A pp l i c at i o n N ot eSurround Soundin Home Cinemas

Dolby 5.1
Number of fullaudio channels

Number ofLFE channels

Home cinema installations commonly include a Dolby or DTS surround sound system. For example a Dolby 5.1 system describes theconfiguration with 5 full audio channels (left, center, right, left surround, right surround) and one LFE (Low Frequency Effects) channel. The LFE channel originates from the cinema world with its rumbling bomb explosions and dinosaur stomps. These special soundeffects are generated by a sub woofer covering the frequency range
20 Hz - 120 Hz.
Follow these guidelines to optimize the surround sound performance of a home cinema installation.
1. Tips on Room Design for a Home Cinema
Listeners enjoy best sound performance from the direct speakersound. Any additional reflections from walls, floor or ceilings candeteriorate the surround sound image and even influence the perception of direction of the sound source. The basic guideline forthe architectural room design is to follow the common cinema
approach:• The side walls should not be parallel, they should widen fromthe front to the rear. This design leads any sound reflections
from the side wall directly to the rear wall.
• Sound absorbing treatment on the rear wall
• The ceiling should not be parallel to the floor• Carpets on the floor, curtains on the walls• Prevent any hard, solid reflecting materials, such as glass• Seating areas should be made with cushioned seats instead ofhard wooden chairs• Reverberative response of the room. An average-sized homecinema should typically have an RT60 time between 0.3 and0.6 seconds.

Exel Line: Digirator DR2 with
XL2 Audio and Acoustic Analyzer

www.nti-audio.com• Background noise level. Recommended Noise Curve level
NC 30-35

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2. Tips on Speaker Positioning for a Home Cinema
Align the speakers along the perimeter of a circle centered on
the main seating position (the golden seat). In case of space restrictions, for best sound performance the speaker arrangement
should be as close as possible to the circle alignment.			

Dolby 5.1 Layout

The Center channel speaker (C) is the most important speakeras it produces 80% of what we hear, which is dialog, and should
therefore be anchored above or below the screen.
Left (L) and Right (R) produce musical swells or things moving off
and onto the screen.
The optimal speaker height is slightly above the listener’s ears,
i.e. 130-140 cm.
The LFE sub woofer positioning need not be within line-of-sight of
the audience as bass sound is non-directional. However the LFE
positioning is important with respect to the Room Modes (seelater in this document). Commonly the LFE is situated on the floor
near the center speaker.
3. The Tools and Measurement Positions
Requirements:• Digirator DR2 with extended file set (from the DVD)• XL2 Audio and Acoustic Analyzer• M2211 or M2230 Measurement Microphone• Microphone stand• XLR audio cable
Exel Surround Sound Set

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Some surround systems complete a basic commissioning process automatically. The resulting sound performance is based on
default settings. This guideline allows you to customized settings
to suit your personal perception of sound quality.

Home Cinema Test Configuration

Measurements may be taken at the golden seat and repeated at
the four corner seating positions.
The NTi Audio Digirator DR2 generates digital surround sound test
sequences for verification and adjustment of professional Dolbyand DTS installations.
The XL2 Audio and Acoustic Analyzer in combination with theM2211 or M2230 measurement microphone measures the acoustic output.
All data captured by the XL2 may be documented on your PC for
backup and later verification.

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4. Overview of the Recommended Tests

The recommended tests for a home cinema installation

System• Presencea quick check to establish that sound is present at each speaker at a reasonable sound pressure level.
• Polarity
ensure that all speakers have the correct polarity.
• Gain Structure
optimize the signal level throughout the entire signal chain.
Room• RT60determine if the room may need more absorption treatment orreflection redirection.
• Noise Curves
determine if external sound sources (e.g. room’s ventilationsystem) can be reduced or removed, or if the room may need
to be more insulated (e.g. from street traffic).
• Room Modes
discover resonances (particularly at low frequencies).

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A pp l i c at i o n N ot eQuality• Delayssound should arrive at the golden seat from all directions at thesame time.
• Frequency
Measure						the Average Frequency Response for the room. Mea
sure the frequency response at several points and get
an average picture of the room.
or				

at the Golden Seat. Mmeasure the frequency 		
response at the golden seat position.
Adjust				the equalization to smooth the frequency response 		according to the X-Curve. Measure again.
Verify						that the shape of the spectrum at each audience 		member’s listening position contains no significant 		
deviations from the X-Curve.

• Levels
all speakers should provide appropriate sound pressure level.
5. How to Use the Tools
Presence
Check that all channels are connected and set at the typical listening level.
• Connect the Digirator to a system input and selectDLBY -> D_51 ->Ch_i_l (identification for left channel),
Ch_i_ c (center),
Ch_i_ r (right),
Ch_i_ rs (right surround),
Ch_i_ ls (left surround),
Ch_i_ lfe (lfe),
consecutively.
• Sit in the golden seat and listen.
• Adjust the gain for each speaker as necessary.

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A pp l i c at i o n N ot ePolarity
The polarity test verifies the correct cable and speaker polarity.
Incorrect polarity may result in a completely wrong stereo image
and cancel out some frequency ranges.
• Digirator: Select DLBY -> D_51 -> Pola_all• XL2: Select Polarity -> Signal Source -> XLR Input
Opposite polarity resultsin cancellations• With the measurement microphone connected directly to theXL2 Audio and Acoustic Analyzer, position the microphone 10to 20 cm in front of the left speaker, pointing at the center ofthe speaker cone.
• Measure the polarity of each speaker, saving a screenshot of
the XL2 Polarity Details screen for each.
It is a fallacy to assume that all speakers should have positive
polarity as some loudspeaker manufacturers intentionally design
speakers with negative polarity for better sound performance.
Rather, speakers of the same model should have the same polarity frequency pattern.

XL2 Polarity Details Screenwith polarity frequency pattern

If the polarity measurements are not correct, verify that the measurement microphone is consistently positioned relative to eachspeaker during the test, then invert the speaker cabling and measure gain.

Gain Structure
Review the entire signal chain and ensure that the gain structureis optimally configured so as not to unnecessarily decrease thesignal-to-noise ratio or introduce clipping. Try to run all components at unity gain*. If you need to interrupt the signal chain tomeasure, use the XL2 RMS/THD function.
• Digirator: Select DLBY -> D_51 -> Sine_1k• XL2: Select RMS/THD+N• Connect the audio line at the desired measurement point
directly to the XL2 Audio and Acoustic Analyzer.
* Unity gain (also known as “0 dB”) is where both input and output are at the same voltage level.

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RT60 (Reverberation Time)To measure RT60 you need to inject Pink Noise at 35 dB above
the noise floor in all of 8 frequency bands. It is best to utilize anomni-source such as a dodec. We will measure at each of the fiveseating positions to obtain an average RT60 result for the roomand measure at least three times at each position.

On the XL2 select RT60. With a quiet room, press the SET function to measure the room noise floor.

Digirator: Select DLBY -> D_51 -> Pnoise

Now use ear protection.
Play the Pink Noise and turn up the system volume until the blackbars cover the grey bars. (You may want to boost the LFE).

Press the PLAY button on the XL2.
Allow the Pink Noise to play for 5 seconds and then mute the Digirator. The black bars decrease and the AVRG number increases
by 1. Play the Pink Noise again for 5 seconds.
Repeat to measure at least 3 cycles, then press the STOP button on the XL2.
To see the results, press the Page button.
Measure again with the mic at the next seating position.
For tips on measuring RT60, see Appendix A
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Noise Curves
Noise Criterion (US) and Noise Rating Curves (Europe) define thelimits of octave band spectra for background noise that meet thedesign requirements of your home cinema installation.
Background noise can be caused, for example, by the room’s ventilation system or automobile traffic in a nearby street.
For a home cinema, the recommended level is NC 30-35 or NR35-40.
We will measure at each of the five seating positions. To measure, connect the measurement microphone to the XL2 Audioand Acoustic Analyzer.
Ensure that the room is quiet.
On the XL2, select “Noise Curves” from the Main Menu.
Choose your required Noise Curve Type and Standard.
Press the Play button to measure.
After 30 seconds, stop and save the measurement result.
Save also a screenshot of each result.
Measure again with the mic at the next seating position.
Drop _Report.txt file into MS Excel and insert screenshot too.
Room Modes
Resonance is the tendency of the air or items in the cinema room
to oscillate with greater amplitude at some frequencies than atothers.
Room modes are the collection of resonances that exist in aroom.
The input of acoustic energy to the room at the modal frequencies causes standing waves. These standing waves result in theloudness of the particular resonant frequency being different atdifferent locations of the room.
An acoustic resonance usually occurs at more than one resonancefrequency; especially at harmonics of the strongest resonance.
Excessive resonance can have a negative effect on the quality ofthe system, so it’s a good idea to get a feel for the resonances in
the room before you tune your system.
To test for excessive resonance, the room should be injected withenergy at each frequency. It is usually sufficient to examine within
the frequency range 20 Hz to 300 Hz. Play a 5-second chirp between these frequencies. Walk around the room and you will hear

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the resonant frequencies. Also during this test you determinethat there are no audible rattles or buzzes caused by mechanicalinstallations in the room.
TIP: The frequency bands with the longest RT60 times may indicate where to find resonant frequencies.
A cure may be as simple as re-positioning the LFE speaker, or
you may need to add absorbent material to the room to damp
such resonances.
Equalization of the sound system to compensate for the unevenfrequency response caused by room resonances is of very limited use as the equalization only works for one specific listening
position and will actually cause the response to be worse in otherlistening positions.
Delays
The sound from all surround sound speakers should arrive at thesame time at the golden seat. To achieve this, signals will have tobe delayed for those speakers that are physically located closer
to the golden seat.
• MR-PRO: Select DELAY (delay test signal)• XL2: Select Delay measurement function• Connect the MR-PRO to the XL2 with an XLR cable to synchronize the clocks of the two devices.
XL2 Screenshot Delay• Now connect the MR-PRO to an analog input of the system,and play the delay signal consecutively through each speaker.
• Measure with the XL2 at the golden seat position and adjust
the delay time for each speaker appropriately.
Note: Do not connect the measurement microphone to the XL2
as the internal Voice Note microphone is used during this test.
Frequency
The next quality test is to shape the frequencies of the sound.
The frequency response differs at every seating position. In order to enhance the quality of sound for all seating positions you
should optimize the shape of the frequency spectrum while ensuring that each audience member’s listening experience contains no significant frequency peaks or troughs. You then averagethese measurements to get an overall picture of the room.

XL2 Test “TemplateHomeCinema“

www.nti-audio.com• XL2: Select 1/12 Oct function -> Load Test “TemplateHomeCinema” (1/3 OCT resolution, EQ value, single mode, SET
00:00:20, XCurve_SMPTE202M tolerance)
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A pp l i c at i o n N ot e• Digirator: Select DLBY -> D_51 -> Pnoise_l (pink noise signal atleft channel)• Connect the microphone to the XL2 Audio and Acoustic Analyzer using an XLR cable.
• Consecutively position the measurement microphone at eachof the seating positions; the golden seat, front-left, front-right,back-left and back-right, using a microphone stand to set themicrophone to the typical height of the listener’s head. i.e.
130-140 cm, and pointing the mic in the direction of the speaker from where the sound is being produced.
At each position,• Stand away from the microphone so as not to influence theresults by you absorbing or reflecting sound.
• Press the play button on the XL2. The equivalent (EQ) frequency response will now be measured. The measurement willstop automatically after 20 seconds.
• If the limit light is red at the end of the measurement, adjust
the equalization of the system to smooth the frequency response to within the tolerance band and remove peaks andtroughs*. Start measuring again from the first position.
OTHERWISE• Capture EQ and store it with an appropriate name, such asGold for the golden seat, FrLf for front-left, FrRt for front-right,
BkLf for back-left and BkRt for back-right.
• Calculate the Mean value and display it.
• Save Test with an appropriate name, such as Speaker_Left_Tol,
for later retrieval of the Mean value and the tolerance data toyour PC.
• Exit tolerance mode and Save Test again with an appropriate
name, such as Speaker_Left_Data, for later retrieval of themeasurement data at each seat position to your PC.
• Repeat the entire process for each of the other 4 speakers.
• Connect your XL2 to your PC, open the plain text files thatwere saved (\Projects\MyProject) and paste the data into areport. e.g. “NTi Audio XL2 Frequency Report.xlt”.
* A note on equalization: Equalization of the sound system tocompensate for the uneven frequency response caused by room
resonances is of very limited use as the equalization only works
for one specific listening position and will actually cause the response to be worse in other listening positions.

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A pp l i c at i o n N ot eLevels
All speakers should provide the required sound pressure levelmeasured at the golden seat.
• Connect the measurement microphone to the XL2 Audio andAcoustic Analyzer using an XLR cable.
• Position the microphone at the golden seat, using a microphone stand to set the microphone to the typical height of thelistener’s head. i.e. 130-140 cm• XL2: Select SLMeter, 123, LCS.

XL2 Screenshot Sound Level Meter• Connect the Digirator to a system input and selectDLBY -> D_51 ->Pnoise_l (pink noise signal at left channel),
Pnoise_c (center), Pnoise_r (right),
Pnoise_rs (right surround), Pnoise_ls (left surround),consecutively.
• Measure. If you do not get a stable result, increase the overallsound pressure level. Adjust the sound pressure level of eachspeaker to the required relative level.
• Note the typical LZS sound pressure level at 1 kHz displayed in
the RTA mode. This reading is required for the LFE level setting.
The LFE channel is tailored for the rumbling bomb explosions anddinosaur stomps. The LFE sound pressure level shall be about
10 dB higher than the surround level within the typical LFE frequency range 20 Hz - 120 Hz.
• Digirator: Select DLBY -> D_51 -> Pnoi_lfe (pink noise signal atLFE channel)• XL2: Select the SLMeter, RTA Mode, 1/3 oct., LZS• Measure the sound pressure level of the LFE sub woofer inthe lower frequency bands 20Hz - 120Hz. The setting shouldbe about 10 dB higher than the reading at 1 kHz of the fullaudio channels.

Level alignment of LFE and surround sound speakers
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Appendix ATips on measuring RT60
Use ear protection during the RT60 measurements.
The pink noise used to generate the signal for the RT60 measurement must be at least 35 dB above the noise floor in eachfrequency band. To achieve this, it is often necessary to boost
the lower band frequencies with equalization or by turning up theamplifiers to the LFE speakers.
Within each cyclic RT60 measurement it is necessary to saturate
the room with as much sound energy as possible within eachfrequency band. This “saturation” occurs when a sound is played
for a continuous period and that sound has “had enough time toreflect on all possible surfaces”. The RT60 value for the room is agood indication of the time required for this saturation to occur.
Thus for the pink noise we choose a cycle time of at least theRT60 value of the room.
What is the Correlation Percentage?
After the pink noise is turned off, the XL2 starts to measure thetime taken for the energy in each frequency band to decay. It may
be the case that sound in the environment does not decay in a
linear manner. This is indicated by a low correlation percentage.
For example, the room may consist of more than one area of different sizes, or a door may be open, or objects in the room may
rattle at certain resonant frequencies and sound pressure levels,or the background noise may be sporadic. All these may interfere with the linearity of the decay during the RT60 reading giving
you either an incorrect RT60 value or no value at all. Some suchinconsistencies, such as the room consisting of more than one
area of different sizes, may not be fixed. Others may be fixed. For
example, if you can not completely remove the background noise(traffic noise from outside), then measure many RT60 cycles anddelete the outlying cycles.
What is the Uncertainty Percentage?
Pink Noise is by nature a fluctuating signal. Over a longer periodof time, Pink Noise contains all frequencies. Over a short period,certain (particularly low) frequencies are not always present. The
possibility of the absence of such frequencies is indicated by theuncertainty percentage. Uncertainty is thus higher at lower frequencies and with shorter RT60 values. To reduce this uncertainty
we measure at least 3 times at each measurement position.

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