How We Test Loudspeakers...
Rene St. Denis sets up a
loudspeaker in the NRC's Anechoic Chamber. All speakers
are measured in the same chamber with identical
equipment to ensure consistent and comparable results.
measurements are performed by the prestigious National Research
Council of Canada. The NRC’s facilities include a modern
anechoic chamber and precision measuring devices, along with staff
with decades of experience conducting these tests. All
measurements are performed separate from the subjective evaluation
-- the body of the review.
In all, we perform a
total of seven tests displayed on four charts to give perspective
into the measured performance of the loudspeakers under
All small- and
medium-sized loudspeakers are measured at a distance of 2 meters
(6.5 feet). Where appropriate, larger loudspeakers are measured
from a distance of 3 meters (9.75 feet) to allow for proper driver
1 - Frequency Response and Sensitivity
Four measurements can be
seen on this chart:
- 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
20hKz, 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
- 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
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.
2 - Listening Window
- Listening window
- Averages five frequency response measurements and plots them
as a single frequency response. The five frequency response
measurements that are averaged for the Listening Window are:
on-axis, 15 degrees left and right off-axis, 15 degrees up and
Purpose: Gives increased perspective of on-axis
loudspeaker response in listening position. Takes into account
subtle variations of on- and off-axis response on both the
horizontal and vertical plans.
What it tells you: Averaging multiple measurements is
important because subtle frequency response changes occur in
small increments on- and off-axis, both laterally and
vertically. This measurement is especially useful because it
allows for small variations in the listening position and ear
height and can be a more useful determinant of real-world
listening than the standard on-axis measurement. Like any
frequency response one should take note of the bandwidth (the
upper and lower frequencies the speaker extends to), as well
as the smoothness of the response across all frequencies. Dips
in response mean a speaker is "less-loud" at that
point, while peaks mean it is "louder" (i.e. more
sound energy). Depending on the frequency it may result in a
more distant or forward quality.
3 - Total Harmonic Distortion + Noise (THD + N)
- THD+N variation
with frequency at 90dB - Measured at 2 meters (equivalent
to 96dB at 1 meter) from 50Hz to 10kHz. The top curve of the
chart shows the frequency response of the loudspeaker at the
determined SPL level (i.e. 90dB) while the bottom curve shows
the distortion component of the signal (values below 40dB
should be ignored because they are too close to the noise
floor of the test equipment to be of use).
Both curves are reported in dB which can be read off the
vertical axis. In order to convert to a percentage one must
read the top line (frequency response) and then determine the
dB difference between that line and the bottom line (THD+N
line). Translation from dB to % is as follows:
Equal (or 0dB difference) = 100 %
-10dB = 31.6%
-20dB = 10.0%
-30dB = 3.16%
-40dB = 1.0%
-50dB = <0.5%
Please note: an SPL level of 90dB measured
anechoically is very loud and considered far beyond normal
listening levels, particularly for small loudspeakers. To give
more information for real-world listening levels, if it
appears that the speaker is being strained beyond its output
abilities at this level we will provide a second measurement
at at lower SPL (the SPL level will be printed with the
Purpose: Measures THD+N output at discrete frequency
intervals for above-normal listening levels. Please note that
90dB output at a 2-meter distance is equivalent to an SPL
level of 96dB at a 1-meter distance.
What it tells you: Audibility of distortion varies as
to type of distortion and also the frequency at which it is
occurring. Distortion measurements for loudspeakers are
usually many times that of electronics (i.e. amplifiers,
receivers, etc.). Furthermore, certain types of distortions
are more audible than others and the audibility of that also
depends on the frequency. Our distortion measurements give a
general indication of how much distortion is occurring for a
given output level at above normal listening levels.
Distortion levels will be less (sometimes much less if the
speaker is being stressed beyond capabilities at 90dB) at
4 - Impedance Magnitude Variation With Frequency
magnitude variation with frequency - Measured across
audible frequency spectrum.
Purpose: Measures impedance at discrete
frequency intervals to indicate load placed on amplifier to
drive the loudspeaker.
What it tells you: In general, the lower the
impedance is the harder it will be for the amplifier to supply
enough power to properly drive the loudspeaker. The larger the
peaks are in the impedance chart, the more difficult the
loudspeaker load is and the more control the amplifier will
need to have over the loudspeaker to get good optimum sound.
The easier the loudspeaker load, the flatter the impedance
plot will be and the closer to 8 ohms it will stay. There is
no one thing in the impedance curve that tells the entire
story of how difficult the loudspeaker load will be, however,
in general, there are a couple things to look at including: 1)
The minimum impedance levels (in particular, take note of
frequencies below 200Hz which many consider harder to drive
than the same impedance at higher frequencies), and the size
of the narrow peaks in impedance.
Many stereo and A/V receivers have the smallest power supplies
on a watt-per-channel basis so they tend to perform best when
connected to loudspeakers which do not go below 6 ohms and do
not have large prominent impedance spikes. Many tube
amplifiers also benefit from avoiding loudspeakers with large
impedance peaks. Occasionally there may be speakers for
special applications, like high sensitivity loudspeakers for
low powered tube amplifiers where the loudspeaker
intentionally has an impedance higher than 8 ohms. This will
likely be discussed in those reviews.