Horn Response, Hiss and Equalization

Bell Response

When looking at a horn’s frequency response, a dip in high frequencies compared to mid frequencies might lead you to believe you’ve “lost” decibels (dB) and need to boost the highs with additional amplification.

However, this way of thinking is not accurate and we will see here why.

To gain a more comprehensive understanding of the concepts discussed here, particularly about horn loading (energy) and constant directivity, we recommend referring to our articles dedicated to these subjects.

Essentially, the bell response of a horn arises from the combined influence of its directivity pattern and acoustic loading effect. While constant directivity horns typically maintain a consistent directivity behavior up to around 7-8 kHz, the acoustic loading effect is inversely proportional to frequency. This means it has minimal impact on high frequencies.

Flattening Frequency Response with EQ

Typically, to achieve a flat on-axis frequency response, we choose a reference frequency above which we want a flat response. This is often set at 15 kHz.
Beyond this point, we allow the natural roll-off of the driver to occur, or use equalization (EQ) again if there’s a rise.

Negative or positive EQ can be used to achieve a flat response. The direction (positive or negative) isn’t crucial, the final result will be the same.
The primary goal is to avoid clipping (in the source, DSP, or amplifier) and maintain a proper gain structure to our desired maximum power output.

Minimum-phase EQ (IIR) is a valuable tool for correcting on-axis horn response because it addresses both the frequency response and the phase response simultaneously.

When a significant dip or peak occurs at a specific frequency, minimum-phase EQ (IIR) not only corrects the level at that frequency but also adjusts the phase response in a way that complements the level correction. This combined effect helps to achieve a more linear overall frequency and phase response.

Here are two EQ examples for an X-Shape X25 horn

BMS 5530 one:

18Sound 1095N one:

Our article about audibility can help to understand one of the reasons why we do nothing after 15 kHz in the 1095N case.

This region is also where the breakup occurs, that is the second main reason why we don’t try to increase volume here.

In each case 15 kHz is our anchor point, EQ are different but the main judge will be, as alway, the distortion after having EQ it flat below 15 kHz, as we have seen on our 1" compression driver test.

If we push constant directivity very high on the same horn, way after 7/8 kHz, the effective lose of energy in 12/17khz area will be in fact minimal, arround 1 or 2dB maximum.
Its due to the fact that only directivity will impact it and not the loading effect.

S/N ratio and Hiss

Hiss is a low-level noise often noticeable in high-sensitivity components in relation to the S/N (Signal-Noise) ratio of the system.

A common concern when comparing horns, especially regarding EQ usage, is increased hiss.

It might seem that extensive EQ required for some horns, compared to others, along with the gain adjustments needed to achieve a flat response, would exacerbate hiss, reduce the signal-to-noise ratio (S/N ratio), or even consume more power.

However, this vision is partial. Here’s why:

• Power Amplifier: No change in S/N ratio or Hiss.

  • Our reference point is 15 kHz. Constant Directivity Horns stop exhibiting constant directivity behavior around 7-8 kHz.

  • Therefore, directivity (as an energy repartition) above this point has minimal impact on the energy at 15 kHz (as 15 kHz is roughly one octave above 7-8 kHz). Directivity at 15 kHz is dictated by the driver, not by the horn.

  • Acoustic loading is inversely proportional to frequency. Consequently, it has minimal influence on 15 kHz, almost none.

• DSP: Slight decrease in S/N ratio.

  • While extensive EQ adjustments might slightly increase noise as they take a part of DSP’s dynamic range, however systems operate well below their maximum dynamic range (typically between 96 and 125 dB).

In conclusion, while EQ adjustments might influence how the DSP perceives the S/N ratio, the impact on hiss is still minimal.

L-PAD

To reduce hiss and protect the driver, an L-pad to reduce driver sensitivity is often recommended, it’s a simple resistor network placed just before the driver:

You can simulate the appropriate L-pad values using VituixCad software, considering the driver’s impedance and the horn on-axis response. For the X25 horn, the L-pad configuration might involve a combination of a 1.5-ohm resistor in parallel and a 12-ohm resistor in series, depending on the desired dB reduction to match the mid-woofer level.

It’s important to note that the L-pad resistor network doesn’t degrade audio quality, It simply absorbs some of the amplifier’s energy which will therefore not reach the compression driver or tweeter.

Conclusion

For a given driver, the amount of energy at 15 kHz is primarily determined by the driver itself, regardless of whether the horn has constant directivity or even a reduced coverage area.

In essence, differents constnat directivity horns have roundly the same hiss levels and S/N ratio when using the same driver.

The bell response is not a problem and is related to both directivity and loading effect:

• It’s the proof that the horn is constant, as a flatter response can indicate a narrower directivity horn.

• A part of it in medium and low-end is the loading effect, an energy “bonus” that will reduce overall distortion, allowing to make a lower frequency crossover.

• While EQ adjustments might influence how the DSP perceives the S/N ratio, the impact on hiss is still minimal.

• From the amplifier’s perspective, there’s no change in S/N ratio. Even in situations where we push constant directivity beyond its limits, the decrease in S/N ratio on a well-designed horn is minimal (typically 1-2 dB).

• The L-pad allows for adjusting overall sensitivity and counteracts the amplifier’s inherent noise.

• For very advanced users, passive equalization (RC network) enables limiting EQ application, thereby gaining on the DSP’s digital dynamic range.

• It also depends on compression driver characteristics.