speaker system is with a microphone, at one point at 1 meter on axis, and a tone sweep across the frequency spectrum and plotted. This measurement provides vital information for detailed research but limited for how it sounds.

Small speaker systems (passive) have a wide polar response dispersion at low frequencies, but narrow beaming dispersion at high frequencies. An ideal sound system will have even dispersion at all frequencies. To achieve this the diameters of speakers must change each 2-3 octaves, this requires 4 different size speakers to cover the sound spectrum. Theoretically a single speaker would have to change diameter from (1in - 24ft) or (20mm - 8m) to maintain similar level and polar dispersion over the frequency spectrum.

Spectral energy response includes energy response, frequency response, polar response and efficiency for the whole acoustic energy delivered from a speaker system, giving a closer understanding of how a system behaves.

Cone Speakers are approx 2% efficient. Power and efficiency is dependant on magnet, voice coil size, mass and area of cone, suspension compliance, damping and frequency. The fine art of musical instrument and speaker making, is combining efficiency and power, responding with clarity and evenness, complying within the 3 octave rule. A musical instrument and speaker may be efficient but uncontrolled, which means the sound is colored, the notes uneven and without clarity. Many cheap musical instruments and speakers behave this way. An instrument or speaker may be in-efficient, having a flat response, the notes even, but lacking dynamic expression and responsiveness, requiring to be played harder to be heard.

Primary inter-modulation is where a single speaker is used beyond 3 octaves, or full range. The large cone movement (excursion) for low-frequencies, modulates the middle and higher frequencies, causing them to sound dirty. Lobe and node distortion, is caused by high-frequencies creating secondary vibrations and chaotic resonances within the speaker cone, causing it to sound harsh and screechy.

Dynamic power response is the ability for the sound fidelity to remain intact between low and high power. It is not possible for one amplifier driving different speakers (passive), or one speaker used beyond 3 octaves to achieve this accurately. The majority of sound systems are passive, mainly due to cost and the fashion for systems to be small. For a system to sound consistent and accurate at all power levels, it must be active. Each speaker driven by its own amplifier, and matched in efficiency, power and dispersion. This also eliminates cross interference within and between the speakers (inter-modulation).

The construction of speakers is approached in the same way as musical instrument making. Fine tolerances and attention to detail make large differences to performance. Large musical instruments and speakers suit low frequencies and vice versa. The majority consist of paper or plastic moulded into a cone shape, loosely suspended in a frame so as to easily move back and forth to vibrate the air. Glued to the back of the cone is a coil of wire (voice coil) within a strong magnet field. Passing electricity through wire causes a magnetic field around the wire, which attracts or repels, causing the cone to move back and forth. The larger the magnet and voice coil the greater the power and efficiency, if well made. Externally vibrating the cone will cause the voice coil to generate electricity.

The energy of the magnet is conducted through the mild steel pole plates and pole piece, and concentrated (north - south) across the gap. The smaller the gap, the more intense the magnetic field, the greater the efficiency. The slightest variations in alignment, during manufacture, cause large variations in performance. No two speakers or musical instruments can be identical.

Passing electricity through wire causes a magnetic field around the wire. Changing polarity of the electric current through the wire also changes the polarity of the magnetic field created around the wire. The interaction of the two magnetic fields causes the voice coil to be pushed out of the gap, forward or backward, depending on the polarity of the electricity through the voice coil. Most voice coils are double layered, wound with enamel coated copper wire, around a former, then bonded with an epoxy compound and backed in an oven. Voice coils can also be wound with rectangular aluminium wire, to achieve less weight and greater accuracy. A cone speaker is approx 2% efficient, therefore approx 98% of the electrical energy is wasted as heat.

At bass frequencies the voice coil has to move back and forth a long distance, especially at high power, compared to the high frequencies. During movement, the percentage of voice coil in the gap, must remain constant. The voice coil can be long and the pole plate thin, or the voice coil short and the pole plate thick, to achieve the same outcome. There are argued advantages and disadvantages both ways. Mid and high frequency speakers cones only move a small distance, compared to the large movement of bass speakers. The voice coil length and pole plate thickness are similar.

Mid and high frequency speakers are approx 6-10db more efficient than bass speakers. In small passive systems the mid and high frequency speakers are attenuated to match the less efficient woofer. In active systems the bass speaker is driven with higher power.

On the same diameter speaker, a small voice coil has less control over the cone compared to a large voice coil. With a small voice coil the cone is able to be more resonant, compared to the same size cone with a large voice coil. Some small voice coil speakers may appear to be more efficient, but this extra efficiency may be only at the one resonant bass note. At frequencies above this resonant bass note the speaker may be less efficient, compared to the same cone with a larger voice coil. Cost and performance of speakers increase with voice coil size. The larger the voice coil the better the control over the cone, and therefore improved damping and linearity. Large voice coils are expensive to make, (limit approx 4in 100mm). Assembling the speaker is also more difficult, tolerances taken to greater accuracy. The larger the voice coil diameter, the larger the area of the magnetic gap. To keep the flux density of the magnetic energy (per unit area) in the gap the same, (for the speaker to retain the same efficiency), magnet size and therefore mass of the speaker must be increased. The major advantage of larger voice coils is greater power-handling.

The maximum distance the cone can move limits bass power. To maintain the same acoustic output, the cone must move 4 times the distance for each octave decrease and vice versa. 1/4 distance for each octave increase. Two times, due to cone mass changing direction. As frequency decreases, air slips sideways decreasing resistance. As frequency increases, air doesn't have time to slip sideways, increasing resistance against the cone (radiation resistance). At wavelengths greater than 10 times the speaker diameter, cone movement is so great it becomes non-linear and inefficient. Some manufacturers make the suspension tight to limit cone movement, compromising bass performance for a higher power rating. At high frequency, power is limited by excessive heat that can destroy the voice coil.

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