Klipsch Forte III loudspeaker Measurements

Sidebar 3: Measurements

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

The Klipsch's specified sensitivity is an extraordinary 99dB/2.83V/m; my estimate was lower, at 95.2dB(B)/2.83V/m, but this is still 8dB higher than the average sensitivity of the speakers I have measured over the past 30 years. This speaker will play loudly with a mere handful of watts. Klipsch specifies the Forte III's nominal impedance as "8 ohms compatible." I found that the speaker's impedance magnitude (fig.1, solid trace) dropped below 6 ohms only in the upper bass and in the valley between the twin low-frequency peaks that define the reflex woofer loading. The minimum value is 3.65 ohms at 125Hz. However, the electrical phase angle (dotted trace) is sometimes extreme, and there is a current-hungry combination of 5 ohms and –49° phase angle at 90Hz. Despite its high sensitivity, the Forte III will work best with amplifiers that are comfortable driving 4-ohm loads. And the very large difference between the average impedance in the lower midrange and that in the treble means that the speaker might sound a touch bright with tube amplifiers having high output impedances.


Fig.1 Klipsch Forte III, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

There are small discontinuities in the impedance traces, particularly around 125Hz and 400Hz, that would imply the presence of resonances of various kinds. When I investigated the enclosure's vibrational behavior with a plastic-tape accelerometer, I found a low-level resonant mode at 120Hz and a stronger one at 398Hz on the side panels (fig.2), and another at 434Hz on the top panel. Given the speaker's high sensitivity, these modes might not result in audible midrange congestion, however.


Fig.2 Klipsch Forte III, cumulative spectral-decay plot calculated from output of accelerometer fastened to sidewall level with midrange unit (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

The Forte III's impedance-magnitude plot has a saddle in the bass centered on 43Hz, close to the frequency of the lowest note on the four-string electric bass and double bass, suggesting that this is the tuning frequency of the drone. The blue trace in fig.3 indicates that the output of the woofer, measured in the nearfield, has the expected minimum-motion notch at 43Hz, while the output of the passive radiator (red trace) peaks between 30 and 60Hz. Some midrange peaks are visible in the passive radiator's output. However, because this radiator is mounted on the rear panel, these modes shouldn't color the Forte III's midrange.


Fig.3 Klipsch Forte III, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield woofer (blue) and port (red) responses, both plotted below 350Hz.

The crossover between the woofer's farfield output on the tweeter axis (fig.3, blue trace above 350Hz) and that of the midrange drive-unit (green trace) occurs slightly higher than the specified 650Hz. The filter slopes are asymmetrical, the output of the horn-loaded midrange unit rolling off very rapidly. The farfield response of the midrange unit and the horn-loaded tweeter, taken without the grille, has a flat trend but with some small peaks and dips, a suckout centered on the 5.2kHz upper crossover frequency, and a slight excess of energy in the top audio octave.

The black trace below 300Hz in fig.4 is the complex sum of the nearfield woofer and passive radiator responses taking into account acoustic phase and the different distances of the two diaphragms from a nominal farfield microphone distance. The rise in the mid- and upper bass is primarily due to the nearfield measurement technique, which assumes that the drive-units are mounted in a true infinite baffle—ie, one that extends to infinity in both the horizontal and vertical planes. But this rise does suggest that the Forte III's maximally flat low-frequency alignment will give a lot of bass in all but large rooms. Note that, despite its size, the Klipsch doesn't offer a lot of bass extension, it being tuned for sensitivity rather than extended lows.


Fig.4 Klipsch Forte III, 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.

Higher in frequency in fig.4, the black trace shows the Klipsch's farfield response, averaged across a 30° horizontal window centered on the tweeter axis. Although the overall balance is even, several small suckouts and peaks are still evident between 1 and 20kHz. The plot of the Forte III's horizontal dispersion (fig.5) suggests that the suckout in the crossover region gets deeper to the speaker's sides. However, the speaker's radiation pattern is otherwise well-controlled and even up to 12kHz, above which the tweeter becomes fairly directional. In all but small rooms, this will tend to balance the slight excess of top-octave energy seen in figs. 3 and 4. In the vertical plane (fig.6), the on-axis crossover-region suckout appears to fill in 5° below the tweeter axis but deepens above it. As the tweeter is just 32" from the floor and the average (seated) listener's ears are 36" high, the Forte III might benefit from a slight amount of tiltback. However, as the suckout is very narrow, it might not be too much of an issue when it comes to treble sound quality.


Fig.5 Klipsch Forte III, 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 Klipsch Forte III, 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 Forte III's step response (fig.7) is complicated. All three drive-units appear to be connected in inverted acoustic polarity, with the tweeter's output—the sharp down/up spike at 3.8ms—arriving first at the microphone. The output of the midrange unit is the lazier downward spike just before 4.5ms followed by the slow rise of the woofer's output. The decay of the midrange unit's step smoothly blends with that of the woofer, suggesting optimal crossover design. The difference in arrival times of the tweeter's output and that of the midrange unit can also be seen in the Klipsch's cumulative spectral-decay plot (fig.8). This is fairly clean in the region covered by the tweeter, but some delayed energy can be seen at lower frequencies.


Fig.7 Klipsch Forte III, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).


Fig.8 Klipsch Forte III, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).

Multiway loudspeakers with horn-loaded upper-frequency drivers but a flat baffle can't be made time-coincident without the use of digital signal processing. Does this matter? Perhaps a more conventional speaker with time-aligned drive-units would give better-defined stereo imaging, but I keep coming back to the Forte III's astonishingly high sensitivity, one of the highest I have encountered. At typical listening levels, the drive-unit diaphragms will hardly be moving, which implies low distortion.—John Atkinson

Klipsch Audio Technologies
3502 Woodview Trace, Suite 200
Indianapolis, IN 46268
(317) 860-8100

Bogolu Haranath's picture

May be KM could also review the JBL L-100 Classic speakers ......... L-100 Classics are also about the same price, $4,000/pair :-) .........

Indydan's picture

According to one review.


Bogolu Haranath's picture

I read that Hi-Fi News review ......... We don't know what amp that reviewer used ........ Hi-Fi News measurements of JBL don't look bad ........ Hi-Fi News gave that JBL 'editor's choice' award, anyway ....... It would be nice to get a 'second opinion' from Stereophile :-) .........

Jason P Jackson's picture

On one hand, I'm surprised JBL are satisfied leaving a $4k design measuring that way. On the other hand, I've worked with a manufacturer who insisted on limiting the performance of a less-expensive design, as to avoid the design being "intrusive" toward the upmarket models. Why else would JBL be satisfied with a 5db uptick at 2k? Ouch.

Bogolu Haranath's picture

Those measurements were taken with the MF and the HF controls set at 0 db ......... KH mentions that in the measurements section ......... L-100 comes with HF and MF adjustable controls ....... Listeners can adjust those to suit their preference :-) ...........

dalethorn's picture

It's not like the wizard of oz. You don't get points for linking to something you don't understand.

Jason P Jackson's picture

The comment section of an online magazine is hardly the place for "points". Hahaha.

Ortofan's picture

... a full-range horn-loaded design.
As mentioned in the review, a "steep-filter passive network crosses over at 650Hz and 5.2kHz" to the horn-loaded mid-range and tweeter.
So, the main radiator of frequencies up to 650Hz - about one and one-half octaves above middle C - is the 12" paper cone woofer.
In that event, how much of the apparent sound quality from the Forte III is a function of the horn-loaded mid-range and tweeter versus the relatively conventional passive-radiator-loaded paper cone woofer?

Bogolu Haranath's picture

May be 'best of both worlds' :-) ........

Bogolu Haranath's picture

JBL also has several different models, which use similar type of cone woofers and horn midranges and horn tweeters ....... JBL also uses that 'hybrid' type of configuration in several of their pro models :-) ......

Ortofan's picture

... rave reviews of such "hybrid" speakers because they are horn-loaded, when that applies to just the tweeter (and midrange) driver(s).

Bogolu Haranath's picture

Ortofan, La Scala is for you ......... Fully horn loaded .........Talk to Anton ....... See below :-) ........

Ortofan's picture

... the following speaker is the one for me:

Bacek's picture

LaScala bass horn works only till about 100Hz. Below that works like "direct" radiator.

David Harper's picture

kind of amazing that in 2019 an old fashioned conventional horn loaded dynamic speakers-in-a-wooden-box can be described by a reviewer in such wonderfully imaginative terms of sound quality. And only four grand! For 700 bucks a pair of the new maggie LRS speakers would,I suspect,reveal the primitive nature of the aformentioned Klipsch horns.

Bogolu Haranath's picture

We're talkin'about LOUD speakers heer :-) .........

Anton's picture

It's nice to see Klipsch regarded as "Hi Fi" again.

In the millennium prior to this one, Klipsch was anathema among the Hi Fi hoi polloi.

I don't have precise recollection of when Klipsch was welcomed back into the fold, perhaps when Sam Tellig heard a pair during his travels and mentioned it in a column?

I root for Klipsch. Horn loading and high efficiency strike me as the beating heart of getting at 'the absolute sound.'

Thanks for a fun review!

I impulse bought a pair of new Heresy's last year to use in a small loft and love them (observations in the press about occasional HF intensity are correct, by the way,) and my son uses a pair of La Scala speakers with a PS Audio Sprout to FABULOUS EFFECT.

It's nice seeing Klipsch back in the pantheon.

ken mac's picture

Glad you enjoyed the review. I love the Forte IIIs.

Anton's picture

I wanna go hear the new Klipshcorns and La Scala AL 5 speakers, as well!

I also lurk at the Klipsch forums, they have a very dynamic tweak/upgrade community!

About 5 years ago, our club played with a pair of Klipsch corner horns and we attacked two large plywood "corners/wings" to the back of the speakers and then played then in free space, it worked very well!

I am on the Klipcsh website's mailing list...if they ever discount the heritage line like they do the regular line, I am taking a deep dive!


David Harper's picture

and best of all they're really really LOUD!!!! So loud that you may not ever want to listen to anything again.

Bogolu Haranath's picture

May cause 'hyperacusis'? :-) .........

Robert Francis's picture

Nicely written article Ken, I remember hearing a set of Klipschorns at Kenny's Castaway's on Bleecker St., back in the late 80's. From the first note to the last, wow, those babies really sang out and opened my ears to a new level of reproductive high fidelity sound that I didn't think was possible at the time. Of course the living musical waves that resonated out from the large plywood cabinets could only be amplified by a fine vintage, sweet-sounding, and artfully crafted McIntosh tube amp that was well cabled.

The Klipsch/ McIntosh combo creates a sublime depiction of a musical landscape that still lives on within my inner ear. I'm grateful for holding dear to those beautifully sounding sense memories that I recall when an occasion arises. Plain and simple, music, I mean high quality sounding music makes life truly enjoyable in this paradise known as earth. Thanks Ken. You truly are a gifted writer, and you really helped me out all those years ago. Kudos, to you my friend.

Ken? Hmm... I think I subleased your place back in 1990 on MacDouglal Street, I remember a roommate who played a great jazz trumpet as the aroma of Mamoun's spicy fried falafel balls wafted through your place enticing me to indulge... Though, perhaps you're a not the Ken I once knew. Anyway, keep up the great work. I too love the fine art of audiophile grade music as much as you and your esteemed colleagues at the venerable Stereophile magazine enjoy. Cheers.

ken mac's picture

That was surely me, and my apartment, then, thankfully I no longer reside above Mamouns. And the trumpeter was Ken Watters, a fine musician. I remember your name but not your face. Sorry. Thanks for the kind words. I never knew Kenny's had that setup!

Robert Francis's picture

Yeah, it took me awhile to figure out where that amazing sound was coming from at Kenny's. As one walked through the set of double doors, the bar was on the left, and on the right was the jukebox that connected to the McIntosh amp that was tucked discretely on a lower shelf in the sound booth area.

The speakers were up on a platform toward the ceiling at the end of the bar... What a bunch of misfit characters at that place, it reminded me of an old pirate ship. I worked the day bar, and served hot dogs and a beer for a buck, and at night I managed the floor at the Bitter End, a couple blocks to the east. That was a really fun time, we were saving the Bitter End from a rogue outfit of real estate scavengers from Long Island who had plans to suck the life's blood out of Bleecker by gentrifying the spirit out of the place. Needless to say we were set to make a final stand and so Kenny Gorka, Paul Colby and the crew called in reinforcements in a successful attempt to make the Bitters into NYC's only official landmark status dive bar. People like Bonnie Raitt, David Crosby, Judy Collins, Joan Biaz, Kris Kristopherson, Bill Cosby, George Carlin, etc... all showed up night after night to attract the needed PR and I was lucky enough to have been there at that time. What a blast of great memories from the past.

Ortofan's picture

... work/sound with the First Watt SIT-3?

ken mac's picture

would know.

Ortofan's picture

... doesn't like - save for some speakers he once owned several decades ago.

Bogolu Haranath's picture

Is he the opposite of Mikey? :-) .......

ken mac's picture

...products he doesn't like. If he gets in something for review that truly stinks, he returns it. He doesn't want to waste his or the reader's time. I believe that is a correct characterization....

Bogolu Haranath's picture

Wonder how they sound like with the new darTZeel NHB-468? ........ See AnalogPlanet :-) ..........

Bogolu Haranath's picture

SIT-3 may work even better with 101 db sensitive Klipsch La Scala AL5 ........ See Hi-Fi News measurements :-) ..........

Bogolu Haranath's picture

AD could review the La Scala AL5 ....... Should work well with his favorite SET tube amps :-) ........

Timbo in Oz's picture

No, not true.

cyclebrain's picture

"which implies low distortion."
Why don't you do speaker distortion measurements?
Would be interesting to see speaker efficiency vs distortion.
Maybe don't want to show orders of magnitude of distortion between speakers and electronics.

Bogolu Haranath's picture

Hi-Fi News does speaker distortion measurements in addition to the other speaker measurements :-) .......

smileday's picture

I guess it is difficult to measure in low frequency without an expensive anechoic chamber.

Bogolu Haranath's picture

You can read JA1's previously published articles in Stereophile about how he does loudspeaker measurements ....... You can also watch JA1's previous video presentation of how he does loudspeaker measurements at the RMAF :-) ..........

smileday's picture

We need the exact level of the fundamental to tell harmonic distortion. In the bottom end frequencies, the level of the fundamental is inaccurate. It is guessed.

johnny p.'s picture

I just wonder if Ken ruffled any feathers with those who own the O/93. Several on the 'phile staff do and Ken reports that the Forte is more transparent than the Devore. I believe it, but this would be amazing, at the Forte's price.

ken mac's picture

...the generous warmth and easy soul of the O/93s. Those are major qualifiers.

smileday's picture

I guess Klipsch had Stereophile review Forte III instead of Cornwall III, because Forte III has smoother 'horizontal' off axis response.

However, there are online reviewers saying that Forte III is very sensitive to positioning: toe angle, tweeter height, etc.

I guess the discrepancy between JA's off-axis plots and human experience is due to the incompleteness of two sets of off axis measurements: horizontal and vertical.

We naively guess all off axis responses from the two sets: horizontal and vertical. It may not work that simple in the real world.

If a speaker has an ideal point source as a tweeter and an ideal point source as a mid-range driver, the two sets, horizontal and vertical, can give us a good guess about all off axis responses. Reality, however, is more complicated.