Dual woofer, MTM, MEH? 2, 2.5, 3 ways speaker?

MTM: A Trade-Off in Speaker Design

The MTM (Mid-Tweeter-Mid) speaker configuration offers advantages as high SPL capacties, but it also comes with challenges related to vertical lobing.

This article will describe specific mounting impact on the directivity, as mentionned in other article we listen at the so called critical distance where direct and reflected sound (from walls) are roughly balanced (50/50).

This impact on the directivity will also impacts the listening in-room experience, even when we are positioned in front of the speakers.

This article need a good comprehension about vertical lobing.

The Lobing Issue:

Placing two woofers in an MTM design increases the distance between their acoustic centers compared to a single woofer. This larger spacing can lead to vertical lobing, causing cancellation effects at certain frequencies, particularly when listening off-axis (from the side).

Here’s an image representing the difference in vertical off-axis response of a MTM design with Faital 12FE120 25cm spaced to let 12cm for the tweeter or the waveguide:

Addressing the Issue in MTM:

While MTM presents a lobing challenge, there are ways to mitigate it:

• Lower Crossover Frequency: In MTM designs, using a lower crossover frequency for the woofers can help reduce lobing. However, this lowers the overall bass response and requires careful design considerations.

• 2.5-Way Design: If you’re set on an MTM layout, a 2.5-way crossover design can be a solution. In this design: - The bottom woofer is crossed over with the tweeter at a higher frequency. - The other woofer is crossed over at a lower frequency and compensated baffle step loss.
  This approach helps maintain a good on-axis response while minimizing lobing effects and keeping high SPL capacities.

  

Alternatives to MTM:

It’s important to consider the context when choosing a speaker configuration.

• Subwoofer Integration: If you’re using a subwoofer, the benefits of dual woofers or MTM become very less significant, even in 2.5 voices.

• Long-Range Listening with high SPL usage: For long-distance listening with high sound pressure levels (SPL), a 3-way speaker system might be more suitable, with or without subwoofer.

TMM

Older conceptions utilized a wide horn, approaching 60cm in width, with two 15" woofer horzontally placed such as the TAD TSM 1:

In this configuration, achieving directivity matching could only be accomplished with dual 15" woofers placed side-by-side. Theoretically, this arrangement, with the 15" drivers positioned as closely as possible, would leverage horizontal lobes to create a more restricted directivity pattern.

Finite Element Analysis (FEA) simulations were employed to model real woofers within this arrangement.

While these simulations yielded results, the narrowing of the directivity pattern was excessive, as the 15" drivers centers remained relatively far apart:

If an internal radius is introduced to bring the woofers closer together by “pinching” the box from the inside:

Negative interactions occur, resulting in a significant deterioration in performance.

Alternatively, an external radius can be employed to bring the woofers closer by “pinching” the box from the outside:

While this approach introduces some “blurring” of energy due to the driver orientation, the resulting directivity pattern is still excessively narrow. Since the “pinching” is external, the negative throat effect observed with the internal pinch is no longer present.

This system can function, but it necessitates a large horn with a 1m mouth and an 80° flare angle cut off at 400Hz, or a 70° flare angle cut off at 450Hz.

Modern coaxial compression drivers can reach 400/450Hz, but not at high output levels, as the voice coil would overheat. This type of arrangement appears more promising with 12" or 10" woofers, allowing for closer driver spacing.

Further simulations demonstrate that with a 1.4" driver crossed over around 600Hz to a 60/65cm 90/80° horn, the dual 10" configuration appears to be the most suitable:

We have to keep in mind that a change is the center to center spacing beetween both woofers have a huge impact on the polar response, it should be simulate in FEA before.

The solution can be to push to restrict vertical directivity in this way, using a “quadro” configuration:

This solution is useful for creating a radiation surface between one 15" woofer and two horizontal woofers.

We have simulated this arrangement with four Faital 5FE120, which fit well with the X-Shape X25 or X28 in a TMM/MM arrangement. MMTMM must not be used, similarly, an 7/7.5-inch quadro in a TMM/MM configuration below an X-Shape X40 at a crossover frequency of 700-750 Hz is interesting.

It’s important to note:

• The improvement due to the lobes will be mainly present if the driver size and position are carefully chosen to match the horn directivity

• The effect mainly applies around the crossover in comparaison to a regular well implemented TM arrangement, see next chapter why.

• Lobes created by multiple drivers (MM or MM/MM) suppress the impact of mid range narrowing through their own cancellation.

• When the enclosure width is significantly larger, using two 15" woofers will result in an excessively narrow radiation pattern, even with slight adjustments, it’s an old design that show some limits.

• MTM can be used in “quadro” woofer arrangement (MMTMM) but the generated vertical directivity will make a good vertical directivty match impossible, it’s not recommended.

Midrange Narrowing as a directivity control device

For a better understanding, we encourage the reader to consult our article on midrange narrowing effect.

Regular case:

A single 15" driver’s directivity is close to this kind of mounting if we respect some principles: the box must be just large enough to put the driver inside; otherwise, the midrange narrowing will be shifted lower.

This means that the midrange narrowing itself behaves like a lobe. Similar to the lobes we simulate upper, it needs to be positioned correctly to achieve proper directivity matching. We need the midrange narrowing of the 15" woofer box around the crossover to match the directivity. This does not apply to compression driver horns where the midrange narrowing must be minimized or eliminated.

In-Wall case:

In this case, the wall behaves as a 180° (or less in a corner) horn, providing very good constant directivity control at low frequencies. However, at the crossover frequency, it can cause a significant divergence between the directivity of the horn and the woofer. A flat wall will give around 140° when the horn should be close to 90°.

The temptation is huge to create a kind of corner at 90°, but as the woofer wavefront is not planar, it will not follow it, and we will simply add throat resonances on a large scale.

One of the solution can be this kind of device:

The central part has no effect. The principle here is to take control on the horizontal part of the wavefront, guide it slightly, and simultaneously induce a midrange narrowing to achieve the desired directivity that matches high-frequency horn directivity.

It’s important to note that this device makes sense only in in-wall applications due to the wall directivity effect. In free-air, the solution described earlier in “Regular case” is at least as efficient and significantly simpler.

But there is a far simpler way: Just respect the ‘regular case’ and use a wall composed of absorbing materials, not an hard hemispherical one or it will create a lot of reflections (early reflections) directly as it propagates, even if everything is fluid or/and 180°.

In all cases, the front wall must be absorbing anf flush with the speaker box. It also plays a major role in room acoustics.

If the in-wall cannot be made of absorbing materials, using the ‘regular case’ in a non-in-wall configuration will be far better.

MEH and Synergy horn

Synergy Horns are a fine solution to reduce size and make portable speakers for PA usage. However, their directivity can be quite uneven. This is not a significant issue outdoors without walls, but indoors, when we listen to a combination of direct sound and sound reflected from walls, it can result in an uneven and less than ideal listening experience.

Here are some “not cheated”, and hard to find, polar plots of MEH, one of the polar was in 4π and was reduced to 2π (Half Space) to better illustrate what happens:

We need to pay attention to scale. A “half-space” option in VituixCAD displays the polar response over 180° instead of 360°. If we stay in the 360° mode, the scale is affected, and the polar response may appear more consistent than it actually is.

The same issue exists with the color scale, where using solid colors for a 2 or 3 dB range instead of a gradient can hide certain problems.

Lastly, an even simpler issue is with polar maps that start at 0 dB or use the same color for ranges like +6 dB to -2 or -3 dB (and sometimes even down to -6 dB). This completely obscures the true response of the horn.

The yellow color should also be on the -6dB and not -10dB to not hide everything in the red color.

Coaxial

Here, we will examine the measurement of the KEF R3 in half-space, widely considered one of the best coaxial speakers on the market, featuring a 100% fluid profile.

The coaxial arrangement avoids lobing between sections at the crossover. However, the mid-woofer section now serves as a waveguide for the tweeter, introducing challenges in maintaining consistent directivity, as the tweeter can only be guided around its natural radiation pattern due to its non-plane wave behavior.

This limitation is overcome by the ReShape Waveguide, which enables true plane wave behavior.