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:

The Horn’s Influence

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

Bell Response and Energy Balance

The interplay between driver characteristics, 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.

The reasons of the bell response are:

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

When we see these high frequency lower than medium in comparison we tend to thinks that we have “lost” dB in high frequency, we will see that in fact it doesn’t work like this in Horn Response and Hiss article,

In most cases we in fact have “gain” in the midrange area, it’s the high frequency level that doesn’t have moved.

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.

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:

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.