Polk Legend L100 loudspeaker Measurements

Sidebar 3: Measurements

I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Polk Legend L100's frequency response in the farfield, and an Earthworks QTC-40 mike for the nearfield responses.

Polk specifies the L100's sensitivity as 85.5dB/2.83V/m; my estimate was actually a little higher, at 87dB(B)/2.83V/m. The L100's impedance is specified in the manual as 4 ohms, with a minimum magnitude of 3 ohms, though on Polk's website it is specified as 3–4 ohms. The solid trace in fig.1 shows that the impedance magnitude remains above 4 ohms from the bass through to the low treble but does drop to just below 3 ohms between 4kHz and 7kHz. The electrical phase angle (dashed trace) is occasionally high, and I used the formula in a 1994 JAES paper by Eric Benjamin to calculate the "equivalent peak dissipation resistance" (EPDR, footnote 1). The L100 has minimum EPDRs of 1.73 ohms between 127Hz and 140Hz and 1.25 ohms between 3.3kHz and 3.6kHz. This loudspeaker will work best with amplifiers that are comfortable driving loads below 4 ohms. This is probably why KM found that the Polk speakers proved a better match with some amplifiers than others.

920Polkfig1

Fig.1 Polk Legend L100, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

There are small discontinuities in the traces in fig.1 that imply resonances of some kind. When I investigated the enclosure's vibrational behavior with a plastic-tape accelerometer, I found strong resonant modes at 410Hz on the side panels (fig.2) and at 363Hz on the top panel. A strong mode was also present at 685Hz on the "Enhanced Power Port" diffuser. All the modes were of relatively high Q (Quality Factor) and so might not be as audible as their amplitudes would imply.

920Polkfig2

Fig.2 Polk Legend L100, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of side panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

The saddle centered at 58Hz in the impedance magnitude trace suggests that this is the tuning frequency of the port on the rear panel. The minimum-motion notch in the woofer's output (fig.3, blue trace) lies at that frequency, and the port's output (fig.3, red trace) peaks in textbook fashion between 40Hz and 100Hz. There is the usual boost in the upper bass in both the woofer and port outputs, which will be due to the nearfield measurement technique; the actual bass alignment is maximally flat. The port's upper-frequency rolloff is relatively clean but is disturbed by a fairly broad peak centered just below 700Hz. This frequency is suspiciously close to that of sharp discontinuities in both the impedance magnitude and phase-angle traces. This midrange peak in the port's output is probably due to an internal air-space resonance rather than to the enclosure's vibrational behavior. However, I note that KM wasn't bothered by any coloration that would have resulted from the midrange peak in the port's output. Perhaps this is due to the fact that the port both faces away from the listener and is obscured by the cone-shaped diffusor.

920Polkfig3

Fig.3 Polk Legend L100, anechoic response on tweeter axis without grille at 50", averaged across 30° horizontal window and corrected for microphone response, with the nearfield responses of the woofer (blue) and port (red), respectively plotted below 300Hz and 1kHz.

The Polk's farfield response, taken without the grille and averaged across a 30° horizontal window centered on the tweeter axis, is shown as the black trace above 300Hz in fig.3. The balance is superbly flat from the midrange through the mid-treble but with a slight rising trend visible above 6kHz. This behavior might correlate with KM finding the upper-midrange and treble to be somewhat forward-sounding, Repeating the measurement with the grille reduced the on-axis output in the top two audio octaves by 1–2dB.

Fig.4 shows the Polk's horizontal radiation pattern. (The off-axis responses are normalized to the response without the grille on the tweeter axis, which thus appears as a straight line.) The contour lines in this graph are evenly spaced between 1kHz and 5kHz, which suggests stable stereo imaging. I suspect that the apparent peak around 700Hz far to the speaker's sides is due to the microphone picking up more of the port's output. As I have found with other ring-radiator tweeters, the L100's dispersion dramatically narrows in the top octaves, which will work against the audibility of the excess of on-axis energy in the same region. This behavior also suggests that the Polk's treble balance can be fine-tuned by experimenting with toe-in.

920Polkfig4

Fig.4 Polk Legend L100, 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.

In the vertical plane (fig.5), again with the off-axis response normalized to the tweeter-axis response, a suckout centered on the specified crossover frequency of 2.9kHz develops immediately above the tweeter axis. This suckout doesn't appear until 15° below the tweeter axis, which means that the L100s should be used with stands that are sufficiently high so that the listener can't see the tops of the cabinets.

920Polkfig5

Fig.5 Polk Legend L100, 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 L100's step response on the tweeter axis (fig.6) indicates that both drive-units are connected in positive polarity. The tweeter's step arrives first at the microphone, and the decay of its step doesn't quite smoothly blend with the positive-going start of the woofer's step. This means that the optimal integration of their outputs occurs just below the tweeter axis. Again, a high stand needs to be used with this speaker. Other than small amounts of delayed energy in the midrange, the Polk Legend L100's cumulative spectral-decay plot (fig.7) is superbly clean overall. (As always with my CSD plots, ignore the small ridge just below 17kHz, which is due to interference from the computer monitor's line-scan frequency. Note also that this graph has not been compensated for the measuring microphone's departure from a flat response.)

920Polkfig6

Fig.6 Polk Legend L100, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

920Polkfig7

Fig.7 Polk Legend L100, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).

Overall, the Polk Legend L100 offers excellent measured performance, indicating equally excellent audio engineering.—John Atkinson


Footnote 1: EPDR is the resistive load that gives rise to the same peak dissipation in an amplifier's output devices as the loudspeaker. See "Audio Power Amplifiers for Loudspeaker Loads," JAES, Vol.42 No.9, September 1994, and stereophile.com/reference/707heavy/index.html.
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COMMENTS
thatguy's picture

Great review and once again my bank account is fortunate that my affinity for SE Tube amps limits my speaker choices.

Anton's picture

Of course, now I need to know more about that L200 and that new KEF LS50!

What a great time for standmounts at humanly kind prices!

Thanks for a killer review that is foundational in its category.

James.Seeds's picture

You lost me at "Made in China"

Ortofan's picture

... minimum amount one must pay, today, for a new pair of speakers that are not manufactured in China?

The Dynaudio Emit M10 is made in Denmark and is priced at $799/pr.

Is there anything else that is less expensive?

tonykaz's picture

a person could buy a used pair of Magnapan MG3s w/ribbon tweeter and existing support from the manufacturer.

Just purchased a pair in Miami for under $799.

Are we suggesting that transducer performance has significantly improved over these last 3 or 4 decades? I can't see how it has, in any meaningful way!

What has changed is the cost/price structures and designs featuring lower cost materials. Shipping Costs have skyrocketed mandating designs that optimise shipping & warehouse efficiencies.

China offering labor at 1/10th to 1/20th the Cost of Domestic Labor allows Manufacturers a Quarterly Profit attractive to Venture Capitol.

This Manufacturer's decision makers are selling/diluting their Brand Name for short term gain. ( expect long term pain and low residual dollar values )

Tony in Venice

AndreasB's picture

Focal Chorus 706 (and matching S700 stands).
Cheap and quite great. Made in France! (assembled in France and shipped from France using European MDF for enclosure)
I have them and I can't think of anything comparable for the price. They are of higher build quality than my JBL L890s made in Mexico.
I know that the bottom driver is made in China though but is a Focal exclusive too, not off the shelf. I believe the tweeter is even manufactured in France.

Ortofan's picture

... discontinued models, then we can do even better on price.
The Focal Chorus 605 is still available in new condition for $300 or $349/pr.

https://www.safeandsoundhq.com/collections/focal/products/focal-chorus-605-two-way-bookshelf-speakers-pair

https://upscaleaudio.com/products/focal-chorus-605-bookshelf-loudspeakers-pair

https://www.focal.com/us/high-fidelity-speakers/chorus-600/chorus-605

jmsent's picture

..assembled in Denmark? Dynaudio is owned by Goertek, a large Chinese company, and does a lot of its driver and cabinet manufacturing in China. Given the cost structures of manufacturing in Scandinavia, I'd bet that the majority (if not all) of the parts for that speaker are sourced out of China, and the rather simple final assembly work is done on a small line in Denmark. The loudspeaker company I worked with in Scandinavia also did this kind of OEM work for a number of well known brands.

Ortofan's picture

... "made in XYZ" label really means "assembled in XYZ from foreign and domestic components".

For example, the British-made Harbeth speakers include tweeters sourced from a company based in Scandinavia and crossover capacitors (other than those for the limited-edition Anniversary models) sourced from a company in Hong Kong.
So, do their speakers deserve to be called "handmade in England", as it states on their website?

In any event, the Dynaudio speakers are labeled "made in Denmark" and the Focal speakers are labeled made in France" - for whatever that may be worth.

jmsent's picture

Emit
Designed, engineered and assembled in Denmark

Found in both the user guide and the literature.
Where does it say "made"? I always thought that this required that a majority percentage of the individual parts be made in that country. E.g., Made in USA requires at least 50% to my understanding. I'd bet it's at least that high for DK, if not higher.

Ortofan's picture

... the Dynaudio Emit M10 is "Made in Denmark"?

Did it ever occur to you to look on the speaker?
Obviously, not.

https://images-na.ssl-images-amazon.com/images/I/71H2o4tFwFL._AC_SL1500_.jpg

https://www.avprofessionals.in/wp-content/uploads/2020/04/emit_m10_satin_black_back.png

tonykaz's picture

it's Legalese sort of language.

Legal advisors are nudging EVERYONE into the Non-descriptive phrasing of doubt. All except China where there is no doubt that Chinese stuff is made in China, they say it on everything they make. Everyone else hides the china sourcing. ( for dam good reasons: Made in USA equates to 35% better Sales )

The Made in China label signifies the Race to the Bottom.

There are quite a few super high quality Chinese Brands being sold here and globally but they don't have the Glamor Branding of Traditionals like Polk and they don't get reviewed by our better reviewers, Mr. HR the exception.

If Stereophile had a specialist reviewer only reviewing/ reporting on Asian Brands it would be an exciting thing. China has some outstanding gear at Schiit level price points .

Tony in Venice

Electrophone's picture

I asked my local Hifi Specialist to check the labels on the back of his Demo Emit 20s. It says „Made in China“. He also told me that nowadays all cabinets for Dynaudio loudspeakers are manufactured in China, even for the fancier Evoke and Contour loudspeakers.

tonykaz's picture

It's probably great for Wall Street but it's Hell on our Main Street.

It feels like an old friend giving us the middle finger, after all these years.

Tony in Venice

dougspeterson's picture

As JA noted ring radiators despite their extended response become beamy.
Therefore I am still with the KEF LS50 and its wide, uniform and Extended response as the
ultimate mini monitor.

Is Stereophile going to look at the new Meta version?

Long-time listener's picture

"As JA noted ring radiators despite their extended response become beamy."

I suspect that means they might not be as good for near-field listening, but am not absolutely certain.

These, and the L200s, are getting good reviews. But it seems I'm seeing more reviews for the 100s. Are they considered better?

What happens if you take of the back plate and just let the port fire straight out?

Just a couple of questions.

Charles E Flynn's picture

@dougspeterson:

Yes. Please see John Atkinson's comments at https://www.stereophile.com/content/recommended-components-fall-2020-edition-loudspeakers on September 26, 2020 - 8:29am and on September 28, 2020 - 11:00am.

Long-time listener's picture

If Polk has really managed to improve their drivers over their previous series, why couldn't they just offer an improved lsim703, which everyone seems to love? Why do speaker companies always throw away their good old designs? For example, why did Revel go from making the nearly-perfectly-measuring M20 to speakers with smaller cabinets and much less perfect measurements? I don't understand this idea of "progress."

Archguy's picture

The LSiM series is justly lauded, but nothing sells like a whole new model with new reviews in the audio press. Time will tell if 'this year's model' is actually an improvement or just another lateral move with short-term profits in mind.

prerich45's picture

It's the same reason Def Tech redesigned their speakers as well, the LSiM had Sandy's name is all over them (LSiM Series). Those and Def Tech Mythos, seem like precursors to the Golden Ear brand. Polk and Def Tech moved away from them. Speaker brands change - sometimes for the better, others for the worse. Some have died because of nostalgia - it didn't look and adhere to the principles of the old stuff even though they measured well (Thiel anyone, happened to Infinity too). That's what happens.

remlab's picture

for someone who used a laser vibrometer in the design process to specifically eliminate those resonances(And bragged about it). I would love to hear the designer's take on John's measurements regarding this issue.

beave's picture

I'm more troubled by the port resonance than the cabinet resonance. I'm guessing it's more likely to be audible.

remlab's picture

..it's technological resources, the port resonance and cabinet vibrations are pretty interesting observations for a relatively expensive design. Audible or not, it implies some sloppiness in the engineering process. If a company has already invested in the technological resources, it's not the least bit costly to do it right.

Trevor_Bartram's picture

Not to pick on Polk in particular (as most first world companies seem to be guilty of this) but it really troubles me (having worked in the UK & US defence industry) that high value intellectual property* is being given to a potential foe. This didn't happen during the Cold War, why is happening now** (was it Nixon in China?), perhaps someone can educate me?

*You may say it's just a loudspeaker but to get to this level requires the same skills that can be applied to other less peaceful devices.

**I fully realize this has been steadily going on for forty years but I've never seen an explanation as to why.

dial's picture

Wall street do that to save one $ a piece. In Europe they cut trees and send them to China so they get panels in return. And the same like IKEA talk of ecology or climate getting warmer (?!)

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