Four measurements :
On-axis frequency response - Measured directly in front of the speaker face (2 or 3 meters).
Purpose: Shows the forward-firing output of the loudspeaker across the audible frequency spectrum.
What it tells you: In comparison to the 15 degree and 30 degree measurements we do, this measurement should be the flattest and have the widest bandwidth. Bandwidth refers to the upper (highs) and lower (bass) frequencies that the loudspeaker under test will reproduce. Most good speakers today will extend easily to 20kHz and beyond, although bass performance will vary widely. Full-range is considered 20Hz to 20kHz, but only the largest loudspeakers can approach 20Hz and even some very large speakers will not be "flat" at 20Hz. Many subwoofers cannot reproduce 20Hz at the same sound pressure level as they reproduce 50Hz. One should recognize that since these measurements are performed in anechoic chamber, they will generally show less bass than what you can expect in a real room.
Although all frequency response measurements will have some bumps, in general, good speakers will have a smooth and even response within its bandwidth without many severe dips or bumps. Dips indicated less output at that frequency while bumps indicate more. The audible result of the dips and bumps in the response curve will depend on the frequencies where they occur. A bump in the upper bass may make the speaker sound boomy. A dip in the midrange can make the speaker sound recessed
Off-axis frequency response (15 degrees) - Measured horizontally at 15 degrees off-axis from the loudspeaker face (2 or 3 meters).
Purpose: Measures output of loudspeaker at 15 degrees from the center position across the audible frequency spectrum. This mimics the sound that you would get at your listening position with the speakers toed-in somewhat, but not directly aimed at your ears.
What it tells you: Ideally this should be very close to the on-axis response, although it will likely vary downward, particularly at higher frequencies. Speakers that have off-axis frequency response that matches the on-axis response are said to have good dispersion characteristics.
Off-axis frequency response (30 degrees): Measured horizontally at 30 degrees off-axis from the loudspeaker face (2 or 3 meters).
Purpose: Measures output of loudspeaker at 30 degrees from the center position across the audible frequency spectrum. This measurement is useful for predicting how strong the early reflections from the side walls of the room will be. There will likely be more high frequency roll off than the 15-degree off-axis measurement, but the curves should complement each other and not vary radically.
What it tells you: Like the 15-degree response, this one should ideally be close in shape to the on-axis response. However, this one will likely be lower than the 15-degree response. Like all response measurements one should look examine the bandwidth and the smoothness of the response across that range. If the off-axis response at 30 degrees is very close to the on-axis response the speaker would be considered as having excellent off-axis response.
Sensitivity - Averaged response from 300Hz to 3kHz for input signal of 2.83V.
Purpose: Expresses the output level of the loudspeaker with standard input voltage.
What it tells you: How much power will be needed to drive the speaker to achieve any given listening level. A sensitivity of 92dB and above is relatively high, so the speakers will require less power for any given listening level, while a sensitivity of 85dB and below is low, which means the speaker will require more amplifier power for the same listening level as the
more sensitive speaker. Sensitivity does not correlate with speaker quality and should only be used to determine how much amplifier power one will need to drive a speaker to sufficiently loud levels.
