Loading and Acoustic Impedance

Acoustic Impedance and Efficiency

• Acoustic impedance: It represents the resistance a material or medium offers to the propagation of sound. It’s analogous to electrical impedance in AC circuits.
  The higher the acoustic impedance, the greater the resistance.
• Efficiency: It refers to the ratio of the sound power output of a driver to the electrical power input.

A horn can increase the acoustic impedance seen by the compression driver, allowing it to operate more efficiently at low frequencies. This means the driver can produce the same sound level with less movement of its diaphragm.

How a Horn Increases Efficiency at Low Frequencies

The loading of a horn depends on several factors, including:

• Directivity of the horn, as it impact his surface propagation and deep

• Size and shape of the horn throat and horn in general

• Frequency of the sound

Here’s a breakdown of the key points:

• Impedance Matching: A horn acts as an acoustic transformer. It gradually increases the acoustic impedance seen by the compression driver from the throat (narrow end) to the mouth (wide end). This impedance transformation helps to match the impedance of the driver diaphragm to the impedance of the air.

• Low Frequency Efficiency: At low frequencies, the impedance mismatch between the driver diaphragm (high impedance) and the air (low impedance) hinders efficient energy transfer.

• Matching the Air Load for Efficiency: Air has a relatively low acoustic impedance compared to the diaphragm of a compression driver. This mismatch can lead to inefficient energy transfer from the driver to the air. By increasing the impedance seen by the driver, the horn helps to better match the impedance of the air, resulting in more efficient transfer of energy at low frequencies.

• Reduced Diaphragm Movement: With improved impedance matching, the driver needs to move less to produce the same sound pressure level at low frequencies. This reduces distortion and improves the overall efficiency of the driver. Low Frequency Efficiency: At low frequencies, the impedance mismatch between the driver diaphragm (high impedance) and the air (low impedance) hinders efficient energy transfer.

Think of it like this:

• Imagine pushing a heavy object (high impedance) on soft sand (low impedance). It takes a lot of effort (large diaphragm movement) to move the object a small distance.

• Now, imagine placing a board (horn) under the object. The board increases the effective impedance of the sand, making it more similar to the object. Now, pushing the object requires less effort (smaller diaphragm movement) to achieve the same movement distance. This analogy helps illustrate how loading with a horn essentially increases the “stiffness” of the air for the compression driver at low frequencies, allowing it to operate more efficiently with reduced diaphragm movement for the same sound output (dB).

Important Note:

It’s important to remember that this principle primarily applies to low frequencies. At higher frequencies, the horn’s effect on impedance matching becomes less significant, and other factors like diaphragm size and material come into play.

More informations about global energy in horn: Horn and energy

A point about Constant Directivity Horns:

As energy is not “free”, a constant directivity horn cannot be straight on axis as the energy is dispashed off axis to be constant, that we need, so the on axis response will show a bell response curve.