Equivalent volume : Vas | Bass-reflex boxes | Power rating | D’Appolito | Double magnet | Closed cabinets | Fiberglass | Ferrofluid | HexaCone | Impedance | Calotte speaker | Coaxial speaker | Carbon fibers | Force factor | Copper | Magnesium | Magnet | Magnetic field | Membrane | Volume in dB | Polypropylene | QES FACTOR | QMS FACTOR | QTS FACTOR | Resonant frequency | Sandwich cone | Voice coil | Short-circuiting ring | Seal | Subwoofer | Frequency response | Efficiency factor
Equivalent volume: Vas
Measures the flexibility of the membrane suspension based on the membrane area..
Bass reflex boxes
A bass reflex box is also known as a Helmholtz resonator. Technically speaking, this kind of acoustic resonator consists of a volume with an opening. The air enclosed in the cabinet has a spring rigidity that depends on the volume; the air inside the opening and in the (bass reflex) tube also has mass. By coordinating the spring rigidity of the enclosed volume with the suspended moving mass (air volume in the bass reflex tube), you can define a specific resonant frequency. This resonance is created by a loudspeaker that is built into the cabinet. By carefully choosing the resonant frequency, you can amplify the low-range playback while also reducing the membrane displacement.
The nominal power rating in watts indicates how much electrical power can be fed into the loudspeaker during continuous operation (DIN 45573). The DIN standard establishes the exact parameters. The musical power rating indicates how much power can be fed in at short-term high levels from the loudspeaker without causing damage or strong distortions (DIN 45500). The power rating is not a quality criterion. It does not even tell you how much volume a loudspeaker will produce.
After looking at studies performed by Siegfried Linkwitz about multi-path systems and their dispersion behavior, American Joseph D’Appolito came up with an interesting solution. He adopted Linkwitz’ idea of using a satellite subwoofer system. However, he thought the low power rating of the satellite systems was a disadvantage (according to Linkwitz). He investigated ways to increase the maximum acoustic pressure of the satellite system without any loss of quality. Like Linkwitz, D’Appolito believed that a deep mid-range chassis with a diameter of 130 mm was the ideal. However, he discovered that the small chassis did not allow him to realize the acoustic pressures that he needed to faithfully reproduce digital signal sources. D’Appolito investigated how using two 130-mm chassis components affected the overall system. The conventional placement, in which one chassis is stacked on top of the other and then the tweeter is placed on top of that, significantly worsened the dispersion behavior. D’Appolito discovered that he could achieve almost perfect dispersion behavior if the tweeter was placed in the middle of the box with one mid-range speaker above it and one below it. This solution by D’Appolito means that the crossover is closely related to the overall behavior of the combination. If the assembly uses a third-order all-pass filter (18 dB per octave), the crossover frequency forms a single main dispersion beam that allows the listener to change listening position throughout the entire vertical area without losing sound. An assembly based on D’Appolito’s theory consists of two identical stacked mid-range speakers with a tweeter mounted between them. The mid-range speakers have a parallel function, and the structure is controlled by a third-order all-pass filter. It’s very rare to see a third-order filter in the currently available construction plans. This apparent discrepancy can be explained if you take into account the fact that the filter’s all-pass function is not created electrically, but acoustically.
The name ‘double magnet’ is somewhat misleading. The loudspeaker does not have multiple magnet systems, but rather one large magnet ring consisting of two single magnet rings stacked on top of each other.
This simple trick allows us to create high-performance, cost-effective magnet systems with relatively compact structures.
In principle, a closed cabinet is simply an infinitely large baffle board. The baffle board has been folded into a completely closed cabinet. If the cabinet is air-tight, the spring rigidity of the enclosed air in the cabinet can be precisely specified.
Closed cabinets have a lower power rating for the bass range than bass reflex boxes, but in exchange they offer better impulse responses. Closed cabinets are also suitable for loudspeakers with higher Qts values over 0.5.
How it works: the woofer works with a finite volume.
The air volume is like an additional spring for the loudspeaker.
The linearity of the frequency response in the bass range depends on the overall quality of the installed speaker.
If the Thiele/Small parameters for the speaker chassis are known, closed cabinets can be calculated very simply and precisely using the BoxCalc program.
The most cost-effective Q value is considered to be 0.707. Here, the frequency response as well as the transient and decay responses act like a second-order Butterworth filter. Below the installed resonance, the frequency response drops off by 12 dB per octave.
In this illustration you can see how various Q values affect the frequency response in the resonant frequency range.
Fibers created through pulling, centrifuging or blowing glass. The fiber diameter is 0.003-0.03mm. Fiberglass-reinforced plastics are distinguished by their good rigidity-to-weight ratio.
Ferrofluid (FFL) is an oily, magnetic liquid that is inserted into the air gap of a tweeter.
This liquid increases the mechanical insulation of the spring-mass system (voice coil, seal, membrane); in other words, it reduces the Qms value. As a result, it largely suppresses the impedance increase at the resonant frequency.
The result: Significantly higher mechanical capacity!
Speakers transform only a trace amount of the fed-in energy into sound; much more of it is turned into heat and represents a dissipation loss.
This heat can now be transmitted much more directly to the magnet system via the ferrofluid. In other words, the air coil is cooled much more intensively than it would be without the FFL. For systems without FFL, heat can only be drawn off via the air in the air gap.
The result: Significantly higher thermal capacity when FFL is used!
The HexaCone membranes in the Eton HEX chassis are 305 times lighter, but 70 times more rigid than conventional membranes. This is achieved through two layers of Kevlar with one layer of Nomex honeycomb between them. The result is very good bass playback quality.
The nominal apparent resistance of the loudspeaker. The impedance is not constant throughout the entire frequency range, something that needs to be taken into consideration when designing the frequency crossover.
A calotte is a cutout from a section of a sphere. This dome is set directly on the voice coil. Calottes are different from cone loudspeakers in that the cone membranes are omitted.
• The calotte is very rigid.
• The low membrane thickness guarantees a large diffusion angle.
• The membrane mass is relatively small, which is important for achieving a good efficiency level.
However, calottes can only be manufactured in certain sizes. They are very useful for tweeter systems. Common membrane thicknesses are between 10 and 25 mm for tweeters and between 34 and 70 mm for mid-range systems. Still, even with the latter it is hard to reconcile the rigidity of the material with a reasonable membrane mass. In addition, the edge clamps are placed under a great deal of pressure in terms of linearity.
Calotte membranes are made of plastic, woven fabric or metal.
Developers have always been fascinated by the idea of placing the acoustic centers as close together as possible, or even unifying them into a single source.
The first step was creating coaxial speakers with a stacked tweeter, the way they are still found in car speaker systems. New magnetic materials such as neodymium made compact tweeter systems a possibility, which meant that the tweeter could be installed in the center of a woofer system. The main benefits of the coaxial system are stable phasing and a constant envelope delay. This is often responsible for a homogenous sound pattern.
Thin fibers made of pure carbon, used to reinforce plastics. Carbon fibers are also used in membrane construction.
Force factor W x L
The product of magnetic induction and voice-coil wire length in the air space tells us how “strong” a loudspeaker’s driver is. As a rule, speakers with large W x L products have small Q values, in other words large damping effects.
This gleaming reddish, soft and very malleable heavy metal is the best electrical and thermal conductor after silver. Copper is obtained through pyrometallurgy, by roasting it together with iron minerals.
The refining process produces refined copper. Pure copper is obtained through electrolysis. The voice coil of a loudspeaker is usually made of (enameled) copper wire. As a good conductor, copper is also used for manufacturing phase plugs (Seas Excel Line), among other things.
A base, silver-colored, highly reactive light metal. Magnesium dissolves in acid and burns with a blinding white light (used in flashbulbs). It is obtained from magnesium chloride through fused-salt electrolysis. Magnesium membranes are very rigid and have a low specific weight.
A magnet is a body that creates a magnetic field around it. Magnetic field lines emanate from the “north pole” of the magnet, and enter the magnet at the south pole.
In loudspeakers, ferrite is normally used to create the magnetic field.
The metal components (pole plate, pole core and base plate) are used to conduct the magnetic field to the air space.
A field created by magnets or by moving electrical loads that transmits forces between magnets or electrical flows.
The magnetic field is described by the magnetizing field strength.
The membrane of a sound converter can take different shapes.
These are described as:
• cone membranes
• flat membranes
The cone – a flat area, for instance a piece of paper, formed into a funnel – offers outstanding rigidity.
If, as in a loudspeaker, this cone is moved back and forth at varying frequencies, the cone tends to vibrate unevenly above a certain frequency; rather than vibrating in one piece, its motion is broken up. This results in partial vibrations, which create various sound centers. These then overlap and either erase or amplify one another. “NaWi” (non-developable) membranes can help here. This membrane is not a cone, but has a hyperbolic membrane cross-section. The membrane cannot be developed. In other words, the membrane could not be shaped into a flat piece of material if you were to cut it apart.
Volume in dB
The volume in dB describes the logarithmic relationship between two values. In loudspeakers, these are usually acoustic-pressure relationships.
One example: A 6 dB loss or increase in the acoustic-pressure level means that the volume is either cut in half or doubled.
Polypropylene is a plastic material with a high level of internal damping that is used in manufacturing membranes, primarily for woofers and mid-range speakers.
Some of these membranes are reinforced with fillers such as chalk. That allows the hardness, rigidity and internal damping to be precisely defined and set.
Loudspeakers with polypropylene membranes have very high production consistency for all quality manufacturers.
Advantages compared to paper:
- better damping characteristics
- more docile frequency response at the upper end of the transmission range
- lower tendency to create partial vibrations
- very high production consistency
- neutral sound behavior in the mid-range
Disadvantages compared to paper:
- membrane is less rigid
- greater mass is moved with a medium-sized chassis (16 cm)
- poorer impulse behavior in the bass range
The electrical damping value Qes measures the damping created by the effect of the driver on the voice coil in the magnetic field.
The mechanical value Qms measures mechanical damping. This mechanical damping is created by friction between the centering membrane and the seal.
The Qts value (according to Thiele and Small) explains the damping behavior in the loudspeaker. Small values mean high damping levels.
The Qts consists of the electrical value Qes and the mechanical value Qms.
Qes x Qms
Qes + Qms
The Qts factor is decisive when it comes to the transient and decay response.
The resonant frequency is the frequency at which a mass can be made to vibrate by itself with the lowest possible energy.
A loudspeaker is a vibratory system of masses and springs. The membrane and the voice coil represent masses that are suspended from springs, consisting of the centering membranes and the seal.
In addition, mechanical friction forces and electrical damping are also created by the interactions between the magnet system, the voice coil and the amplifier. At a certain frequency, the mass – the membrane and the coil – can be moved with very little energy application.
Visually, you can see this in woofers because they show a large deflection at a certain frequency. This resonant frequency is a specific value depending on the chassis. An impedance graph shows the resonant frequency as more or less the first strong increase in impedance.
A sandwich membrane consists of a multi-layered laminate made from polypropylene with varying degrees of rigidity. It provides optimal internal damping and high rigidity with a low membrane weight.
The voice coil is an essential part of the driver system in a loudspeaker.
How it works: An electrical conductor, the voice coil, conducts an alternating current and creates a magnetic field with fluctuating polarity and intensity. If a fixed magnetic field is placed in opposition to this first magnetic field, the result is either repulsion or attraction, depending on the direction of the current.
The voice coil itself is usually designed as a hollow cylinder over which several layers of coiled wire have been applied. The greater the number of coils, the stronger the induced magnetic field.
Short circuiting ring
A ring that creates a short circuit in the magnet system. It reduces distortions caused by the movement of the voice coil.
The seal is the part of the speaker that connects the membrane to the basket and makes it air-tight. The importance of the material should not be underestimated, depending on the intended use. A well-chosen seal can prevent unintended effects or compensate for faults in the membrane.
The following materials are used:
- impregnated cloth
- foamed polymers
Today, the basic shape is a semicircular rim that points either inward or outward.
The lower the frequency, the longer the wavelength. Since the delay between the right ear and the left ear becomes smaller and smaller, relatively speaking, as the wavelength increases, lower frequencies are harder to locate.
Based on this finding, developers had the idea of reproducing the low range using only one speaker box, known as a subwoofer. The main speakers, which reproduce the entire musical event, then no longer need large membrane surfaces or large cabinets. These systems are known as satellites.
People can only locate the source of a sound above about 150 Hz, so the normal separation frequency between the subwoofer and the satellites is about 100 Hz to a maximum of 200 Hz.
The placement of the subwoofer in the room is not especially critical, but it is not as unimportant as is sometimes claimed. Every room has places where its natural resonances are activated more or less efficiently. The key is to test it out. If possible, the subwoofer should be equidistant from the satellites. This has more to do with the physical perception of the pressure waves than with the acoustic perception.
Frequency response (range)
This is the frequency range at which the sound level of the speaker is linear, within a tolerance of +/- 4 dB. Below 100 Hz and above 8000 Hz, the volume can drop off more strongly (DIN 45573).
The efficiency factor of a speaker system indicates how much of the fed-in electrical energy is transformed into acoustic energy by the amplifier.
The higher the efficiency factor, the louder the speaker, given a specific constant amplifier output. However, the efficiency factor cannot be used to estimate the maximum possible sound level.
Normal efficiency factors are between about 0.15% and 2.5%. Thus only a very small portion of the fed-in energy is transformed into sound. The remaining energy dissipates as heat and has to be diverted to the magnet system via the voice coil.
The following formula can be used to convert the efficiency factor to the mean sound pressure:
SPL = 112 + 10 log * efficiency factor