Test Bench: Eminence’s Tour Grade NSW4018-8 18” Woofer

July 22 2020, 09:10
In this Test Bench, we characterized a very high power handling 18" woofer from Eminence’s recently announced Tour Grade pro sound product line, the NSW4018-8 (see Photo 1, 2 and 3). The Tour Grade speaker family consists of two 18” options, including the ferrite magnet FSW4018-8, the neodymium magnet NSW4018-8, the massive neodymium magnet 21" NSW6021-6.
 
Photo 1: Eminence’s Tour Grade NSW4018-8 woofer shown from the top.
Photo 2: Eminence’s Tour Grade NSW4018-8 woofer shown from the bottom.
Photo 3: Eminence’s Tour Grade NSW4018-8 woofer shown from the side.

According to Chris Rose, president of Eminence, the new line was developed over two years to support the undisputed growth of professional audio. Touring sound professionals want to move more air, more efficiently without failures in a wide range of applications and venues. Senior Design Engineer Matt Marcum explained, “These products were designed from a clean sheet of paper using American ingenuity with the intent of there being NO compromises. This resulted in new products with unsurpassed reliability and best in class performance. If you make a living re-creating sound, you need these tools. Anything less could literally be playing with fire. Look at the NSW6021-6. Its 6” voice coil handles more power and boasts a cleaner stroke than any other 21” pro sound option in the world.”

“Each speaker in the TOUR GRADE line features a true double spider rather than a silicone sandwich to ensure it produces unparalleled linear throw,” according to Jerry McNutt, Product Design Manager. McNutt also noted that, “each speaker in the series features a super strong X5 pulp material—a new USA developed technology.” Needless to say, the Tour Grade pro sound product group represents a new landmark line from Eminence.

Applications for the new Eminence NSW4018-8 include use as a bass driver in multiway systems or as a dedicated subwoofer in bass reflex designs. The feature set for the NSW4018-8 is like most high-performance pro sound drivers, rather substantial. Starting with the frame, the NSW4018-8 uses a proprietary and robust six-spoke cast-aluminum frame incorporating six 38 mm × 7 mm rectangular vent holes in the area below the spider mounting shelf for enhanced voice coil cooling. This series of cooling vents allows air to move past the voice coil and across the front side of the neodymium motor assembly.

Additional venting is provided by 12 round 6 mm diameter peripheral vents in the top (back) of the black painted motor return cup. There is also another 6 mm diameter pole vent. Gone are the days of huge pole vents that short circuited air flow past the voice coil in the gap area. Small pole vents keep drivers cooler!
 
Figure 1: Eminence Tour Grade NSW4018-8 woofer 1 V free-air impedance plot.
Table 1: Comparison data for Eminence’s Tour Grade NSW4018-8 woofer.

The cone assembly consists of a straight profile thick “X5” pulp paper ribbed cone along with a 5.75” diameter convex paper dust cap. Compliance is supplied by pleated coated polycotton M-type surround and from two 7” diameter flat cloth spiders (dampers) mounted back to back on an 8 mm height aluminum standoff at the base of the frame. The entire cone assembly (cone, surround, and spider) is water resistant.

The NSW4018-8 motor is based on a neodymium ring magnet structure. The neodymium magnet motor was FEA-designed using a 102 mm (4”) diameter voice coil wound with round copper wire on a non-conducting fiberglass former. Motor parts, such as the return cup and the rear heatsink plate that form the spider vents, are coated with a black heath-emissive coating for improved cooling. Factory-rated power handling for this driver is 1600 W AES, 3200 W program, and 6400 W peak power. And, the voice coil is terminated to a pair of chrome color-coded push terminals.
 
Figure 2: Eminence Tour Grade NSW4018-8 computer box simulations (black solid = vented 1 at 2.83 V; blue dash = vented 2 at 2.83 V; black solid = vented 1 at 100 V; blue dash = vented 2 at 100 V).
Figure 3: Group delay curves for the 2.83 V curves shown in Figure 2.
Figure 4: Cone excursion curves for the 100 V curves shown in Figure 2.

I commenced testing of the NSW4018-8 using the LinearX LMS analyzer and VIBox to create both voltage and admittance (current) curves with the driver clamped to a rigid test fixture in free-air at 1V, 3V, 6V, 10V, 15V, 20V, 30V, and 40V allowing the voice coil to progressively heat up between sweeps with a 200 Hz sine wave. Note that the NSW4018-8 was still quite linear at 40V and certainly could have probably been tested at 50V plus, but I generally call it quits with high-efficiency pro sound drivers at 30V to 40V due to the sound pressure level (SPL) in my parameter test room, and that includes wearing hearing protection.

Following my established protocol for Test Bench testing, I no longer use a single added mass measurement and instead use the measured Mmd data (262.6 grams for the NSW4018-8). I post-processed the 16 550-point stepped sine wave sweeps for each Eminence Tour Grade NSW4018-8 sample and divided the voltage curves by the current curves to generate impedance curves.

I imported the data, with the phase derived using the LMS calculation method, along with the accompanying voltage curves, to the LEAP 5 Enclosure Shop software. Because Thiele-Small (T-S) parameters provided by the majority of OEM manufacturers is generated using either the standard model or the LinearX LEAP 4 TSL model, I additionally created a LEAP 4 TSL parameter set using the 1 V free-air curves. I selected the complete data set, the multiple voltage impedance curves for the LTD model, and the single 1 V impedance curve for the TSL model in the Transducer Model Derivation menu in LEAP 5 and created the parameters for the computer box simulations. Figure 1 shows the 1 V free-air impedance curve). Table 1 compares the LEAP 5 LTD and TSL data and factory parameters for both of NSW4018-8 samples.

The LEAP 5 parameter calculation results for the NSW4018-8 correlated well with the Eminence factory datasheet. Note that since the LTD measurements are multi-voltage designed to produce better high voltage excursion curve simulations, I am really only comparing factory TSP to the TSL parameters, which again, in this case look good. Also the published coil length and gap height dictate a 13.97 mm Xmax, so the quoted 15.21 mm factory Xmax is, like a lot of other pro sound manufacturers, specifically accounting for the gap area fringe field, which I certainly understand and have no problem with.

Following my established measurement protocol, I configured computer enclosure simulations using the LEAP LTD parameters for Sample 1. Eminence’s box volume recommendation for this driver is 3.5 ft3 to 12 ft3. Given that I arbitrarily set up two computer box simulations into LEAP 5, the first a vented box with a 3 ft3 volume (15% fill material) tuned to 50 Hz and a larger vented alignment with a 6 ft3 volume tuned to 35 Hz, also simulated with 15% fiberglass damping material.

Figure 2 displays the results for the NSW4018-8 in the two vented enclosures at 2.83 V and at a voltage level sufficiently high enough to increase cone excursion to Xmax + 15% (16 mm for the NSW4018-8). This produced a F3 frequency of 47.5 Hz (F6 = 42.8 Hz) for the 3 ft3 vented alignment and -3 dB = 31.5 Hz (F6 = 29.5 Hz) for the 6 ft3 vented simulation. Increasing the voltage input to the simulations until the maximum linear cone excursion was reached resulted in 125.8 dB at 100 V for the 3 ft3 box and 123 dB for the same 100 V input level for the larger 6 ft3 vented box. Figure 3 shows the 2.83 V group delay curves. Figure 4 shows the 100 V excursion curves. Please note that the drivers were not excursing beyond 16 mm at 100 V, so I would conclude that the NSW4018-8 is more thermally limited than excursion limited, which is a good thing.
 
Figure 5: Klippel analyzer Bl(X) curve for the Eminence Tour Grade NSW4018-8.
Figure 6: Klippel analyzer Bl symmetry range curve for the Eminence Tour Grade NSW4018-8.
Figure 7: Klippel analyzer mechanical stiffness of suspension Kms(X) curve for the Eminence Tour Grade NSW4018-8.
Figure 8: Klippel analyzer Kms symmetry range curve for the Eminence Tour Grade NSW4018-8.

Klippel analysis for the NSW4018-8 is normally provided using our DA2 Klippel analyzer (courtesy of Klippel GmbH), and performed by Patrick Turnmire (owner of Redrock Acoustics and author of the SpeaD and RevSpeaD transducer development software). However because Turnmire was unavailable this month and since Eminence has their own DA2, they graciously provided me with the KBDX files for the NSW4018-8, which is what I used to produce the Bl(X), Kms(X), and Bl and Kms symmetry range plots given in Figures 5-8.

The Bl(X) curve for the NSW4018-8 (see Figure 5) is very broad and symmetrical with some small amount of offset. Looking at the Bl symmetry plot (see Figure 6), this curve shows an inconsequential 0.11 mm coil-out offset at the 8 mm, just half way to the Xmax + 15% position. Figure 7 and Figure 8 show the Kms(X) and Kms symmetry range curves for the NSW4018-8. The Kms(X) curve is also mostly symmetrical in both directions accompanied by an amount of coil-in offset. Looking at the Kms symmetry range plot, the coil-in offset at the same 8 mm excursion point of the driver is an also inconsequential 0.36 mm.

Displacement limiting numbers calculated by the Klippel analyzer for the NSW4018-8 were XBl at 70% Bl = greater than 10.5 mm and for XC at 75% Cms was also greater than 10.57.3 mm , which means that for the NSW4018-8, both factors were roughly equal contributors for prescribed distortion level of 20%. For an 18” woofer, this driver has “excursion for days,” as my dear departed friend Chris Strahm (founder of ATI and LinearX and absolutely one of the best engineers in the industry from my experience) would have said.
 
Figure 9: Klippel analyzer L(X) curve for the Eminence Tour Grade NSW4018-8.
Figure 10: Eminence Tour Grade NSW4018-8 factory 0° on-axis frequency response.
Figure 11: Eminence Tour Grade NSW4018-8 SoundCheck distortion plot.

Figure 9 gives the inductance curves Le(X) for the. Inductance will typically increase in the rear direction from the zero rest position as the voice coil covers more pole area, which is what is happening here. The inductance swing for this driver is 0.47 mH coil-in to 0 and 0.09 mH coil-out from 0, which is very low. Following the Klippel testing, I normally perform frequency response measurements on and off-axis, however, I do not have the logistic capability of doing that with 18” and larger diameter woofers, and also because 18” and larger drivers are generally crossed over below 500 Hz where there generally isn’t much that is relevant in terms of frequency variation. However, I have included the factory on-axis frequency response (see Figure 10).

For the remaining tests, I employed the Listen, Inc. SoundCheck AudioConnect analyzer and SCM 1/4” microphone to measure distortion and generate time-frequency plots (courtesy of Listen, Inc.). For the distortion measurement, I rigidly mounted the NSW4018-8 in free-air, and set the SPL to 104 dB at 1 m (9.67 V), using a pink noise stimulus. I measured the distortion with the Listen microphone placed 10 cm from the driver. This produced the distortion curves shown in Figure 11. Again, since I do not have test enclosures for 18” plus drivers in my test box inventory, I did not perform the usual SoundCheck time domain plots (cumulative spectral decay and Wignerville). Eminence is one of the older names (founded in 1966) in the pro sound industry and has a justifiably highly regarded reputation. The data presented for the NSW4018-8 certainly speaks to that fact. For more information, visit the Eminence website at www.eminence.com. VC

This article was originally published in Voice Coil, May 2020
related items