Bass reflex

Bass-reflex enclosures are a special form of loudspeaker cabinets for bass speakers. In such a box the case is not closed, but with an opening - provided - the bass reflex port. The backward emitted from the speaker sound component is used to " amplify " the bass reproduction to. The channel serves as a resonator, with the internal volume and increase the efficiency of the box in the region of the resonance frequency of the loudspeaker.

Objective

As with transmission lines or backloaded horn speaker enclosures is here the rear radiated sound portion of a speaker - in the context usually referred to as chassis - used to increase the playback relevant to the human ear the low frequency range. In simpler terms, provides a box of this type " more bass ". In fact, it increases the sound pressure at the chassis resonance frequency at the expense of lower frequencies; It thus depends on the parameters of each speaker, as far as such a use is reasonable.

Resonance tuning

Tuning the resonance is performed by the length or the cross-sectional area of the channel ( ultimately the reciprocating mass of air a resonator, together with the elasticity of the air volume in the box). Usually due to change in length of the bass reflex tube must be made to adapt to the so-called Thiele - Small parameters of the speaker and the enclosure volume. Voting can be done by experiments, mathematical approximation formulas (vote by Hodge ) or by means of computer simulations. 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 (short: BR enclosures ) allows (also called chassis or driver) speakers to use 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 close to the natural resonance.

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

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.

Calculation of the reflection channel

The general formula for the calculation of the Helmholtz resonator having a circular cross-sectional channels in the assumed diameter d of the tube is of a length l (in each case in cm):

Where f is the desired tuning frequency (in Hertz) and VB for effectively net internal volume of the housing ( in liters).

In BR boxes usually a housing quality Qtc = 0.707 is sought ( Butterworth Charakteristk: optimal balance between frequency response and step response). The corresponding internal volume results from the total Q factor Qts of the chassis used and its equivalent volume Vas (in liters):

The channel length L (in cm) is here with an assumed cross-sectional area Af (in cm ² ) is determined using the resonant frequency fs of the chassis:

However, the control mode of the speaker affects the result because Qts of the electrical quality Qes and Qms the mechanical quality of the chassis gives:

When using a passive crossover for example, add up the internal resistance and the chassis resistor Re to rsum (all in ohms), thereby changing Qes as follows:

What the above BR -channel length calculation means that ultimately must be replaced by Qt Qt *; therefore summarized by:

This correction applies primarily for passive speakers. With active control ( = no other components between the amplifier output stage and chassis ) affects only the output resistance of the amplifier, which in standard transistor output stages has little effect - in contrast however to tube amplifiers.

Principle BR channels can have any cross-sectional shapes ( as long as they remain the same over the entire length ) and also curved, bent, etc.. Too small or too narrow channels but air turbulence while at large channels cause ( whistling ), including be audible to higher radiated from the membrane back of the chassis frequencies " permeable". It is possible to use multiple channels. Only the variable D must then be replaced d ∙ √ n, where n is the number of channels: as long as they have identical dimensions, the above BP -channel formula also applies to their respective lengths.

Note that VB ( as to the input of this section above), the effective net internal volume of the housing respectively. This means on the one hand, that the space requirements of embedded crossovers or protruding chassis magnet in the later case production must also be added as that of BR- channel itself the other hand, the effect which is introduced damping material - depending on quantity - a virtual magnification of VB around 10 to 30%.

Benefits

• Significantly higher sound pressure level ( 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 due to large deflection.
• Different Abstimmvarianten ( Hooge, Thiele / Small, Novak, Bullock, ...); Frequency response and case size varied for a given chassis and space conditions designed.

Disadvantages

• Larger group delay or worse transient response
• Steeper drop of the transfer function is below the lower cut-off frequency
• If the chassis transmits frequencies whose wavelength is in the range of the tunnel ( bass reflex tube ), it comes to tunnel resonances. This problem occurs in virtually all bass reflex speakers that reproduce with the same speakers and mid frequencies.
• In case of insufficient dimensioning of the tunnel, there are interfering flow noise.
• When radiation of sound below the resonant frequency of the entire system occurs due to lack of spring stiffness of the air cushion to large cone excursion while canceling out sound of the speakers and the tunnel ( acoustic short circuit ). The large deflection leads to non-linear distortion and intermodulation.

• 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 usually smaller than passive speakers and then subsequently electronically equalized frequency response.

This electronic equalization includes virtually always a protective filter 2nd to 4th order below against frequencies. Such filters gradients should be implemented with passive filters ( speaker points in passive speakers ) little or are too material intensive.

The reduction of the housing volume is limited by the required low frequency higher electrical power - it must be further ensures that the mechanical override before the electrical overload occurs. Furthermore, the Resonatortunnel must not be too long (approximate value ~ 1/Vs ).