Loudspeaker Efficiency

Quote (BSTJ 1928):

One of the simplest methods of determining the power efficiency of a loud speaker is to measure the electrical impedance, first, when the receiver is in operating condition, and, secondly, when the diaphragm is constrained from moving so that no back e.m.f. is generated. The difference between these impedances is known as the motional impedance.

The resistance component of this motional impedance when multiplied by the square of the current gives the power that is generated by the motion of the diaphragm. If there is a negligible amount of power lost in viscosity and mechanical hysteresis, the ratio of the motional impedance to the free impedance can be taken as the efficiency of the receiver, i.e., the ratio of the acoustic power output to the total power input. 

This method of measuring efficiency is well known to the art, but for most commercial receivers the efficiency is so low that the motional impedance cannot be determined with a high degree of accuracy over an extended frequency range. 

While the quote describes a method for estimating loudspeaker efficiency using motional impedance, it's essential to consider advancements in measurement techniques and address its limitations:

Original Method:

1. Measure electrical impedance: Twice - once with the speaker operating normally and once with the diaphragm fixed (no "back EMF").

2. Calculate motional impedance: The difference between these impedances.

3. Estimate efficiency: Approximate by assuming negligible mechanical losses and using the motional impedance's resistive component.

Modern Perspective:

• Limited accuracy: This method has limitations, and its accuracy, especially for low-efficiency speakers, might not be reliable across a wide frequency range.

• Alternative methods: Modern loudspeaker testing relies on specialized techniques like anechoic chambers and sound power measurements for more accurate efficiency evaluation.

Key Points:

• Motional impedance provides valuable insights into the speaker's mechanics and electrical interaction.

• It can be indicative of energy transfer efficiency, but isn't a direct measure.

• Other factors like mechanical losses and radiation impedance also influence efficiency.

Conclusion:

The quoted method offered a historical approach to efficiency estimation using motional impedance. While valuable for understanding the concept, modern measurement techniques provide more accurate and reliable results for loudspeaker efficiency evaluation.

High-efficiency champions:

1. Horn loudspeakers: These classic designs utilize folded horns to amplify sound waves, achieving high efficiency (even exceeding 100%!). However, their size and limited frequency response can be drawbacks.

2. Planar magnetic speakers: Their large, thin diaphragms driven by magnets across their entire surface lead to efficient sound production. They often excel in low-bass reproduction but can be expensive.

3. High-efficiency dynamic speakers: Utilizing lightweight materials and specialized designs (e.g., large magnets, powerful voice coils), some dynamic speakers achieve impressive efficiency. However, these often prioritize efficiency over other aspects like frequency response and distortion.

Factors influencing efficiency:

• Diaphragm material and size: Lighter, larger diaphragms tend to move more easily, requiring less power for the same sound output.

• Magnet strength and voice coil design: Stronger magnets and efficient voice coil designs improve energy transfer, boosting efficiency.

• Enclosure design: Some enclosures, like horns, can amplify sound-waves, contributing to higher efficiency but impacting other aspects like frequency response.

Remember:

• Trade-offs exist: High efficiency often comes with compromises in size, frequency response, or sound signature.

• Modern speakers: Today's focus goes beyond raw efficiency, prioritizing balanced performance across various factors like fidelity, power handling, and sound quality.