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The midwoofer-tweeter-midwoofer loudspeaker configuration (called MTM, for short) was created by Joseph D'Appolito as a way of correcting the inherent lobe tilting of a typical mid-tweeter (MT) configuration, at the crossover frequency, unless time-aligned.〔(AES 74th Convention, September 1983 )〕 In the MTM arrangement the loudspeaker uses three drivers: Two mid-range (or mid-woofer/woofer) for the low frequencies and a tweeter for the higher frequencies, with the tweeter being placed between the mid-range drivers (as shown in the second image below). D'Appolito initially configured his design using a 3rd order (18 dB/oct or 60 dB/dec) crossover, D'Appolito has since amended this original recommendation in favor of 4th order topology. However, this does not impart any significant effect on the MTM design's unique characteristics. There are speakers where the tweeter appears below the larger driver and though electrically it is identical to the MT configuration, it is customarily denoted as "TM". The only other difference is the lobe tilt which is exactly the opposite of the MT configuration. The direction in which the lobe tilts (i.e., the vertical orientation or angle of the lobe) is a function of the difference or offset between the acoustic centers of the two drivers. == Lobe tilting of MT or TM configurations == Since it is rare for a tweeter and mid-range (or woofer) to have their diaphragms or acoustic centers in the same physical plane, it follows that sound waves emitted by them (of the same frequency) will not reach a particular listening position in the on-axis plane at the same time. Conversely, the MT or TM combination's on-axis plane is not coincident with the physical on-axis plane - it is tilted w.r.t. the physical plane. The overall effect is that at the crossover frequency (when both drivers are reproducing the same frequency) the sound from both drivers does not acoustically sum ideally at the on-axis listening position. There will, however, be some off-axis listening position where the acoustic sum is ideal - but the listening position itself may be such that it is not practical. Thus, with a typical TM or MT loudspeaker where the drivers are not time-aligned, the main lobe is tilted away from the horizontal.〔() "Article by Rane discussing the Linkwitz-Riley crossover, time-alignment, lobing and correction of lobe tilt in TM or MT speaker configurations."〕 The above image shows the lobing pattern of a typical TM loudspeaker. As can be seen, the lobe tilts downwards towards P' which is not the same as the on-axis listening position P. This is one of the reasons for physically offsetting the mid-range and tweeter such that the tweeter is physically behind the mid-range, or tilting the speaker upwards, to achieve an on-axis response that is coincident with the physical on-axis plane. This process is known as time-alignment. Other ways to time-align are to introduce a phase shift in the tweeter signal (lag or lead, depending upon the offset between the drivers) so that it mimics the physical offset. All these methods, however require some amount measurement and effort to get right. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Midwoofer-tweeter-midwoofer」の詳細全文を読む スポンサード リンク
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