Monitor Audio Platinum PL300 II loudspeaker Measurements

Sidebar 3: Measurements

I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Monitor Audio PL300 II's frequency response in the farfield; and, for the nearfield frequency responses, an Earthworks QTC-40, which has a ¼" capsule and thus doesn't present a significant obstacle to the sound. For logistical reasons, I measured a different sample of the PL3000 II (serial no.100047) than those auditioned by Robert Deutsch. I left both rear-panel ports open for the measurements and left the grilles in place.

My estimate of the PL 300 II's voltage sensitivity was 87dB(B)/2.83V/m, which is 3dB lower than the specified figure of 90dB/W/m. The speaker's electrical impedance magnitude and phase are shown in fig.1. The PL300 II is specified as a 4 ohm load with a minimum value of 4.2 ohms at 111Hz; the minimum magnitudes in my measurement (solid trace) are 4.12 ohms at 119Hz and 4.1 ohms at 2.2kHz. Though the impedance amplitude remains above 6 ohms for much of the audioband, the electrical phase angle (dotted trace) is larger than usual at some frequencies, meaning that Monitor's rating of 4 ohms is correct.


Fig.1 Monitor Platinum PL300 II, electrical impedance (solid) and phase (dashed) (5 ohms/vertical div.).

The impedance traces are free from the small discontinuities that would imply the existence of enclosure panel resonances, and the cabinet did indeed generally feel inert to the traditional knuckle-rap test. But the top panel and the concave rear panel behind the midrange driver did feel more lively; using a plastic-tape accelerometer, I found a single strong mode at 410Hz (fig.2) on these surfaces. As RD didn't comment on any midrange congestion, I suspect that the resonant areas of the cabinet are too small to affect sound quality.


Fig.2 Monitor Platinum PL300 II, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of top panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

Fig.3 shows the combined farfield outputs of the tweeter and midrange unit (green trace above 400Hz) and that of the woofers (blue above 350Hz) on the tweeter axis, with the nearfield responses of the midrange unit (green below 400Hz), woofers (blue below 350Hz), and ports (red). (The two ports behaved identically, as did the two woofers.) The output of the ports peaks a little lower in frequency than the minimum-motion notch in the woofers' response, which occurs at 34Hz, the frequency of the saddle between the two impedance peaks in the bass in fig.1. The smooth higher-frequency rolloff of the ports is disturbed by a couple of peaks, but these are well down in level.


Fig.3 Monitor Platinum PL300 II, acoustic crossover on tweeter axis at 50", corrected for microphone response, with summed nearfield responses of ports (red), woofers (blue), and midrange unit (green), all plotted in the ratios of their radiating diameters.

The farfield response of the woofers rolls off sharply above the crossover to the midrange unit, which appears to be slightly lower in frequency than the specified 500Hz. The midrange driver rolls in more gently, but its output and that of the tweeter are impressively flat between 1 and 12kHz. However, there are some small peaks and dips in the top octave, which may be due to reflections from the vestigial grille.

The black trace in fig.4 shows the PL 300 II's anechoic response, averaged across a 30° horizontal window on the tweeter axis, with the complex sum of the nearfield responses plotted below 300Hz, taking into account both acoustic phase angle and the different distances of the radiators from a nominal microphone position. The slight rise in output in the upper bass is due to the nearfield measurement technique; the PL300 II's output is superbly flat overall, though the peaks and dips between 12 and 30kHz are again visible.


Fig.4 Monitor Platinum PL300 II, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with complex sum of nearfield responses plotted below 300Hz.

Fig.5 shows the Monitor's horizontal dispersion, normalized to the tweeter-axis response. The contour lines are smooth and even, though there is a slight off-axis flare at 4.5kHz (cursor position), and the speaker's output drops off sharply to the sides above 13kHz. The PL300 II might sound a little lacking in air in very large or dry rooms. In the vertical plane (fig.6), a suckout develops at the upper crossover frequency more than 5° above the tweeter axis. The balance remains unchanged up to 5° below the tweeter axis—which, as that axis is 40" from the floor and a typical seated listener's ears are 36" from the floor, is a good thing.


Fig.5 Monitor Platinum PL300 II, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.


Fig.6 Monitor Platinum PL300 II, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 15–5° above axis, reference response, differences in response 5–10° below axis.

Turning to the time domain, the PL300 II's step response on the tweeter axis (fig.7) reveals that the tweeter and midrange units are wired in inverted acoustic polarity, the woofers in positive polarity. More important, the decay of each unit's step blends smoothly with the start of the next step lower in frequency, suggesting optimal crossover design. However, if you look closely, this graph does suggest that the listening axis is just below the tweeter. The cumulative spectral-decay plot (fig.8) reveals a generally even decay across the audioband, any delayed energy being well down in level.


Fig.7 Monitor Platinum PL300 II, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).


Fig.8 Monitor Platinum PL300 II, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).

I am not surprised that RD was impressed by Monitor Audio's Platinum PL300 II; so was I.—John Atkinson

Monitor Audio Ltd.
North American distributor: Kevro International Inc.
902 McKay Road, Unit 4
Pickering, Ontario L1W 3X8, Canada
(800) 667-6065

brenro's picture

This is the size loudspeaker I'm currently shopping for my listening room. My current front runner is the Revel Performa3 F208. As soon as I find them I'll audition these Monitors but at three times the price are they that much better?

findcount's picture

bro....don't bother.....just grab American speakers like Ryan, Aerial Acoustics, Silverline among a host of many many others.......they'll be far better than Monitor Audio

w1000i's picture

I doubt they can outperform the platinum, and what I think set the platinum apart from other is there midrange/woofers

findcount's picture

oh yeah......if you want those AMT tweeters......just get Elac or Legacy

deano2812's picture

Clearly you have nothing interesting or knowledgeable to say.I think you will find ELAC is a German company not American.

findcount's picture

amazing Monitor Audio hasn't gone out of business..........they haven't a chance to compete with the good brands in USA

Puresound's picture

I never read such nonsense for a long time. Most brands in the USA are inferior. Wanna bet? In drivers and response Europe is a long distance further. Take a look at this video

In other things in audio US companies are the best. But in drivers and tweeters mannn they can lear a lot of the Europian companies. When you use amps and sources which are able to create a 3 dimensional stage, Monitor Audio speaker will take a big distance from their competitors. The silly thing is that most Monitor Audio speakers are demoed with 2 dimensional amps. This is based on the fact that the knowledge and insight in audio has become rather poor

deano2812's picture

Why would you assume MA would go out of business? I think this shows a relative uneducated view of the world. I bet you don't even own a passport. You will find that MA operate in almost 100 countries, far more than most U.S brands. MA also happens to be the strongest sales in the U.S of any foreign speaker brand,except for B&W. I would check your facts before you spout such drivel.

volvic's picture

The Energy Veritas 2.8 which I missed purchasing a few years back. Have heard many Monitor Audio speakers including their higher priced ones and always loved them. Enjoy!

funambulistic's picture

Every time I listen to Monitor Audio, I have been overwhelmed. My favorite (cost vs performance) line is the Silver line (RS6, RX6 - whatever they are calling it now). Each time I audition them, the little voice in my head says "Do it!" but I love my current speakers too much. It would take an in-home audition to steer me away.

Puresound's picture

First of all many Americian loudspeakers look like S.In the US many speakers are not the like the best looking ladies. The Pl-300 is even the one of the Platinum series who is the least good looking. This has to do with the proportions of the loudspeaker. But in material use and in technique they are superior to all in their price range. The new AMT tweeter they use can reveal a level of realism what I never auditioned with any dome diamond and beryllium tweeter. It brings highend to a much higher level. This is based on the fact that it can easily let you hear the differences of height of voices and instruments. But what is most unique is that it is able to let you hear so much more diversity in sound. This is based on the technique and material use. In the last months I had clients with Wilson Audio speakers. We in Europe see them as ugly women. But this is looks and some men like ugly women. So who am I to judge!! Based on all parts you judge sound for the new Platinum outperformed the Wilson Sasha with ease. I know for sure that magazines never will do shootout of these togheter. Because Wilson will go crazy!! The tweeter of the Wilson Audio speakers show still harshness in the high freq. The AMT tweeter shows us that this is all gone. The biggest difference is based on the fact that in stage depth and width the New Platinum outperforms the Wilson in this part. Use Pass Labs amps and the Platinum will show you a wider and deeper stage. This has nothing to do with taste. The stage is bigger, so the level of this part is better, end of discussion. The Platinum goes so much further than this alone. It outperfroms the Wilson in the level it can show you the differences in height of instruments and voices. It is able to let you hear instruments at the height you hear it in real. Wilson even uses paper (are you still living in the 80's?) in their mid-drivers. The low and mid drivers of the new Platinum are superior in speed and response. It has so much more controle but it also reveals more layers. With recordings of a double bass the Sasha showed us difficulties in layering and control. With the new Platinum series the realism in control and layering is of a new level. The energy of a bass drum or the strings of a double bass is fully free from the speaker. The same recordings with the Sasha showed us that the energy still was comming from the (very slow) drivers. Audio is ruled by money. Let me put it this simple: would you like to hear the truth or the thing you would like to hear? Audio need to become much more open and honest. Battle between Platinum and Wilson Audio. Or a battle between the new Platinum series with the 800D3 series. This is how audio needs to be shown and presented. Because it is al about shootout. The thing I love most in audio is that the best and most convincing sound always will win. So let the best win!!

leec's picture

Thanks for acknowledging that speaker looks are a design thing and in no way related to the much maligned WAF BS the generic audiophile community subscribe to. Simply put, speakers like the PL300 MII, KEF Blade, Raidho and a few others dispel the old image of a speaker having to look like a speaker: in a fugly box. From the vast majority of "man-cave" listening room pictures I see posted, most of the owners belong in a cave. Monitor Audio should be applauded for producing reference quality sound in a package that can be incorporated into a real-world living space.