Years of experiments and studies conducted by Harman Research Scientists https://www.routledge.com/Sound-Repr.../9781138921368
has led to a series of 70 measurements which, after post-processing, provide an excellent indication of a loudspeaker's sound quality. This group of measurements, dubbed "Spinoramas," have proven to be so meaningful in characterizing the sound of loudspeakers in rooms that they have been codified as the industry standard method with which to measure loudspeakers as ANSI/CEA-2034-A. https://www.techstreet.com/mss/products/preview/1868536
These 70 measurements, measured as a sphere surrounding the loudspeaker, are post-processed to provide six curves that, taken together, correlate closely with carefully controlled double-blind listening tests. http://www.aes.org/e-lib/browse.cfm?elib=3833
The on-axis frequency response is and has been the most commonly used loudspeaker measurement. However, when used by itself it is a questionable indicator of the speaker's sound quality. If it is poor, the speaker will not sound good, but the on-axis response can be good, while the speaker does not sound good. The measurements described below are necessary to more fully characterize the sound of a loudspeaker. It is no wonder that speaker measurements have had such a poor reputation regarding their relationship to sound quality, given that they have generally been of the simple on-axis variety.
This updated measure of a speaker's direct sound output is composed of an average of nine frequency response measurements on-axis, at ± 10° vertical, and at ± 30° horizontal off-axis angles. Since this measurement is a spatial average, it attenuates small fluctuations that are merely the result of acoustical interference that is far less sonically significant than it appears to be based upon a single on-axis measurement. These relatively acoustically-benign local interference phenomenon change with tiny adjustments in the microphone position, and their visual distraction can mask actual performance problems--resonances in particular. Resonances are "bumps" in the response that can be significant sonic problems. Since resonances tend to radiate over a wide area, they will remain visible in the spatially-averaged Listening Window response, while insignificant fluctuations that would change with tiny microphone position adjustments are suppressed. Loudspeakers with smooth and flat listening window responses tend to excel in Harman's Double-blind Listening Tests.
First, or Early Reflections
Most of the sound we hear in rooms is reflected. The second-loudest sound, after the direct sound, is the first reflected sound from the loudspeakers. In fact, Harman research has discovered that the first reflection from side walls, both from the wall adjacent as well as the opposite side wall are critically important. The acoustic output of a loudspeaker far off-axis horizontally is very significant, and should match the response of the Listening Window as much as possible. This goal is technically challenging, but is essential for optimum timbre, as well as to provide a sense of seamless coherency. Revel's waveguides, along with optimum engineering choices such as crossover points and slopes, relatively small midranges and tweeters that can be safely used to lower frequencies than typical designs contribute to far off-axis responses that are close to the direct sound, as seen in Listening Window measurements. http://www.aes.org/e-lib/browse.cfm?elib=6079
Sound Power is composed of the weighted average of all 70 measurements. Each measurement is weighted to properly represent an area of a sphere. Sound Power is a measure of the total sound radiated by a loudspeaker. One of its uses is to detect resonances, since response aberrations seen in both the Sound Power and other curves are likely true resonances. Harman research has determined the threshold of audibility of resonances. All Revel speakers are designed to keep resonances below the threshold of human audibility using the most sensitive stimulus. http://www.aes.org/e-lib/browse.cfm?elib=5163
The Directivity indices describe how the speaker's radiation changes as a function of frequency. The Sound Power Directivity Index is defined as the difference between the Listening Window curve and the Sound Power, while the First Reflection Directivity Index utilizes the Listening Window curve and the First Reflections curve. Both curves should be smooth and change gradually. A DI of 0 dB indicates an omnidirectional speaker, while a larger DI indicates greater directivity.