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 a 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 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 as we move up the frequency range. More details here : Loading and acoustic impedance

• Horizontal and vertical directivity: The horn’s directivity pattern, vertical and horizontal, plays a role in on-axis energy. For example, reducing vertical coverage, can redirect some off-axis energy back to the on-axis (at the price of a vertical positioning more sensitive) region as well that reducing horizontal coverage too.

• 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

Bell Response and Energy Balance

The interplay between driver size, horn loading and horn directivity creates a phenomenon known as the “bell response”.

This describes a perceived roll-off in high frequencies. However, it’s important to understand that this doesn’t necessarily mean there’s less high-frequency energy overall.

Regardless of directivity:

• Larger diaphragm compression drivers produce more low-midrange energy compared to smaller ones. So in comparaison the high frequency can look lower than a tinier diaphragm when in fact that is mainly the medium range that is upper.

• The horn’s loading effect enhances this low-midrange presence.

This can make the high frequencies seem less prominent in comparison, creating the bell response.

High Frequencies and Physics

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

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. .

The 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 impact of phase response 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 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.