Loudspeaker enclosure

A loudspeaker housing is used to mount loudspeakers . Depending on the design, it has a decisive influence on the reproduction characteristics and / or the load capacity of the sound transducers used. It is mainly used in the low frequency range to avoid an acoustic short circuit , with the sound emitted from the rear being used in different ways.

The unit of loudspeaker (s), housing and usually required loudspeaker crossover (in addition to any built-in amplifier, see active box ) is called a loudspeaker box.

The loudspeaker housing essentially has three functions:

• Lineup . The chassis are brought into a suitable position by setting up the loudspeaker box. This applies to beam angles as well as distances to the ground. In the case of non-floor-standing loudspeakers (which can also be large bookshelf loudspeakers), the loudspeaker stand or wall mounting is part of an optimal setup.
• Sound guidance of the rear sound . In order to reproduce frequencies whose wavelength is greater than the membrane diameter, it is necessary to hinder the exchange of air between the front and back (so-called acoustic short circuit ) or to make it suitable using resonators. This point is dealt with under loudspeaker housing (1).
• Sound guidance of the sound from the front . The immediate vicinity of a chassis is decisive for the frequency response and the direction of sound radiation. This point is dealt with under loudspeaker housing (2).

Schematic representation of a loudspeaker in a closed housing

Housing shapes

(Open) baffle

Open baffle

In order to prevent the pressure equalization between the front and back, it would be best to build the chassis into an infinite baffle . Like installation in an infinitely large box, this construct is used for theoretical considerations (e.g. resonance frequency on an infinite baffle). In practice one has to be content with finite baffles .

Finite baffles only partially prevent the acoustic short circuit . Below a certain cut-off frequency, there is more pressure equalization between the front and back, which leads to a drop of 6 dB / oct. Together with the behavior below the resonance frequency of 12 dB / oct, this leads to an asymptotic behavior of 18 dB / oct at the lower end of the transmission range.

Finite baffles are common in playback devices with built-in speakers, e.g. B.

• Tube receiver ("steam radio")
• Mono transistor radios
• portable playback devices with speakers
• Television sets with picture tubes
• Guitars / instrument amps

Open baffles are also used in modern dipole speakers or speakers in a retro design.

Open baffle (folded)

When folded, the dimensions can be significantly reduced, but these constructions are more sensitive to cavity resonances and may have to be acoustically dampened.

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

Advantage: No influence on the housing → loudspeaker

Disadvantage: In order to transmit frequencies below 100 Hz, baffles have to be very large.

Sensible coordination with active speakers: Bass area with equalizer to absorb the 6 dB drop.

Closed housing

Closed housing

The chassis is installed in an airtight housing (if possible). Compared to baffles of moderate size, the acoustic short circuit is reliably prevented. However, the spring effect of the closed cavity adds another component to the chassis, which increases the resonance frequency. Chassis with a low free-air resonance are therefore used for installation in closed housings. The interior can be dampened to reduce standing waves. Ample damping with heat-storing material that converts the largely adiabatic compression into a largely isothermal compression is ideal. In air this means an effective increase in the internal volume by a factor of 1.4.

Asymptotic behavior: 12 dB / oct below the resonance frequency

Advantages:

• Easy construction
• Easy to vote
• Playback of low frequencies with less distortion, albeit with reduced amplitude
• No flow noise
• More linear course of the phase response and the group delay in the lower application range
• Clean sound through active equalization (frequency response correction) of the speakers possible

Disadvantage:

• Overall, the speaker has a lower level because the sound radiated from the rear of the chassis is converted into heat.
• Lower maximum level. near the resonance frequency of the system
• Temporary drop in the free field amplitude frequency response.

Sensible coordination:

• For active boxes:
  • A comparatively linear frequency response can be achieved via the amplifier (possibly even taking into account the bass parameters in the negative feedback of the amplifier).
  • The system is usually tuned in such a way that the thermal and mechanical load capacity in the low frequency range occurs at the same time.

  Chassis with a low overall Q factor (Q _ts ) (i.e. more deflection at the resonance frequency) enable compact loudspeaker boxes.

• For passive boxes:
  • Usually one agrees on a good compromise between impulse behavior and the lowest possible frequency response.

  This is approximately achieved with a Q _{ts of} the entire system of 0.707 (0.6… 1.0).

  Loudspeakers with small Q _{ts also} lead to compact loudspeakers here, but with less precision in the bass reproduction.

Bass reflex housing

Bass reflex tube

Bass reflex enclosures are a special type of loudspeaker enclosure for woofer speakers. In this so-called bass reflex box, the volume is not closed, but rather connected to the outside by a channel. The air mass in this channel and the housing volume form a resonator (also called a Helmholtz resonator ).

The resonator causes an increase in the sound radiation in the range of its series resonance frequency. In practice, on the one hand, the sound from the rear of the membrane is used instead of being discarded as with the closed housing. However, the membrane hardly moves on the reflex resonance, so that the counterforces of restraint and mass inertia that consume power only play a subordinate role.

The resonance is matched by the volume used, the length and the opening area of ​​the channel. This can be done through experiments, mathematical approximation formulas (according to Hoge) or with the help of computer simulation. So-called equivalent circuit diagrams according to Thiele / Small are used in the simulation. The goal is as linear a frequency response as possible down to the lower limit frequency. The use of bass reflex enclosures makes it possible to use loudspeakers (also called chassis or drivers) with relatively powerful electrodynamic drives. The resulting increased efficiency in the midrange is made up for by the reflex system.

The effective range of the resonator is in the range 0.75 · fb… 2 · fb, in this range the maximum level is improved by at least 2.5 dB. Although the stroke is reduced in the range between 0.75 · fb and 0.9 · fb, it is still very high there, so that frequencies above 0.9 · fb can be regarded as the optimal working range.

Asymptotic behavior: 12 dB / oct below the resonance frequency, 12 dB / oct below the tunnel resonance frequency, 24 dB / oct if both apply.

advantages

• Significantly higher sound levels (up to 13.5 dB) possible in the area of ​​the lowest octave, or
• Expansion of the power bandwidth by 1.1 octaves (factor 2.2)
• Stronger bass reproduction in chassis with more powerful drives, whose frequency response would otherwise drop prematurely due to mutual induction.
• different tuning options (Hooge, Thiele / Small, Novak, Bullock, ...); Frequency response and housing size can be designed in a variety of ways with the given chassis and space.

disadvantage

• Steeper course of the transfer function below the lower limit frequency
• Higher group times
• If the chassis transmits frequencies whose wavelength is in the range of the tunnel, tunnel resonances occur, typically around 500Hz to 2kHz. This problem occurs with practically all woofers.
• If the tunnel is insufficiently dimensioned, disturbing flow noises arise.
• When sound is emitted below the resonance frequency of the overall system, the lack of spring stiffness of the air cushion leads to excessive membrane deflections and simultaneous cancellation of sound from the front and rear.

Sensible coordination

With passive boxes

There are different tuning variants from filter theory that lead to a largely linear bass frequency response. These can be used as a first approximation for the design.

With active boxes

• The tunnel resonance frequency defines the power frequency response; if the lower power bandwidth is defined, this is added to . ${\displaystyle 1{,}1\times f_{\mathrm {min} }}$
• Active loudspeakers are usually built smaller than passive loudspeakers and then subsequently equalize the frequency response. This equalization almost always includes a 2nd to 4th order protective filter for frequencies below fmin (which would also look good in a passive box, but is material-intensive there).
• The reduction in the housing volume is limited by:
  • Increased electrical power in the low frequency range - it must also be ensured that the mechanical overload occurs before the electrical one
  • The resonator tunnel must not be too long (this is ~ 1 / Vs long).

Bandpass housing

Single ventilated bandpass case

The name indicates that the transmission range is limited to both low and high frequencies. A frequency band is reproduced. The level drops above and below the transmission range by 12dB / oct. from, which corresponds to a filter of the second, i.e. a total of fourth order. Systems with an effective frequency band of 30 Hz to about 100 Hz are typical.

The chassis works between two chambers. As with the simply ventilated bass reflex housing, the front chamber is coupled to the environment via a ventilation channel; the rear chamber, like a closed box, has no connection to the environment.

The front chamber forms a Helmholtz resonator like a bass reflex box : the mass of the air in the duct and the spring stiffness of the volume determine the resonance frequency, the membrane movement is greatly reduced at the resonance frequency. This element is the only part of the structure that effectively radiates sound. The volume of the rear chamber only contributes to the spring stiffness of the membrane.

Accordingly, the efficiency and the achievable maximum level are hardly higher than with a reflex box. Efficiency advantages can only be achieved with very narrow bandpasses. Then the group delays are correspondingly high, which make integration into the overall system more difficult.

One advantage is the lack of an acoustic short circuit below the Helmholtz resonance, as occurs with a reflex box. As a result, the transmission range can start at comparatively low frequencies without restricting the maximum level that can be reached too much. But because the efficiency drops at the same time, a higher electrical output - and the corresponding thermal load capacity must be provided in the chassis.

Another possible advantage is the mechanically built-in band limitation to higher frequencies, which saves a filter train in the crossover.

Double ventilated bandpass case

Double ventilated bandpass case

In the case of the simply ventilated bandpass housing, the energy radiated into the rear chamber is not used, similar to the closed housing. With the double-ventilated band-pass housing, the energy of both chambers is decoupled and radiated via a Helmholtz resonator. For a wider transmission range, it makes sense to dimension the two chambers differently and to coordinate them differently. This allows the transmission range to be expanded. Just as with the single ventilated bass reflex cabinet, however, there is a steeper drop of 36 dB / oct below the transmission range (one speaks of the 6th order bandpass), which is caused by the acoustic short circuit.

Double-ventilated bandpass enclosures can only be used sensibly for the transmission of narrow frequency ranges (max. 1½ octaves). Compared to bass reflex and mono bandpass tuning, only to closed housings, they have i. A. good efficiency and high maximum levels, but also stronger distortions of the phase response / group delay.

Asymptotic behavior: 12 dB / oct below the resonance frequency, 12 dB / oct below the tunnel resonance frequency, 24 dB / oct if both apply (and another 12dB / oct due to the characteristics of the chassis itself). Upward 12 dB / oct drop.

Multi-chamber bandpass housing

Multi-chamber bandpass housing

Much more complicated systems are also possible, the best known representatives of which are the multi-chamber bandpass systems.

Their general problem is the long group delay (the resonators need some time to set each other into oscillation). Furthermore, there is usually a very problematic frequency curve. Similar to the bandpass systems, higher efficiencies are possible at the expense of poor pulse reproduction. Often, several chassis are used. In contrast to mono and double reflex bandpass filters, the bandwidth of the system can be made larger, which is what mass manufacturers often use in combination with very small satellite speakers (e.g. Bose ).

Subsonic filter

It makes sense to equip the bass chassis with a band pass instead of a low pass. Frequencies below the transmission range otherwise lead to considerable mechanical loads on the chassis, since the ever larger wavelengths require enormous deflections of the membrane. Manufacturers of many loudspeaker chassis indicate the maximum possible deflection in millimeters.

Passive subsonic filters:

• Mostly 1st, more rarely 2nd order high passes. The necessary sizes (200… 500 µF, 20… 50 mH) are usually expensive and particularly problematic with high outputs. Few manufacturers use it. Bass equalization is only possible to a limited extent; it is associated with a reduction in the impedance in the effective range (series resonant circuit). Another problem is the temperature sensitivity of the tuning due to the effects of the TSP parameters on the filter.

Active subsonic filters:

• Subsonic filters are common in powered speakers. The effort is low, mostly a bass equalization is integrated.

Advantages:

• The speakers become more resilient
• Less distortion, especially in the presence of low-frequency interference ( record )
• static or dynamic bass equalization possible
• Lower group delay above the transition area.

Disadvantage:

• additional effort
• Increase in group delay in the transition area (with analog filters)
• Steeper drop in amplitude frequency response, complete lack of deep bass in compact speakers

Passive membranes

In addition to ventilated tubes, passive membranes can also be used as mass elements for Helmholtz resonators. With them it is much easier to reach the masses necessary for deep votes.

Advantages over tunnels:

• No flow noise
• No natural resonances or pass frequencies, as occur in tunnels of normal length
• More precise bass reproduction
• Deep coordination is also possible for those in cases where a tunnel would become impractical

Disadvantages compared to tunnels:

• Higher cost
• Larger space required on the outside of the box
• Only useful for pure bass boxes ( subwoofers ), since higher frequencies penetrate through the comparatively thin membrane

Use of multiple chassis for one frequency range

Series and parallel arrangement

Row and anti-row arrangement

Parallel arrangement with common and anti-parallel arrangement with separate chambers

Often several chassis are used for the same frequency range. This can happen for a number of reasons:

• It increases the load capacity, because both electrical and mechanical stress (does not apply to acoustic row arrangement) is reduced
• When arranged in a row, Vas is reduced, which is helpful for reducing the volume of the boxes. At the same time, however, the efficiency drops
• With parallel arrangement, the efficiency increases at low frequencies
• Anti-parallel and anti-row arrangement: Distortion components in particular are reduced
• Appropriate control allows the radiation characteristics to be modeled better (loudspeaker line, suppression of rear sound)

Note: Separate chambers are useful for anti-parallel arrangement; they reduce large-signal operating point shifts.

Emitter line

Emitter line

Emitter moving with the shaft

All chassis are controlled in phase in radiator rows, the resulting radiation beam is just 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 line and 8 m listening distance, the necessary delay is 117 µs.
• Decoupling the outer chassis at higher frequencies: Normally the lobe becomes lower and lower towards higher frequencies as the size increases relative to the wavelength. Targeted uncoupling (up to 1 chassis) can reduce or eliminate this effect.

Emitter moving with the shaft

A further rotationally symmetrical radiation with attenuation of rear and side components is obtained by a loudspeaker line that is controlled by a “traveling wave” in the direction of the listener.

Spherical speakers

The spherical shape brings advantages in terms of sound reproduction inside and outside the housing.

1. 

   Course of the sound waves in rectangular and round loudspeaker housings
   There is less interference inside a spherical loudspeaker housing than in rectangular speakers. More precisely, fewer standing waves as there are no parallel walls.
2. In addition, the housing can be designed with a constant and homogeneous wall thickness, this distributes pulses more evenly over the entire housing.
3. 

   Natural oscillation of rectangular and round loudspeaker housings
   The sphere is the geometric body that has the smallest surface area in relation to its volume. Less area means reduced unwanted vibrations in the housing.
4. With the spherical shape, sound propagation emanating from the speaker membrane is not hindered by corners and edges. This minimizes the undesirable phenomenon of the sweet spot : the effect that a balanced sound is only achieved at a certain listening position.

Disadvantages: Advantages such as the low natural vibration only come into play with a seamless spherical shape. However, this is a challenge in production, especially with wooden or porcelain ball speakers.

Horn loudspeaker

see main article: Horn (loudspeaker)

Horn loudspeakers in the sense of loudspeaker boxes are used when all frequency ranges (including the bass range) use long horns. Depending on the depth of the bass reproduction, such horns are large to huge.

Sketch: folded horn (W-Bin)

A distinction is made between front-loaded and back-loaded (lowther) horns. Front-loaded horns work with the back on a pressure chamber, the front is coupled to a horn. Back-loaded horn, after the inventor also Lowther called -Hörner, irradiate the front sound from directly or via a (short) waveguide, the back is coupled to a long horn, which takes over the bass reproduction.

Example of a horn with a rear chamber and pressure chamber

                    Druck- rückwärtige
                    Kammer Kammer
 _______________________________
|  /         \ /    \ /   |     |
| /           V   ,  V |   \ ## |
|/            |   |  | |    >## |
|       ,     |   |  | |   / ## |
|       |     |   |  | |__|_____|
|       |     |   |  | |       /
|       |     |   |  | |      /
|       |     |   |  | |     /
|       |     |   |    |    /
|       |     |   |\___/   /
|       |     |   |       /
|       |     |   |      /
|       |     |   |    _/
|       |     |   |  _/
|       |         |_/
|       \        _/              Hornmund
|        \______/
|
\
|\
| \_
|   \_
|_____\___________________________

Example of a direct radiating lowther horn:

 ____________________________
|  /         \ /    \ /  :::|
| /           V   ,  V | ## /    direkt-
|/            |   |  | | ##<   strahlendes
|       ,     |   |  | | ## \   Chassis
|       |     |   |  | |::::|
|       |     |   |  | |:::/
|       |     |   |  | |::/
|       |     |   |  | |:/
|       |     |   |    |/
|       |     |   |\___/
|       |     |   |   /
|       |     |   |  /
|       |     |   | /
|       |     |   |/
|       |         /             Hornmund
|       \        /
|        \______/
|
\
|\
| \_
|   \_
|_____\_________________________

Advantages:

• High to very high efficiency
• High final sound level achievable
• Low diaphragm excursions of the drivers also in the bass range
• A high and constant directivity factor down to the lower of the fundamental range

Disadvantage:

• The size of the horn mouth must be in the range of the maximum wavelength to be emitted; below this frequency the horn loses its efficiency, the damping of the driver by the air load collapses. Therefore horns are huge for the usual lower limit frequencies (40 Hz ... 80 Hz). In the past, people believed, through an incorrectly applied symmetry argument, that the size could be reduced significantly if horns were inserted in the corners or edges of a room. According to modern simulation methods, this is not the case. Rather, the required housing volume at 40 Hz cut-off frequency is around 1000 liters, regardless of the driver size, the length of the horn should be 3 to 4 meters, and the mouth surface should not be less than half a square meter. It should be noted that the cross-sectional expansion should run symmetrically - the usual screw shape is less suitable, especially at the end.
• A steep high-pass filter should be used to protect the chassis because the air load disappears below the transmission range. However, this only applies to mathematically correct horn loudspeakers. With built according to heuristics , each individual case must be considered separately because there is no clearly defined lower limit frequency. The air load then consists of rather irregularly distributed resonances.
• The sound emitted by the horn has considerable signal transit times.
• Discontinuities in the radiation resistance and the heuristics used in the construction lead to comparatively very strong ripples in the amplitude frequency response of more than ± 10 dB.

With (projected) permanent installation in large rooms (light film theaters, concert halls), size no longer plays a major role. The construction is currently made possible by simulation programs. The direct solution of the wave equation with the formulas derived from it takes a back seat.

Web links

Commons : Loudspeaker enclosures  - collection of pictures, videos and audio files

• Subwoofer Design Theories
• Loudspeaker enclosure: the closed enclosure, for beginners

Footnotes