ProAc Response D2R loudspeaker Measurements

Sidebar 3: Measurements

I used DRA Labs' MLSSA system to measure the ProAc Response D2R, using a calibrated DPA 4006 microphone for the farfield behavior and an Earthworks QTC-40 mike for the nearfield responses. Though ProAc specifies the Response D2R's sensitivity as 88.5dB, presumably for 1W at 1m, my estimate was somewhat lower, at 85dB(B)/2.83V/m.

The ProAc's specified impedance is 8 ohms; the solid trace in fig.1 reveals that, other than two regions in the midbass and lower midrange, the impedance remains above 8 ohms for the entire audioband. The minimum value is 6.22 ohms between 170Hz and 180Hz, and while the electrical phase angle (dashed trace) is high at some frequencies, the impedance magnitude is also high at those frequencies. The Response D2R will therefore be a very easy load for the partnering amplifier to drive. Note that the average impedance is higher in the treble than it is in the midrange. As a result, the response will be tilted up a little when the speaker is partnered with a tube amplifier having a high output impedance.

320PD2Rfig1

Fig.1 ProAc Response D2R, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

When I investigated the enclosure's vibrational behavior with a plastic-tape accelerometer, I found two resonant modes, at 250Hz and 281Hz, on the top panel and sidewalls (fig.2). Given that these modes are both high in level and relatively low in frequency, I would expect this behavior to add some congestion in the midrange.

320PD2Rfig2

Fig.2 ProAc Response D2R, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of sidewall (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

The saddle centered on 45Hz in the impedance magnitude plot suggests that this is the tuning frequency of the ProAc's port. This was confirmed by the fact that the nearfield response of the woofer (fig.3, blue trace) has its minimum-motion notch at that frequency. (The back pressure from the port resonance holds the cone stationary at this frequency.) The nearfield response of the port (red trace) broadly peaks between 30Hz and 90Hz, and its upper-frequency rolloff is clean. (I have only plotted the port's output up to 400Hz; above that frequency, the measurement was affected by crosstalk from the woofer, which is just above the port on the front panel.)

320PD2Rfig3

Fig.3 ProAc Response D2R, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield responses of the woofer (blue) and port (red), respectively plotted below 350Hz and 400Hz.

There is a small suckout just above 1kHz in the woofer's output, and it crosses over to the tweeter (fig.3, green trace) at 2.3kHz with what appears to be a third-order, 18dB/octave, low-pass slope. The rolloff is disturbed by some small peaks, however. The tweeter rolls out very rapidly below the crossover frequency, and its top-octave output is 3–5dB too high in level compared with the average level of the woofer. This can also be seen in the Response D2R's farfield response, averaged across a 30° horizontal window centered on the tweeter axis (fig.4, black trace above 300Hz), along with a slight lack of energy between 900Hz and 2kHz. (The latter may well have affected my sensitivity estimate.)

320PD2Rfig4

Fig.4 ProAc Response D2R, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the complex sum of the nearfield woofer and port responses plotted below 300Hz.

The black trace below 300Hz in fig.4 shows the complex sum of the D2R's nearfield woofer and port outputs. The peak in the upper bass will be due to the nearfield measurement technique, which assumes that the radiators are mounted on a baffle that extends to infinity in both horizontal and vertical planes. The D2R's low-frequency alignment is actually maximally flat and down by 6dB at the port's tuning frequency, which is close to that of the lowest open string on the double bass and four-string bass guitar.

The plot of the Response D2R's horizontal dispersion, referenced to the response on the tweeter axis, is shown in fig.5. (Because the tweeter is mounted asymmetrically on the front baffle, I have plotted the differences in response on the tweeter side of the baffle to the front of the graph and the differences on the other side to the rear.) The speaker's output drops off to the sides above 10kHz, which will tend to balance the on-axis peakiness in the top audio octave in medium-sized and large rooms. The lack of energy in the low treble in the tweeter-axis response is maintained to the speaker's sides, which, all things being equal, will make the Response D2R sound a little polite. In the vertical plane (fig.6), as the ribbon tweeter is relatively long—2.75"—its dispersion is limited in the high treble, though there is a little more top-octave output 5° above the tweeter axis. A suckout develops in the crossover region 10° above and below the tweeter axis. The D2Rs should be used on stands that place the listener's ears close to the tweeter axis. (ProAc recommends stands that are at least 18" high.)

320PD2Rfig5

Fig.5 ProAc Response D2R, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90–5° off axis on other side of baffle, reference response, differences in response 5–90° off axis on tweeter side of baffle.

320PD2Rfig6

Fig.6 ProAc Response D2R, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45–5° above axis, reference response, differences in response 5–45° below axis.

In the time domain, the Response D2R's step response on the tweeter axis (fig.7) indicates that the tweeter and woofer are both connected in positive acoustic polarity. The decay of the tweeter's step, which arrives first at the microphone, smoothly blends with the start of the woofer's step, indicating optimal crossover topology. The ProAc's cumulative spectral-decay plot (fig.8) is clean over most of the audioband, though undulations in the decay of the woofer's step response are associated with some delayed energy at the frequency of the suckout in the on-axis output. The decay of the tweeter's output above 10kHz is initially clean, but some low-level hash develops.

320PD2Rfig7

Fig.7 ProAc Response D2R, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

320PD2Rfig8

Fig.8 ProAc Response D2R, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).

The ProAc Response D2R's measured behavior suggests that experimentation with toe-in and taking care over vertical listening axis will result in the most even tonal balance.—John Atkinson

COMPANY INFO
ProAc Limited
US distributor: The Sound Organisation
1009 Oakmead Dr.
Arlington, TX 76011
(972) 234-0182
ARTICLE CONTENTS

COMMENTS
Bogolu Haranath's picture

KM could try the NAD M10 ($2,750, reviewed by Stereophile) with some of these bookshelf/stand-mount type of speakers ...... The built-in Dirac Live could improve the sound even further, for the already good sounding speakers :-) ........

georgehifi's picture

Is it me or does the older look the better in the tests

https://www.stereophile.com/content/proac-response-two-loudspeaker-measu...

Cheers George

Bogolu Haranath's picture

What measurements of Response Two look better to you? ......... Could you be more specific? ........ I'm just curious :-) .........

georgehifi's picture

A couple,
Frequency Fig3, new is tilted up quite a bit at both ends.
Vertical response Fig5.
Cumulative spectral-decay plots Fig8.

Main one to me would be the frequency +5db boost at 100hz and a rising response from 3khz to 20khz, sucked out between 900hz to 4khz, compared to a very flat +-2.5db from 50hz to 20khz on the old one. The new is typical of a bookshelf trying to be a floor stander, with compromises, where the old with that tweeter and with a sub would be far better proposition.

Cheers George

Bogolu Haranath's picture

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Bogolu Haranath's picture

You are right about the measurements of this new model Response D2R ....... In addition, FR from about 100 Hz to 20 kHz looks little bit like the 'Fletcher-Munson curve' ......... The dip you mention from about 900 Hz to 4 kHz is the famous 'BBC dip' in the presence region ....... There is a serrated FR from about 10 kHz to 20 kHz which could be perceived as treble harshness and hardness by some young listeners ........ This is seen as rippling of the FR from about 10 kHz to 20 kHz in Fig.6, also :-) ........

Bogolu Haranath's picture

Regarding the measurements of the old model Response Two, there are some problems there too ....... There are a couple of dips in the FR in the midrange from about 200 Hz to 1 kHz ....... There is also a 'BBC dip' in the presence region from 1 kHz to 3 kHz ........ There is also another dip in the FR from about 3 kHz to 10 kHz and another dip from 10 kHz to 20 kHz ....... Almost all of the treble region has somewhat serrated appearance in the FR in the old model ........ We can see this as rippling in the cumulative spectral decay plot, especially in the mid and upper treble region in Fig.8, in the old model ........
The KEF LS-50 has better measurements than either of these above mentioned ProAc models ......... However both the ProAc models have larger size mid/woofers, and can go lower in the bass frequencies ......

BTW ..... The name ProAc rhymes with the name Prozac :-) ........

Trevor_Bartram's picture

Many mini-monitors have the 3-6dB bass boost, it is intentional, in that if the bass drive unit is placed one quarter wavelength (at the boost frequency) from the room's rear wall (primary reflector), partial cancellation will occur in the region resulting in a smooth in room response. I don't believe the review mention bloated bass, so placement must have been correct.

Ortofan's picture

... a silk dome tweeter, would measure (and sound) in comparison?

Bogolu Haranath's picture

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Bogolu Haranath's picture

Some of these ribbon tweeters used in some of the various speaker models, seems to have similar problems like these ProAc D2R model ........ One good example is the GoldenEar speaker models reviewed by Stereophile :-) ........

JHL's picture

...is a ribbon, not an AMT.

Bogolu Haranath's picture

My comments were about ribbons :-) .........

JHL's picture

...you don't know the technology any more than why anyone would then jabber about it.

Bogolu Haranath's picture

I mentioned about RAAL ribbons ....... See, below :-) ........

JHL's picture

...none of that jabbering is even remotely relevant.

Bogolu Haranath's picture

There are distortions in some of the ribbons, like the ones used in ProAc and GoldenEar ......... RAAL ribbons are better with less distortions :-) ........

JHL's picture

...the GE doesn't have a ribbon, it has a small AMT. As for the RAAL, implementation is everything, which in the case of a small loose diaphragm is *really* everything.

Even so, this is irrelevant jabbering. So too is making unfounded assumptions about domes.

Bogolu Haranath's picture

Why is it irrelevant when we talk about distortion? :-) .......

JHL's picture

...if we're analyzing distortion and its audibility or if we're jabbering about technology without knowing what it really is and what it does.

Presumably this review goes to the relative success of this product at reconnecting a listener to an original musical event, not whether another product entirely has a "ribbon" in it when it does not. Same for domes when we make completely unfounded and irrelevant remarks about them. Nobody cares, nor should they.

Bogolu Haranath's picture

GoldenEar (GE) specs say 'High velocity folded ribbon (HVFR) tweeters' :-) .........

JHL's picture

...can also call it a ham sandwich. Nobody stopping us.

*Flat* drivers exist in at least four different technologies, ribbons being one and AMT's being another. They work differently, they're used differently, they measure differently, and they sound different.

Jabbering about them as a lump sum is irrelevant to the point of a review like this by a number of orders.

ChrisS's picture

...think "Sheldon".

Bogolu Haranath's picture

More specifically 'Sheldon Comics' may be? :-) .......

ChrisS's picture

....more like Sheldon in TBBT.

Endearingly.

Bogolu Haranath's picture

'Dunning-Kruger effect', (illusory superiority) may be? (see, Wikipedia) :-) ........

ChrisS's picture

...that either.

Bogolu Haranath's picture

BG/BGR 'Boy Genius :-) ......

ChrisS's picture

Other people need to understand what "Sheldon" is about.

Bogolu Haranath's picture

'Wunderkind' ....... May be like Mozart who started composing at the age 5 :-) ........

ChrisS's picture

...of Sheldon's.

Bogolu Haranath's picture

So, JHL is one of them? :-) .......

ChrisS's picture

Follow my posts.

Bogolu Haranath's picture

I think JHL is more like 'Patrick Star' :-) ........

ChrisS's picture

...fun!

That's all we need to know.

Bogolu Haranath's picture

Bart Simpson could also fit the description :-) ...........

Bogolu Haranath's picture

May be a little bit of Bugs Bunny too :-) .......

Bogolu Haranath's picture

Distortions about domes is also relevant topic for discussion ......... Unfortunately, your comments are incoherent :-) ........

JHL's picture

It just ... need they, moar cow bell ;o) ........

Bogolu Haranath's picture

Q: Why do cows wear bells? .....
A: Because their horns don't work :-) .........

Bogolu Haranath's picture

Wilson Audio uses silk dome tweeters in their top models Alexia2 and Alexx speakers ....... Those tweeters have some rippling problems too, in the mid and upper treble regions ....... See the Stereophile measurements :-) .......

Bogolu Haranath's picture

Wilson Audio all models reviewed by Stereophile use silk dome tweeters ....... All those tweeters have problems, especially in the mid and upper treble region, seen in the measurements :-) ......

Bogolu Haranath's picture

Revel Performa F208 ($5,000/pair) which has Aluminum dome tweeters has better measurements than some of the above mentioned tweeters :-) ........

Bogolu Haranath's picture

Also, KEF LS-50, which has Aluminum dome tweeters has better measurements :-) .......

Bogolu Haranath's picture

Alta Audio speakers (reviewed by Stereophile) use RAAL ribbon tweeters ........ The tweeter itself shows fairly clean measurements :-) .......

Another speaker reviewed by Stereophile, ADAM Audio speaker ribbon tweeters, also measure well :-) ........

Bogolu Haranath's picture

Recently reviewed Sonus Faber Olympica 1 uses 'damped-apex silk-dome tweeters' ........ Those tweeter measurements look very good, including clean spectral decay plot ......... May be Wilson could switch and use that type of tweeters :-) ........

JHL's picture

JA, instead of 'acoustical polarity', would it be more precise to refer to driver wiring as inverted or non-inverted 'DC polarity'? The acoustic or AC signal in the DUT show the drivers to be in-phase, however depending on design their DC wiring can take a number of forms...

Bogolu Haranath's picture

Like 'Optimus Prime', perhaps? :-) .......

JHL's picture

...can't stop yourself, can you.

Bogolu Haranath's picture

I got that 'can't stop virus' from you ...... It is 'contagious' :-) ........

ChrisS's picture

...engaging with "Sheldon".

He really can't help himself.

Bogolu Haranath's picture

I just don't want JHL to build a nuclear power plant with the DC wiring wrongly connected :-) ........

ChrisS's picture

...some people don't appreciate your humor or your comments and get annoyed at you.

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