DSWPRO

Powered Subwoofer

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Overview
The Polk Audio DSWPRO subwoofers are the result of Polk Audios 15 years of experience developing superior performance powered subwoofers. We drew upon several exclusive Polk technologies to produce subwoofers with the best performance, convenience and value at the price. They were designed with serious listeners in mind listeners who are serious about the accurate reproduction of music and movie soundtracks at life-like volume levels in a home environment. The deep, precise bass of the DSWPRO woofers will remind listeners of the finest theater systems and live performances, not just a Home Theater Sub.
Features and Technologies
Polk Room Optimizer (PRO)
The response of any loudspeaker is influenced by its proximity to room surfaces boundaries. In the case of subwoofers adjacent surfaces have a profound effect on frequency (spectral) balance as the graphs below indicate. Figure 1a is the response of a subwoofer placed far away from room surfaces. Notice that the response is very smooth and flat but with a steep roll-off of deep bass. Figure 1b is that same subwoofer placed midway on a wall. Notice that the deep bass response is improved but there are other changes as well. Figure 1c is the same sub placed in a corner. Here the effects are more dramatic with a peak that would make for a poor listening experience.
Objectives of the DSWPRO Series Subwoofers:
Flat frequency response. Superior subwoofer performance depends on many factors but the cornerstone is flat frequency response. Excellent performance on all other aspects of design cannot overcome major errors in frequency response. Woofers with large response peaks may sound flattering on some program material but over a broad range of music types will come up short in listener satisfaction. Adaptability to placement conditions. Complicating the issue of accurate response are boundary effects. The DSWPRO woofers were designed to minimize the most pressing problems associated with placement and boundary effects. Highly accurate music reproduction. It is relatively easy to design a subwoofer that plays movie sound effects at thunderous levels, but difficult to design one equally adept at music reproduction that is tight, detailed and musical. Extended low bass response. There is no sense having a subwoofer unless it goes low. Real low. High SPL. In order to cover the wide dynamic range of live music and be fun to listen to, a subwoofer needs to be able to play loudvery loud. Low distortion. The key to obtaining bass you want to listen to for hours and adds to rather than subtracts from the music is maintaining low distortion at listening levels from whisper to live concert levels. Convenient operation. DSWPRO subwoofers were required to be easy to set up and adjust at the whim of the user. Convenient size. Consumers want subwoofers that are easy to place and fit into a room so the DSWPRO subs had to deliver big bass from modestly-sized enclosures.

120 115

100 95

80 freq 140

Figure 1a

125 120

Figure 1b

Figure 1c 2

The Best Bass Anywhere
Polk Room Optimizer (PRO) is a set of four preset equalization curves designed to compensate for these boundary effects. The four buttons near the bottom of the remote control correspond with the four primary placement options: mid-wall, mid-room, corner and in-cabinet to introduce the proper EQ compensation to yield ideal frequency response irrespective of location. All the listener has to do to get flat, accurate bass response is to choose the woofer placement that works best for his/her lifestyle and push the appropriate button on the remote. Figures 2a (mid-room), 2b (mid-wall), 2c (corner) and 2d (in-cabinet) are the actual EQ curves applied to the DSW600 for the four PRO settings.
Raw Curve 23852.2Hz 0.0db
-10 db -LF 10 Frequency Hz 100 HF 1000 1/2 Fs 48000

Figure 2a Mid Room

Figure 2c Corner

Figure 2b Mid Wall

Figure 2d In Cabinet
Feed Forward distortion suppression technology
No matter how big or how powerful a powered subwoofer may be it is always possible to push it beyond its limits to the point where it becomes distorted, makes objectionable noises or even damages itself. It is always a challenge when designing a powered subwoofer to come up with a method of limiting the maximum output in such a way that avoids excess distortion or damage but preserves as much of the dynamic capability and maximum output capacity of the subwoofer as possible. Distortion and, eventually, damage to a subwoofer comes from one of three major sources:

40 100

P1refn 80 Ztrefn
Too much power from the amplifier causes over heating of the voice-coil, higher distortion and ultimately failure of the woofer driver. Too little power available from the amplifier causes high distortion in the signal fed to the woofer driver, generates excess heat and may eventually burn out the amplifier. Excessive movement of the driver cone caused by over driving at low frequencies leads first to high distortion, then to objectionable mechanical noises and finally to mechanical damage to the woofer driver as it literally tears itself apart.
Figure 3Frequency Response (Red) and Impedance (Blue) for a hypothetical subwoofer. (For clarity the Impedance curve (blue) has been offset upward by 40 ohms)

100 Hz

Usually powered subwoofers are designed with amplifiers that have more than enough power to over drive the woofers mechanical system. So, #2 above is normally not a problem. Advances in voice-coil technology have also made voice-coil burnout much less likely and it is common practice to match woofer drivers and amplifiers to make burnout very unlikely. That leaves #3 as, by far, the most challenging problem. Mechanical distortion and damage can occur almost instantaneously regardless of how well matched the woofer driver and amplifier may be and may be triggered by any number of complex signal combinations which can be very difficult to predict. As a result, most attempts to protect the system or reduce distortion limit the output of the woofer far below its theoretical maximum output in an effort to guarantee a safe level of operation. The difficulty comes from the fact that even though a woofer may have flat frequency response it takes different amounts of power from the amplifier to produce the same amount of sound at different frequencies. This means that combinations of signals at different frequencies may overload a woofer while the same signal level at a single frequency would be okay. For example, lets consider a hypothetical 10" powered subwoofer in a 2.5 cu ft. ported box. Figure 3 shows the frequency response and impedance of the system. As you can see, the frequency response (Red) is quite flat to below 30Hz, but the impedance is far from flat. It rises from around 4 ohms at 30Hz to 36 ohms at 20Hz and 60 ohms at 50Hz. (please note that the impedance curve has been offset upward by 40 Ohms for clarity) That means this woofer will draw only 1/9th as much power from the amplifier at 20Hz compared to 30Hz and only 1/15th the power at 50Hz. So, if the amplifier has enough power to drive the system to maximum output at 30Hz it will have way too much for the system to handle at 20Hz and 50Hz. 4

Pmaxrefn

Figure 4Max. hypothetical subwoofer acoustic output at 10' with 500 watt amp This demonstrates both why it is necessary to have some form of limiting or protection and why it is so difficult to do so in a way that still makes full use of the performance potential of the woofer. One common method of protection and distortion reduction is to apply compression or limiting to the amplifier signal over the entire bandwidth reproduced by the subwoofer. The basic method is to find some maximum amplifier output signal level that will not over drive the woofer at any frequency or combination of frequencies. The problem with this broad-band approach is that the same limit is used for all frequencies and for all combinations of frequencies. So, the entire system is limited to a safe level that is far below the actual maximum output at most frequencies. Figure 4 shows the maximum acoustic output of our hypothetical woofer versus frequency assuming a 500 watt amplifier. As you can see, the maximum output at 30Hz is about 6dB greater than at 40Hz to 50Hz and 25dB greater than max output at 20Hz. Broad band limiting would dictate that we roll off the low frequency response of the
amplifier below 30Hz to protect the woofer at the lowest frequencies even though the woofer can deliver significant low bass output (See Figure 3). We would also have to limit the amplifier to a safe level well below the max output in the 40Hz to 50Hz range. So, at normal listening levels we would lose all of the low bass capability of the woofer and at high levels we would lose at least 6db of dynamic range since the limiter setting is determined by the frequency range with the lowest maximum output. It is no surprise that this form of compression or limiting tends to make a subwoofer sound muffled, lacking in extension and lacking in punch. Another commonly used method is to apply limiting or correction based on the actual movement of the woofer driver cone as measured by a position sensor. These are known as servo feedback systems. The position sensor is coupled to circuitry that compares the input signal to the cone movement and provides a correction signal to the amplifier input signal. In theory the correction signal modifies the amplifier output signal in a way that will correct for any mechanical distortion in the acousticoutput. And, in practice servo feedback systems often produce excellent test results with steady state test tones. However, real program material is dynamic and the servo system cannot correct for anything until after something has already gone wrong. So, the more heavy handed the servo correction algorithms the more sluggish the system sounds as it tries to measure, compare and correct in a dynamic signal environment. In addition, the correction signal may go the wrong direction by asking the woofer to correct itself by moving beyond its mechanical limits. Therefore, broad band limiting is still required to protect the woofer driver against correction signals that might seriously overload the system. So, although a servo system can produce impressively low harmonic distortion numbers for steady state test tones its ability to handle the dynamics of real world audio signals can be seriously compromised due to the fact that it is correcting for something that has already gone wrong and, of course, still requires the broad band limiting we discussed above.

From a music or movies performance point of view the best limiting or protection system would be none at all. Obviously we cant take that risk. But, second best would be a system that limits only that which absolutely needs to be limited and which does its job proactively rather than reactively. Polks proprietary Feed Forward distortion suppression system anticipates problems and fixes them before they happen. It uses our ability to predict how the woofer will respond to complex signals to respond with the absolute minimum intervention to minimize distortion and chance of damage without compromising dynamics or transient response and while actually increasing maximum output. In essence the digital processors of the DSWPRO subwoofers are programmed with a mathematical model that knows in advance how the woofer acoustic system will respond to the output signal from the amplifier. The processor monitors the amplifier input for signal combinations that would cause distortion or damage and alters only that part of the signal required to maintain low distortion and safe operation. It does this before the signal ever gets to the woofer driver so there is no feedback correction time delay and no need for broad band limiting. Also, since Feed Forward is proactive rather than reactive, the limits can be set much higher and closer to the theoretical maximum output of the woofer at each frequency without risk of excessive distortion or damage. For example, in our hypothetical woofer of Figure 4 the Polk Feed Forward system would know in advance that the output capability is lower at 50Hz. Even if limiting at 50Hz were required, Feed Forward would still preserve full dynamics above and below that frequency. It would also preserve low bass capabilities at normal listening levels while providing appropriate protection from low frequency mechanical damage at higher levels. As a result, when the program material calls for greater dynamics Feed Forward allows the system to deliver without an artificial cutoff. Unlike other distortion reduction or protection systems Feed Forward is virtually undetectable in operation. It delivers all of the excitement and impact or subtlety of the program material, as the case may be, while maintaining lower distortion and higher maximum output levels.

Active Driver

The active subwoofer drivers used in DSWPRO models are robust designs with several premium features not normally seen in subwoofers in this price range. The DSW Series cone is injection molded Polypropylene stiffened by an oversized inverted dust cap. The cone is bonded to the basket with a soft Nitrile Butyl Rubber surround that acts to further suppress cone resonance. If an unwanted resonance develops in the cone it travels through the cone like a wave in a pool. Like a water wave reflecting off the side of a pool, if the cone resonance wave encounters a rigid boundary, it will reflect back through the cone (Figure 5). In addition to contributing to the drivers superior performance, this surround is impervious to Ultraviolet rays (sunlight), temperature extremes and humidity for ultimate long life and reliability. Our surround has a smooth, rounded transition from the roll to the cone attachment point to spread out stress over a wide area (Figure 6). Surrounds that meet the cone interface at severe angles concentrate stresses at that interface and are generally more prone to tearing and cracking (Figure 7). The most important part of a woofers suspension is the spider. The spider is a spring that resists and controls cone movement, keeps the coil centered in the voice coil gap and supplies the restoring force to move the cone back to the center rest position. It also prevents the voice coil from smashing against the back of the pole piece on its backwards travel and from flying out of the magnetic gap as it moves forward. It is a vitally important part. DSW drivers use flat spiders. Theyre not really flat but at the outer-diameter terminus they are flat rather than having a knee like traditional spiders (Figure 8) that can break down and become non-linear at high drive levels. But whats really cool about the DSW spider is that it is a Progressive Roll design-the roll sizes and spacing varies across the spider (Figure 9). This design improves the linearity of the spiders spring-the stiffness is the same whether the cone is moving forward or backward. Well see why that is important and the proof in the Klippel Optimization Chapterbelow.

Figure 5

Figure 6

Figure 7

Figure 8
Voice Coil, Former & Magnets
The voice coil is four layers of.45mm Copper wire wound around a 2" diameter low-mass glass/epoxy former rated to 220 C (428 F). Dual magnets are employed to soft shield the product. DSW Series subwoofers may be placed reasonably close to CRT video displays and monitors without causing magnetic interference.

Klippel Optimization

Of even greater importance than voice coil material, magnet type or maximum excursion rating is the location of the voice coil in the magnetic structure and how it moves within that structure. In order for a driver to reproduce complex sounds accurately, the cone must move in exactly the same distance and manner in both directions of travel. To illustrate what we mean, take a look at Figure 10. If the cone moves 0.5 mm in the forward direction and
Figure 9Blue line indicates the progressive roll spider

Figure 10

only 0.4 mm in the backward direction in response to a given signal, the sound will be distorted.In order for the driver to move accurately in both directions equal force must be applied to the cone and the cone must be able to move as far and as freely in both directions. Youd think that would be easy but it isn't. Thankfully Polk Audio owns a Klippel Distortion Analyzera device that uses a laser and sophisticated software to analyze the behavior of drivers in motion under high power conditions. With this state-ofthe-art tool we can optimize the motor structure, voice coil alignment and suspension to yield the best possible performance. Figure 11 is a graph of the BL Product of the 10" driver from the DSW500. BL Product is the product of the motor's magnetic flux density times the length of voice coil in the gap. Put more simply it is a measure of the electromotive force imparted to the woofer cone. The important thing isnt whether you have more or less BL Product, but whether th motor is large enough for the drivers moving mass and whether the BL is applied uniformly. Ideally you would like to have constant BL Product through the entire excursion range. The perfect subwoofer would have

Force Factor BI (X)

(00:35:06)
-Xprot < X < Xprot Xp- < X < Xp+ BI (-X)
straight line graph plot. In the real world drivers have limitations. In Figures 11 through 13 the center 0.0 vertical line represents the cone at the center rest position. To the left of center line is backward cone movement; to the right is forward cone movement. In this kind of graph you're looking for a symmetrical curve, centered on the 0.0 line and a broad, flat shape. In Figure 12 the solid line represents the BL product of a competitive subwoofer driver. The dashed line represents the inverse of the actual BL product. The more different the two lines are the worse the BL Product symmetry. In this case we can see that the BL product curve is not even close to flat and the motor force is not the same in both directions of cone travel. From this data we can predict that this driver would have high amounts of distortion and low overall accuracy of sound. In Figure 11 we see that the DSW PRO500s driver has a symmetrical, broad, flat BL product curve indicating its high degree of movement linearity, low distortion and overall accuracy.

(00:38:20)

BI [N/A]
0.0 -15 << Coil in -10 -5 -0 X [mm] -Coil out>> 15
-10 << Coil in -X [mm] -Coil out>>

Figure 11 a

Figure 12
Klippel Distortion Analyzer
Another important result of Klippel analysis relates to the drivers suspension stiffness or springiness of the suspension. Again youd like to have a symmetrical, broad, flat curve indicating that the suspensions stiffness is consistent over a broad range of cone travel in both directions. Greater or lesser stiffness is immaterial, what is important is consistency. Figure 13 is the DSW400 drivers suspension vs. excursion plot showing symmetrical and linear suspension stiffness over a broad excursion range.
Stiffness of suspension Kms (X)

(00:34:59)

-Xprot < X < Xprot 2.5 Xp- < X < Xp+ Kms (-X)

Enclosure & Venting

DSWPRO enclosures are fabricated of 3/4" low-resonance MDF stiffened by braces positioned mid way on three of the side panels. Venting is accomplished via a slot load port whose MDF walls serve to further stiffen the enclosure and reduce panel vibration. The enclosure is finished in matte black paint that is unobtrusive and easy to care for. The enclosure can be re-painted after masking a few parts. If a DSWPRO owner wants a Puce subwoofer, he or she can have it. The large surface area slot port exhibits lower air turbulence at its mouth than tubular ports for lower chuffing, port noise and audible distortion. The driver and port face toward the floor to further minimize the audibility of port noise. Four rubber-clad feet provide stability on wood, tile and other hard surfaces. The rubber overshoes can be removed to reveal metal spikes that provide ideal stability on carpeted floors. If the user wants to position a DSWPRO subwoofer inside a cabinet, forward-firing driver and vent will yield better performance than the DSWs standard downward firing orientation. Simply remove the feet/spikes from the bottom of the enclosure and screw them into the threaded inserts on the amplifier side as shown in Figure 14.

Kms [N/mm]

0.0 -15 << Coil in -10 -X [mm] -15 Coil out>>

Figure 13

Slot Port

Stiffening Braces

Figure 14

Amplifier

DSWPRO subwoofers are powered by Class D amplifiers which offer high power and cool, reliable operation. The low operating temperatures of the Class D amps make DSWPRO subs ideal for use in-cabinets and other confined spaces.* The DSW400 amp puts out a robust 180 Watts (360 Dynamic), the DSWPROWatts (400 Dynamic) and the DSWPRO 600 packs a wallop at 250 Watts (500 Dynamic). The DSW amps provide instantaneous shut down for excessive heat, short-circuit, current/power overdraw, clipping, and under/over supply voltage conditions, making them rock solid reliable. When the protection circuit is engaged the rear-panel pilot light glows red to alert the user. The protection is reset by switching the main power switch (on the amplifier) off and on. A full array of inputs and outputs allow great hookup flexibility. A preamp-level LFE input bypasses the adjustable low-pass filter. Preamp-level L&R inputs pass the incoming signal through the adjustable low pass filter. The amplifier allows both LFE and L&R inputs to be used simultaneously. 5-way binding post-style loudspeaker terminals for convenient, secure wire connections with a variety of terminator systems from bare wire through banana plugs. * For safety and reliability a DSWpro subwoofer should never be placed in a totally sealed enclosure. Some provision must be made for fresh air to enter the enclosure. Night ModeHome theater enthusiasts face a dilemma when listening under conditions where loud playback may disturb others (late night listening, apartment dwellers, etc.) Setting the average system level to moderate volume helps keep the others from complaining but low level sounds are rendered inaudible. Setting the system volume high enough to hear low level details renders the average and peak levels to the point where family members and/or neighbors object. Engaging the Night function, compresses the dynamic range of the DSWPRO subwoofer. Average level can be set high enough to reveal low level detail while limiting the peak output to one-half volume. The Blue front panel LED glows at half intensity when the Night mode is engaged. Resetresets volume control to half volume (20), resets Phase to 0 and EQ to the default PRO mid-room EQ setting (4 confirmation blinks).

Remote Control

The supplied remote control offers a bevy of cool and convenient features to help users get the best possible sound from their DSWPRO subwoofer. One of the coolest features of the DSWPRO is that the Blue light behind the logo badge gives the user feedback as to what functions have been engaged via a series of blinks. Powerturns the amplifier on/off. When the power is on, the light behind the logo badge glows Blue. LEDturns front panel LED on/off. In darkened room conditions even the gentle blue glow may be distracting; now you can toggle it on/off with ease. Volumeadjusts volume up/down with confirmation blinks. Phase0, 90, 180, 270 adjusts phase to one of these four settings. One of the most critical settings for getting seamless blending between a subwoofer and main speakers is the phase setting. With the four phase angles selectable on the remote a user can now do trial and error listening and phase adjustments without leaving the listening position. This beats the heck out of running back and forth between sub and chair. The blue logo pod light gives 1 confirmation blink for 0, 2 blinks for 90, 3 blinks for 180, 4 blinks for 270. Mutemutes amp output and the front Blue logo light flashes continuously to remind the user that Mute is engaged.
PerformanceHow to Know When You Have It
All of the technical wizardry would be for naught if the DSWPRO subwoofers didnt performand they do. But before we show you how well they perform, lets talk about how most consumers (and sadly, many salespeople) evaluate the performance of powered subwoofers. They use the Inches and Watts method. Their assumption is that the bigger the driver size and/or the higher the amp rating, the better the subwoofer. Bad assumption. These two facts tell you absolutely nothing about how loud the subwoofer will play, how deep it goes, or how accurate it sounds. One could at least assume that the powered sub with the largest amplifier would play the loudest, right? No way. Let us show you why: How loud a system plays is a function of the available clean power and the efficiency of the driver/cabinet system. Efficiency (and a similar spec called Sensitivity) tells you how much of the input power gets used to make the sound you hear. A speaker system with an efficiency rating of 90dB would sound audibly louder than another system with a rating of 87dB. Since every doubling of power gets you 3dB greater sound output, it follows that a 90dB efficient system driven by 100 Watts would play just as loud as an 87dB system

Presetsengages mid-room, mid-wall, corner and in-cabinet EQ settings as described on pages 2 and 3. The blue light blinks 1 through 4 times corresponding with the selected EQ setting.
driven by a 200 Watt amplifier and a 90dB efficient system driven by a 200 Watt amplifier would play 3dB louder. And yes Virginia, woofer size on its own is not a good predictor of efficiency and by the way, there is no Santa Clause. So, all you have to do is look up the Efficiency rating of the powered subwoofer, then look up the power spec and youre all set, right? Ah, not that easy. First since powered subwoofers are self-powered closed systems, no manufacturer publishes an efficiency spec. Second, power specs are often, ahem, exaggerated by manufacturers. Third, how loud a subwoofer plays on average does not tell you anything about how low it goes in frequency. We could easily design a subwoofer system that plays very loud down to 50Hz but that doesnt qualify it to be called a SUB-woofer. A sub has to have solid output down to 30Hz or lower to qualify.
Lets look at the CEA 2010 results of 4 subwoofers in Figure 16. based on the Inches and Watts method of evaluation, Subwoofer D is clearly the best woofer here as it has 50 more Watts than its nearest competitor and has a large woofer. But when you look at the CEA2010 numbers you see that in fact Subwoofer D has the poorest performance! Subwoofer A with its mere 10" woofer and 250 Watt amplifier outperforms all of the other models and trounces the performance of the mighty Subwoofer D.
So, Is That All You Have to Know?
Wouldnt it be great if every manufacturer published honest CEA2010 numbers? Youd never have to get out of your easy chair. Not so fast Virginia. One limitation of the CEA2010 standard is that it does not have the force of lawmanufacturers cannot be compelled to publish the data. As of 2007 it is unclear which manufacturers will publish the data. Polk Audio will, as always, stand up for Truth, Justice and the American Way and will publish CEA2010 numbers on all new models including the DSWPRO models (Figure 17). Also, CEA2010 numbers tell you a lot but not everything. If you can accommodate any size woofer, dont care about looks or convenience and you never listen to music, you can simply use verified CEA2010 number to make a subwoofer choice. But for most of us there are other things to consider such as size, features, cost and sound quality. It is easy to make a huge box that goes low and plays loud but most listeners are looking for space efficient, attractive subwoofers that are easy to use as well as powerful while other listeners are looking for subwoofers that are tight, fast and detailed enough to blend seamlessly with high-end audiophile speakers in music-only systems. For all of these people CEA2010 numbers will be but one of many factors to consider when choosing a powered subwoofer. At the end of the day there is no substitute for understanding your needs and listening to the models under consideration before making a final choice. Figure 17

CEA 2010 to the RescueLoud & Low
Recognizing the dilemma, the Consumer Electronics Association (CEA) formed a committee of engineers to study the problem and devise a new measurement standard that consumers could reliably use to make informed choices. Polk engineers served on this panel along with engineers from other quality minded manufacturers. They realized that what is most important is what comes out of, not what goes into the subwoofer. They established a standard way of measuring the undistorted output of a subwoofer at various frequencies and reporting the results in two easy to digest numbers. The standard is called CEA 2010 but we call it Loud & Low because it tells you how loud and how low a subwoofer actually plays. Lets take a look at Figure 15, the measurements of a subwoofer using the CEA2010 method. Sound Pressure Level (SPL) measurements were made at 6 frequencies: 20Hz, 25Hz, 31.5Hz (the ultra low bass range) and 40Hz, 50hz and 63Hz (the low bass range). Then the three SPL numbers for each range are averaged to come up with Low Bass and Ultra Low bass numbers that the manufacturer will publish. By comparing CEA2010 numbers you will at least be able to predict how different subwoofers compare in terms of how loud they play. Figure 15 Frequency 20 Hz 25Hz 31.5Hz 40Hz 50Hz 63Hz Figure 16 Woofer Size Advertised Power 10 250W 12 200W 12 250W 12 300W Low Bass (40-63Hz) 117.9 dB 114.0 dB 114.7 dB 113.9 dB Ultra Low Bass (20-31.5Hz) 100.8 dB 99.9 dB 93.9 dB 92.2 dB Max SPL 87.8 dB 104.3 dB 110.3 dB 115.2 dB 118.1 dB 120.3 dB CEA 2010 Rating 100.8 Ultra Low Bass
DSW PRO CEA2010 Specifications
CEA2010 DSW400 DSW500 DSW600 Low Bass (40-63Hz) 114 dB 118 dB 121 dB Ultra Low Bass (20-31.5Hz) 91 dB 96 dB 100 dB

117.9 Low Bass

Subwoofer A Subwoofer B Subwoofer C Subwoofer D