Understanding the Difference Between Two Similar Loudspeaker Specs
Some people may say that I’m a sensitive guy. But that’s a different story. Of course, sensitive is one of many words in the English language that has multiple meanings. I was born in the USA and I’m still trying to figure out the English language (at least the American version).
Now seriously. The last few months we have been discussing sub drivers and enclosures (a.k.a. bass boxes) that are suited to the requirements of concert sound production and P.A. (public address) systems. We delved into the history of Hoffman, his Iron Law (sub enclosure type trade-offs), Thiele-Small parameters of woofers (to predict woofer performance) and subwoofer venting tradeoffs.
This month, let’s look at the similarities and differences between two loudspeaker specifications: “efficiency” and “sensitivity.” Efficiency is usually a more colloquial term used by consumers, to informally compare two loudspeakers or generally describe how much power a loudspeaker or subwoofer needs to achieve a desired loudness level. But sensitivity is a much more commonly published and specific specification.
The Keele Report
Just before I joined the AES (Audio Engineering Society) as a student member, famed speaker design engineer Don Keele presented his 1976 AES paper titled “An Efficiency Constant Comparison Between Low-Frequency Horns and Direct-Radiators,” Preprint No. 1127 (M-1). I met Don a few years later, but it would be several more years before I understood the impact that he had on the installed live-sound industry. D.B. Keele Jr. developed and patented the constant directivity HF (high-frequency) horns for both JBL and Electro-Voice. Mr. Keele published over 35 technical papers on loudspeaker design, measurement methods and other related topics.
It is that Keele 1976 AES paper that I still find quite interesting ,characterizing the difference in efficiency and maximum output between direct radiating bass boxes and horn-loaded bass boxes. Keele’s paper showed a comparison of measurements between a direct radiating ported array/enclosure of four 15” woofers vs. a dual 12” bass horn of the same cubic volume. They had the same low-freq. efficiency, but at 500 Hz., the sensitivity was slightly different. The ported array of four 15” woofers played 15 Hz lower, and 1.5 dB louder due to its higher power handling. I only recently discovered Keele’s ‘76 AES White paper during an Internet search for technical papers on this subject. Having access to this AES paper early in my career would have better informed my design process throughout my live-sound production and P.A. system design career. Almost a half century later, I still cannot find much else in the way of credible P.A. loudspeaker measurements, comparing the difference in efficiency and sensitivity between direct radiating bass boxes and horn-loaded bass boxes. Mr. Keele did share with me some recent white papers detailing more advanced/modern methods of loudspeaker measurements, but they are more suited to those involved in live-sound loudspeaker R&D.
To help clarify the differences between efficiency and sensitivity, I consulted several of my live-sound/P.A. loudspeaker design textbooks. One such textbook described sensitivity as “an indication of a loudspeaker’s efficiency.” That is just one example of why many sound pros use these terms interchangeably. The massive Handbook for Sound Engineers — 1988 third edition with several authors and over 1,200 pages — included a section called “Sensitivity and Efficiency.” It talked about how big loudspeakers are more efficient than little loudspeakers (and how big loudspeakers have better dynamic range), but it did not explain the differences between efficiency and sensitivity, nor did it define loudspeaker efficiency.
M.J. Lawrence’s High-Quality Loudspeaker Engineering textbook had 27 chapters. It included the fundamentals of loudspeaker drivers and loudspeaker systems along with some early history on the development of cinema and P.A. loudspeakers, and the associated terminology with many equations. It also included a few short sections on system alignment and on efficiency, but it failed to cover the meaning of sensitivity — surprising, as sensitivity is the more commonly published specification. Mr. Lawrence also gave due credit to some early fathers of the P.A. system industry, including Dr. E.C. Wente and Dr. H.F. Olson.
Don and Carolyn Davis (formerly of Altec-Lansing) created the first comprehensive training curriculum for P.A. (or sound reinforcement) system design calculations and intelligibility predictions. In Chapter 13 of their extensive P.A. system design textbook, Sound System Engineering, Davis and Davis had a section called Loudspeaker Sensitivity and Efficiency (with extensive math) that explains how loudspeaker sensitivity is NOT the same as efficiency. They specified “In the free-field (i.e., near the loudspeaker in an auditorium), a sound level meter placed at 4 feet on-axis in front of a loudspeaker will read the sensitivity of the unit being measured. Well beyond critical distance (in the reverberant field), the meter will read the relative efficiency (or radiated power level) of the unit being measured.” Further explanation goes into the impact of directivity index and loudspeaker Q on sensitivity. They also went through the math to explain how in the reverberant field of a venue, the LF (low-frequency) section of a loudspeaker array can actually be more efficient than the more sensitive high-frequency (horn) section of an array.
David Lee, loudspeaker and system engineer at BASSBOSS, explained online that “sensitivity and efficiency aren’t the same thing, but they can tell you the same thing. You can use one to find out the other. If you convert sensitivity to efficiency (assuming an 8-ohm speaker and 2.83 V applied), you will find that a loudspeaker with a sensitivity of 92 dB is 1% efficient. That’s right, 1%. For each 3 dB increase in sensitivity, the efficiency doubles. So 95 dB sensitivity is 2% efficiency, and so on.”
Efficiency is not a specification relevant to directional full-range (main) loudspeakers (more on sensitivity of main loudspeakers next), but let’s first look again at the efficiency of bass boxes and subs to help compare the difference between loudspeaker efficiency and sensitivity specifications.
Loudspeaker Efficiency is the ratio of total acoustical output power to electrical input power, measured in watts, expressed as a percentage. Loudspeaker efficiency is more relevant to bass/sub boxes and takes into account LF energy radiated in all directions, as subs are not typically very directional; it is measured by placing the bass box in an auditorium (or reverb chamber) whose reverberation time versus frequency has been measured. See Chapter 19 on Loudspeaker Measurements in M.J. Lawrence’s High-Quality Loudspeaker Engineering for instrumentation required and the formulas on how to calculate measured loudspeaker efficiency in a reverberant space.
Loudspeaker Sensitivity is the on-axis dB SPL output measurement (directly in front) of a loudspeaker — typically full-range — at a specified distance, a stated frequency, impedance and input level, typically in a non-reverberant environment. A traditional measurement distance was 4 feet, but now a standard measurement is 1 m, either at one Watt of input. Current measurement standards allow using 2.83 V into the loudspeaker, regardless of its impedance. A loudspeaker’s sensitivity can be raised by confining available acoustic power into a smaller area; such as by reducing the dispersion angle of the HF horn(s), by increasing the size of the LF array and/or by measuring the loudspeaker in half space (on the ground).
In review, all loudspeakers can be considered relatively inefficient, in that they generate a lot of wasted heat at high power. But more sensitive loudspeakers tend to be more efficient, requiring less power, play louder and generate less heat, so their components can last longer. With amplifier power so cheap these days, why else would you need more sensitive loudspeakers? Well, if your amp or mains wattage is a constraint, and/or if rigging weight or sightlines are maxed out keeping you from making the array larger, more sensitive loudspeakers can certainly give you more sound volume (dB SPL) from each Watt of amplifier power available.
Note on Sensitivity from Andrew Jones, Lead Loudspeaker Designer of Pioneer/TAD
“Whenever sensitivity is quoted, the nominal impedance must also be stated. This will prevent the manufacturer from cheating in cases where the lower impedance speaker is able to draw more current making the speaker appear more efficient. Of course, the efficiency has NOT increased, but the sensitivity has, which is why it is important to include nominal impedance into the spec. Plus, all speakers are designed to be driven from voltage source amplifiers and have a flat frequency response when so driven. Therefore, sensitivity does give a direct indication of how much louder one speaker will sound on a direct comparison (disregarding the capability of the amplifier to drive a speaker that cheats on impedance).”
Some Concert Loudspeaker Industry Challenge Questions
- Do direct radiating bass boxes typically have a much smoother frequency and phase response than other types of bass boxes with increased sensitivity?
- Do bass boxes and bass horns with increased sensitivity have more limited bandwidth than direct radiating bass boxes?
- What is the dispersion of horn-loaded woofers compared to direct radiating woofer arrays of the same mouth size?
- Why do horn-loaded woofers commonly sound better than direct radiating woofers of similar size and brand (especially in reverberant rooms)?
- Is power compression significantly different between various type bass boxes of the same size?
- How do tapped-bass horns and modern TL bass boxes compare with low-frequency horns?
- Can band-pass bass boxes ever be as high-impact as horn or direct-radiating bass boxes?
- With high-power woofers (and the low-cost of amplifier power), is distortion now more relevant, and is bass box efficiency no longer relevant?
- Is modulation distortion much lower in bass horns than in direct radiating bass boxes?
For help with live-sound design in large spaces, visit David Kennedy’s consulting sound design web site at www.D-K-A.com.
