Acoustic Speaker System Design

Electrical Engineering Senior Design Project

Submitted to Dr. A.B. Kunz

May 4, 2000

By: Matt Berg

Daniel Cram Jason Karby

Craig Pelletier Carl Rochon

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Project Management

Team Assignments Matt Berg � Testing Software and Drivers Daniel Cram � Project Manager and Cabinet Construction Jason Karby � DSP Programming and Drivers Craig Pelletier � Crossovers and Cabinet Design Carl Rochon � DSP Algorithms and Cabinet Design Note: All members participated in all facets of the Design and Implementation process. The areas listed above are what each person was placed in charge of. Resources Anechoic Chamber Computer Software Liberty Audio Suite Microsoft Office

Motorola DSP debugger software Netscape Navigator MATLAB Visio

Hand Tools (drill, saw, etc.) Machine Shop (Harry Kleiman) Oscilloscope Soldering Station Spectrum Analyzer J.R. Van Pelt Library Wood Shop (Albert Dowdle)

Acknowledgments

The members of our team would like to thank the following people for all of their contributions that they have made to this project throughout the course of this year. Technical Contributions Dr. A. Barry Kunz � Faculty Advisor Dr. Tim Schulz � DSP Professor John Miller � Laboratory Supervisor Roland McKinstry � Shop Supervisor Albert Dowdle � Woodworking and Metal Machining David DiCarlo Motorola Tech Support

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Donations Dr. A. Barry Kunz � Amplifiers and Computer David Stephens Vifa Drivers Donation Harry Kleiman � Machine Shop use (Kleiman Pump and Well Drilling) TABLE OF CONTENTS

Team Assignments 2

Resources 2 Technical Contributions 2 Donations 3

EXECUTIVE SUMMARY 4

INTRODUCTION 5

1.0 CABINET DESIGN AND CONSTRUCTION 6

1.1 Bass Cabinets 6

1.2 Tweeter and Mid-Range Cabinet 8

2.0 DRIVERS 8

2.1 Bass Drivers 8

2.2 High and Mid-range Drivers 8

3.0 CROSSOVERS 9

4.0 AMPLIFIERS 11

6.0 TESTING SOFTWARE 12

7.0 DIGITAL SIGNAL PROCESSING 13

8.0 BUDGET 14

APPENDIX 15

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Executive Summary What is one of the common problems in a conventional speaker system? Most systems produce a very good sound in certain �sweet spots� of the room they are in. This is great if you are listening in that sweet spot, but what if you want the sweet spot to be in different parts of the room? This is the task that our group chose to tackle. In a conventional 2-channel speaker system the sweet spot generally lies in between the speakers at some distance in front of them. If you are listening from a position directly in front of one of the speakers however, you tend to only hear that speaker and the other is drowned out. This gives the listener more of a mono, one-channel sound instead of the intended stereo 2-channel sound. We decided to attempt to solve this problem using a Digital Signal Processing (DSP) scheme called Wide Beam Forming (WBF). WBF imparts a delay on one channel of the signal while increasing the magnitude(volume) of the other channel. This allows the listener to hear both channels at the same time with an apparent equal volume from each channel. This allows the �sweet spot� to be moved to the listener instead of the listener having to move to it. We started planning our project by taking into consideration the recommendations from the previous year�s project. Their suggestions were to use a 2-channel system, try to reduce the amount of reflections imparted on the sound by the environment, and to have a flat frequency response of 3 dB from 20 Hz to 20,000 Hz. To satisfy these recommendations, we searched for a driver design that would provide the least amount of reflections. After looking at last year�s data and doing some listening tests, we found that the driver that would best fit our criteria was the KEF Uni-Q tweeter/mid-range driver. This driver consists of a tweeter and mid-range driver in a coaxial design that has the tweeter positioned inside the middle of the mid-range driver. This design is unique because it uses the mid-range cone to horn load the tweeter. We then needed to choose a woofer driver and cabinet design. Once again we used the results from our listening test and decided that a bass reflex box design with two 8 inch woofers for each channel, similar to the KEF design, produced an excellent bass sound. We chose the Vifa 8 inch drivers because of their flat frequency response and the fact that they were available at no cost to us from the local chapter of AES. After assembling our speakers we performed frequency response tests on them and encountered a few problems. Our choice of a passive crossover design created 2 problems. The first was the low crossover point (150Hz) which forced us to use large inductor values which saturated at certain frequency levels causing distortion to be sent to the tweeter. Secondly, the tweeter and mid-range drivers proved to be much more efficient than our woofers which failed our requirement of a flat frequency response. To solve these problems we decide to use an active crossover in a bi-amped system. This allowed us to use two separate amplifiers, one for the woofer and one for the mid/tweeter, therefore we could adjust the output gain individually to the 2 subsystems to match our 3 dB requirement. Switching to the active crossover also solved our distortion problem since we no longer needed to passively filter the amplified signal going to the woofer. Once these problems were resolved we re-tested the system and found that it performed within our specifications. Our overall frequency response was within 3 dB and our tweeter dispersion was about 40 degrees at 3 dB which is much narrower than other speakers we tested. Our last step was to implement the DSP board into the system. We started by forming and testing our algorithms in Matlab to simulate the Wide Beam Forming theory. We implemented this theory successfully into our simulation. Our problem arose when we had to translate our Matlab code into the DSP Assembly language code for our Motorola boards. We consulted experts in Assembly from the MTU Computer Science department, an MTU EE Graduate Student, and even Motorola. We feel that the problem may be in the age of the DSP board itself. Though the specifications of the board indicate that it should be sufficient, the quality of the output was not high enough to be satisfactory for a high-end stereo system. At the time of this report, we were successfully producing delay with the DSP board, and were very close to creating complete wide beam steering. We would recommend that next years project use the narrow dispersion system we have designed to continue the effort to put our theory into practice. Not having to build the speaker system would give them the advantage of spending most of their time implementing the DSP techniques.

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Introduction Welcome to the Team 14b(Dispersion) final report. Enclosed in this document you will find a review of our senior year Electrical Engineering (EE) design project. Every year in the EE department, senior students are divided up into teams to work on a design project for the entire year. This particular team is the Acoustic Design Team. The team members are Dan Cram, Craig Pelletier, Carl Rochon, Matt Berg, and Jason Karby. Originally the team was ten people but we were divided up into two different groups based upon two design areas. Our team is comprised of students specializing in Communications, Digital Signal Processing (DSP), or both.

Once we sat down and got to know each other, we had to come up with a design hypothesis. After brainstorming for a little while we finally decided on a particular idea. We wanted to try and design a speaker system that would have a movable �sweet spot�. A sweet spot is the place in the room where most of the sound of the speakers is directed, thus resulting in the optimum sound. None of us had ever seen a speaker system where this was capable. At this point we didn�t know if this was possible, but we felt that with some of our particular backgrounds in DSP, it was feasible. Taking this into consideration we came up with the following idea:

Problem Statement: To design a 2 channel acoustic speaker system that utilizes dispersion control and produces a moveable "sweet spot" that can be adjusted according to the position of the listener and the variables of the room. We intend to explore several possible methods of dispersion control including horn loading, DSP, and cabinet design. We also intend to design the system to limit the amount of reflection of sound in the room off of the walls. We will do this by trying to make our dispersion pattern as narrow as possible. Our project will focus on processing the signal between the amplifier and the speaker to achieve the desired effect. After looking into last year�s acoustic design project, we knew that our ideas did have

some possibilities. Their team did try to use Wide Beam Forming (WBF) in the project. Although, they didn�t totally succeed, their analysis, results, and suggestions provided a starting point for us in the area of DSP. This will be further discussed in the DSP portion of this report.

Unlike many of the other senior design projects we were not developing our product for a

customer. We felt that this was much to our advantage rather than to our loss. The reasons we felt that this was a good thing were because we were able to completely come up with all of our own ideas and concepts. We weren�t limited in any facet by a sponsor. The field of acoustic design, is as much of an art as it is a science. Thus, any distraction of creativity could potentially be bad.

Once, our project hypothesis was set, our team was given a budget of $1000 for the

entire year. As with any project our small budget forced us to prioritize what we wanted to actually purchase and what things we could build ourselves or possibly get donated.

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There were many steps taken to complete this project, the following report is a summary and documentation of those steps.

1.0 Cabinet Design and Construction

The subject of acoustic speaker cabinet design is indeed very broad. There are countless design alternatives and variations. Cabinet design is just as much an art form as it is an engineering problem. The most common designs are an acoustic suspension or a ported box. Most other designs are variations on these two ideas. Acoustic suspension boxes are completely sealed and usually offer very accurate response, while ported boxes tend to be more efficient, but sometimes sacrifice accuracy. The decision to use one cabinet for the lower frequencies and a separate one for the midrange and high frequencies was based on the necessity for an efficient, yet accurate cabinet design for the lower frequencies.

1.1 Bass Cabinets

The biggest challenge in reproducing low frequencies is to generate sound that is not distorted while achieving a �3 dB frequency response that goes down to 20 Hz. This is difficult because low frequencies have long wavelengths and require a much larger amount of air to be moved than high frequencies. Taking this into consideration, the decision was made to design a bass cabinet based on a design used by KEF Audio (UK) Limited in their Reference III� loudspeaker system. This decision was made for several reasons. First, research by Kraft, Marchi, Niemiec, Shabino, and York presented in �Design of Advanced Acoustic Speaker Systems, (1999), showed that the KEF Reference III� provided a very flat frequency response and a narrow dispersion pattern. The narrow dispersion pattern is necessary for our beam steering theory in order to limit reflections, especially in the higher frequencies. Next, a pair of KEF Reference III��s were provided by Dr. A.B. Kunz for a listening evaluation by our entire group. All group members agreed that the Reference III��s provided high sound quality, and their design was one that could be replicated effectively. Finally, this bass cabinet design provided the group a chance to learn many aspects of cabinet design because it combined a number of acoustic methods. Each bass cabinet uses two Vifa P21WO 8� drivers. Each driver is situated in its own chamber, which is ported into one common chamber. The common chamber is ported to outside of the cabinet. See Appendix __ for a diagram. The volume for the top and bottom enclosure was calculated using the following standard equation from McComb, Evans, and Evans� book, �Building Speaker Systems�, Master Publishing Inc, 1991. :

V Q Vb ts as= × ×2 15.87

V litersb = × × =0 30 15 105 49 72. ..87

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Where Qts = the resonance magnification of the driver, Vas = compliance of the driver. Once the volume was known, the actual dimensions are somewhat arbitrary. Our dimensions were chosen with a narrow, but deep design because a narrow front panel will generally improve the imaging of the speaker. The volume of the middle enclosure is arbitrary and was chosen to accommodate correct bracing and porting techniques while allowing for maximum building efficiency. There are a total of three ports in each cabinet. The top and bottom sections are each ported into the middle section with a 2 inch port tuned for a single 8� driver. These ports were placed in opposite corners of the cabinet to avoid interfering with each other (See figure 1). Their volume and length were calculated using the top and bottom section volume and the driver characteristics as follows:

Where 1.16 is the tuning frequency factor found in the McComb book, and 28 Hz is the free-air resonance of the speaker. The Tuning Frequency and the volume of the enclosure were then used to find the length for a port with a 2 inch diameter to be 4.9 inches. The middle section of the cabinet has a larger 4 inch port common to both drivers and ported to outside of the cabinet. Its length was found to be 5 inches for a tube with a 4 inch diameter. Bracing is essential in speaker construction in order to eliminate unwanted vibrations caused by the movement of the speaker drivers. Several bracing techniques were employed in the construction of the bass enclosures. A central brace was machined for us by Albert Dowdle that connected the two 8� drivers to each other. This brace ensured that each driver was feeling the same vibrations and cancelled out opposite vibrations. Also, a diagonal brace was placed in both the top and bottom sections of the cabinet to further reduce vibrations and to break up any resonances inherit in the box shape. Figure 1 shows the locations of these braces. Finally, the construction itself was done to minimize vibrations. The material we chose was 3/4 inch, medium-density particleboard. This provided us with a dense wood that was both more cost efficient and easier to work with than hardwoods that would give a similar result. The cabinet was built using lap joints, screwed, and glued for maximum strength, then caulked to ensure an air tight seal. Finally, each cabinet was filled with fiberglass insulation for baffling. Speaker baffling creates an acoustic illusion that the cabinet is larger that it actually is.

TuningFrequency Hz Hz= × =116 28 32 48. .

Figure 1 � Bass Box Internal

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1.2 Tweeter and Mid-Range Cabinet

The cabinet for the tweeter and midrange required a much simpler design. This is partly due to the fact that it is easier for a speaker driver to produce higher frequencies because less air needs to be moved. The combination of the midrange and tweeter into one unit as with the KEF Uni-Q� driver further simplified the design. (See Appendix and Section 2.0 for details on the KEF Uni-Q� driver). The volume for the cabinet was calculated for a 6� driver, which is the size of the midrange. (The tweeter is negligible in calculating enclosure volume). The volume was found to be 0.5 cubic feet. The midrange-tweeter enclosure was constructed very similarly to the bass enclosure, using 3/4 inch, medium-density particleboard, lap jointed, screwed, caulked, and glued joints, and fiberglass insulation for baffling.

2.0 Drivers

2.1 Bass Drivers

The bass drivers we selected were Vifa 8� woofers (see Appendix for spec sheet). We made our decision by looking at the Madisound catalog and comparing the available drivers. We were looking for a good flat response, but cost was also a factor. Luckily enough, the MTU Audio Engineering Society (AES) had these specific drivers on hand for us to use at no cost. The exact specifications of the bass drivers weren�t as important to us since WBF isn�t as effective at lower frequencies due to the non-directionality of the sound waves.

2.2 High and Mid-range Drivers After listening to a pair of KEF speakers, the Uni-Q coaxial driver was selected as our upper and mid-range driver. We were also advised by Dr. Kunz, an audiophile, to use these drivers because of their special design and narrow dispersion patterns. The concept of coaxial design is a tweeter built directly into the cone of the mid-range driver. This creates a horn-loaded effect for the tweeter. Although these drivers cost approximately $400 for the pair we decided that they were worth the price after listening to them because of their amazing sound characteristics, such as the dispersion pattern and the clean sound quality. Although these KEF drivers are difficult to find in the United States, we managed to find a U.S. distributor that had a pair. For comparison to our KEF system, we build a tweeter/mid box from our donated drivers from Vifa, thanks to David Stevens. Even before testing, it was clear that the KEF speakers should outperform the others, the testing results proved this, and this is better explained in the testing section of this report. Due to our relatively low budget, driver cost was a serious factor. In the audio industry, cost is proportional to quality; what you pay for is what you get. We decided to spend a large portion of our budget on our Uni-Q drivers. We feel that these drivers greatly enhanced our results.

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3.0 Crossovers

Designing a multi-driver system, such as the one we had decided to build, requires a crossover network. Different size speakers can more efficiently reproduce different frequencies. We decided to go with a common 3-way system, high-range, mid-range, and low-range. For this system we needed a 3-way crossover network to separate the full range signal from the music source into the desired frequencies of our selected drivers. The next decision was whether to use a passive or active design. -Active Vs. Passive Design

The biggest distinction between an active and a passive crossover is where they are located in the audio signal path. An active network is located between the signal source and the amplifier. The frequencies are filtered out at the signal level and then passed to separate amplifiers, one for each range of frequencies. Passive crossovers are located between the amplifier and the drivers. These networks are comprised of inductors, capacitors, or some combination of each. Some points that were considered while deciding which design to use were as follows:

1. How much money did we want to spend? Electronic crossovers on the average cost around $100. A passive network can range from $5 to $10 if you build it yourself, to as much as $50 for a professionally designed and packaged system. Also, active crossovers require multiple amplifiers, which can significantly add to the price of the system.

2. How much flexibility did we want? Active crossovers, with their continuously variable crossover points, gains and small size, are considerably more flexible than their passive counterparts.

3. What type of performance were we looking for? The efficiency, isolation, damping, and distortion ratings of active networks are better compared to passive systems.

After evaluating all the options we decided the most economical choice would be the passive network. The network itself is quite inexpensive and only requires one amplifier. We purchased some prefabricated circuit boards from Madisound. Later we found out that the same company would design the network for us for about 60 dollars. Considering we already had an amplifier donated by Dr. Kunz, we could fit this in our budget. After receiving the crossovers, we connected them to our system and quickly noticed that there was a problem. It seemed that the lower frequencies were getting passed to the hi-range driver, which did not produce a pleasant sound and could possibly cause damage. After talking with Madisound about possible problems we finally sent it back to them for evaluation. It seems that the iron core inductor, used for the network, was saturating thus causing the impedance of our network to drop below that which the amplifiers were rated for. This can be seen in the inductance plot of our passive network, as seen in the figure on the next page.

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We had suspected an iron core inductor would saturate easily but an air core inductor with the same value would have been physical to large to use. The large size of our inductor was due to the fact we wanted a relatively low crossover frequency of 150Hz.

During this process of dealing with the passive crossover Dr. Kunz donated another amplifier, so we now had two amplifiers. This allowed us to use a bi-amped system with an active crossover. One of our group members had an active crossover network he agreed to let us use. After testing the crossover we found it had a desirable characteristic as seen in this plot. Also, the crossover was adjustable which now gave us some freedom to experiment with different crossover frequencies and gains. We now have two amplifiers so we decided to designate one for the low range and the other for the high, and mid ranges, which were separated with a passive crossover. As can be seen in the MTX Crossover Characteristic graphic we chose to set the gain for the Low-Pass at +3dB and the gain for the High-Pass at �1dB. This selection of gains allowed us to compensate for the differences in efficiencies of the KEF drivers and our Bass drivers.

Inductance Plot of Passive Crossover

MTX Crossover Characteristic

-5-4-3-2-1012345

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Frequency (Hz)

Gai

n (d

B)

High-PassLow-Pass

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4.0 Amplifiers

In discussing our options for the amplifier section of our system we could design our own amplifier or use a manufactured model, which would probably produce a better sound than one we could design. Our advisor, Dr. Kunz, expressed the purpose of our project was not to �re-invent the wheel�, but to produce a good sounding acoustic speaker system. We were very fortunate to have top-of-the-line vintage amplifiers donated to us. Dr. Kunz donated four McIntosh units to us a pre-amp, 2 power amplifiers (MC245 and an MC2100), and a tuner/amplifier. We chose to use the pre-amp along with the 2 power amplifiers for our final system. Our system was connected in the following configuration

High-end Amplifierf>150Hz

Low-End Amplifierf<150Hz

Source Pre-ampActive Crossover

Passivecrossover

DSP Board

Computer*

* The computer is connected in order to program the DSP board

High-end Amplifierf>150Hz

Low-End Amplifierf<150Hz

Pre-amp Active Crossover

Passivecrossover

Computer**

** The computer is connected in as a source for testing

System Configuration

Tweeter

Mid-range

Woofer

Tweeter

Mid-range

Woofer

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6.0 Testing Software

The software used to test the speakers is Liberty Audiosuite (version 3.12). Once the speaker was setup in the anechoic chamber, a white noise signal was sent through to represent frequencies from 20Hz to 20kHz. Using Liberty Audiosuite requires that the computer have a separate DSP sound card, as the normal sound card will not work. In our case, an Echo card was install and used. Liberty Audiosuite was able to find the card and use it once it was installed properly.

The speaker was then rotated 10 degrees and another frequency sweep was taken. This was repeated until the speaker was turned 180 degrees. These plots were then combined together using a program called Polar 3.0 (created by Tim Sandrik) and Microsoft Excel to create a 360 degree polar response plot. Our goal was to build a speaker that had a narrow dispersion pattern, so we built a different cabinet for the high/mid-range drivers, and tested it the same way. By looking at the two plots it can be seen that the KEF drivers had a much narrower dispersion pattern, and hardly any high frequency content near the 180 degree point. Meanwhile, the other cabinet gave results showing that the dispersion was much wider and the signal was spread further into the back. The physical setup of frequency testing the speakers involved the anechoic chamber, a microphone (with pre-amp), and rotating wheel to mark 10 degree intervals (picture?). The pre-amp came from Liberty Instruments for these specific testing purposes. The amplified signal was

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then used as an input into the Echo card. The output of the Echo card was sent through our amps and crossovers and into the speaker in the anechoic chamber. One team member would stay in the closed anechoic chamber and rotate the speaker when necessary and another would take measurements with the computer. Finally, we gave a demonstration of our speaker to the MTU Audio Engineering Society. Some of their comments are listed in the appendix.

7.0 Digital Signal Processing The main focus of this project was to move the �sweet spot� of two channel audio signals around a room. To do this, simple wide beam forming theory was implemented. The theory uses gain changes and delay to adjust the sound field. The algorithm breaks down as follows. First, the distance from the listener to each channel�s audio source is calculated. The gain of the farthest source is then amplified by the ratio of the farther channel and the closer channel. Meanwhile, the closer channel is delayed by the difference of the two distances divided by the speed of sound. An example is shown in the figure. Ch1 and Ch2 are the channel sources, L is the listening position, and d1 and d2 are the distances to Ch1 and Ch2, respectively. In this case, the gain for channel 1 would remain the same, while the gain for channel 2 would be multiplied by d2/d1. Also, channel 1 would be delayed by (d2 � d1)/(speed of sound). To implement these processes into our audio signal, digital signal processing was used. Several different options for DSP hardware were available. After a discussion with David DiCarlo , a representative from Motorola, the decision was made to use Motorola�s DSP56030EVM board. This evaluation board is designed primarily for audio, contains the processor and codec, and is capable of processing two channels. The codec can sample at rates up to 48 kHz, which is acceptable for audio applications. The codec runs at 100MHz, and can perform 100 million instructions per second. When it came to implementing the algorithms we devised with the DSP board we ran into some difficulties. Our assembly language programs did not create the delay we needed. Rob Buse from MTU�s computer science department, Kyle Cooper from MTU�s EE grad program, and David DiCarlo from Motorola, were all consulted to help devise the code needed to produce delay with our DSP board. At the time this report was written, our group was successfully

L

Ch1 Ch2

d1 d2

Figure � Example of gain/delay calculation

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producing delay in one of the channels of our system. Given a little more time, we feel that wide beam steering could be accomplished with our system.

8.0 Budget The following table summarizes our expenses for the project. Team 14b Budget MTU Overhead Charge $100 Drivers $398 Cabinet Materials $75 Crossovers $74 Miscellaneous Supplies $162 TOTAL Expenses $809 Allowed Budget $1000 Balance $191

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APPENDIX

1 Asymmetric surround 2 LF suspension �spider� 3 Metal chassis 4 LF front steel plate 5 Cloth dome HF diaphragm 6 LF voice coil 7 LF rear steel plate 8 HF voice coil 9 LF pole piece 10 LF magnet assembly 11 HF magnet assembly 12 HF waveguide 13 LF cone 14 Baffle

23.5"

11.5"

45.5"

Brace

Port

Port

Middle Port

Bass Cabinet Design Actual Base Cabinet

KEF Uni-Q Driver

Legend for Uni-Q Diagram

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IMAGE GALLERY Testing Facilities

Passive Crossovers

Anechoic Chamber Testing Computer Setup

Passive Crossover Boards Distortion seen in Tweeter @ 150Hz

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Test Station Setup Diagram

Motorola�s DSP56030EVM board

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Listening test comments from MTU AES: �Overall clean sounding system. Perfect balance of bass to treble. Sound seems to wrap around you. Little to no distortion. I like it a lot.� - anonymous �All in all a very good sounding, well rounded system.� - Erik Szyperski �-Very detailed mid-range - Deep bass (Good round sound) - Bass is a little boomy at higher volumes� - anonymous �Clean, crisp high-frequency response. Solid, well defined Bass. Highs don�t rolloff.�

- Andre LaRouche �They are the best things I have ever heard in EERC 214.� - Jeremy