Loudspeaker Builder

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Loudspeaker theory and information for builder

Transcript of Loudspeaker Builder

LoudspeakerBuilder.ca - (Thiele-Small Parameters)But still the most common cause of speaker failure is simple abuse; cranking it up beyond its power rating while asking the speaker to produce frequencies lower than it's frequency rating. So be sure to take into account the suggested usable frequency range and the Xmech parameter in conjunction with the power rating of the speaker to avoid such failures.The Thiele-Small Q's - The control of those peaks at resonant frequency (Fs) is done with the speaker's suspension (spider, surround) balanced off against the opposing force of the voice coil and magnet. The measurements used in describing the control (dampening) the movement of the speaker's suspension are the Qms, Qes and Qts. If the manufacturer does this right they can often put the resonate frequency of the speaker outside; either above or below its intended frequency range use which helps results in a more "flat" frequency response speaker.Q - The relative damping or system losses of a loudspeaker in an enclosure. The ratio of stored to dissipated energyQms - Is the measurement of control from the mechanical suspension system at resonance (Fs) which include the spider and the surround. They allow and control the movement of the speaker cone.Qes - Is the measurement of control from the electrical suspension system at resonance (Fs) which include the voice coil and the magnet; the "engine" of the speaker.Qts - The opposing forces from the mechanical and electrical suspensions acting against each other is the total Q of a speaker in free air at resonance (Fs).Qmc - The Q of a speaker in a sealed box considering only the mechanical resistance.Qec - The Q of a speaker in a sealed box considering only electrical resistance.Qtc - Put a speaker into an enclosure and you then change how that speaker will act due to the resistance of the air pressure inside of the enclosure. When the speaker cone moves in or out the air pressure within the enclosure will put a resistance on its movements. The size and type of enclosure you build will depends upon the Qtc value you desire.To decide upon your loudspeakers enclosure size you will need some loudspeaker software or good math skills and the math formulas; I prefer the software approach. In the software programs you will be asked to enter the required Thiele-Small parameters about the drivers such as the Q's, Fs, Vas, etc, some also ask enclosure type, number of drivers, etc. Then for the program to calculate the enclosure size it will want to know the Qtc value you want. The Qtc value you choose is a personal preference. A value of 0.707 is what most designers aim for, it will give you the flattest frequency response (accurate sound reproduction) and the lowest possible F3 (widest usable frequency range). Some people may not like this sound and want to enhanced base response so they may aim for 0.8 or higher.In general high quality accurate loudspeakers Qtc are around 0.707, while loudspeakers that are designed to enhance the base may range from 0.8 to a max of 1.1. The more you move away from 0.707 anything over that will slowly start to sound boomy and unnatural and the base response will become more restricted.If you want loud clean base go with larger drivers in larger enclosures, don't overwork a smaller speaker and try to increase tA.N. Thiele and Richard H. Small defined most of the relationships and terms we now use to describe what happens in a speaker and between a speaker and a particular enclosure's type and size. Their work has become the standard for speaker measurement criteria and is known as the Thiele-Small parameters. All speaker manufactures use the Thiele-Small parameters in describing their products which allow you to do a direct characteristics comparison of different speakers as well as give your the necessary information for designing the crossover network and enclosure.The most commonly used Thiele-Small parameters are listed below:EBP - Is used loosely to decide what type of enclosure will be best for any given speaker. It is calculated by dividing the Fs by the Qes. A result closer to 100 is usually best suited for an vented enclosure while an EBP closer to 50 will usually require a closed box design. This is just the "rule of thumb", some well designed high quality system violate this rule so use the EBP as a guide if the speaker manufacturer doesn't make a recommendation.Fs - This is the free-air resonant of a speaker; it's the frequency that the speaker wants to vibrate at. This is a result of the weight of the moving parts (cone, etc) in balance with the stiffness of the speaker's suspension. At a speaker's Fs the speaker will over emphasize (make louder) that frequency and cause crossover points to change due to impedance variances. For accurate sound reproduction these frequency peaks must be controlled (kept flat).Fb - The enclosure resonance (bass reflex).Fc -The enclosure resonance (sealed enclosure systems).Fp -Is the free-air resonant (Fs) frequency of a passive radiator.F3 - Is the frequency where the response (loudness) is down from the reference level by 3 dB. Anything below this frequency is often too quiet to be useful; so F3 help defines a speaker's useful range. Look at the graph down below at the Qtc 0.707. The reference frequency is 90 dB, look along the line and look where it crosses the 87 db. The F3 for this speaker in a box with a Qtc of 0.707 is aprox 45 HzLv - Is the length of the speaker enclosure's port.Pe - Is defined as the maximum continuous (RMS) power-handling capability of a speaker.Power Handling - Is rated on how much power a speaker can handle without causing damage. The most important consideration is the speakers ability to get rid of excessive heat. Factors that effect this include magnet and voice coil size and their ability to handle heat, venting, and the adhesives used in voice coil construction.Mechanical factors are also considered, such as the power required to cause;1. The coil to hit the back plate or come out of the gap2. The cone buckling from too much outward movement.3. The spider bottoming on the top plate.

A speaker that has a "flat" response is a speaker that usually reproduces sounds accurately. A speaker that doesn't have a "flat" response is said to "color" (distort) the sound it reproduces. Some people actually prefer certain forms of "coloring" over the sound of an accurate "reference" type speaker. Home theater center channel speakers often sound best if the midrange frequencies are slightly louder, this will tend to improve the on screen dialogue; others prefer a loudspeaker with louder lower frequencies (boom box type). Even so it's generally better to have the loudspeakers response as linear (flat) as possible and if more base or midrange is desired you may add the distortion with an equalizer or the tone controls on the amplifier.This graph below is what a perfectly flat loudspeaker response chart would look like; it has the same amplitude (loudness) across every frequency. If nothing extra is added or removed (coloring) the loudspeaker will accurately reproduce the original sound.; thus this is what most loudspeaker builders will aim for even though it's impossible to achieve.

With some knowledge and skill obtaining a response like the one below is about as good as it gets. A loudspeaker is considered linear (flat) if it stays within a range of about two dB's from highest to lowest. The frequencies outside of the blue lines have become too quiet and won't affect the overall sound of the loudspeaker so they are not considered within the loudspeakers frequency range.

Their are many variables that influence the amplitude / frequency response that a loudspeaker will produce; these variables include the choice of loudspeaker parts, type of enclosure, cabinet material, crossover network type, crossover slope, the materials / furniture in the room and the room's size and shape. To compensate for room acoustics an equalizer or the tone controls on the amplifier can be used to "color" the sound produced by the loudspeaker.Why Use A Speaker Crossover Network?No one single driver is capable of reproducing the entire sound bandwidth faithfully. Because of this we divide the task among two or more drivers to get better sound quality. A typical three-way loudspeaker consists of a woofer, midrange and a tweeter. Each one of these components is designed to do a certain bandwidth of the sound spectrum. The woofer typically reproduces all the sounds under around 500Hz, the tweeter may take all the frequencies above 6000Hz and the midrange does everything in between. By dividing up the frequencies onto three different speaker components the overall sound quality is dramatically improved over that of a single component loudspeaker.The crossover network is responsible for dividing up the sound bandwidth into specific frequencies and sending them to the proper speaker component; this is done by the use of coils, capacitors and resistors. Coils filter out higher frequencies and capacitors filter out the lower frequencies.Speaker Crossover Network DesignThis is without a doubt the most complex and difficult part of building your own loudspeakers. No matter what you do or how much you learn, or how much money you spend you will never build yourself anything near to a perfect passive crossover network. Capacitors and inductors just don't have what it takes to build the "brains" necessary to have a world class perfect crossover network. If you seek absolute perfection look into Active (Electronic) crossover networks, for the rest of us passive is usually good enough.You really don't need any crossover network "brains" for a single driver type loudspeaker since there is no division of the f requencies between different drive units. Add a tweeter with the driver and things change a little, you now have a two-way design and the minimum amount of "brains" now required is a simple c