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RBH Sound PM-8 Monitor Measurements & Conclusion

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pm8 outdoor testing.jpg 

The RBH Sound PM-8 was measured in free-air at a height of 7.5 feet at a 1-meter distance from the microphone, and the measurements were gated at an 11-millisecond delay. In this time window, some resolution is lost below 250 Hz and accuracy is completely lost below 110 Hz. Measurements have been smoothed at a 1/12 octave resolution.

pm8 spin o rama.jpg 

The above graph shows the direct-axis frequency response and other curves that describe the speaker’s amplitude response in a number of ways. For more information about the meaning of these curves, please refer to our article Understanding Loudspeaker Measurements Part 1. While there are some ripples throughout the midrange, the overall response of the PM-8 hews pretty closely to a central amplitude line, so this speaker has a good tonal balance much as I heard in my listening sessions. What the PM-8 doesn’t have is any broad peaks or dips that would color the sound. The narrow ripples are much less audible and don’t contribute much to the sound character. This kind of response is what one would hope for in a studio monitor. It will reproduce the source signal with little coloration added. We do see that the directivity is a bit imperfect, but it is not wildly imbalanced. For the most part, everything shows very good performance. 

pm8 3D horizontal waterfall response.jpg

pm8 2D horizontal waterfall response.jpg

The above graphs depict the speaker’s lateral responses out to 90 degrees in five-degree increments. More information about how to interpret these graphs can be read in this article: Understanding Loudspeaker Review Measurements Part II. Here we get a better look at the slight directivity incongruities that caused some roughness in the directivity indexes in the lower midrange. The off-axis sags a bit after 50 degrees or so. However, keep in mind this is not some major flaw, and I only point it out for academic curiosity rather than a damning critique. It may well be audible if you were listening outside of a 50-degree angle, but realistically no one is going to listen at that angle. It is possible that it could have some discernible impact as an acoustic reflection versus a speaker with perfect directivity, but even then I doubt very much that the difference would be serious since this directivity flaw isn’t very severe. Something else that we do get a clearer view of is how much the tweeter beams at high frequencies. This is seen more with dome tweeters than AMT tweeters, but the PM-8 does squeeze the high treble at off-axis angles. It’s not a serious flaw either since most of the frequency range where the beaming becomes substantial is above 10 kHz. There isn’t a whole lot of content that high, and it isn’t a range that is meaningfully subject to acoustic reflections. If you want to be met with treble flat to 20kHz, you should be listening within a 20-degree angle of the on-axis response.

pm8 Polar Map.jpg 

The above polar map graphs show the same information that the preceding graphs do but depict it in a way that can offer new insight regarding these speakers’ behavior. Instead of using individual raised lines to illustrate amplitude, these polar maps use color to portray amplitude, and this allows the use of a purely angle/frequency axis perspective. The advantage of these graphs is they can let us see broader trends of the speaker’s behavior more easily. For more information about the meaning of these graphs, we again refer the reader to Understanding Loudspeaker Review Measurements Part II.

From the above polar map, we can see that the PM-8’s dispersion pattern gets a bit ragged at off-axis frequencies but the bulk of the projected sound remains confined within certain angles. Much of the raggedness is higher-Q peaks and dips which aren’t all that audible, as was explained before. So, while the off-axis response doesn’t exhibit perfect uniformity, it isn’t likely to be seriously deleterious as acoustic reflections.  

pm8 3D Vertical waterfall response.jpg 

The above graph shows the PM-8’s response behavior along its vertical axis where zero degrees is directly in front of the tweeter, negative degree values are angles below the tweeter, and positive degree values are angles above the tweeter. There is a lot to like about this response. Typical speakers tend to have a large gap in the vertical off-axis response around the crossover frequency. This is because as the microphone moves off-axis, the difference in distance between the drivers becomes significant, and that distance difference causes phase cancellation within the range of shared frequencies which they reproduce. The shallower the slope of the crossover design, the larger the cancellation null that occurs in the vertical off-axis response. Sometimes these nulls begin to set in at just ten degrees above or below the on-axis angle. Because of these cancellation nulls, it is usually recommended to be listening to the speaker where the tweeter is level with the listener’s ear, where the drivers normally have equal distance from the listener.

As a consequence of their steep 48dB/octave crossover slopes, the PM-8s exhibit very little crossover cancellation. In fact, the vertical off-axis response is unusually good. These speakers may well be usable on their sides, in contrast with many other two-way speakers in this class. Many studio monitors use waveguides in an attempt to limit vertical dispersion based on the idea that acoustic reflections off of desktops or mixing consoles degrade the sound. Implicit in that idea is that the vertical dispersion patterns of most two-way designs are badly flawed. RBH Sound takes a different approach: they do not limit the vertical dispersion but rather ensure that it has good correlation to the on-axis response. In other words, there is no need to confine the vertical dispersion if the reflections that it produces are benign. Aside from the matter of reflections, PM-8 users benefit from this excellent vertical dispersion by being able to listen at lower or higher angles and still be met with a full sound as opposed to many other speakers of this type. 

pm8 low frequency responses.jpg 

The above graphs show the PM-8’s low-frequency responses that I captured using groundplane measurements (where the speaker and microphone are on the ground in a wide-open area). This graph depicts the low-frequency responses of each of the presets that affect the low frequencies. The purple curve for presets 1, 4, 5, and 8 shape the response for situations where no boundary or room gain is expected (or also if the user just likes a lot of bass). The blue curve for presets 2 and 6 is the low-end response intended for situations where room gain will give an acoustic boost to low-frequency output, and that response attempts to compensate for the boost. The green curve is the response for presets 3 and 7, which set a high-pass filter for use with subwoofers. While room acoustics will invariably screw up the low-end response, the PM-8 puts out a nicely smooth or linear response for whatever preset it is used in.

pm8 preset response differences.jpg 

The above graph exhibits the changes that can be made to the PM-8’s treble response using the preset knob settings. The red curve represents presets 1 through 4 and is intended for further distance placements that can attenuate higher frequencies. The green curve represents preset 5, which is also intended for far-field placement but shelves the high frequencies by 3dB. The blue curve represents presets 6 through 8 and is the response intended for near-field placement. Preset 5 gives the most neutral anechoic response from these options, but those who are looking for a more detailed, forward sound might enjoy presets 1 through 4 more, and those looking for a slightly more relaxed sound use presets 6 through 8.

Conclusion

pm8 single.jpgAs always, before wrapping this review up, I will briefly go over the strengths and weaknesses of the product under review. As always, I will start with the weaknesses, although there aren’t many weaknesses here. However, at this price point, few weaknesses should be expected. One issue that might be a nuisance for some users of the PM-8 who need a near-field monitor is there is a slight background hiss when the speaker is turned on which is not affected by the gain, so it has the same loudness level irrespective of anything else. Many active monitors, especially those with class-D amplifiers, seem to have this background noise which sounds like a soft white noise. It is not loud, and you do have to be in a near-field distance to hear it, but when you are, it is audible. Unlike other studio monitors though, these speakers are so large that I doubt many people are going to use them as desktop speakers, so this complaint might be a moot point (although I did use them as desktop speakers for my PC system for a while and the noise wasn’t intrusive enough to bother me). 

One other small flaw is the PM-8’s directivity control, which isn’t quite as good as some other monitors that we have seen which use waveguides. However, as I mentioned before, it isn’t bad, and the adverse effects of their imperfect directivity control aren’t likely to be an audible matter, just an academic one.

pm8 outdoors7.jpgOne caveat is that they are large monitors, but you can’t get their level of performance from a small enclosure, so that is not a reasonable complaint. However, those looking for some speakers for a tight space may want to make a mock-up of the PM-8’s size first to see if they will fit before ordering a pair. Otherwise, if you want the RBH Sound with the PM-8 technology, you'll have to wait until they offer a 6" version in a PM-6 model.

With those nitpicks out of the way, let’s now discuss the PM-8’s strong points. Their strongest attribute is, of course, their sound. They have a very balanced and detailed sound along with an outstanding dynamic range. The on-axis response is very good, the off-axis response is decent, and, unlike many of the monitors that we have looked at, they aren’t picky on the vertical axis. You can have these aimed a fair bit above or below you and you will still be met with a full and linear sound. They are certainly neutral enough to use for studio work. What you put in them is what they give out without adding or omitting anything. They project a superb soundstage with excellent imaging abilities. Their ability in bass can match that of a full-sized tower speaker. Even though they are stand-mount speakers, they can dig well below 40 Hz with real dynamics and low distortion, and their 32Hz low-end in their specified +/-3dB window is very real.

The PM-8s have outstanding dynamic range...they project superb soundstage with excellent imaging abilities.

The ability to alter the response with the preset knob is also an advantage for those who want to tailor the sound to their tastes or who want to use a subwoofer in a system without bass management. Being able to high-pass filter at 80Hz along with the XLR outputs make it easy to incorporate subwoofers without needing extra processing gear. Many studio gear setups don’t have easy bass management like a typical home audio AVR, so those features can be advantageous.

pm8 pair7.jpg

RBH Sound PM-8 Active Studio Monitor

 

The RBH Sound PM-8s are the swiss-army knife of hi-fi speakers...

The PM-8’s build quality is also very good, befitting a speaker of this price point. The accent panels are a pretty significant surcharge at an additional $400 per speaker, and individual buyers will have to decide whether it is worth it to them. The accent panels really do give these speakers some serious heft though, and make them feel and look more like a high-end home audio speaker than a studio monitor. Buyers should keep in mind that the accent panels are made from engineered stone and can be had in a variety of colors and stone mixtures, and inquiries should be made to RBH Sound to see what can be had. Outside of the accent side panels, the textured laminate surface is certainly a cut above the usual plastic or vinyl finishes that most monitors have in both appearance and durability. Those who want their studio gear to look high-end, as well as sound high-end, will want to give a close look to the PM-8s.

pm8 logo2.jpgAs I mentioned earlier in this review, it looks to me like RBH Sound isn’t just making a studio monitor but a high-fidelity speaker with the flexibility to take on a lot of roles: near-field monitor, far-field monitor, or an upscale home audio standmount speaker. It is the swiss-army knife of hi-fi speakers, but that does engender a pricey speaker since it has to accommodate the aesthetic requirements of home audio, the dynamic range needed for far-field monitoring, the connectivity needed for pro-audio gear, and the linearity needed for studio production work. It does all of these things well, and that, to me, makes its cost very reasonable.

Those who are interested in the PM-8s can try it as an in-home demo from RBH Sound for 30 days with the option to return it for a full refund for any reason (although they will have to cover return shipping costs). Buyers get a five-year warranty on the cabinet and drivers and a three-year warranty on the amp. I quite enjoyed the RBH Sound PM-8s, and I think anyone looking for a high-performing loudspeaker that can meet the needs of a wide range of situations has a rock-solid choice in the PM-8s (get it? “Rock-solid” since they have engineered stone side panels… I’ll be here all week, try the veal, and don’t forget to tip your waitress).

The Score Card

The scoring below is based on each piece of equipment doing the duty it is designed for. The numbers are weighed heavily with respect to the individual cost of each unit, thus giving a rating roughly equal to:

Performance × Price Factor/Value = Rating

Audioholics.com note: The ratings indicated below are based on subjective listening and objective testing of the product in question. The rating scale is based on performance/value ratio. If you notice better performing products in future reviews that have lower numbers in certain areas, be aware that the value factor is most likely the culprit. Other Audioholics reviewers may rate products solely based on performance, and each reviewer has his/her own system for ratings.

Audioholics Rating Scale

  • StarStarStarStarStar — Excellent
  • StarStarStarStar — Very Good
  • StarStarStar — Good
  • StarStar — Fair
  • Star — Poor
MetricRating
Build QualityStarStarStarStarStar
AppearanceStarStarStarStar
Treble ExtensionStarStarStarStarStar
Treble SmoothnessStarStarStarStarStar
Midrange AccuracyStarStarStarStar
Bass ExtensionStarStarStarStarStar
Bass AccuracyStarStarStarStarStar
ImagingStarStarStarStarStar
Dynamic RangeStarStarStarStarStar
Fit and FinishStarStarStarStarStar
PerformanceStarStarStarStarStar
ValueStarStarStarStarStar
About the author:

James Larson is Audioholics' primary loudspeaker and subwoofer reviewer on account of his deep knowledge of loudspeaker functioning and performance and also his overall enthusiasm toward moving the state of audio science forward.

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Recent Forum Posts:

ryanosaur posts on November 12, 2020 05:46
Another cool review. Thanks Shady!
shadyJ posts on November 12, 2020 00:57
41576

RBH Sound raised a few eyebrows at the 2019 CEDIA show when they unveiled the PM-8 studio monitor which is a product for a very different market than RBH’s traditional focus of home audio. Perhaps it shouldn’t have been as much of a surprise since the requirements of studio monitors and home audio loudspeakers have a multitude of similarities. The only significant differences are that monitors need to have the appropriate connectivity to work in a studio environment, and that monitors more than home audio loudspeakers should strive to have an accurate sound that is faithful to the source signal. One would hope that typical home audio speakers would have some accuracy, but often they can be designed to sound ‘good’ rather than accurate. That isn’t necessarily a bad thing, but it can complicate things especially when accuracy has been shown to sound ‘good’ in tests of human listening preferences. Seeing as how accurate sound also happens to sound good, it stands to reason then that studio monitors with ostensibly accurate sound should also be very good for recreational listening. That is what we will be looking at today since we have a pair of the PM-8s in for review…

READ: RBH Sound PM-8 Studio Monitor Review
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