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One of the simplest forms of mount for a loudspeaker is called the baffle, open baffle or doublet. The baffle or open baffle consists of a flat board onto which the loudspeaker is mounted.
Not only does the open baffle provide a methods of mounting and protecting the loudspeaker, but it also enhances the performance. Large baffles give a significant improvement in performance.
Speaker open baffle basics
When a speaker is mounted on a baffle board it increases the distance the front and rear sound waves must travel before they meet.
When the front and rear waves meet they can cancel each other out. If the baffle is small, the distances travelled before cancellation are also small and hence only the higher frequencies will not cancel. Lower frequencies with longer wavelengths will still cancel. Hence if the baffles are made larger then lower frequencies will not cancel and the bass response will be better.
The speaker o its own is a second order system and as a result the response rolls off at a rate of 12 dB per octave. The roll off for most open baffles starts well above this and reduces by half every octave, i.e. 6 dB per octave.
Whilst the concept of the speaker open baffle or doublet seems attractive, a very large baffle is required to ensure a flat response down to well into the bass region. For a circular baffle of radius r, the rear wave must travel 2r to reach the front. Therefore there is a roll off in output at frequencies where the wavelength is longer than the baffle diameter, i.e. 2r. This can be calculated:
f = frequency where roll off starts
c = speed of sound
r = radius of open baffle
Therefore a baffle of almost 8 metres radius is needed to give a flat response to 45 Hz. However the roll off has a more gentle approach than other systems. A one and a quart metre baffle starts to roll off at just under 300Hz, but is only 12 dB down at 70 Hz.
The low frequency performance can be enhanced by adding a rim around the baffle. Extending this further back (e.g. by up to a meter or more on a metre radius baffle) improves the bass performance significantly although it does give a peak in the response before it falls away. In some instances this can be by as much as 8 to 10 dB.
Speaker open baffle peaks and troughs
The time taken for the sound waves to reach the edge of the baffle can mean that some frequencies will cancel whilst others may reinforce. This can give a very coloured sound.
The solution to this is to mount the loudspeaker off-centre. This removes the sharp peaks, smoothing them out as the centre mount means that the peaks and troughs occur the same in each direction, assuming a circular baffle. Even with a square or rectangular baffle, there are multiple directions where the peaks and troughs occur.
The bass from an open baffle system can sound very natural, especially of the baffle is large. The slow roll off and extended bass makes for a good combination. However the size constraints mean that the speaker open baffle is really only applicable to the specialist end of the hi-fi market. For most practical installations an open baffle system is not really applicable. That said, open baffles are used for some more standard applications, but they cannot offer particularly good bass response.
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