Loudspeaker enclosure

A loudspeaker enclosure is used to mount speakers. Depending on the design it influences the reproduction characteristics and / or the carrying capacity of the transducer used. Especially in the low frequency range, it is used to avoid acoustic short circuit, wherein the light emitted from the rear sound is used in different ways.

The unit of speaker (s ), housing and usually more necessary crossover ( accompanied by any built-in amplifier, see Aktivbox ) is called the speaker.

The speaker enclosure essentially has three functions:

Formation. The chassis are made by putting the speaker into a suitable position. This affects angle as well as distance to the ground. For non- standing speakers (which can also be large bookshelf speakers ) belongs to an optimal table was still the speaker stands or wall mounting.
Sound management of the rear sound. In order to reproduce frequencies whose wavelength is larger than the diameter of the diaphragm, it is necessary the exchange of air between the front and rear to interfere (so-called acoustic short circuit ) or via suitable resonators to make. This point is discussed in speaker enclosure (1).
Sound management of the sound of the front. The immediate surroundings of a chassis is critical to the frequency response and the direction of the sound radiation. This point is discussed in speaker enclosure (2).

4.1 Series and parallel arrangement
4.2 radiator line
4.3 With the wave mitlaufender spotlight

Housing forms

( Open ) Baffle

In order to prevent pressure equalization between the front and back, it would be best to incorporate the chassis in an infinite baffle. This construct is how the installation in an infinitely large box for theoretical considerations used (eg resonance frequency on infinite baffle ). In practice, one must be content with finite baffles.

Finite baffles prevent the acoustic short circuit only partially. Below a certain frequency limit it is increased to equalize the pressure between the front and back, which leads to a drop of 6 dB / oct. Together with the behavior of the resonant frequency of less than 12 dB / oct, this leads to an asymptotic behavior of 18 dB / oct at the lower end of the transfer portion.

Finite sound walls are common in playback devices with built-in speakers, for example,

Tube receiver ( " steam radio " )
Mono transistor radio
Portable media players with speakers
Televisions CRT
Guitar / Instrument amplifiers

Open baffles are also used in modern Dipollautsprechern or speakers in retro style.

Folded can significantly reduce the dimensions, but these designs are more sensitive to cavity resonances and must be acoustically damped if necessary.

Asymptotic behavior: 6 dB / oct from occurrence of the acoustic short-circuit, 12 dB / oct below the resonant frequency, 18 dB / oct when both criteria are met.

Advantage: No influence housing → Speakers

Disadvantage: To transmit frequencies below 100 Hz, noise walls must be very large.

Closed housing

The chassis is mounted in a (largely) hermetically sealed housing. Compared with baffles moderate size of the acoustic short-circuit is thus reliably prevented. However, the chassis receives the spring effect of the sealed cavity, add a further component, which increases the resonance frequency and the resonance quality. For installation in a closed enclosure is therefore used chassis, which have a low free air resonance and overall quality. The interior needs to be attenuated to reduce standing waves, ideal is an ample damping heat-storing material that converts the largely adiabatic compression in a largely isothermal compression. In air, this means an effective increase of the internal volume by a factor of 1.4.

Advantages:

Easy to set up
Simply tune
Reproduction of the lowest frequencies, it is possible, albeit with reduced amplitude
No flow noise
Low distortion of the phase response and the group delay in the lower area of application
Small, active equalized boxes possible

Cons:

Compared with a bass reflex system, there is no octave, in which the diaphragm deflections can be effectively reduced by a resonator, the deflection increases with 12 dB / oct at.
This results in higher non-linear distortion and lower maximum level resulting in the vicinity of the resonance frequency of the system
Time decay of the free-field amplitude frequency response at chassis with strong drive.

Meaningful vote:

In Active Speakers: Usually, true to the system down so that the thermal and mechanical strength at low frequencies occur simultaneously. Chassis with small Qts lead to very compact speakers.

Usually you tune to maximum linearity of the frequency response, ie it reduces the volume as far until you reach a Qts of 0.707 ( 0.6 ... 1.0).

Speakers with small Qts also lead to compact speakers, but with time, decreasing bass response.

Bass reflex enclosure

Bass-reflex enclosures are a special form of loudspeaker cabinets for bass speakers. In this so-called bass-reflex box, the volume is not complete, but is connected by a duct to the outside. The air mass in this channel forms (also known as a Helmholtz resonator ) and the housing volume of a resonator. The resonator causes an increase in noise emission in the area of its series - resonant frequency.

Tuning the resonance is performed by the length of the channel, and requires a sensitive adjustment of the so-called Thiele-Small parameters of the speaker and the volume of the housing. This can be accomplished by experiments, mathematical approximation formulas (vote by Hodge ) or with the aid of computer simulation. In the simulation, so-called equivalent circuits are used. The target is a linear frequency response as possible to the lower limit frequency. The use of vented enclosures allows speakers ( also called chassis or driver ) to use with in relation to the size of their baffle opening relatively strong electrodynamic actuators. Such speakers have in closed systems, a low efficiency at low frequencies below the natural resonance.

The determined by the bass-reflex enclosure so-called tuning frequency compensates for the weak efficiency in the range of low frequencies - the speaker produces the same deflection of the membrane, a higher sound intensity. This systems allows higher efficiency combined with lower cutoff frequency than with equally large sealed enclosures.

The effective range of the resonator is in the range 0.75 * fb ... 2 * FB, in this area the maximum level is improved by at least 2.5 dB. In the range between 0.75 and 0.9 * fb * fb stroke is indeed reduced, it is there nevertheless still very high, so that as the optimum working range frequencies above 0.9 fb * can be viewed.

Benefits

Significantly higher noise levels ( up to 13.5 dB) in the lowest octave possible, or
Expansion of the power bandwidth of 1.1 octaves ( a factor of 2.2 )
More powerful bass reproduction with chassis with powerful drives, whose frequency response usually falls early by mutual induction.
Different Abstimmvarianten ( Hooge, Thiele / Small, Novak, Bullock, ...); Frequency response and case size varied for a given chassis and space conditions designed.

Disadvantages

Higher group delay variations
Steepening of the transfer function below the lower cut-off frequency
When the chassis transmits frequencies whose wavelength is in the region of the tunnel, there will be resonance tunnel. This problem occurs in virtually all woofers. In case of insufficient dimensioning of the tunnel, there are interfering flow noise.
For emission of sound below the resonance frequency of the overall system occurs due to the lack of the spring stiffness of the air cushion to be large cone excursions at the same time erasure of sound from the front and back.

Meaningful consultation

In Passive Speakers

There are several Abstimmvarianten from the filter theory, leading to largely linear bass response. These may be used as a first approximation to the design.

With active speakers

The tunnel resonance frequency determines the frequency response performance, at a fixed lower power bandwidth is calculated to this.
Powered speakers to build mostly smaller than passive speakers and then subsequently equalized frequency response. This equalization includes virtually always a protective filter 2 ( would be good to face even one passive, but there is material- intensive) to 4th order against frequencies below fmin.
The reduction of the housing volume is limited by: at low frequencies increased electrical power - it must be further ensures that the mechanical overload occurs before electrical
The Resonatortunnel must not be too long ( this is ~ 1/Vs long).

Bandpass enclosure

The chassis is carried out between two chambers. The front chamber is coupled to the same environment as in the simple vented bass reflex enclosure with a ventilation duct, the posterior chamber is closed as in a closed box and has no connection to the environment. From the mass of air in the ventilation duct and the spring stiffness of the enclosed front volume results in a so-called Helmholtz resonator, which completely takes over the sound radiation. (Also known as tuning frequency) of the efficiency in the region of natural resonance is higher than that of a free radiating chassis. The volume of the rear chamber has the same function as in a closed box, it should allow movement of the membrane and prevent the acoustic short circuit.

Bandpass enclosure are for transmission of narrow frequency ranges (within one octave) used meaningfully. A high efficiency further reduces this frequency range, a wide range reduces the efficiency. In conjunction with the properties of the chassis, the level falls above and below the transfer portion in order 12dB/Oct. , which corresponds to a respective filter of the second, a total of the fourth order.

Double vented bandpass enclosure

In the single- vented bandpass enclosure radiated into the rear chamber energy is not used similarly to the closed housing. In the double- vented bandpass enclosure, the energy of both chambers is coupled and radiated via a Helmholtz resonator. For a wider transmission range, it is useful to vary in size to dimension the two chambers and vote differently. This allows the transmission range can be extended. As with the simple vented bass reflex enclosure there is however below the transmission range to a steeper slope of 36 dB / oct ( one speaks of the bandpass 6th order ), which is caused by the acoustic short circuit.

Double vented bandpass enclosure are used meaningfully only for the transmission of narrow frequency ranges (within 1 1/2 octaves). Compared with bass reflex and bandpass mono- voting, only to sealed enclosures, they have iA a good efficiency and high maximum level, but also stronger distortions of the phase response / group delay.

Asymptotic behavior: 12 dB / oct below the resonant frequency, 12 dB / oct below the tunnel resonance frequency, 24 dB / oct, if both are true ( and more 12dB/oct by the characteristics of the chassis itself). Towards the top, 12 dB / oct waste.

Multi-chamber bandpass enclosure

There are also much more complex systems are possible, the most famous of which are the multi-chamber bandpass systems.

Their general problem is the high group delay ( the resonators need some time until they have mutually vibrated ). Furthermore, there is still usually a very problematic frequency response. Similarly as with the band-pass systems are possible higher efficiencies at the expense of poor transient response. Often working with multiple chassis. In contrast to mono- and dual -reflex bandpass filters, the bandwidth of the system can be made larger, which in particular mass manufacturers in combination with very small satellite speakers often use (eg Bose).

Subsonic Filter

It is useful to provide the bass chassis with a bandpass instead of a low-pass. Frequencies below the transmission range otherwise lead to considerable mechanical stresses on the chassis, as the ever increasing wavelengths require enormous deflections of the membrane. Manufacturer of many loudspeaker chassis give the maximum possible deflection in millimeters.

Passive Subsonic filter:

Most of the high-pass first, rare 2nd order. The necessary sizes (200 ... 500 uF, 20 ... 50 mH ) are usually costly and problematic especially at high power. Use only a few manufacturers. Bass equalization is possible only to a limited extent, it is connected with a reduction in the effective range of the impedance ( series resonant circuit ). Another problem is temperature sensitivity of the vote by repercussions of the TSP parameter to the filter.

Active Subsonic filter:

In active speakers subsonic filter are common. The cost is small, mostly a bass equalization is integrated.

Advantages:

The speakers are more resilient
Reduced distortion, especially in the presence of low-frequency disturbances ( LP )
Static or dynamic bass equalization possible
Reduced group delay above the transition region.

Cons:

Additional expense
Increase in the group delay in the transition region ( in analog filters)
Steeper drop in the amplitude frequency response, total lack of deep bass with compact speakers

Passive Radiators

For mass elements for Helmholtz resonators are used in addition to ventilated tubes and passive radiators in question. With them it is much easier to achieve the necessary votes for deep masses.

Advantages over tunnels:

No flow noise
No resonances or pass frequencies, as they occur in tunnels conventional length
More precise bass reproduction
Depth Voters also those in cases possible where a tunnel would assume impractical proportions

Disadvantages compared to tunnels:

Higher costs
Larger space on the outside of the box
Only for pure bass speakers (subwoofer ) is useful, since higher frequencies penetrate the relatively thin membrane outward

Using a plurality of chassis for a frequency range

Series and parallel arrangement

Multiple chassis are often used for the same frequency range used. This can happen for several reasons:

It increases the capacity, because both electric and mechanical loading (does not apply acoustic array ) reduced
For the series arrangement, Vas, which is helpful for a reduction in the boxes volume reduced. At the same time, however, the efficiency drops
For parallel arrangement increases efficiency at low frequencies
Anti-parallel and anti- series configuration: Straight distortion components are reduced
By suitable control of the emission can be better modeled (Speaker line, oppression rear sound )

Note: For anti-parallel arrangement of separate chambers are useful, they reduce large-signal operating point shifts.

Radiator line

In radiator lines all chassis are driven in phase, the resultant beam is as wide as that of a single chassis, but vertically compressed. Possible modifications:

Delay of the inner chassis: The ideal listening distance (all chassis in phase ) moves from the infinite to a finite distance. For a 1.6 m high and 8 m line intercept distance the necessary delay is 117 microseconds.
Decoupling of the outer chassis at higher frequencies: Normally the club to high frequencies will always go lower, as the size increases relative to the wavelength. By selectively uncoupling ( up to 1 Chassis ) can reduce or eliminate this effect.

With the wave mitlaufender spotlight

A further rotationally symmetric radiation with damping of the rear and side portions are obtained by a speaker line that is driven by a " traveling wave " in listener direction.

Horn speaker

Main article: Horn (speaker)

Of horn speakers in the sense of speaker box is when all frequency ranges ( the bass range) use long horns. Depending on the depth of the bass reproduction are those horns large to huge.

We distinguish between front-loaded and back- loaded ( Lowther ) horns. Front -loaded horns work with the back of a pressure chamber, the front is coupled to a horn. Back- loaded horns, named after the inventor also Lowther horns, emit sound from the front directly or through a ( short ) Waveguide, the back is coupled to a long horn, which takes over the bass reproduction.

Example of a horn with rear chamber and pressure chamber

Print back chamber chamber _______________________________ | / \ / \ / | | | / V, V | \ # # | | / | | | | > # # | |, | | | | / # # | | | | | | | __ | _____ | | | | | | | / | | | | | | / | | | | | | / | | | | | / | | | | \ ___ / / | | | | / | | | | / | | | | _ / | | | | _ / | | | _ / | \ _ / Horn mouth | \ ______ / | \ | \ | \ _ | \ _ | ___________________________ _____ \ Example of a direct radiating Lowther horn:

____________________________ | / \ / \ / ::: | | / V, V | # # / direct - | / | | | | # # < Radiant |, | | | | # # \ Chassis | | | | | | :::: | | | | | | | ::: / | | | | | | :: / | | | | | | :/ | | | | | / | | | | \ ___ / | | | | / | | | | / | | | | / | | | | / | | / Horn mouth | \ / | \ ______ / | \ | \ | \ _ | \ _ | _____ \ _________________________ advantages:

High Endschallpegel accessible
Low diaphragm strokes, the driver also in the bass range

Cons:

The size of the horn mouth must be in the range of the maximum radiated wavelength, below this frequency loses the horn of efficiency, the damping of the driver by the air load collapses. Therefore, for common lower cut-off frequencies (40 Hz ... 80 Hz) horns huge. Previously it was believed by a false Applied symmetry argument, one could reduce the size significantly, if you're getting horns in the corners of rooms, or rooms edges. This is not the case, according to modern simulation methods. Rather, the required cabinet volume at 40 Hz cutoff frequency is independent of the size of the driver to 1000 liters, the length of the horn curve should be 3 to 4 feet, and the oral surface should not be less than half a square meter. It should be noted that the cross-sectional widening should extend symmetrically - the usual screw form is less suitable at the end.
It should be used due to the disappearing below the air load transfer portion, a steep high-pass filter to protect the chassis. However, this applies only for mathematically correct built horn speaker. Must be built according to the case of heuristics, each individual case will be considered separately, because there is no clearly defined lower limit frequency. The air load is then composed of rather irregularly distributed resonances.
The radiated over the horn sound has considerable signal propagation times.
Discontinuities of the radiation resistance and in the construction applied heuristics lead to comparatively very strong ripple in the frequency response of more than / -10dB.

In ( a configured ) fixed installation in large rooms ( light movie theaters, concert halls ) size does not matter anymore. The construction is being made possible by simulation programs. The direct solution of the wave equation with the derived formulas fades into the background.

Helmholtz resonance

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