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Monoprice Monolith K-BᾹS Bookshelf Speakers Measurements and Analysis

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K-BAS-testC.jpg

The Monoprice K-BᾹS speakers were measured in free air at a height of approximately 9 feet and gated at 14 ms. At this window gate, some resolution is lost below 140 Hz, and accuracy is completely lost below 70 Hz and so that range should be ignored. The microphone was placed 1 meter away from the speaker at a height level with the tweeter.

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Monoprice K-BAS Frequency Response Curves

The Monoprice K-BᾹS speaker measures well, although there are some slight imperfections. Most visible is a 3 dB, high-Q resonance around 700 Hz. This resonance may have high enough of a Q and low enough amplitude to be inoffensive, and in my listening I did not notice anything amiss in this region. What is likely to be more audible is the 2 dB dip from 3 kHz to 6 kHz, but since this doesn’t show up in the curves involving off-axis responses, I don’t think it is a problem. The scale of this graph and the low amount of smoothing (1/24 octave) make it a bit unsparing of any irregularities. The ‘Listening Window’ is exceptionally smooth above 800 Hz, which means this speaker should have a wide ‘sweet spot’ at least out to 30 degrees to the sides of the speaker, and it should be good for situations that need good coverage for that broad a listening area. The ‘Total Sound Power’ curve and its corresponding directivity index would seem to indicate some extra off-axis energy from 3 kHz to 6 kHz, which may mean those frequencies will be more reflected off-axis than neighboring frequency bands. However, the ‘Early Reflections’ curve is very smooth, and this bodes well for the end results since ‘Early Reflections’ can be a good predictor of the room sound, so extra off-axis energy from 3 kHz to 6 kHz is not something to be overly concerned about. For a good explanation of these curves and their significance, we refer the reader to this Audioholics article: Objective Loudspeaker Measurements to Predict Subjective Preferences.

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 Monoprice K-BAS Bookshelf Speakers horizontal frequency response +/-90 degrees from axis: 3D view

K-BAS-dispersion-profile.jpg 

Monoprice K-BAS Bookshelf Speakers horizontal frequency response +/-90 degrees from axis: 2D view

Looking at the K-BᾹS speakers’ horizontal dispersion pattern, we see a relatively uniform off-axis dispersion pattern. After 40 degrees off-axis, a dip develops at 12 kHz, but that is too high a frequency for that to be a problem, especially that far off axis. At 90 degrees we see a bit more energy forming from 3 kHz to 6 kHz, and this does explain the bump in the ‘Total Sound Power’ curve in that region, but up to that point the response in this region is nicely flat. This is the dispersion pattern of the woofer starting to tighten up before playback is handed off to the tweeter, which will have wider dispersion in this frequency range. The fact that we have to wait until 90 degrees for that to become evident means the woofer and tweeter are well-mated at the 3 kHz crossover point. Overall, this is a very nice response, and bodes well for those who need a speaker that sounds good in more than just one listening position.

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Monoprice K-BAS Bookshelf Speakers Polar Map

The colorful chart above is really just presenting the same data as in the two horizontal dispersion graphs above it, only shown from a top-down view and using color to indicate amplitude instead of raised lines. Presented in this way, we can more easily see certain trends from the other graphs. For example, we do see a slight narrowing of dispersion above 6 kHz. We see a hint of the 3 kHz crossover point from the slight waist-banding around that region, but it is pretty well controlled. We can more easily see the uniformity of the dispersion out to 40 degrees; past that point, there is more energy under 6 kHz, but it keeps a relatively smooth dispersion down to 1 kHz. The bottom line is essentially the same as we have seen so far; good coverage out to 40 degrees off-axis, so very good speakers for covering a wide area with decent sound and not bad control of sound outside of that point.

 K-BAS-vertical-dispC.jpg

Monoprice K-BAS Bookshelf Speakers vertical frequency response +/- 90 degrees from axis: 3D view

The above graph displays the dispersion of the K-BᾹS speakers on its vertical axis. Before you panic, it should be said that the vertical dispersion isn’t nearly as important as the horizontal dispersion. This is a very typical pattern for this kind of bookshelf speaker. We see a narrow strip of a regular response on-axis, but lobing effects quickly create deep nulls as the response goes past ten degrees off axis in either direction. This is the result of the woofer and tweeter cancelling out each other’s sound around the crossover point; this is inevitable with this type of driver layout. On this scale, the negative degrees are below the tweeter, and positive degrees are above the tweeter. What this graph says is that, as with most speakers, the K-BᾹS speakers should not be placed on their side and are best listened to with the tweeter roughly at ear level.

K-BAS-ImpedanceC.jpg 

Monoprice K-BAS Bookshelf Speakers Impedance and Phase

I blasted these speakers pretty hard and didn’t sense any obvious distortion or compression.

The impedance graph of the K-BᾹS speakers show a fairly benign electrical load. Our own measurements seem to be a couple of ohms above that of Monoprice’s. They claim an impedance minima of 4.2 ohms, but we measured a 6.2 ohm minima, although our profile shape matches theirs. Either way, most amplifiers will have no problem with these speakers; even an entry-level AVR should be able to run them fine. According to our measurements, these can easily be characterized as an 8-ohm speaker. Our sensitivity measurements showed the K-BᾹS speakers to have a 84 dB for 2.83v at 1 meter. Monoprice has these speaker spec’d at 87 dB sensitive, but they do not say if that was taken at 1 watt or 2.83v, although either way would not match our results. If that spec was taken at half-space, it would match our results, and sometimes manufacturers measure sensitivity at half-space without declaring it in the specs. We have to stress that, while that 84 dB sensitivity might seem low, it is very typical for a bookshelf speaker of these specifications, and many manufacturers have ways of stretching the sensitivity measurement. This is a speaker that can get loud, but it is not a speaker that one would want to use as mains for a big, dedicated theater room. I blasted these speakers pretty hard and didn’t sense any obvious distortion or compression. However, there is only so much a bookshelf speaker with a 5.25” woofer can do. I think these would get more than loud enough for most people’s tastes.

 K-BAS-THDc.jpg

Monoprice K-BAS Bookshelf Speaker total harmonic distortion

The above graph depicts the total harmonic distortion of the K-BᾹS speakers at 90 dB and 95 dB sweep levels. This level of distortion looks very good for this loudness level, especially in lower frequencies. The only thing in here that could come close to being audible is the bump almost touching 3% around 700 Hz, and it would have to be using a pure tone in laboratory conditions to be heard. The third harmonic is the dominant harmonic throughout this distortion profile, but the second harmonic does crop up at the 700 Hz bump, which is almost entirely second order harmonic distortion. The ridges of distortion in lower frequencies would be totally inaudible, since human hearing is much less sensitive to distortion in that band. The beefy midwoofer driver looks to be paying off in these tests. This speaker could have taken higher levels than this and still retained its composure, but the point was already made. This is a fine showing, and the K-BᾹS speakers can stay clean at relatively loud levels.

 

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Recent Forum Posts:

Motrek posts on February 13, 2017 19:25
For $220 less, you can get a pair of Elac B6s which are super-well reviewed and have advertised bass extension down to 44Hz.

I'm actually in the market for something like this. I watch TV with a pair of Ascend CBM-170 SEs which have amazing midrange and treble output but are noticeably lacking in bass (58-20KHz +/- 3dB). It would be nice to buy some affordable speakers with more bass output but I'm not sure how to decide between, e.g., the Elacs and these Monoprice speakers…
shadyJ posts on February 06, 2017 22:36
Dennis Murphy, post: 1171434, member: 29480
Thanks again I still don't really understand why this curve is indicative of the weighting the ear would give, which intuitively and in my experience is dominated by the earlier arrival times. But I do see why it would be a useful means of determining whether an observed on-axis peak is benign or a serious resonance. However, i don't think it would take 70 measurements to establish that, and a waterfall plot should also give you that information.
Floyd Toole explains it with a bit more depth in his papers and lectures. For example, this one, from about the 20 min mark to about the 40 min mark he discusses these curves and how they affect room acoustics, etc. By the way, Floyd did mention to me that Sound Power is not the most useful curve in that bunch, so I wouldn't worry about it too much, but I do think it has some merit, so I include it in that graph.
Dennis Murphy posts on February 06, 2017 21:49
shadyJ, post: 1171379, member: 20472
Here is what Dr. Floyd Toole writes about Sound Power in his book:
“Sound power: is intended to represent all the sounds arriving at the listening position. It is the weighted average of all 70 measurements, with individual measurements weighted according to the portion of the spherical surface that they represent. Sound power is a measure of the total acoustical energy radiating through an imaginary spherical surface with the radius equal to the measurement distance. Thus, the on-axis curve has very low weighting because it is in the middle of other, closely adjacent measurement points (see the perspective sketch at the top of the figure), and measurements further off axis have higher weighting because of the larger surface area that is represented by each of those measurements. Ideally, such a measurement would be made at equally spaced points on the entire surface of the sphere, but this simplified spatial-sampling process turns out to be a very good approximation. The result could be expressed in acoustic watts, the true measure of sound power, but here it is left as a sound level, a frequency response curve having the same shape. This serves the present purposes more directly. Any bump that shows up in the other curves and persists through to this ultimate spatial average is a noteworthy resonance.”
Thanks again I still don't really understand why this curve is indicative of the weighting the ear would give, which intuitively and in my experience is dominated by the earlier arrival times. But I do see why it would be a useful means of determining whether an observed on-axis peak is benign or a serious resonance. However, i don't think it would take 70 measurements to establish that, and a waterfall plot should also give you that information.
shadyJ posts on February 06, 2017 17:13
Dennis Murphy, post: 1171371, member: 29480
Thanks. That does seem counter-intuitive in terms of the relative audible importance of on-axis vs extreme off-axis sound. I guess the technique is just supposed to tell us how relatively directional a speaker is?
Here is what Dr. Floyd Toole writes about Sound Power in his book:
“Sound power: is intended to represent all the sounds arriving at the listening position. It is the weighted average of all 70 measurements, with individual measurements weighted according to the portion of the spherical surface that they represent. Sound power is a measure of the total acoustical energy radiating through an imaginary spherical surface with the radius equal to the measurement distance. Thus, the on-axis curve has very low weighting because it is in the middle of other, closely adjacent measurement points (see the perspective sketch at the top of the figure), and measurements further off axis have higher weighting because of the larger surface area that is represented by each of those measurements. Ideally, such a measurement would be made at equally spaced points on the entire surface of the sphere, but this simplified spatial-sampling process turns out to be a very good approximation. The result could be expressed in acoustic watts, the true measure of sound power, but here it is left as a sound level, a frequency response curve having the same shape. This serves the present purposes more directly. Any bump that shows up in the other curves and persists through to this ultimate spatial average is a noteworthy resonance.”
Dennis Murphy posts on February 06, 2017 16:51
shadyJ, post: 1171350, member: 20472
You can get the standard online, but its a tad pricey, something like $120 last time I looked.

The weighting numbers makes the direct axis angle very small and insignificant compared to the angles at and around 90 degrees. This is because the surface area of the sphere that the direct axis and near direct axis represents is very small. The weighting gives more weight to angles as they approach 90 degrees, with 90 degrees being given the most weight.

Thanks. That does seem counter-intuitive in terms of the relative audible importance of on-axis vs extreme off-axis sound. I guess the technique is just supposed to tell us how relatively directional a speaker is?
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