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DD Audio PM151 and ABC10 Measurements & Analysis

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A full suite of measurements were conducted on the PM151 and ABC-10.  The PM151 measurements closely followed the Audioholics Loudspeaker Measurement standard except the amplifier in the PM151 powered monitor was used for all tests.  Loudspeaker sensitivity and impedance were not measured, as they are not relevant for powered monitors.  However, a max sweep test was run to provide the reader with an indication of how loud the system plays.  All tests were conducted with the 80Hz high pass filter turned off and the volume knob set to 100%.  

PM151 Powered Monitor Measurements

PM151 On-Axis

DD Audio PM151 On-Axis Frequency Response

The on-axis frequency response above 300Hz was measured with an input signal required to produce approximately 90dB at 1 meter.  The microphone was placed on-axis 1 meter from the loudspeaker at a height that was half of the distance between the tweeter and the woofer.  The low frequency response was spliced into the high frequency response at approximately 300Hz.  The low frequency response was measured using the ground plane measurement technique at 2 meters.

The PM151 on-axis frequency response slopes downward as frequency increases.  The difference between the bass and the 2 kHz to 10 kHz range is nearly 6dB.  This creates a warm sounding speaker.  Since there is nearly a 5dB reduction between 1kHz and 2kHz, this explains my desire to add a few decibels back above 2kHz.  A loudspeaker with a downward slope can still sound great but I believe the magnitude of change between the critical 1kHz to 2kHz is drastic over a short change in frequency.  After analyzing the frequency response I added a -3dB parametric equalizer centered at 700Hz with a Q of 0.7.  This created a more agreeable response for vocals.  That said, this is knit picking an already good sounding powered monitor loudspeaker system that costs a mere $279.  Given the response, the PM151 is definitely geared toward the home user, since this is not a typical response for true studio monitors but rather a pleasing response that does not offend or take too much away from recordings.  

  PM151 Max SPL Sweep

DD Audio PM151 Max SPL Sine Sweep

I ran a max SPL sweep test in-room to get an idea of what this speaker is capable of.  This test was run on-axis at 1 meter with only the powered PM151 monitor.  I increased input signal until the PM151 stopped producing an appreciable increase in decibels.  This response is in-room and includes the effects of room modes and reflections so the roughness in the low frequencies should be ignored.  The output above represents a single speaker.  Mutual coupling of a pair will produce close to 100dB at 1 meter throughout the entire usable audio range of the PM151.

PM151 Listening Window

DD Audio PM151 Listening Window

The top listening window measurement is a composite of all of the frequency response at the angles listed on the graph.  The additional responses give you an idea of how the loudspeaker will sound when a listener moves out of the sweet spot.  This includes 15 degrees above and below the loudspeaker.  This can be very useful in helping with desk placement.  For example, you might prefer the -15 degree vertical response meaning you can increase the listening height of the PM151s on your desk and alter the frequency response at the listening position significantly.

PM151 Polar Response

DD Audio PM151 Polar Response

Polar response measurements are obtained by measuring the frequency response of a loudspeaker at various points around a circle with the loudspeaker baffle in the center.  This measurement shows the response from -90 degrees to +90 degrees on the circle where 0 degrees is directly in front of the loudspeaker baffle.  Measurements were taken every 7.5 degrees and put together to form the above graph.  The DD PM151 polar response is pretty even out to 30 degrees.  Beyond 30 degrees, the polar response varies significantly with frequency.  Uneven polar response in a reflective room can lead to issues with imaging.  Due to the 500Hz to 1kHz dispersion characteristic of the PM151, I would recommend sitting closer to the speaker and move them as far away from walls and surfaces as practical.  The polar response graph is saying that the 500Hz to 1kHz range is only down approximately 5dB at 90 degrees off-axis, which means a slew of midrange sound is available at the side of the speaker.

PM151 Harmonic Distortion

DD Audio PM151 Harmonic Distortion

The harmonic distortion graph was generated using a 90dB stepped sinusoid sweep measured at 1 meter.  This test was run from 30Hz to 20kHz.  The PM151 performed well considering the speaker size and price.  The higher distortion numbers in the low frequencies are of little consequence because this speaker is primarily responsible for 70Hz to 20kHz.  The peak in the 3rd harmonic at 3kHz is notable but not likely noticeable.

PM151 Cumulative Spectral Decay

DD Audio PM151 Cumulative Spectral Decay

The cumulative spectral decay graph shows how various frequencies die out as a function of time.  A transient perfect speaker would stop playing a frequency the instant the input frequency changed.  Since that defies the laws of physics, we have to give the speaker time to stop moving at a given frequency.  This graph is useful in finding issues with cabinet resonance, etc.  There is a bit more energy in the 500Hz to 1kHz range than expected.  Given the polar response graph in conjunction with this graph, I suspect that the ½ inch MDF cabinet is radiating sound in this range.

PM151 Group Delay

DD Audio PM151 Group Delay

The group delay graph shows the rate of change of the slope of a loudspeaker’s phase.  As a rule of thumb, values below 1.6ms in the mid to high frequencies will likely not affect perception of sound quality.  Increasing group delay in the low frequencies is not as objectionable as it is in the mid to high frequency ranges.

 

ABC10 Powered Subwoofer Measurements

ABC10 Crossover Settings

DD Audio ABC-10 Crossover Settings Effects

All measurements were conducted at 2 meters using ground plane technique.  The above graph shows the effects of the 30Hz to 90Hz adjustable low pass filter.  The graph shows that the crossover frequency does not really line up with the setting on the back.  A setting of 50% yields a response that should integrate well with the PM151 monitors according to this measurement.

ABC10 Subwoofer Bass Boost

DD Audio ABC-10 32Hz Boost Setting Effect

The ABC-10 subwoofer has a 32Hz bass boost setting.  This setting makes the subwoofers -3dB point below 30Hz and yields a pretty flat response throughout the subwoofers usable range.  Depending on content and room gain, some users may opt to turn off the 32Hz boost.

ABC10 Compression

DD Audio ABC-10 Power Compression

The ABC-10 was subjected to a long-term compression test where long sinusoid sweeps at increasing power are run through the subwoofer in quick succession.  Immediately following this test, the output is measured again at a nominal output of 90dB.  The ABC-10 performed well and did not show any signs of thermal compression at 90dB.  The thermal compression test shows how the output response varies as a function of sound pressure level.  The ABC-10 was able to output just over 105dB at 50Hz.  Adding input that should result in 110dB generated additional output below 30Hz but was unable to increase the acoustic output above 30Hz.  Therefore, the maximum power sweep generated nearly 100dB at 30Hz and over 105dB at 50Hz.

ABC10 Harmonic Distortion

DD Audio ABC-10 Harmonic Distortion at 90dB

The ABC-10 subwoofer harmonic distortion at 90dB is low remaining below 4% across the entire audio range.  The ABC-10 performance related to harmonic distortion is excellent.

ABC10 Group Delay

DD Audio ABC-10 Group Delay

The group delay measurement for the ABC-10 was very well behaved. The subwoofer never approached the 1 cycle of delay threshold throughout its usable bandwidth.

 

 

 

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Recent Forum Posts:

shadyJ posts on July 14, 2014 22:17
Kudos for using a polar map. The more I learn about polar map charts, the more useful I think they actually are. Here is a informative presentation by Earl Geddes on the matter, who is, as far as I know, the chief proponent of Polar Maps as a measurement of loudspeakers, he gets into polar charts at around 13:00, but you will want to watch it from the beginning for context:
https://www.youtube.com/watch?v=fE2nWVwFT9k&feature=youtu.be
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