Introduction This document has been revised & re written Jan 22, 2001 In this demonstration, we show how
ETF5.x software may be used to optimize low frequency acoustic response of
rooms through trial and error measurements with various speaker placements. We
then determine the best room positions for foam absorbers, and use the ETF5.x
software to conduct "before and after" measurements to determine if
they are effective in achieving the desired high frequency response control. We will describe how and why each measurement is taken, and how to interpret the results. The ETF help topic "Getting Started" is designed to get the new user to both test ETF5.x with their hardware and to take a new measurement in the shortest possible time. To understand more about ETF, open the example files and surf through them. If a question arises, click help and an explanation of the current graphic will appear. The software provides all of the
measurement taking functions required to optimize room response using the
methods outlined below. The software and a few foam absorbers are the minimum
requirements to complete a project as in this example. This experiment has two purposes: Illustrate effective
methods that create audible differences not only for the "golden
eared" professional, but for the ordinary individual who normally may not
be interested in optimizing a sound system. Illustrate methods that
are both easily understood and cheaply implemented through features in ETF5.x software We have designed this article to be read and understood by an intelligent layman with an interest in, but not an extensive knowledge of, acoustics and audio systems. Tutorials posted on the support page of this site will explain the basic physical principles at work in this example. This article no more than scratches the surface of the large and complex field of small room acoustics. More knowledgeable readers will recognize where, for the sake of clarity and simplicity, we have skipped over some details and made generalities that are not actually valid in all conditions. All readers and ETF users are encouraged to learn more about this fascinating field. The Master Handbook of Acoustics (McGraw Hill) and our tutorials (support page) are a great place to start. BENEFITS In setting up the acoustics of home
theatre systems or two channel audio systems, better speaker placement leads to
smoother & more balanced frequency response across the hearing bandwidth
leading to better sound quality. Proper placement methods applied prior to
further corrections such as passive treatments and equalizers can reduce the
requirement for these additional devices. The measurements required to
optimally place the loudspeakers do not take long. Once a few hours are spent
experimenting with ETF a level of comfort with the program leads to taking all
of the measurements in each section shown here in less than an hour. EQUIPMENT We used the following equipment for
this demo: ETF5.x software and
& calibrated microphone Bryston listening room
as shown in the example. PMC MB1 main speakers
on 20 inch stands. Two Bryston 7B power
amplifiers, one for each channel. Bryston 10B-LR sub
woofer crossover. Bryston 3B Mono for
the sub woofer. PMC SB100 sub woofer Bryston BP25 preamp Denon CD player The main speakers are capable of more low frequency output than the sub woofer, so it doesn't really make sense to use it as a part of the sound system. It is, however, quite adequate for use in placement tests, as we plan to add a larger subwoofer to the system later. Note: Most of the measurements in this example can be done with an uncalibrated microphone. A calibrated microphone is necessary for EQ adjustment for high frequencies. Most omni directional electret condenser microphones, such as the one sold by Radio Shack as an SPL meter, work just fine for measurements below 500 Hz, as required for the optimizations shown in this example. Overview: Speaker placement considerations Determining the best speaker
locations in any given room is all about trade offs. The first such trade off
to be considered is practicality versus performance. There are positions in any
room where it is simply not practical to put speakers, such as a doorway,
hanging in front of a picture window, or sitting in the fireplace. Since you're
not going to put the speakers there no matter how good they might sound,
there's no point in measuring speaker performance in these positions. One should also keep in mind that for accurate stereo reproduction, the speakers should be more or less equidistant from the listener and each other. In other words, if a triangle is drawn with the listener and each of the main speakers at each vertex, all sides of the triangle should be close to equal length. It is also important that your main speaker placement should be symmetrical with respect to the walls of the room. If one speaker is 2 feet from the front wall and 3 feet from the left wall, the other should also be 2 feet from the front wall, and 3 feet from the right wall. Once these considerations are taken into account, you'll probably have a pretty good idea of the area's you'll want to place your speakers. ETF5.x can then be used to determine the optimum speaker position within the chosen area. The measurement process consists of 4 steps. (1) Subwoofer placement: The main speakers will be disconnected, and various subwoofer positions tested. (2) Main speaker placement: A single main speaker is tested in various positions. (3)Subwoofer / main speaker integration: The interaction between the subwoofer and the main speakers are analyzed. (4) Absorber placement: The best locations for foam absorbers are determined. These absorbers, typically placed on the walls and ceiling, can significantly reduce the reflections of high frequency sounds, and noticeably improve sound quality at the listeners position. Measurements will be taken before and after absorbers are installed to ensure the absorbers are acting effectively to remove these reflections. Part 1: Sub Woofer Placement The low frequency response is
primarily determined by the dimensions of the room and the placement of the sub
woofer within the room. This is why we can have confidence that the optimal
position found with our small subwoofer will also be the optimal position for
the larger unit we plan to replace it with. Optimum sub woofer placement
testing using ETF5.x will first be illustrated. Before you begin:
SETUP PROCEDURE Test positions for speaker location should include all aesthetically possible locations. These test locations are normally between 1 and 2 feet apart. The nature of the low frequency sounds the subwoofer produces is such that changes in speaker position much smaller than this will not have a major affect sound quality. After looking at the room, we've decided that for practical reasons, we'd like to put the subwoofer in either the front left or back right corner of the room. Using masking tape, we've marked 16 speaker locations to test, 8 in the front corner, and 8 in the back. We'll make two measurements for each location; one with the speaker upright, and another with the speaker on its side. We've somewhat arbitrarily chosen 8 locations in each area simply because ETF allows 8 measurement files to be overlaid for easy comparison of the response for each of the 8 locations. Exact placement of these locations can be recorded using masking tape on the floor. This saves having to get out the measuring tape each time the speaker is moved to a new location. Figure A below, shows the 8 locations to be tested in each of the two candidate areas.  For the subwoofer test, we won't concern ourselves with performance at frequencies above 140 Hz, as the crossover frequency we will determine later is virtually guaranteed to be lower than this. Due to the omni directional nature of sound at these frequencies, a labour saving trick becomes available to us: The sub woofer can be placed in the listening position and the microphone can be moved around the various test points! The sound sensitive end of the microphone is placed where the cone centre for the sub woofer would normally be, and the cone centre of the subwoofer is positioned where the listeners’ head would be. This saves the heavy lifting of the sub woofer and provides very similar test results as if the sub woofer was moved around the test locations with the microphone stationary at the listener. At this point, a few words should be said about positioning and securing the microphone. Ideally, you will have a microphone stand or boom available to you when you make these measurements. If this is the case, you will certainly find it easier to move the microphone around, rather than shifting the speaker over and over. If you don't have a microphone stand, you'll have to jury rig something. Keep in mind that the microphone should be secured at its base, and that no object should be within 5 or 6 inches of the sound sensitive end. You should also take care not to place the microphone on top of any object (such as an empty cardboard box, for example) that may resonate with the speaker sound, changing the rooms’ characteristics. Depending on what you rig up, it may actually be easier to leave the microphone in place and move the speaker itself around to the various positions. For this series of tests, the sub woofer was placed on the sofa with the cone within 1 foot of the normal listener ear location. The microphone, mounted on a boom stand, was placed at the various test locations. For each location, measurements were taken 9 and 24 inches above the floor, representing the subwoofer cone position with the speaker on its side or upright. To generate each measurement, the following steps were followed: After each measurement, check that the peak of the impulse is at 0 ms, if not, adjust accordingly (see help for Impulse Response topic). Have a look at the low frequency example file included in the download. MEASUREMENTS
When the measurements for all the locations have been taken, go to "Overlays" on the top menu, select "new", then add each measurement that you have taken to the overlay using the button at the left of the new overlay form. NOTE: Once measurements have been taken, there are several different ways in which the data can be graphically displayed. Certain representations tend to be better than others for studying different aspects of sound quality or room characteristics. The following measurements illustrate test results with the microphone in the positions shown in Figure A and the sub woofer located at the listener location. THIRD OCTAVE
MEASUREMENTS In this case, we choose to view the data collected as a Third Octave measurement, as this provides an indication of frequency balance while filtering out many room effects (room effects will be looked at in another section, later on). This smoothing highlights the response variation due to the effects of the low frequency sound bouncing off the wall, floor and ceiling (boundary effect cancellations). Each fractional octave measurement should be set to 1/3 octave, 200 ms gate time. We're now going to put together 4 charts; results for the front 8 locations, at 9 and 24 inches above the floor, and results for the back 8 locations, at 9 and 24 inches off the floor. What we're looking for: The following graphs show the sound levels the subwoofer is delivering to the listening position for various frequencies. Ideally, we'd like the speaker to be delivering equal sound levels across all frequencies; that is, we'd like to see a perfectly flat line from left to right. In addition, we'd like the sound level to be as high as possible, as this indicates higher speaker efficiency, and will result in less distortion for a given SPL (sound pressure level).
    In the back of the room, at both 9 and 24 inches above the floor, position 7 has the flattest, and position 1 the highest, frequency response. It appears the best subwoofer location is front location 1 or 5, or rear location 1 or 7. Upon further comparison, we determine that of these four curves, rear location 1 has the most energy, while rear location 7 has the flattest response. Which of these should be chosen? Well, that depends on whether or not a parametric equalizer is going to be present in the system. If you don’t plan to use an EQ, then rear location 7 should be chosen for its flat response. However, if an EQ will be present, it can be adjusted so as to "flatten out" the response from rear location 1, while still maintaining the higher SPL we have seen that position to have. As we plan to use an EQ, we selected rear position 1, with the speaker on its side (i.e.: the cone is 9 inches above the ground.). UNSMOOTHED FREQUENCY
RESPONSE Note: In this section, we
determine room mode excitation frequencies. These frequencies are
characteristics of the room, and can be compensated for using Helmholtz
resonators and/or quarter wave traps. This is a complex topic and
requires lots of experimentation to get to work in practical rooms. This is not
recommended for amateurs. You can set up a very good listening room
without using these low frequency correction devices, and many ETF users never
go through the time and expense of installing them. If you're not
planning on going that far, you may wish to skip to the
next section. So far, we've been looking at the one-third-octave
frequency response. This is a mathematical operation (the mechanics
of which we will not go into here) that serves to smooth out the frequency
response, as well as mask the characteristics of the room in which the
measurements were taken. Now, it's time to examine these room
characteristics, so we're going to display the data we've collected in a
different form; that of unsmoothed low frequency response. What we're looking for: Room excitation modes will be visible as upward spikes below about 100 Hz. We will also be able to spot sharp, drastic dips at other frequencies. These dips are due to the low frequency sounds interfering with themselves after reflecting off room surfaces, called boundary effect cancellations. The good news is that they are much more apparent on the graph than they are to the human ear. In this unsmoothed view, the difference in SPL is also more readily apparent. Figure A8: Overlaid Response This curve set is generated by adding successive *.etf files to a graphical overlay. This measurement set shows that the room resonances are visible for the measurements taken in all locations. All room modes get excited, independent of speaker location because these form the characteristic response of the room.
 Notice the dominant spikes at
the red arrows in all curves in figure A8. These modes
are slightly over excited, and these spikes will exist to varying degrees at
all measurement locations.This is the best graph to use for pinpointing low
frequency modes of vibration. You may find it more instructive to look at the data as a 3D waterfall graph. One of the above measurements taken was used to obtain the 3D graph below.  This graph is a little different
from others we've seen so far in that it has a third axis, representing
time. Moving from the background to the foreground, the lines represent
consecutive slices of time. In this way, we can see that different
frequencies fade away faster than others after the initial sound at 0 ms.
On the left, we see that at
several frequencies, the sound hasn't completely died away even after almost a
third of a second (300 milliseconds). These are the room excitation
frequencies. Much as each bell rings with its own characteristic tone,
these are the frequencies with which this particular room
"rings". The frequencies that ring correspond to the frequencies highlighted by the red arrows in figure A8. The 3d graph is very good for determining the relative sharpness of the resonance. A sharp (also known as "high Q") resonance is one that decays more slowly than others. The sharper the resonance, the more sound quality could be improved by correcting for it. Narrow bandwidth, low frequency room correction devices, such as Helmholtz resonators or quarter wave traps can be very useful in correcting these room deficiencies. As we have stated, though, their installation is not a trivial undertaking. If you decide to install such equipment you should thoroughly research the topic on your own.The Master Handbook of Acoustics (McGraw Hill) and the tutorials on this web site (support page) can provide additional information. CONCLUSIONS
The above tests resulted in two possible optimum sub woofer placements. Front location 5 gives the best response for conventional placement. Back position 7 delivered the highest SPL of any of the locations tested. It is the best choice if equalization is to be used, as the equalizer can be used to optimize response (i.e.: flatten the curve). The end result will be less cone excursion for a given SPL. Other positions may be tested
around the listening position using the methods for further experimentation in
near field sub woofer placement. It has been shown that the difference in response between the best and worst possible sub woofer placement can easily be as high as 10 dB over the range of interest. Careful placement using these techniques can result in a far superior bass response than may otherwise be achieved with arbitrary placement. Careful subwoofer placement is paramount in acheiving high fidelity sound. Part 2. Main Speaker
Placement Before you begin:
SETUP PROCEDURE
In placing the main pair of speakers in a room, there are two criteria one must meet to achieve good stereo sound. The first, as we've mentioned before, is the Equilateral Triangle Rule. The second is symmetry.If one speaker is, say, 3 feet from the back wall, and 2 feet from the side wall, the second speaker should also be 3 feet from the back wall, and 2 feet from the side wall. If one is to meet both of these criteria, possible speaker positions lie, for most rooms, not in an area so much as on a pair of lines, 60 degrees apart, extending away from the listener. In our room, we fudged a little bit, and tested speaker positions in a 2 by 2 foot square, rather than along the line itself. We were able to do this because of the size of the room; various positions in a 2 foot square, just over 10 feet from the listening position, could result in a not-quite-equilateral triangle, but would be close enough no to make a large difference. (Keep in mind that the diagram below is not to scale.) When testing the subwoofer, we were working with low frequency sounds. The nature of these sounds was such that when testing, we could move the speaker up to 2 feet before significant changes in response would become apparent. In this test, we are dealing with higher frequency sounds, which have their own characteristics. Performance in this region is much more sensitive to small changes in position. For this reason, it is appropriate to space main speaker measurements about 6 inches apart. It is also not possible, with these higher frequencies, to use the trick of placing the speaker in the listening position and moving the microphone around. For this series of measurements, the microphone must be fixed at the listeners’ location, and the speaker moved between tests. It is still a good idea to mark out the test positions on the floor with masking tape before commencing, so you don't have to get out the measuring tape every time you move the speaker. For each measurement, the speaker is positioned so that the front centre baffle coincides with each marked point on the floor. We're going to measure speaker performance at 16 locations. As ETF can only overlay 8 results at a time, we have, for convenience while viewing the results, split these 16 locations into 2 sets of 8 locations. We will refer to these as the front set, and the back set. If speaker height is adjustable, this test can be repeated for various speaker heights at 6 inch intervals. Figure B, below, shows the positions of the locations to be measured.  MEASUREMENTS
Two overlay graphs, one for each set of 8 measurements, will be generated with the main speakers at each grid location. This is accomplished in the following manner: Select "Full Range" bandwidth for this measurement. The range we are interested in is between approximately 50 Hz and 550 Hz. We expect fluctuations in response above 550 Hz due to the way in which these higher frequency sounds reflect from the walls and ceiling. We aren't concerned with these fluctuations at this point, as we will be mounting absorbers later that will reduce high frequency reflections. The following measurements were
taken with the loudspeaker centre front baffle at the positions indicated in
Fig B. What we are looking for: Again, we are looking for the flattest line, signifying roughly equal sound levels at all frequencies.
 Location 6 provides the flattest response below 500 Hz for the front set.  The back locations all give rapid
fluctuations in the response below approximately 100 Hz. This may be
problematic, as we have not yet determined our crossover frequency.
All of these locations may lead to poor main/sub woofer integration.
Above 100 Hz, none of the response curves appears any better (that is, flatter)
than our favourite position so far, that of front location 6. CONCLUSION Position 6 from the front set is
the location providing the best response. It provides a very flat
response between 50 Hz and 140 Hz, where the crossover transition is likely to
be. It is interesting to note that front location 6 is the spot that had been used for speaker location before the experiment. This choice was the result of many hours of careful evaluation by a listener with 25 years experience in setting up high-end audio systems. Part 3: Sub woofer/ Main
Speaker Integration Before you begin:
Set Up Procedure: In this section of the example a single main speaker and the subwoofer will be operated at the same time to evaluate the integration of the response between the two units. As you know, the subwoofer puts out primarily low frequency sounds, while the main speakers deliver the higher frequencies. What we want to do here is to establish a crossover frequency below which the subwoofer will operate, and above which the main speakers will operate. These two ranges inevitable overlap, so the second goal is to arrange for the smoothest response in the range of frequencies around the crossover point. Just how much you can do to achieve these goals is, to a large degree, hardware dependent. Before you proceed, you need to know the following about your stereo equipment: (1)Do you have the capability of adjusting the time delay between the main speakers and the subwoofer? This would be a feature of your amplifier or surround sound processor. This feature lets you compensate electronically if your subwoofer is closer or farther from the listener than the main speakers, so that sounds arrive from all speakers at the same time. (2) Do you have the ability to adjust the crossover point on the subwoofer? Some subwoofers allow the user to adjust this, and some do not. If you are able to adjust this, check the owners’ manual to find the crossover point the manufacturer recommends, and use this setting as the default position. (3) Do you have the ability to adjust the phase shift on your subwoofer?; Typically, this would take the form of a dial from 0 to 360, in 30 degree increments, on the back of the speaker. Even if your subwoofer will not allow this, you have the ability to perform a 180 degree phase shift simply by switching the + and - leads on the back of the speaker or the amplifier (but not both, of course!) (4) Use the "Low Freq." bandwidth option for this. If full range is used, the different speaker - listener distance for the subwoofer and main speaker must be the same or the subwoofer must be closer to the listener than the main speakers. The low frequency bandwidth option allows the subwoofer to be up to 25 ft further away than the main speaker for an accurate measurement. MEASUREMENTS All channels in a system should be measured for propagation delay. Appropriate electronic delay can be added to channels having the shortest propagation times so that all channels are synchronized in time for a particular listener position. The new Bryston SP 1 surround sound processor makes this adjustment between channels automatically. Take and save a low frequency bandwidth measurement of the system for each phase adjustment on the subwoofer. If your subwoofer does not allow phase adjustment, take and save one measurement, reverse the inputs on the back of the subwoofer (creating a 180 degree phase shift) and take another measurement. Compare these measurements, in the form of 1/3 octave frequency response, and determine which has the flattest response in the region from half the crossover frequency to twice the crossover frequency. If you are not able to adjust
crossover frequency, you are now finished. Use the flattest response of
the measurements you've taken. If you are able to adjust
crossover, try taking the same set of measurements with the crossover set 10 Hz
above and below the manufacturers recommended setting. Repeat for 20 Hz
above and below the recommended setting if you wish. The 24 dB/octave crossover slope provided by the Bryston electronic crossover made sub woofer / main speaker integration very smooth. We finally settled on a 70 Hz crossover point, which makes the transition region from 70/2 = 35Hz to &0*2 = 140 Hz. The response is held within just over 6 dB in this region.  CONCLUSIONS The response obtained by using the frontal sub woofer position was held within almost 6 dB in the transition region. Response may also have been improved if we'd had the ability to correct for time delay differences between the sub woofer and main speakers. The Bryston surround sound processor does this automatically This result was found to be excellent, subjectively speaking. Part 4: Absorber
Placement OVERVIEW: In part two, we optimized speaker
placement for frequencies below 565 Hz. We mentioned at the time that
much of the variation in response above this frequency was caused by sound
reflections that would be eliminated later by mounting absorbers in strategic
positions about the room. Above about 500 Hz, sound reflects
from hard objects in the room in much the same way that light reflects off a
mirror. We can take advantage of this behaviour. If a mirror is
positioned flat against the wall, ceiling, or other flat, hard surface in the
room, and a person in the listeners position can see one of the main speakers,
we know that high frequency sound coming from the speaker will also reflect off
that surface and reach the listener position, degrading the sound at that
location. In this way, we'll identify trouble spots that might be good
spots for absorbers. We'll then mount an absorber at each of these
positions in turn, after which an ETF impulse response measurement should be
taken to verify the correct position of the absorber and to verify that the
absorber is actually reducing the level of the reflection. If the absorber is
not necessary, it should be removed. Passive treatments, such as
absorbers, have an advantage over electronic processing. The room can
actually sound better without the audio system even turned on, and a
wider range of desirable listening positions become possible. The use of
absorbers will not greatly affect the perceived sound quality in a room, but
they will improve imaging qualities, particularly when used on the
ceiling. Human hearing did evolve to hear sounds from above. We were never attacked from
above, nor did we hunt above so hearing does not process sounds that come from above as
well as it does from at our level & below.
When you hear the difference it makes, you'll understand. Many audiophiles object to a ceiling absorber on aesthetic grounds. The owner of the room in which this test was run was dead set against having a big chunk of foam stuck to his ceiling. He made it clear the absorber was only going up for testing purposes. Once he heard the difference it made to the sound of the room, however, he was an instant convert. The ceiling absorber does not need to be hidden. The appearence is simular to a light fixture in that it doesn't get much visual attention itself. For side wall
absorbers, room layout becomes more important. Many audiophiles consider
that eliminating side wall reflections is good, but not essential. What is
essential is that equal reflections return from each side. This is why it
is recommended that your main speakers be placed at equal distances from the
side walls. If your room is symmetrical side-to-side, with no
doorways or windows near the areas you find to be reflecting spots, side wall
absorbers, while they would still improve sound, are not so important.
If, however, you find that reflections are not equal on both sides, the
wall reflecting more strongly should be equipped with an absorber. As a rule, thicker is better
with absorbers, as thin absorbers simply cannot absorb sounds below a
few thousand Hz. Some absorber manufacturers employ a certain amount of
advertising trickery. Often times one or two inch thick panel absorbers
are specified to being effective to as low as a few hundred Hz. This specification
is derived using the absorber in a completely different manner, and using test
conditions far removed from a typical listening room. Don't be fooled! We recommend absorbers be 6
inches or more to be effective over the entire high frequency region. In
this sample room, we compromised a bit for aesthetic reasons, and used a
ceiling absorber only 4 inches thick. As in speaker placement, this is a
case where practicality must be balanced with performance. SETUP PROCEDURE You will need another person to
assist you in the mirror placement, as well as a chair/ladder they can stand
on. Full range measurements must be used for all measurements in this section Mirror Trick: One person sits in the listeners’ position. From that spot, the listener observes a second participant move a mirror along the ceiling and wall surfaces of the room as well as any other suspect hard surfaces. Care should be taken that the mirror remains flat against the surface. Mirror locations that allow the listener to see the reflection of a speaker should be marked with masking tape. Place the microphone in the
Listeners position, and take a full frequency measurement of the room without
absorbers. Then mount a single absorber, take a measurement, and compare
the results with the "no absorbers" measurement. Take down the
absorber, move it to the next suspect location, and repeat the
procedure. In this way, you can use a single absorber to evaluate the
relative usefulness of absorbers in each location, and thus have the
information you need in order to determine how may absorbers you wish to purchase
for your listening room. MEASUREMENTS Below we will discuss several ways
in which reflections and absorber placement may be evaluated. All these
different ways of displaying the same data have their advantages and drawbacks,
and you may see any of them used in audio magazines and textbooks. In our
experimentation, we found that absorbers were of beneficial effect on the
ceiling, side, and back walls. Since we had enough absorbers on hand, we
installed them all. We used the following:
The "before" measurements
we display below are with no absorbers in place. For the
"after" measurements, all absorbers are in place.   The impulse response is the most
popular way of looking at this response. The large spike at 0ms
represents the original sound arriving at the listener position. The
smaller spikes represent reflections. They arrive later because, due to
their indirect path, they had farther to travel. As sound travels about
1.1 feet per ms (1ms = 1/1000 of a second), we can calculate the extra distance
each of these reflections traveled, and try to deduce which reflections cause
each spike. For instance, we can be fairly certain the first major spike,
at just over 2ms, is the ceiling reflection, as the speaker-ceiling-listener
distance is about 2 or 3 feet longer than the straight speaker - listener
distance. However, it is easier to simply place an absorber and see which
spike (or spikes... some of the smaller spikes are almost certainly reflections
of reflections that will vanish if the first reflection is absorbed). The problem with this
representation is that no frequency information is available. Because
lower frequencies are more spread out in time, and lower in level for the same
energy, this prevents them from being easily seen on the impulse response. The
result is that the energy spikes we see above are mostly from frequencies over
5000 Hz. A thin absorber would remove the spikes from this graph, but not
affect the room sound that much. Looking at the before and after
measurements, we see that all significant spikes have been removed by the
absorbers we've placed. We now know that the absorbers are in the correct
positions, but we still have little information on how much the sound has been
improved.  Figure D3 illustrates the linear (unsmoothed) frequency response and the effect of absorption on this response. Notice the reduction of comb filtering distortion after absorbers are placed. The reduction of comb filtering in itself is not so important, it is the reduction of the reflection that we are attempting to verify. Again, this data representation tells us that reflections, and the associated interference in sound wave patterns, has been reduced, but doesn't tell us much more.
  The above Energy-Time curves (also
referred to in the literature as envelope curves) are perhaps the most useful
in quantitatively evaluating absorber performance. In a room completely
covered in 100% efficient absorbers, we would expect to see sound energy drop
to zero after the initial pulse at 0 ms. This is, of course, impossible
in the real world. All the energy shown after 0 ms is due to reflection
of some sort. We know we can't eliminate it, but we can hope to reduce it
with the placement of absorbers. Notice that for the ceiling
reflection (in the first couple of ms), very little reduction in energy can be
seen in the 500 - 1000 Hz band, though the other frequencies are absorbed
well. This is the penalty we pay for choosing a ceiling absorber only 4
inches thick. The thicker an absorber is, the better it can absorb low
frequency sounds. 4 inches is just too thin to absorb sound from 500 to
1000 Hz, though it seems to have blocked the reflection of the higher bands
well enough. The amount of reflected energy in the room has been reduced significantly. The effect of this amount of absorption was to reduce reverberation time by approximately 50 ms across the range above 1 KHz. The room did not sound different for talking, except when standing under the ceiling absorbers. In this case the room did sound larger. Part 5: Listener
Position Sensitivity and Conclusions The careful set up of this room,
often with only one microphone position used, may lead some to believe that the
listener must be carefully seated for an optimum response. Tests for listener
position sensitivity were carried out by taking measurements at 8 positions
along the back of the couch where listeners might normally be seated. The
results are shown in Figure E1.  As you can see, there is little
variation in response from position to position when at higher
frequencies. Correct absorber placement can take much of the credit for
this. At frequencies below about 400Hz greater sensitivity is shown due
to the nature of the longer wavelengths associated with lower frequencies, but
the greatest variation in response is still only about 6 dB, which must be
considered minor. The response between approximately
30 Hz to 20 KHz is held to almost within a 6 dB set of limits for each listener
placed on a 6-foot wide couch. Objectively, the changes made in this experiment provide substantial improvements to the frequency response measured in the room. The best measured speaker locations
did coincide with the users original placement that was done with many hours of
experimentation by a listener with 25 years experience in setting up high
performance systems. Speaker placement alone has a 10 dB
- 20 dB effect on low frequency response. This can be optimized quickly and
effectively with only ETF5.x software. Click here to Return to Table
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