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This report should be considered a continuation of the article "As Close As You Can Get" as well as an evaluation of the performance of the Stax SR Lambda Pro earphones. In the article, I presented the problems facing anyone who wishes to make measurements of earphones, especially with the idea of trying to correlate them with perceived sound characteristics. While reporting on the Stax SR-Lambda Pro earphones, which I have chosen to be a standard against which listening panel members may make comparative judgments and comments, I will show and explain the tests that I will be using in future reports. I have been very careful not to try to set any new or arbitrary standards; my only desire is to show some measurements that I think can be correlated with listener comments, and that can be used as a quick visual guide to the reader.
The Stax SR-Lambda Pro earphones are electrostatic, setting them apart from most designs, which are dynamic. Stax has been making electrostatic earphones for more than 25 years, and their previous models have all been highly regarded. The Pro is an improved version of the Stax SR-Lambda earphones; the main difference is that the spacing between the diaphragm and the stationary electrodes has been increased to allow higher output, especially in the bass range, and the voltage between the electrodes has been raised from 230 to 580 V to increase the sensitivity. The low distortion of the Stax SR-Lambda Pro earphones is due, in part, to the fact that they employ a push-pull rather than single ended design. A single-ended electrostatic design consists of a moving diaphragm, which produces the sound, and only one stationary electrode. A high voltage is applied between the diaphragm and the stationary electrode. If the voltage on the diaphragm is positive, it provides a constant attraction, which causes the diaphragm to try to move toward the stationary electrode; if it is negative, the diaphragm tries to move away from the stationary electrode. This type of design will always tend to produce more distortion than a push-pull type. In a push-pull design, such as the Stax SR Lambda Pro, there are two fixed electrodes and a moving diaphragm between them; a diaphragm carrying a high voltage of either polarity is neither attracted to nor repulsed from the electrodes and thus does not move toward or away from them until an audio signal is applied. The symmetrical action of this push-pull design reduces even order distortion.
The SR-Lambda Pro earphones are supplied with dedicated electronics, the SRM-1/MK-2 driver unit, which not only provides the high voltage required but also includes an amplifier exactly matched to the 'phones. It has gold-plated input phono jacks which can be connected to the outputs of such program sources as preamplifiers, cassette recorders, and Compact Disc players. Since the input has a high impedance, the sound quality characteristics of the earphones will remain constant and independent of the impedance of the source. This is one of the main reasons why I felt that these Stax earphones would be useful as a reference; if you have a chance to listen to them, you will hear the same kind of sound that the listening panel members and I hear. This is not true for other earphones which, because of their relatively low impedance, are affected by the impedance of the source that drives them. The drawback is that the SRM-1/MK-2 is relatively large and bulky; it is 5 1/8 inches wide x 21 5/16 inches high x 13 1/4 inches deep. The earphones and driver electronics together are $1,199.95. Stax also offers the ED-1 diffuse field equalizer ($799.95) to compensate for the differences between listening to earphones and listening to live sounds in a diffuse field. The ED-1 is in a box of the same size, so if you elect to use it, you will have quite a bit of electronics for your earphone listening. From this it should be apparent that the Stax SR-Lambda Pro earphones are only useful for fixed listening applications.
The imitation-leather earcushions are very soft, and because they fit completely around pinnae without pushing against them, are very comfortable. For listening for more than one hour, the Stax SR-Lambda Pro earphones are the best of any I have used. The 8-foot cord and the headphones' relatively light weight also contribute to the comfort factor, since one can easily move around.
My article, "As Close As You Can Get," showed how the B & K Type 4128 head and torso simulator, or HATS for short, might be used to test earphones. Figure 1 here shows a frequency response curve using the HATS and the B & K Type 2133 frequency response analyzer in its twelfth-octave mode. It is presented as a reference only and as a topic of discussion, not as an absolute measurement. The top curve shows the response without equalization, and the bottom curve shows the response with diffuse field equalization. The dip at 3 kHz indicates that Stax SR-Lambda Pro 'phones do not provide the increased output in this range which would be necessary to match the B & K diffuse field equalization. The inverse diffuse field equalization provided by Stax in their ED-1 is much less than that of the B & K HATS system, which indicates that Stax feels that the SR Lambda Pro earphones do not require a correction for the outer ear canal's characteristics with a magnitude as great as that used by B & K. The SR Lambda Pro earphones may sound a little bright on some program material and slightly dull when the ED-1 is used. Although it provides a reasonable correction for Stax earphones, I decided I would not use the ED-1 with the SR-Lambda Pro earphones when I use them for reference in other reports, because it might not be available when you have a chance to audition them; I want you to be able to hear what we hear when we use them as a reference. If you decide that you might want to buy the Stax SR-Lambda Pro earphones and wonder how the ED-1 diffuse field equalizer affects the sound, I can only say that I think that it does affect the sense of space, making the sound seem a bit more distant.
The bump in the response shown in Fig. 1 at about 100 Hz was caused by the fact that I couldn't achieve a completely tight seal around the artificial ear of the HATS manikin. That artificial ear is stiffer than a real ear and caused the earphone to be held slightly away from the head. When there is an air leak between the front of the earphone diaphragm and the ear, a loss of low frequency pressure coupling occurs, which will cause a reduction in bass extension no matter what type of earphone you might be using. When I asked panel members to evaluate the Stax SR-Lambda Pro earphones, I made certain that the seal was as good as possible, and asked them to push the earphones against their heads briefly while I played some program material that had low-frequency content. One panel member noticed a change in the bass when she did this, and I am certain that her hair was not allowing a complete seal to be made.
HEADPHONE EVALUATION
The panel members were unanimous in praising the quantity and quality of the bass on recordings such as the Sheffield Drum Record (LAB-14), Bravura (Delos, CD3070) Saint-Satins' Symphony No. 3, "Organ," by Charles Munch and the Boston Symphony (RCA 5750-2-RC), and "'Wishing Well" (One Night in Vienna, Schonherz & Scott, Windham Hill, CD-1060). I decided that I could circumvent--most of the problems I discussed in "As Close As You Can Get" by making a very simple measurement that can be used with just about every type of earphone generally available. I placed a half-inch B & K 4133 condenser microphone in close proximity to the diaphragm of a variety of over-the-ear, on the-ear, and in-the-ear earphone types, and I was able to get very consistent results. I am not proposing this as an industry standard for making measurements, and I realize that it is not without problems; the bass output and the square wave and its spectrum will not be displayed in an absolute manner, as I would like them to be, but I think that it will at least allow visual comparisons to be made between different earphones. I will try to point out any discrepancies as they appear.
Since both music and speech are complex mixtures of fundamentals and harmonics, are dynamic, and, in most cases, have a natural asymmetry, I decided to use two test signals that would give an indication as to how earphones reproduce complex tones and transient signals.
Figure 2 shows, at the top, a 500-Hz square wave I chose to represent a complex tone since the square wave is composed of the fundamental at 500 Hz and a series of harmonics which are equally spaced at 1-kHz intervals; the harmonic series is shown in Fig. 3.
Each harmonic also has an exactly defined amplitude and, although it isn't shown, a specific phase relationship.
This tone has a definite timbre, and if the levels of the harmonics are not reproduced accurately, a change in timbre can be perceived. The reproduction of this square wave is shown in the bottom of Fig. 2, and the corresponding spectrum is shown in Fig. 4. It should be emphasized that the output will not be a perfect square wave if the earphone is designed to compensate for its own effect on the outer ear resonance and that resonance's attendant increase in the ear's sensitivity at around 2,800 Hz. The output of the Stax SR-Lambda Pro earphones shows a rise at 2,500 Hz, and this may be correlated with panel members' comments that speech was "very articulate" and that operatic voices were "well projected" and "easy to understand." The sound of trumpets and other brass instruments seemed to be "bright and clear" and to be "projected very well." Of course, this brightness and articulation may also have been assisted by the increased output of the harmonics at 13.5 kHz and above.
The other test signal I decided to use is a 30-kHz cosine pulse; the input pulse is shown at the top of Fig. 5, and the output of the Stax SR-Lambda earphone is shown in the bottom of Fig. 5.
One reason I selected this test signal is that it gives a quick indication of the absolute polarity of the acoustical output; in this case, it is easy to see that the SR-Lambda Pro earphones invert the polarity with respect to the electrical input. Another reason is that it is the test signal I used with my fast Fourier transform (FFT) analyzer to obtain the phase and magnitude transfer functions of the SR-Lambda Pro earphones shown, respectively, in the top and bottom of Fig. 6. The curves in Fig. 6 are meant to be relative, not absolute, measurements. This is also true for data shown in the other figures of this report; they are presented to allow only a relative comparison with data that I will measure for future reports. Notice, however, that the dip in the magnitude response of Fig. 6 occurs at 4,350 Hz; this agrees with the diminished output at 4,500 Hz shown in Fig. 4 and serves as a verification that the output is actually lower in this range. This may correlate with panel members' comments that the sound for full orchestra was "slightly veiled" and "distant."
Fig. 1--Stax SR-Lambda Pro headphones as measured by B & K HATS manikin,
without and with B & K diffuse-field equalization; the bump in response
at about 100 Hz s due to incomplete sealing of the earcushions on the artificial
head. The equalization provided by B & K is greater than necessary for
these phones; see text.
Fig. 2--A 500-Hz square wave and its reproduction by the Stax SR-Lambda
Pro headphones. The output is reasonably good, especially for earphones.
Fig. 3--Spectrum of he 500-Hz square wave shown in Fig. 2, for reference
use. The harmonics are spaced at 1-kHz intervals across the spectrum.
Fig. 4--Spectrum of the 500-Hz square wave as reproduced by the Stax headphones.
The output is a little high at 2.5 kHz, low at 4.5 kHz, and high above 13.5
kHz.
Fig. 5--A 30-kHz cosine pulse and its reproduction by the SR-Lambda Pro.
Note the inverted polarity.
Fig. 6--Phase and amplitude spectra produced by the SR-Lambda Pro for the
30-kHz cosine pulse shown in Fig. 5. This and other curves are meant as relative
measurements only, for comparison with curves in future reports. They are
not meant as absolute measurements; see text.
The fact that the absolute polarity is inverted might not be a problem were it not for the fact that the SR-Lambda Pro earphones are so clear and coherent in their reproduction of transient sounds; since they are, the absolute polarity is easy to hear and definitely affects perceived quality of the sound.
With loudspeakers it is not too difficult to reverse the leads to achieve, at least, a correct starting polarity, which will allow you to mark your records, cassettes, and Compact Discs. It isn't as easy to accomplish with the SR Lambda Pro earphones because the input to the SRM-1/MK-2 has single ended phono jacks, and the output to the earphones is a special socket that mates to a plug on the earphone cord.
Changing this is not a trivial matter. To compound the problem, the ED-1 diffuse field equalizer, which also uses phono sockets for input and output, changes the polarity when it is switched in and out! Since I bought the SR-Lambda Pro earphones and the ED-1 to use as a reference system, I have brought this problem to Stax's attention and hope they will come up with a modification that will allow both new production units and those already sold to be corrected. Until they do, be assured that when the listening panel or I use the Lambda Pro earphones as a reference, I will keep track of the polarity the hard way--by using an external polarity switch.
There are some things about earphones that are really hard to measure in a way that can be correlated to perceived sound quality, and I have discussed them in an oblique fashion by outlining the problems in "As Close As You Can Get." Although the ability of the SR-Lambda Pro earphones to present sounds with articulation, detail, clarity, and integrity of timbre is marvelous, there is nevertheless, if comparison is made to our memory of the "real thing," something missing. It is a sense that the sounds are coming from sources that are truly away from the head. I think this is because earphones do not use the natural directional capabilities of the pinnae but present the sound to them from one direction only.
After you listen to the SR-Lambda Pro earphones for a while, you will tend to forget this phenomenon because they reproduce, in a truly wonderful and very enjoyable way, the feeling of space captured by good recordings.
The only other earphones I would presently consider for use as references are the Etymotic Research ER-1M, which are, at least at present, not available as a production item. They are an in-the-ear type and the most accurate of any I have ever heard. They are not capable of excessive output (which may be a blessing in disguise) and the base is very much affected by the way they are placed in the ear, but these seem to be their only limitations. I decided that the Stax SR-Lambda Pro earphones would be more suitable as a reference because they are close to the ER-1M, are in production, and have been generally available for several years. I would like to thank Mead Killion and Ed Devilbiss of Etymotic Research for allowing me to audition the ER-1M earphones as part of the process of setting up a meaningful report format. If you are serious about earphone listening, I recommend the Stax SR-Lambda Pro as being the best available at the present time. I can also tell you that I bought them and I am not disappointed.
-Edward M. Long
Also see:
Stax SR-Lambda Professional Earspeaker System (Jan. 1985)
Stax Electrostatic Earspeakers (ad, Aug. 1984)