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The infinite baffle is a closed and sealed box that aims to smother the rear sound waves from the loudspeaker. In this way there is no cancellation caused by the rear sound wave from the loudspeaker.
The infinite baffle or closed speaker box provides some significant advantages and well designed, the technique can perform very well.
The infinite baffle technique is used, possibly for the greatest majority of loudspeaker boxes or enclosures that are sold for use with high fidelity and other audio systems. Even though other systems are used, it is the infinite baffle that is the most popular.
Infinite speaker baffle basics
The infinite baffle speaker system is basically a closed box or cabinet. Which absorbs the rearward sound wave. The box is sealed to prevent as much sound emanating from the box as possible and it should also be made from solid material so that vibrations of the panels are reduced as much as possible. The box generally also contains sound absorbing material to further absorb sound and prevent the panels vibrating.
The enclosed air acts as a spring which reduces the compliance and as a result it raises the resonant frequency of the cone.
The compliance increases with the volume of the box as there is more air to compress and this is more easily achieved. This means that for larger box or cabinet sizes, a lower resonant frequency can be achieved with a given loudspeaker unit.
Below the resonant frequency, the response of the speaker system falls at a rate of 12dB per octave so it falls relatively rapidly, reducing the bass notes.
Speaker infinite baffle sensitivity
As might be expected the sensitivity of the infinite baffle is quite low. The speaker has to work hard to compress the air. Also the mass of the bass speaker cone is normally made relatively high to reduce the resonant frequency to provide better bass response.
It is possible to increase the magnetic flux by using a stronger magnet, but this reduces the Q resulting in over-damping. This means that it is not possible to use the higher levels of Q to provide an increase in bass to provide better extended bass response.
However it is possible to increase the volume of the loudspeaker box. This gives a greater amount of air within the enclosure and means that the level of air compression is less. Having a larger volume within the box enables better bass response, but it comes at the expense of requiring more wood for the box making it heavier and increasing the visual impact. Larger loudspeaker boxes may also not fit into every situation.
As with any design there is a balance between the different factors to provide the optimum performance within the requirements for any given situation.
Infinite baffle equivalent circuit
The infinite baffle or sealed baffle loudspeaker can be represented by a series of electrical equivalent components. This representation of the infinite baffle or sealed baffle speaker helps understand how the different elements relate and how the performance can be optimised.
The diagrams shows the mechanical and electrical equivalent circuit for a woofer loudspeaker in a closed cabinet.. The resistor Rab represents the box losses and Ral represents the leakage losses. The loudspeaker cone is represented by the transformer as it converts between the electrical and mechanical domains. The transformer has a turns ratio of 1 : Sd where Sd is the cone area. This converts the mechanical force to pressure and velocity u to volume velocity U. The input force comes from the loudspeaker voice coil.
The physical mass of the speaker cone behaves like an inductor on the acoustic domain, but in the electrical domain it appears as a capacitor. The result of the combined inductance and capacitance is a damped resonant circuit. As the box or closed baffle increases the stiffness on which the moving mass resonates, this increases the resonant frequency.
When using a small box, it is necessary to have a very compliant suspension on the speaker to keep the resonant frequency low. What is termed the BL product which represents the speaker’s electrical / magnetic system is often used within calculations for speaker systems. If this is too high, then the total electrical Q for the speaker becomes too low and the damping becomes too great. The efficiency is higher, but this is at the expense of good bass response. Conversely a BL product which is too low gives rise to higher bass response , but the transient response becomes poor and the bass may tend to boom and sound dull.
The overall Q of a speaker system, i.e. a speaker in a cabinet is known as Qtc. Values of Qtc above unity tend to have boomy bass, whilst values below critical damping which has a value of 0.5 lack life. The critical damping gives a sound pressure level of -6dB at resonance. Often the value of 0.7 is used and this is often called Butterworth alignment and gives a -3dB sound pressure level at resonance.
Internal cabinet air resonances
Apart from the resonance of the bass speaker cone, the box or cabinet will have internal resonances. It is normal to damp these as much as possible by the use of sound absorbent material within the cabinet. Typically special attention should be paid to the points of maximum vibration. This is actually half way along the dimension in question for the fundamental of the resonant frequency.
This material should be placed into the speaker cabinet of box and not fixed to it.
Speaker cabinet panel resonances
One of the major issues within the cabinets for infinite baffle speaker systems is that the panels of the cabinet start to resonate and radiate sound. This acts to cancel with the sound waves from the front of the speaker.
The vibrations can be minimised by using high density panels. Normally high density wood products are used for most domestic speakers, but some high end loudspeaker infinite baffle cabinets have used concrete, metal or cavity panels filled with sand.
Other techniques include lining the panels with a heavy sound absorbent material. Occasionally bracing between opposing panels can be used to stop flexing of the panels, as opposing panels will both move out or in together. Running a bracing strut between them will reduce this effect.
Loudspeaker enclosure volumes
Often when looking at different loudspeaker cabinets / enclosures, one of the specifications mentioned is the volumes. This is generally given in terms of litres.
It is found that the larger the volume of the loudspeaker box or enclosure, the better is the bass response.
Typically a measurement can be made of the internal dimensions of the loudspeaker box. If necessary take the external dimensions and subtract twice the thickness of the panels (one for each side) and then this can be taken as the internal measurements.
Ensure they are in metres or centimetres and then multiply them together. The result is the volume in cubic metres or centimetres, dependent upon the units used for the original measurements.
it is then very easy to convert the volume to litres as 1 cubic metre = 1000 litres, or 1000 cubic centimetres are equal to 1 litre.
High fidelity loudspeaker boxes or cabinets will tend to have a volume of 50 litres or more, with some studio quality loudspeaker enclosures having up to 100 litres or possibly more dependent upon the quality needed and the power of the system.
Loudspeaker enclosure design can be quite complicated looking at the resonances and the like, but often if good quality materials are used, the walls of the loudspeaker box are reasonably thick and rigid, and internal sound deadening wadding and panel damping is used ten the box can perform well.
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