I see speakers that have the tweeter in the middle of the midrange driver. What advantage does that give?
– Roy H.
To produce a sonic signature resembling a single driver, designers often place a tweeter in the middle of two or more other drivers. Many respected audio brands including KEF, Tannoy, Lowther, Quad, Altec Lansing, and Electro-Voice have developed or are still producing such designs.
Ideally, a single driver would reproduce all frequencies from top to bottom. In practical use this is not feasible, so speaker designers use multiple drivers to produce full-range sound. The problem for the designer is that the acoustical output for each driver is different and since they have to be mounted physically separate from each other the displacement causes slight but very audible distortions or colorations in the sound.
Effective crossovers help to blend the sound of multiple drivers, but with only partial success. Imagine if you will the sound of a piano. As a pianist plays the music arrives at the listener’s ears at the same time. If you use two drivers mounted apart from each other, the sound wave from each driver will arrive at the listener’s ears at a slightly different times. These distortions blur the music and are measurable and audible to our ears. Designers, therefore, try to make multiple drivers create an acoustical output that resembles a single source.
One method used by companies such as KEF, is to mount their drivers coaxially so that the point source is better approximated.
By placing the tweeter in the center of the midranges or woofers, the allows all the frequencies arrive at the ear at the same time. The ear and brain will process a point source sound as being more realistic and natural sounding. A further benefit is that the dispersion of the frequencies away from the center of the speaker are even and balanced which helps to produce a better reproduction of the recorded event even if the listener moves away from the “sweet spot” or centered position in front of the speakers.
(Images courtesy of Jim Clements review of KEF Q 300 speaker for The Secrets Team.)
Other speakers are designed with a central tweeter flanked by a pair of woofers. This design is called the “D’Appolito configuration” after the designer, Mr. Joseph D’Appolito.
In 1983, Mr. D’Appolito’s paper “A Geometric Approach to Eliminating Lobing Error in Multiway Loudspeakers,” was presented at the Audio Engineering Society’s conference. The paper noted that all multi-way loudspeakers (any speaker consisting of multiple drivers, not coaxially mounted) exhibited uneven frequency response caused by “lobing.” Lobing occurs when interferences between the different drivers (also called “comb filtering”) causes uneven frequency response, particularly at the crossover frequencies.
D’Appolito noted that by using a central tweeter flanked by a pair of woofers, such lobing could be minimized. This configuration (also often known as “MTM” or “midrange – tweeter – midrange”) extended frequency dispersion at right angles to the driver axis and minimized dispersion along the driver axis.
By using the MTM physical driver configuration combined with steep crossover slopes (D’Appolito suggested 18 decibels per octave) the comb filtering or lobing is minimized.
The MTM configuration is often used with low frequency drivers (woofers) that are not similarly configured. The Snell Illusion Loudspeaker is an example of this.
The lower frequencies exhibit less lobing due to their longer wavelengths, so the bass frequencies don’t really need the MTM configuration.
The advantage of the MTM configuration is that when aligned vertically, as in the photos above, the speaker exhibits a wide horizontal dispersion with smooth frequency response (very little lobing or comb-filtering). The speaker also exhibits limited vertical dispersion, which helps eliminate floor and ceiling reflections.
Of course, occasionally wide vertical dispersion is desired. One example would be a center-channel speaker for home theater. In that case, the speaker can rarely be mounted at ear level. Instead, the video display is at eye/ear level and the center channel speaker must be mounted above or below the display.
In such circumstances, the MTM configuration is also used, but this time with the driver axis horizontal. This arrangement provides wide vertical dispersion so that the sound, as perceived by the viewers is smooth, but with reduced horizontal dispersion. The photo below depicts a MTM configuration center channel speaker.
Center Channel 3
Note that multiple woofers, mid-woofers, midranges, or tweeters can be used in MTM configuration. The benefits of reduced lobing and controlled dispersion have proven very successful for speaker design.
With a successful point source speaker or MTM-configured speaker, the sounds produced will be superior from multiple listening positions in the room. Good speaker design has always been about using the best technology and methods to create the illusion of a live musical event. Placing the tweeter in the center of the midrange or using an MTM configuration are both excellent methods of accomplishing that goal.
Mr. Joseph D’Appolito is author of the book, “Testing Loudspeakers,” and is also the owner of a consulting firm, Audio and Acoustics, Ltd. whose clients include multiple loudspeaker manufacturers.