Mid-range beaming and narrowing
The Challenge of Midrange Response
In loudspeaker design, achieving a smooth and balanced frequency response across the entire spectrum is a continuous pursuit. One specific challenge lies in the midrange frequencies (typically around 1 kHz).
Here, two unwanted phenomena can significantly impact the sound and his power response: midrange narrowing and beaming.
Here we can see a not fully optimised end of profile resulting a narrowed mid-range in 1kHz region :
Whatever the horn, wageguide, integrated on baffle or not, the contour must be fluid in all directions.
Midrange Narrowing: A Dip Caused by Diffraction
Midrange narrowing refers to a decrease in sound pressure at specific midrange frequencies, mainly off-axis.
This occurs due to edge diffraction. As sound waves radiate from the driver and encounter the sharp edges of the speaker baffle or horn mouth, they bend around these obstacles.
The edges of the wavefront naturally attempt to remain perpendicular (≈90°) to the surface they touch, but abrupt changes in the profile can disrupt the coherence of the wavefront by forcing it to bend excessively. This over-diffraction can lead to wavefront peeling (beam) at certain wavelengths, which correspond to the dips observed in the midrange response, affecting both on-axis and off-axis radiation.
Here is an exemple about how to solve it:
Midrange Beaming: Uneven Radiation Due to Over-Diffraction
While midrange narrowing represents a loss of sound pressure in midrange, another effect of edge diffraction is midrange beaming.
This phenomenon arises when over-diffraction at the edges generates standing waves that interfere with the main wavefront.
These reflections arrive with varying delays and out-of-phase components, altering the sound pressure in specific off-axis directions.
The resulting standing waves can produce constructive interference at certain angles, creating peaks in the off-axis response. At other angles, destructive interference occurs, leading to dips.
This creates an uneven sound radiation pattern, compromising the overall soundstage and affecting listeners at various positions.
The Impact on Listening Experience
The combined effects of midrange narrowing and beaming can significantly alter the way we perceive sound. Here’s how:
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Reduced Clarity: The dip in the midrange response, even on-axis, can weaken the presence of vocals, instruments playing in this frequency range (guitars, pianos), and essential details in the overall sound.
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Uneven Tonality: The variations in sound pressure across the entire response (on and off-axis) can lead to an unbalanced listening experience, where the sound might appear brighter or duller depending on the listener’s position relative to the speaker.
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Narrowed Soundstage: The uneven off-axis response can restrict the perceived width and depth of the soundstage, making the sound seem less immersive for listeners positioned off-axis.
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Compromised Power Response: The ideal combination of direct sound from the speaker and reflected sound from the room (reverberant field) at the critical distance (the distance where these two components are roughly equal) is crucial for a natural listening experience. Midrange narrowing and beaming can disrupt this balance, affecting the perceived loudness and fullness of the sound.
Optimizing Baffle Design for Smooth Midrange
To address these challenges and achieve a smooth midrange response, loudspeaker designers employ various strategies:
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Round-Over Returns and Sharp Edges removing: A smooth, curved transition where the horn profile meets the baffle minimizes disruptions to the wavefront, reducing both midrange narrowing and beaming. Finding the ideal radius for the round-over depends on the specific horn profile. It requires balancing diffraction reduction with practicality.
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Fluid Profile Integration: Seamless integration of the horn with the baffle is crucial. The combined profile should be continuous and avoid abrupt transitions on both the front and side of the baffle, minimizing opportunities for edge diffraction.
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Horn and Baffle as One: Ideally, the design strives for the horn and baffle to function as a single unit with a continuous profile. This ensures optimal wavefront propagation and minimizes diffraction effects.
For flat baffles, a simple round-over return might not be sufficient. In those cases, designers may employ profile acceleration and deceleration, this involves a gradual transition from the horn’s curvature to the flat surface of the baffle, minimizing the impact of abrupt changes on the wavefro
Advanced Design Techniques
Modern loudspeaker design tools like Finite Element Analysis (FEA) simulations allow for precise modeling of baffle shapes and their impact on sound radiation. This facilitates the optimization of baffle profiles to minimize diffraction effects and achieve a smooth, balanced midrange response.
Exception: Low midrange horns
Low midrange horns, typically wide horns (around 1 meter) used between 800 and 250 Hz, are a special case:
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They are used in the bottom of their control range where midrange narrowing is present and the wavelength is long. The top range of this kind of horn is not typically used as it would significantly shift the acoustic center relative to the crossover wavelength.
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We need to achieve as much low-frequency control as possible on both axes within a fixed dimension.
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These long wavelengths are less sensitive to abrupt changes in horn shape, especially when combined with a very large mouth.
In this case, it’s more advantageous to exploit the midrange narrowing by introducing a more abrupt change. This will make the horn less usable above 900 Hz but will allow for a deeper horn, resulting in better loading.
This approach will also extend low-frequency control and loading as far as possible but is only applicable in this special case.
Using Midrange Narrowing for Directivity Matching
While midrange narrowing is an issue to be eliminated for horn or tweeter sections, it can also be exploited in the mid-woofer section when its frequency placement is well controlled by the baffle width.
A controlled woofer midrange narrowing will be used to achieve a proper directivity match between the woofer and the horn at the crossover frequency.
Woofer directivity and cabinet width
The directivity of a woofer naturally narrows as frequency increases, as the radiating diameter becomes comparable to the wavelength.
This transition can be reinforced and better positioned in frequency by the enclosure itself.
A cabinet that is just wide enough to accommodate the woofer introduces a midrange narrowing at a predictable frequency range.
When this narrowing coincides with the woofer’s natural directivity transition, it helps shape the polar response so that it smoothly meets the horn’s controlled directivity at crossover.
Importance of controlled narrowing placement
If the baffle is too wide, the midrange narrowing shifts to a lower frequency.
This creates a mismatch with the horn directivity at the crossover region, where the horn is starting to lose pattern control while the woofer is becoming directive to ensure a good directivity match.
Conversely, a well-proportioned enclosure allows:
- the woofer’s directivity to narrow progressively,
- the midrange narrowing to occur at the intended frequency,
- a smooth and continuous directivity transition at crossover.
This approach applies specifically to woofers and mid-woofers.
For horn midrange narrowing must instead be minimized or eliminated, as their directivity is already controlled by the horn itself.
Conclusion
Understanding midrange narrowing and beaming caused by edge diffraction is crucial for achieving optimal sound quality in loudspeakers. By employing careful baffle design techniques and advanced simulation tools, designers can create speakers that deliver a natural and detailed listening experience across the entire frequency spectrum, for both on-axis and off-axis listeners.
