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Multichannel Audio

How do I align the levels of a multichannel system (using pink noise as a test signal)?

The main goal in the alignment of a multichannel system is to set the subwoofer output level the same as the sound output level of the main monitor system. The LFE output of the mixing desk or decoder should be connected to the LFE input on the subwoofer.

For the 7050B, 7060B, 7070A, 7071A, 7073A products:

The LFE input has the same sensitivity as all other signal inputs unless the ‘LFE +10 dB’ DIP switch is ON. The switch is used when there is no +10 dB gain in the LFE channel output. The switch is set to 0 dB when there is already a +10 dB additional gain in the LFE channel output.

What Reference Level to use?

To ensure repeatable results in the finished product, the SMPTE (Society of Motion Pictures and Television) has set standard monitoring levels for cinema post-production work. The SMPTE reference level at the listening position is 85 dB SPL, on C weighted/slow scale. The input signal to the monitors is -20 dB FS (rms) full bandwidth pink noise. The SMPTE RP200 uses an electrical reference level of -18 dB FS.

For music mixes, there are no standardized levels. The level that the engineer chooses is arbitrary and based on personal taste, as is the level chosen by the end user. The level is typically 75 dB SPL for television audio work and 75-95 dB SPL for music production work.

Manual Calibration of the Level and Frequency Response

Monitors are first calibrated to have flat response at the listening position. This is achieved by doing the following:

  • Calibrate the monitor frequency responses using an acoustical measuring system with the subwoofer bypassed or disconnected.
  • Then connect the Genelec subwoofer and adjust the subwoofer level, bass roll-off and phase so that the measured combined frequency response of the subwoofer and the monitor extends flat to the LF cut-off of the subwoofer, paying special attention to the subwoofer to monitor crossover point.

Alternative Level Calibration Methods

If acoustic measurement system is not available for aligning the system then follow the guidelines that can be found in the operating manual for adjusting the frequency response:

Level Calibration using a 1/3 Octave Real Time Analyser, Broadband Pink Noise and an SPL Meter

Connect the Genelec 5.1 system and play broadband pink noise signal (20 Hz – 20 kHz) through the subwoofer and one of the monitors, for example the centre channel monitor. Adjust the acoustic settings in the subwoofer and monitors so that the level in each band on the RTA analyser reads the same value. Then, set the output level of each channel to give the same acoustical level at the listening position.

Level Calibration using Filtered Pink Noise and an SPL Meter

You need to have filtered pink noise to calibrate the levels of the subwoofer and the main channels. You can use a copy of the TMH Corporation 'Multichannel Studio Test Tape' that includes the various test signals required.

  • Pink noise filtered to a passband 500 Hz to 2 kHz is used for adjusting the monitor levels
  • Filtered pink noise from 20 Hz to 80 Hz is used for calibrating the subwoofer level. Note: If the standard recorded level of filtered pink noises is -18 dBFSrms for SMPTE RP200's (-20 dBFSrms for SMPTE) and then the absolute level calibration can be made so that the sound level meter reads a level 2 dB lower than specified for broadband pink noise. This is because there is less energy due to band limiting of the band-pass noise.
  • Connect to monitors and play the 500 Hz to 2 kHz filtered pink noise. Set the SPL Meter to C-weighting and slow reading. Adjust each main channel individually to have the same SPL level at the listening position.
  • Play 20 to 80 Hz filtered pink noise through the subwoofer. The correct adjustment gives a reading 3dB lower than the one for the monitors because the C weighting lowers the reading in the SPL meter at those frequencies. If there is no HP filter in the SPL meter then the reading should be the same as for the monitors.

How do I connect the 7050B subwoofer to my 8020C's in a 5.1 surround system?

The 7050B subwoofer has balanced XLR IN/OUT connector pairs for five main channels and a dedicated LFE input connector for the LFE channel. Connect the signal cables from your source to the female XLR "IN" connectors on the lower connector row. Next connect XLR cables from the corresponding "OUT" male XLR connectors on the upper row to the input connectors of each 8020C monitor.

Turn the volume control knob on all 8020C’s monitors fully clockwise and switch the "Bass Roll-off" dip switch (switch 2) on all 8020C’s to "ON". This switch actuates an 85 Hz high-pass filter on the 8020C’s matching them to the main channel low-pass filter of the 7050B.

Alternatively you can connect to the 7050B a stereo pair of 8020C monitors by routing the signal cables from the source to the input connectors of the main monitors and an another pair of cables from the main monitors' output connectors to the "IN" connectors on the 7050B. In this configuration the volume controls on the main monitors affect the playback level of the 7050B too. The "Bass Roll-off" switch on the main monitors must also be switched to "ON" (switch 2).

 

 

 

 

How do I wire-up and configure a surround sound system?

We recommend running the audio signal cables first to the subwoofer(s) and from there to the monitors. When the subwoofer has less outputs than the number of subwoofers, the usual choice is to run the front monitors via the subwoofer and the rear/side monitors directly. Then, the subwoofer will only aid the front monitors in low frequency reproduction.

When the Smart Active Monitoring (SAM) uses distributed bass management the audio signals can be routed in any order to subwoofers and monitors. The bass management settings will be accomplished in the Genelec Loudspeaker Manager (GLM) software during the setup. The important aspect is that the same audio signal must run to the monitors and subwoofers for all the monitors that need subwoofer support.

How does the subwoofer +10 dB LFE level function work?

The LFE channel is usually recorded 10 dB lower than the main channels so that there is 10 dB of extra level (headroom) available.

Most processors automatically add 10 dB to the LFE channel to restore the level in the LFE channel. The subwoofer sensitivity must be the same as the sensitivity in the main monitors.  Then, the LFE channel level selection is set to ‘0 dB’ gain setting in the subwoofer.

The monitors should have a flat frequency response for accurate monitoring. This is achieved by calibrating the main channel frequency responses as described in the FAQ: How do I align the levels of a 5.1 system (using pink noise as a test signal)?

The block diagram below shows the whole production chain:

 

Some medium format mixing consoles and many smaller consoles do not have the facility to apply the +10 dB gain to the LFE. To overcome this limitation Genelec subwoofers provide a +10 dB LFE gain selection.

How does the subwoofer test tone work?

Subwoofers can output a tone at the crossover to enable testing of the proper phase setting in the subwoofer.

Phase adjustment is done after subwoofers and monitors have been placed in the room. The phase adjustments depends on the physical location in the control room.

I am designing a new studio, what monitoring system is appropriate?

Monitor selection is based on few fundamental requirements

  • What is the listening distance?
  • Should a subwoofer being used or not?

A larger listening distance requires a larger maximum sound output capacity in the monitoring product. A larger listening distance also requires a better directivity control so that the acoustic influence of the room remains low. Genelec publishes a selection guide in its Monitor Setup Guide to enable this evaluation.

If a subwoofer is not used, the low frequency cut-off and the room acoustic influences the monitors and determines how low frequencies are reproduced. Monitors usually work the hardest close to the low corner frequency, so the maximum sound level will also affect the choice. If a subwoofer is used, the subwoofer extends the low frequency cut-off and takes on the workload at low frequencies, so the monitors only have to work down to the crossover frequency of the monitor-subwoofer system.

What is Bass Management?

Bass Management means that a subwoofer is used in the reproduction of the low frequencies in all the audio channels instead of the monitors. Bass Management enables easier installation of monitors in the listening room as the acoustical problems in the room can be managed more efficiently.

 

What is the LFE (.1) Channel?

LFE stands for Low Frequency Effects. It is an additional audio channel in a mix, not needed by the main audio, but adding the possibility for a higher output level for low frequency effects. These are typically related to explosions, and low frequency rumbles. The LFE channel bandwidth is usually band limited to 150 Hz by the reproduction system.

Where do I place my monitors in a surround sound installation?

The audio presentation is only created correctly if the monitor orientation (angle) is correct. The monitor locations are usually specified as angles relative to the listener facing forward.

For stereo and conventional multichannel systems the monitors are assumed to be at the listener’s ear height. If the monitor height is higher or lower, sound perception will change and this will affect the sound image.

3D or immersive sound reproduction systems also use monitors at multiple layers at various heights or above the listener. Then also the vertical orientation angle must be considered and should be correct.

Vertically all the monitors should be at the same height, however, if there is a viewing screen or window in the way, the centre channel can be raised by up to 7° - this is allowed due psychoacoustic limitations in the ability of humans to resolve vertical directions. Lowering the centre channel is not preferred as the negative effects of the floor reflection are increased. The monitors' acoustical axis should be placed at ear height or the monitors can be raised slightly to help reduce floor reflection effects and to give a less obstructed path from the monitor to the listening position. Remember to angle them down so that they still point towards the listening position.

 

 

The acoustical axis should be angled horizontally and vertically towards the listening position for the best flatness of the frequency response.

All of the monitors are positioned at the same distance from the listening position, i.e. the monitors should be placed in a circle with the listening position at the centre of the circle. The Smart Active Monitoring (SAM) products contain an electronic delay to enable accurate distance alignment even in cases where the monitors cannot be located physically at the same distance from the listening position.

The distances from the monitors should be aligned precisely. The timing difference between the two monitors in a stereo system should be smaller than 0.1ms. Sound travels in air 340m/s, so in 0.1ms sound travels 34mm.

 

 

Time delay formula

tdelay = (dmax - dmon) / c

Where:

  • tdelay is the time delay needed to ensure that the monitor sound arrives at the same time as the others
  • dmax is the maximum distance of any of the monitors to the listening position
  • dmon is the distance of the closer monitor to the listening position
  • c is the speed of sound in air at 20°C at sea level = 344m/s

Example:

The centre channel monitor is 2.12m from the listening position. The left and right monitors are both 2.46m from the listening position. The time delay required on the centre channel is:

tdelay = (2.46 - 2.12) / 344 = 988µs (so set the delay unit to 1ms)

Where is the best place to position the subwoofer in a room?

The best location for a subwoofer can be determined by trial and measurements using the appropriate acoustical measuring tools. When measuring a subwoofer system you are looking for a smooth extension of the frequency response from the main monitor low frequency cut-off (85 Hz when using the internal subwoofer bass management crossover) down to the subwoofer low frequency cut-off point.

The subwoofer typically is placed slightly offset from the centre of the front wall. This gives the following benefits:

  • Increased acoustical loading from the front wall and floor improves the sound output in the subwoofer
  • No acoustic cancellations because of the front wall and the floor
  • Typically a relatively flat low frequency response in the subwoofer

If the sound is still not quite right then try the following:

  • Move the subwoofer slightly further away from the room’s centre line. This modifies the way the subwoofer interacts with the room.
  • Add a second subwoofer, located at some distance from the first subwoofer
  • If there are two subwoofers try moving them to the front corners or to the side walls as this sometimes helps with problematic rear wall reflections.

Some additional things to consider:

  • Subwoofers can be flush-mounted; the cabinet takes up less space in the room. No changes in the low frequency radiation load will occur.
  • Subwoofer driver can face the wall. A subwoofer can be placed with its side along the wall. Make sure there is enough space at the reflex port opening (at least 50 mm).  Subwoofers are not directional.
  • Do not place the subwoofer front baffle (driver) more than 0.60 meters from the nearest wall. This avoids acoustic cancellations in the subwoofer output.

Other tricks to try:

  • Place the subwoofer first at the listening position. Move the measuring microphone to different possible subwoofer positions (typically along the walls) to see which position gives the flattest response. Then, move the subwoofer to this position. This is called using the reciprocity principle of acoustics. It is easier to move a small microphone around than a heavy subwoofer!

Which subwoofers do you recommend for use with which monitors?

For all current Genelec products, please refer to our online Speaker Selection Tool.

For older discontinued subwoofers please see below.

 

Table Notes:

  • When using the digital inputs the 2029A and 2029B cannot be used with the 1092A and 1094A analogue crossover filters so the subwoofer is only for use with the LFE channel.
  • The 1034B, 1039A, 1035B and 1036A are all full range main monitoring systems. They will be no acoustic benefit from the use of separate subwoofer systems.
  • Subwoofers can also be used on the rear channels too although it is not shown here. Just select the appropriate model of the rear subwoofer from the 'stereo system' column.
  • If space or finances are limited then the rear channel models type can be compromised slightly by selecting the next sized model down in the Genelec range. For example, use a 1031A instead of a 1032A.

When two subwoofers are positioned close to one another mutual coupling is the fortunate by-product. This is due to the long wavelengths, associated with low frequencies, causing constructive superimposition. For mutual coupling the subwoofers must be place within ½ a wavelength of one another (85 Hz upper crossover frequency ½ wavelength is approx. 2m).