Understanding Energy Distribution in Compression Driver Horns

When it comes to compression driver horns, the energy distribution across frequencies is a crucial factor as a horn is an energy distribution device.

This article explores how the driver itself and the horn interact to shape this energy distribution.

The Driver’s Role

The energy a compression driver produces is determined by several factors:

• Diaphragm Size: Larger diaphragms naturally produce more low-frequency energy, but their breakup can impact very high frequencies .

• Motor Characteristics: The motor’s sensitivity and resonance frequency (Fs) as other characteristics influence the overall energy output and roll-off at low frequencies.

The overall balance and brain

We listen at a so-called critical distance where the direct field (sound coming directly from the speaker) and the reverberant field (sound that rebound on walls) is at 50/50 or 60/40.

If you perceive a sound as aggressive at high volumes but not at lower volumes or an imbalance between treble and the rest of the spectrum when you went further of you speaker , it’s likely due to an imbalance in the energy radiated and perceived by the ears across the frequency spectrum.

Our ears are particularly sensitive to the upper midrange and audible treble frequencies, and excessive energy in these regions can lead to fatigue and discomfort. More details in our audibility article.

To prevent damage to your ears, your brain instinctively limits the volume when the sound becomes too intense in this area. Balancing the energy between the midrange, low midrange, and high frequencies can help mitigate this issue. One effective strategy is to use a loudspeaker with constant directivity, to balance and adapt radiated energy across the spectrum according to your listening distance and acoustic.

This type of loudspeaker is often praised for its ability to deliver high sound levels without causing discomfort. This is because the well-balanced energy distribution allows your brain to “authorize” higher volumes without triggering the pain response. In contrast, direct-radiating loudspeakers, which typically disperse sound over a wider area (120 degrees or more), can create uneven sound fields that may be more prone to causing fatigue.

By using a loudspeaker with constant directivity and optimizing your room acoustics, you can enjoy a more balanced and immersive listening experience, even at higher volumes and gain more impact in the low end spectrum.

The Horn’s Influence

The horn plays a significant role in how energy is distributed:

• Loading: Horns act as a load on the driver, affecting its response at different frequencies. This loading effect is most prominent at lower frequencies and diminishes strongly as we move up the frequency range. More details here : acoustic loading

• Horizontal and vertical directivity: The horn’s directivity pattern, both vertical and horizontal, significantly influences on-axis energy. Decreasing horizontal coverage helps to retain more energy on-axis.

  Reducing vertical coverage will redirect some off-axis energy back to the on-axis region, however, it’s important to note that reducing vertical directivity improves sensitivity only within the bandwidth where vertical and horizontal coverage overlap. As the horn’s mouth height decreases, vertical control is lost earlier, while horizontal control is maintained. When vertical control is lost but the horn is still in use, overall loading is impacted because a portion of the energy escapes vertically.

  This effect is counter balanced with the loading effect see upper, when both horizontal and vertical control are lost, according to horn mouth size, the horn cannot bennefit of loading, sets an absolute limit to the potential loading effect.

• Constant Directivity: Horns designed for constant directivity (consistent sound dispersion across a specific angle) send energy off-axis that will be not more present on-axis, particularly at higher frequencies, as some part of the energy gets redirected off-axis to maintain the directivity pattern.
  For a good listening experience ,psychoacoustic give us a global rule to be constant until 7/8 khz then decrease slowly after.
  More details here : Constant directivity

The interplay between driver characteristics, horn loading and horn directivity creates a phenomenon known as the “bell response” that is explained in Horn Response article.

High Frequencies and Physics

Physics dictates that horn loading has minimal impact on very high frequencies, as it’s related to acoustic loading.

In this region, the driver’s motor characteristics become the primary determinant of energy output as, on best horn about loading, loading is inversely proportional to frequency and ends when horn’ control ends.

As we said upper and in the related article the effect of acoustic loading (so the “loading effect”) is strongly inversely proportional to the frequency.

Horn Low-End Roll-Off

However, the driver’s motor naturally start to produces less energy at lower frequencies from some point, depending on its characteristics and diaphragm size.

This reduction in motor output, combined with the decreasing horn loading effect at lower frequencies, contributes to the natural roll-off in low-end response.

Ultimately, as the horn’s width becomes insufficient to control sound waves, the directivity pattern widens significantly. This signifies the end of the horn’s effective loading capability, resulting in a drop in sound pressure level.

From this low point, whatever we will try to do to load more will not works.

Don’t Waste Energy: Directivity and Efficiency

Adapting the horn’s directivity pattern according to the listening distance isn’t just about improving the listening experience. It also allows us to respect the critical distance, the point where the direct sound from the driver and the reflected sound in the room reach a balance (around 50/50 or 60/40).

By directing energy constantly until 7/8khz with a coverage adapted to our critical distance, we ensure to keep efficiency and, at the same time, achieve clear and accurate sound reproduction. This approach respects the balance between direct and reflected sound within the critical distance.

Here’s where a concept called “midrange narrowing and beaming” comes in. This phenomenon, though primarily affecting off-axis response, can also slightly impact the on-axis response, representing a loss of usable energy and contributing to an imbalance in sound reproduction. You can find more details about this in the linked article: Midrange narrowing and beaming

A Point about Frequency Correction (EQ) and Phase Shift

As described here: How to implement my horn and my speaker, correct the frequency response with IIR (minimum phase filtering) EQ is the way to do:

• Minimum phase filtering also modifies the phase and it’s a very good point: Where there is a deep on a pick on frequency the phase is also impacted in the same way, an IIR EQ will not only correct the response in frequency but also phase response in the same time, and in a good way, we need this phase response impact for linearize it too.

• FIR filtering (linear phase filtering) must be only used for crossover, when we want to do crossover we want to cut the response but without touching the phase, it’s what FIR filtering does, so no EQ in FIR, only crossover.

Conclusion and impact on distortion

Understanding how energy distribution works in compression driver horns allows for informed choices when selecting drivers and horns for our applications, especially when it come to adapt our coverage to our critical distance.

We don’t listen horn without EQ it, even in passive filtering, the horn response should be more or less flat at 30/60cm right in front of him, so the raw on axis response is not a factor of choice.

It’s more interesting to EQ the response flat at 30cm and measure the distortion, in this case the distortion response at high volume will be the crucial factor about the driver as well as temporal measurements.

Considering horns with similar loading capacities and directivity pattern (complete polar map of both axis) that determines how sound is dispersed across different angles at different frequencies:
The on-axis frequency response will show very minimal variation with the same driver.

Loading can give use more SPL at mid-low frequencies, but the starting energy is fixed at the beginning by the compression driver and cannot be changed.