Monoprice Monolith K-BᾹS Bookshelf Speakers Review

[Dennis Murphy]

Hello Dennis,

From the CEA-2034 standard:
"Sound Power
The sound power is the weighted rms average of all 70 measurements, with individual measurements weighted according to the portion of the spherical surface that they represent. Calculation of the sound power curve begins with a conversion of SPL to pressure, a scalar magnitude. The individual measures of sound pressure are then weighted according to the values shown in Appendix C and an energy average (rms) is calculated using the weighted values. The final average is converted to SPL.

Sound Power Directivity Index (SPDI)
The the purposes of this standard the Sound Power Directivity Index is defined as the difference between the listening window curve and the sound power curve. An SPDI of 0 indicated omnidirectional radiation. The larger the SPDI, the more directional the loudspeaker is in the direction of the reference axis."

Thanks. I'm having trouble visualizing the spherical surface, and equating that to what we hear. Hopefully I can access the CEA standard on line.

 

[shadyJ]

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.

 

[Dennis Murphy]

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?

 

[shadyJ]

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]

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]

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.

 

[Motrek]

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...