To set up gain and levels for monitor systems and other mid-sized loudspeaker systems it is recommended to also follow the standards SMPTE RP 200-2002 and SMPTE 202M-1998. These are the standards for loudspeaker setup and alignment in cinemas and control rooms and proofed working perfect to set up systems for loud sound reproduction without distortion and with good signal/noise ratio.
Today more or less all devices are working in the digital domain, but interconnection is often still done in the analog domain. Connecting digital devices via analog interconnection requires knowledge of the equivalent analog level to digital fullscale for each device to get a straightforward and even level management between the individual devices. Especially the level differences between consumer and professional gear can be the cause of large but often undiscovered problems. A consumer device with a "0dB" output of -10dBV feeding a professional device with a nominal "0dB" input of +4dBu or +6dBu will generate an approximate level drop of about 12dB in the professional device. Because of that this device will always be 12dB lower in all levels and therefore also in the dynamic range with a less signal/noise ratio. Worse is a interconnection of a professional device with a nominal output signal of +4dB or +6dB fed into a consumer device that will have probably a nominal input of -10dBV. Here this device is overloaded by 12dB. With a bad signal management (all levels too low, amplifiers to far open) this is by some installers not even discovered ..
|signal chain with analog interconnections between devices
|signal chain with digital interconnections between devices|
All devices in any audio signal chain should be set up in a way that all units have about the same headroom before clipping (0dBFS for digital or the specified clipping point for analog) to guaranty a maximum dynamic range and the same headroom for all units.
Interconnecting digital devices normally the interconnection levels are set automatically (defined by the signal itself). Here it is important to keep the bit depth of the internal processing of the individual devices in consideration. The entire audio quality is always depending on the weakest device in the chain. This will be for instance any 16-bit gear. If it could be chosen any audio processing (general level settings, equalization) should always be done in the device with the highest bit depth.
See: Gain Structure.
The amplifier's continuous power rating should be the about same as the program power rating of the connected loudspeaker(s). Program power rating is often also called 'music power'. If the loudspeaker is spec'd only with it's continuous power rating, the amplifier's continuous power rating should be about twice the loudspeaker's continuous power rating. A 100W continuous power loudspeaker should be drivel by a 200W continuous power amplifier. This is 1) to avoid clipping the amplifier what can destroy the tweeter and 2) to have enough undistorted power reserve at the amplifier to push the loudspeaker to its full capability. The amplifier needs this power reserve to amplify the signal with all its peaks without any distortion.
Here it must also be kept in consideration that signals with higher crest factor need enough power reserve. A signal with a crest factor of 10 dB (regular percussive music) needs about ten times the power for its headroom (10dB = 10 times power): a 100W RMS signal needs a reserve of another 900W to be capable to put out 1000W for peaks within the music!
Using two loudspeakers in parallel, each loudspeaker receives half the amplifier power, four loudspeakers receive a fourth of the amplifier's power each.
An often seen misunderstanding is the idea that a power amplifier's input controls should be all the way open to enable 'full power' and that a turned down input control knob of an amplifier limits the available output power. This is completely wrong! The input control of an amplifier just controls the input voltage. The more the control is turned down th higher the input signal could be without overpowering the input stage of the amplifier (if the amplifier is designed properly). If the input stage of an amplifier can be overloaded despite a turned down input control knob, the level should be 'pre-controlled' in the prior device in the chain.
The output sound power of the amplifier is only depending on the active level of the input stage of the amplifier. (high input level with controls down equal low input level with controls up).
|gain settings and output power|
All used devices in the audio chain should work on the optimum level for good headroom and good signal/noise ratio and the amplifiers should control the listening level produced by the loudspeakers.
The system should be fed with wide-band pink noise at the first input stage after the system is equalized (!) and all levels in the individual units are set (!). The test signal should be set in a way that all devices indicate a processing level of -20dBFS.
SMPTE (Society of Motion Pictures and Television) defined the reference listening level for film mixing as 85dB SPL(C) 'slow'. This acoustical output should be measured with an input test signal of -20dBFS (RMS) full-bandwidth pink noise. SMPTE RP200 defines as the electrical reference level a level of -18dBFS (RMS). (This difference is not interpreted clearly in the literature.) Here the reference level of -20dBFS is used.
Every loudspeaker channel is tested separately with the other channels muted (!) Now the individual amplifier (or the last device before the amplifier) is set to generate 85dB SPL measured with a calibrated SPL meter, set to 'C-weighting' and 'slow', measured at the main listening area.
To avoid wrong measurements caused by strong room modes (standing waves) it is recommended to use a measurement that shows the frequency response of the measured signal (RTA) instead of a SPL meter. In this case the individual bands of the measurement will not show the above mentioned 85dB but 70dB SPL at each individual band.
Look for pink noise sound files at the download area .
|all devices in the audio signal chain at -20dBFS / generated output level 85dB SPL(C)|
This approach guaranties that the listening levels are moderate and all audio devices within the system work on proper levels. By driving the system to the limit a maximum level of 105dB SPL is reached before the entire system would clip.
To set up a Rock&Roll system the same approach can be used but it should be set for at least 20dB more output level measured at the listening area. 105dB SPL would than be the working range and 125dB the clipping point.
The loudspeaker positions have a great influence of the general performance of the speaker system. The wall behind a loudspeaker can cause cancellation and comb filtering. Placing a speaker a quarter of a certain wavelength away from a wall the associated frequency will be partly cancelled out if the wall is reflective enough. If the dip is strong enough (and all other dips of the comb filtering) these anomalies are audible. Dips caused by cancellation cannot be removed by equalization! To avoid these cancellations loudspeakers can be installed as flash-mount speaker systems or installed in a sound baffle (as required by THX).
If no baffle or other flash-mount solution is possible the loudspeakers should be placed as close to the wall as possible. This way the frequencies with cancellations are in a much higher range with a higher number of smaller cancellations. These are not as audible as large dips in the lower frequency range.
If smaller satellite speakers are used that don't reproduce lower frequencies, the wall-distance issue is not as critical as with full range systems (because the critical frequency range is not reproduced by these smaller speakers). The usually maximum 'allowed' wall distance for satellite speakers is about one to two meters. For free standing full-range loudspeakers the low-frequency level gain because of half space, quarter or eighth space positions must be kept in consideration:
|theoretical gain for frequencies below about 150Hz :|
|Q = 1||Q = 2||Q = 4||Q = 8|
The left and right loudspeaker should always be installed symmetrical for accurate imaging. Precise imaging requires exactly similar frequency responses of both loudspeakers. The frequency response of a loudspeaker in a real room is very much influenced by the room itself, the wall distances and all other reflective or absorptive materials. Also all reflecting surfaces in front of the loudspeakers towards the listening position should be avoided as much as possible.
Loudspeakers should be aimed toward the listening position to ensure the acoustical axis of the speaker is directed straight to the listener (ear height). Because loudspeakers are never optimal in all angles they are most optimized in the acoustic axis.
The phase (polarity) of the speaker channels can be checked by listening to test signals. For phase test between the main Left and Right channels or between the Left/Right and Center channel a stereo test signal of 300Hz is used. The stereo signal is fed to the both channels under test. With the speakers in phase the tone will be heard right in the center between the loudspeakers. With speakers out of phase the tone will be heard spread out far left and right. One of the test signals is 'in phase' and one is 'out of phase'. The appropriate tone position should be heard. The test tones can be downloaded here.
|300Hz stereo, In Phase||Play|
|300Hz stereo, Out of Phase||Play|
For the phase (polarity) test between the main channels and the subwoofer a low frequency test signal is used. Different sinus tones can be used depending on the cross over frequency. If a sinus test tone is used it should be as close as possible to the crossover frequency. Sinus tones of 85Hz / 100Hz, 150Hz can be downloaded here. Also pink noise signal (20Hz to 150Hz) van be used and can also be downloaded here. The signal is fed parallel to the main channels one after the other (L, R and Center) and the subwoofer channel. The polarity switch for the subwoofer should be set to the position there the audible level is loudest.
|pink noise signal 20Hz-20kHz||pink noise signal 20Hz-150Hz|