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Page 4
On this page we look at the total power radiated into a closed space by a stereo pair of loudspeakers
separated by a distance of 10 feet. The speakers are assumed to radiate into 2Pi space above the
baffle step and 4Pi space below the baffle step. From left to right we have no baffle step correction,
3dB baffle step correction and a full 6dB baffle step correction. As can be seen, at frequencies with
wave lengths much shorter than the separation between the speakers the power sums as two
uncorrelated sources. However, as the frequency drops and the wave lengths become long the
effects of correlation are evident. The minimum in the left most figure is at 80 Hz, the frequency
where the separation is 0.707 of the wave length. As the frequency drops further, the two sources
are in phase and the total power is now at the high frequency level even though the on axis SPL of
each speaker is 6dB below the high frequency level. Looking at the center plot where a 3dB baffle
step correction has been applied we see flat power response to the frequency where there begins
to be a correlation between the speakers. When a full 6dB correction is applied the power is as
shown in the 3rd figure from the left. This is to be expected since low frequency content on
recording is often monophonic.
While these power calculations are based on being far from the combined source (composed of the
two speakers) they serve to show that the low frequency power radiated into an enclosed space is
not only dependent on how much baffle step correction is applied to each speaker, but also on how
the speakers are positioned. Clearly, 3dB baffle step correction would yield flat power response down
to 50 Hz and below that the baffle step correction can be reduced or eliminated. Other factors must
also be considered though. For example are still faced with room pressurization and room modes
which will further complicate the issue. It is no wonder that clean low frequency reproduction in a
confined space is so difficult to obtain.
Lastly, polar plots are presented at the far right. These polar plots represent the horizontal radiation
patterns generated by the stereo pair of speakers at the frequencies noted, 250, 500, 750, and
1000 Hz. The speakers are considered point sources in the calculations. I have presented them
because often large panel speakers are faulted because of their horizontal beaming characteristics
as the wave length of the radiated sound approaches the width of the panels even though quite often
such beaming is not as clearly evident when listening as would be imagined. These figure show that
even with point source speakers the interference patterns can result is considerable beaming when
both speakers are reproducing the same signals. The here point is that, just as with the power
response, the radiation characteristics are those of the complete system and it is
incorrect to make a final judgement base don the characteristics of a single source.
On the next page we take a brief look a comparisons between MT and MTM configurations.
separated by a distance of 10 feet. The speakers are assumed to radiate into 2Pi space above the
baffle step and 4Pi space below the baffle step. From left to right we have no baffle step correction,
3dB baffle step correction and a full 6dB baffle step correction. As can be seen, at frequencies with
wave lengths much shorter than the separation between the speakers the power sums as two
uncorrelated sources. However, as the frequency drops and the wave lengths become long the
effects of correlation are evident. The minimum in the left most figure is at 80 Hz, the frequency
where the separation is 0.707 of the wave length. As the frequency drops further, the two sources
are in phase and the total power is now at the high frequency level even though the on axis SPL of
each speaker is 6dB below the high frequency level. Looking at the center plot where a 3dB baffle
step correction has been applied we see flat power response to the frequency where there begins
to be a correlation between the speakers. When a full 6dB correction is applied the power is as
shown in the 3rd figure from the left. This is to be expected since low frequency content on
recording is often monophonic.
While these power calculations are based on being far from the combined source (composed of the
two speakers) they serve to show that the low frequency power radiated into an enclosed space is
not only dependent on how much baffle step correction is applied to each speaker, but also on how
the speakers are positioned. Clearly, 3dB baffle step correction would yield flat power response down
to 50 Hz and below that the baffle step correction can be reduced or eliminated. Other factors must
also be considered though. For example are still faced with room pressurization and room modes
which will further complicate the issue. It is no wonder that clean low frequency reproduction in a
confined space is so difficult to obtain.
Lastly, polar plots are presented at the far right. These polar plots represent the horizontal radiation
patterns generated by the stereo pair of speakers at the frequencies noted, 250, 500, 750, and
1000 Hz. The speakers are considered point sources in the calculations. I have presented them
because often large panel speakers are faulted because of their horizontal beaming characteristics
as the wave length of the radiated sound approaches the width of the panels even though quite often
such beaming is not as clearly evident when listening as would be imagined. These figure show that
even with point source speakers the interference patterns can result is considerable beaming when
both speakers are reproducing the same signals. The here point is that, just as with the power
response, the radiation characteristics are those of the complete system and it is
incorrect to make a final judgement base don the characteristics of a single source.
On the next page we take a brief look a comparisons between MT and MTM configurations.
Tech Design.....
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Consideration of Power Response in Speaker Design:
Crossover Choice and Setup.
___________________________
Consideration of Power Response in Speaker Design:
Crossover Choice and Setup.