Snell Type B loudspeaker Manufacturer's Comment

Kevin Voecks Responds

Editor: Thank you for the opportunity to comment on Mr. Harley's review. I am pleased that he noted the Type B's ability to play at high volume levels "without any sense of strain or congestion," and that "It was especially rewarding to hear full-scale orchestral climaxes reproduced with effortlessness and ease." Mr. Harley also noted that "The B's tonal balance was quite smooth through most of the band. The treble was well balanced to the rest of the spectrum..." I would have to agree, with his system and setup as described, that the Bs had excessive bass and a slight midrange anomaly. I specified to Mr. Harley that the Type Bs, as is the case with all Snell Acoustics loudspeakers, were designed to be used with low output impedance amplifiers.

I also specified that the loop resistance of the cable should be no more than 0.1 ohm. A dynamic woofer and passive filter (crossover) must be terminated with a known impedance. If they are terminated with a significantly higher impedance (the combination of the amplifier's output impedance, and that of the connecting cable) than anticipated, the frequency response at low frequencies will rise dramatically, the speaker will "ring" at low frequencies, and the filter network will not operate at its intended frequency and slope.

Fig.1 Snell Type B, error in front woofer response due to 2.5 ohm amplifier output impedance

Fig.2 Snell Type B, error in rear woofer response due to 2.5 ohm amplifier output impedance

Fig.1 shows the error (difference) of the actual measured amplitude response of a Type B front woofer when used with a typical solid-state amplifier as compared with an amplifier with the same 2.5 ohm output impedance as Mr. Harley's reference amplifier, as specified by the manufacturer. Note that a 3dB response rise, centered around 30Hz, is introduced. I would certainly agree that this is too much bass!

Fig.2 shows the measured amplitude response error of the rear woofer. Utilizing an amplifier with a 2.5 ohm output impedance produces a greater than 2dB rise.

Fig.3 Snell Type B, cumulative spectral-decay plot of front woofer using a typical solid-sate amplifier

Fig.4 Snell Type B, cumulative spectral-decay plot of front woofer using an amplifier with 2.5 ohm output impedance

Fig.3 shows the cumulative spectral decay of the front woofer with a solid-state amplifier. The cursor indicates that the signal stops at about 20ms at 43Hz. Fig.4 shows the cumulative spectral decay, with the same scale as fig.3, of the front woofer when driven by an amplifier with the output impedance of Mr. Harley's amplifier. Note that the speaker's output at 43Hz continues to about 53ms, more than twice as long as the output continues when used with an amplifier with a low output impedance.

Fig.5 Snell Type B, error in on-axis response due to 2.5 ohm amplifier output impedance

Fig.5 shows the response difference at midrange and high frequencies between a typical solid-state amplifier and an amplifier with the output impedance of Mr. Harley's reference amplifier. With a 2dB error, it is not surprising that Mr. Harley heard a midrange anomaly!

One could design a loudspeaker that is optimized for a high output impedance, but then it would not perform properly with a low output impedance source. Since the vast majority of modern amplifiers have very low output impedances, it makes sense to design speakers for use with that type of amplifier. (For an excellent explanation of the effects of high output impedance or high-resistance speaker wire, please refer to "Why Amplifiers Don't Always Sound Right: Output Impedance & All That," by Robert E. Greene in The Absolute Sound, Issue 71, May/June 1991.)

Mr. Harley states that his room has optimum dimensional ratios for room-mode distribution. I have confirmed that using Snell Acoustics' CARA room-analysis program. Unfortunately, that is not enough. The placement of the speakers and listener(s) within this room is critical with any loudspeaker. Looking at the recommended loudspeaker and listener placement from Snell Acoustics' LEO program, and Sitting Duck Software's The Listening Room, it is clear that the loudspeaker placement used by Mr. Harley would emphasize low frequencies, and this would occur in the same frequency range most emphasized because of the incompatible amplifier output impedance.

Additionally, if Mr. Harley has not changed his listening position since he described it in Vol.13 No.12 (December 1990), it is also incompatible with the goal of hearing the smoothest possible low-frequency response. This might be desirable if small, limited low-frequency capability loudspeakers were in use. While this kind of placement of speakers and listening position increases the quantity of bass, it certainly decreases the quality by making the low-frequency amplitude response much more irregular. When the speakers are set up as Mr. Harley has done, my calculations indicate a 10 decibel increase in amplitude response at 40Hz, compared to the response at the listening position when set up according to our program's recommendation. This is in addition to the large response rise caused by mismatched amplifiers. (For an excellent primer on listening rooms, see Tom Norton's "Enough Room?" in Stereophile, Vol.14 No.10, October 1991.)

I invite readers to listen to the Type Bs for themselves, with compatible electronics and proper room placement. Thank you.—Kevin Voecks, Chief Engineer, Snell Acoustics

John Atkinson responds: Readers should note that, as mentioned in the review, Robert Harley's impressions of the Snell B were gained using a solid-state amplifier, the Rowland Model One, as well as the tubed VTLs.

While respecting Kevin Voecks's more-than-competence as a design engineer, he has somewhat overstated the case regarding the interaction between the amplifier's output impedance and that of the loudspeaker. The VTL 225's output impedance actually measured significantly lower than that quoted by KV or VTL, at 1 ohm at 20Hz (see Vol.14 No.10, p.197), meaning that KV's conjectured response changes in his figs.1 and 2 are exaggerated. Plugging the correct value of 1 ohm into the equation for the response variation, and using my measured figures for the Snell's impedance of 22.8 ohms at 28Hz and 4.7 ohms at 42Hz, gives an apparent bass boost of 1.3dB rather than 3dB. While this interaction exists, therefore, I don't feel it to be "large," to use KV's words, even though, as Martin Colloms mentions in his article on bass reproduction in this issue, the human ear is "more sensitive to small variations in bass level than [it is] in the midrange."

Substituting the correct measured figure of 0.9 ohms in the midrange for the VTL's output impedance means that the 2.5dB error at 300Hz shown in KV's fig.5 is also exaggerated. The correct difference in the Type B's output between 300Hz and 1200Hz when driven by the VTL amplifier is 1.1dB, which, while audible, will not be major, in my opinion.

I therefore don't feel that either the impedance interaction or the specifics of RH's room were the primary reasons for his value judgment that the Snell B has too much bass. (I also noted the B's overgenerous bass during auditioning in the Stereophile listening room, again using solid-state amplification.) As I note in this issue's review of the Spendor S100, a loudspeaker that has its bass tuned for maximal flat low-frequency extension under anechoic conditions will tend to sound bass-heavy in a real room unless that room is extremely large. I suspect that this is the case here.

Because of this possible mismatch between the Type B's bass alignment and its balance in moderate-sized rooms, I have asked Larry Greenhill, whose room is extremely large, to do a "Follow-Up" review on the Type B. In the meantime, as KV says above, readers should listen for themselves.—John Atkinson

s10sondek's picture

JA1, thank you very much for posting these historic reviews.

One small request: would it be possible to dig up and append the Manufacturer's Comment from Kevin Voecks? It would really help provide another perspective to the dialog between LG, PWM, and RH.

I will add that reviews like this are very special and important, for several reasons: 1/ It is a NEGATIVE review, in which the reviewer explicitly states that the product is NOT recommended. This is exceedingly rare in Stereophile's pages over the last two decades, so it is useful to see an example of one; 2/ It contains 3 different perspectives from Stereophile reviewers -- which goes to show how opinions and value judgements can vary -- dramatically (and the manufacturer's comment would add a valuable 4th), and 3/ The primary full-length subjective review by RH somewhat contradicts the objective measurements provided - the measurements show 'near textbook' engineering and performance, while the subjective perspective shows dissatisfaction, an example of how great measurements are merely a necessary but not sufficient condition for great subjective performance in a loudspeaker.

I'll add one more general comment, which is that the review language of this prior era seems more analytically pointed than that which is commonplace today. It is easier for me to grasp how a component sounds using that more straightforward language than the more evocative poetry that appears now. I've read things recently in this magazine along the lines of "the singers sounded like they were dancing barefoot in the candlelight in a state of intense reverie." I mean, the latter is descriptive but I have no idea how that impression would map to my experience listening to the component in my system with my music. For all I know, that may translate to all music sounding crude, blurry, and dimly-lit ... served with a side of painful calluses that respond poorly to repeat salicylic acid treatments and long foot soaks.

John Atkinson's picture
s10sondek wrote:
JA1, thank you very much for posting these historic reviews.

You're welcome. When Stereophile launched its website 15 years ago, one of the goals was to post every review to the free on-line archive. We're well on the way to reaching that goal!

s10sondek wrote:
One small request: would it be possible to dig up and append the Manufacturer's Comment from Kevin Voecks? It would really help provide another perspective to the dialog between LG, PWM, and RH.

I'll retrieve the comment from the archived files and post it in a day or so.

John Atkinson
Technical Editor, Stereophile

s10sondek's picture

Thank You for adding Kevin Voeck's Manufacturer's Comment, JA1!

Wow, what an invigorating dialog around this loudspeaker! You've got the systematic formulation of hypotheses regarding the bass response from various parties; and then subsequent validation/invalidation of them through measurements and simulations (amplifier output impedance, frequency response, etc) and experimentation within LG's listening room. Talk about thorough. And in the end, you get a sense of resolution or explanation as to why different people had different perceptions as to the speaker's bass performance.

Coming out of all this, I really get a good sense of how this speaker would sound in various kinds of rooms and driven by different kinds of amplification. What a treat. Furthermore, reviews like this give me a more general framework for how I can think about other loudspeakers that I may be listening to or reading about.

Thanks again JA1 for posting this Gold from the Stereophile Vaults.

John Atkinson's picture
Is now posted:

John Atkinson
Technical Editor, Stereophile