Effective surround monitoring
Nov 1, 2005 12:00 PM, By Alex Kosiorek
During the past few years, I have seen the topic of surround sound grow from a small topic to one of prominence at radio and audio engineering conferences. It is considered by some to still be in its infancy, but with others it has taken flight. Certainly, we are still at the beginning stages of surround and it will continue to evolve. In radio broadcasting, there are still many factors to be resolved. However, no matter where you are in the conversation, we all need to understand the basics of how to produce, distribute and broadcast surround sound for it to be successful.
Let's start with the monitoring system. It is important to review and ascertain how to setup a surround sound monitoring system appropriately for the medium to which it is applied. Without this understanding, it is possible to produce material that won't exhibit the full benefits of surround or won't translate well.
First comes the speaker placement, which in home theater systems will be all over the map. Inconsistency is guaranteed. Room dimensions and speaker placement will be different in every household. The consumer will set up his system to fit his physical and aesthetic environment. The speakers themselves are unknown variables. Home theater systems use five or six satellite speakers, ranging from less than four inches to full-range (two- or three-way) speakers. The front speakers may be of different size and quality than the rear speakers. Correspondingly, the accompanying subwoofer can range from six to 18 inches. However, it is assumed that most consumers will acquire a system with smaller satellites and a subwoofer. Regardless, it is unlikely that many consumers in this environment will have a consistent sweet spot.
The automobile is a sharp contrast. Though the physical environment may not be ideal and is smaller than the home, the speaker placement is much more consistent. The advancements in automobile speaker design and amplification systems also make the sweet spot more consistent. An interesting similarity between home theater systems and the automobiles is that the size of the speakers and subwoofer become uncannily similar. Although the subwoofer can and will vary in size in the vehicle, it can produce low frequencies with more ease because of the size of the environment.
Figure 1. The optimum speaker placement as recommended by the ITU.
In most cases, the surround content will be music, talk and sports, not movies. Therefore, we will immerse the consumer in a sound field where the sound will be heard all around him coupled with localized events. This is different than movie mixing, where the sound usually has a central focus toward the front with a less immersive experience.
The consumer's listening environment and the content play a role in how a monitoring system is set up in the control room. A radio surround monitor setup will be different than that for film mixing. Knowing that, a few things can be said for how to produce content for surround. The first is that neither a mixing engineer nor those monitoring broadcasts should be locked to a sweet spot. The other is that the mixing engineer or broadcaster should listen on a quality monitoring system and a consumer-grade system. These should be radically different from one another in size and quality.
In an ideal situation, surround will be monitored in a room with appropriate low-frequency absorption and without two walls that are the same length. Consult an acoustician who is aware of all the variables of surround-sound monitoring. Because budgets and other constraints may not allow an ideal room to be constructed, at least pay attention to room reflections. Make every effort to minimize first-order reflections from any given monitor. Also, use as much diffusion as physically and financially possible. Reflections, especially first-order, mask problems and even create audible problems that don't originally exist. Furthermore, consumers will not have any particularly dependable pattern of first-order reflections in their environments, so we don't want reflections in our monitoring environment to create an anomaly that could result in an exaggerated problem to the consumer.
Let's address the monitor types. As I said before, install at least two surround-monitoring systems. The main system should be a professional, full-frequency system. The other should be a consumer-grade home theater system. We will address the professional system from this point forward, with the assumption that the consumer system will be placed along side the professional one, matching speaker placement and levels.
All the monitors in a surround sound system should be direct radiator speakers (not dipoles). All the monitors should be of the same make and model and provide the fullest frequency response possible. Some believe that the rear monitors do not need to have the same frequency response as the front monitors. While acceptable with the first generation of surround delivery systems, modern delivery methods and media provide full frequency response to all monitors. The selection of the correct monitors may be limited by budget, but remember, the monitoring system is most crucial, so invest wisely.
Monitor placement has some complex elements, but there are some simple rules to follow:
- All monitors should be the same distance away from the ideal listening position (generally six to nine feet).
- The monitors should be at the same height as the listener's head.
- The monitors should be on stands.
The monitors should be positioned to give the listener the most pleasant and accurate surround experience possible. The movement of any monitor, even by a few inches or degrees can radically change how the sound is perceived.
For the most part, 5.1 (five main speakers and one subwoofer) will be used for radio broadcast, so that will be the setup addressed here. The ITU-R BS 775-1 recommended setup for small or midsize rooms as shown in Figure 1 says the following:
- The center monitor should directly face the listening position.
- The front left and right monitors should be 30 degrees off center, angled slightly towards the listening position.
- The rear speakers should be 110 degrees off center, angled toward the listening position.
Don't alter the front angles as the 30-degree spacing for the front also provides excellent stereo monitoring when the surrounds are not used. However, when it comes to the rears, I concur with The Recording Academy's Producers and Engineers Wing's recommendations, which suggest extending the rear speakers beyond 110 degrees for certain program material. About 110 degrees is appropriate for surround mixes to make the listener feel like he is in the audience, such as a classical recording. However, with some of the popular recordings where the listener is placed in the middle of the ensemble, an extended angle of the rear monitors (e.g. 120 degrees) provides a more immersive and dramatic experience. The key is to find a rear angle that works for the particular format and room and stick with it.
Subwoofer placement often depends on the room configuration. Locate the subwoofer in the front of the surround sound field and at least a foot away from a wall. Placing a subwoofer in a corner is not advised, as extreme bass build-up will occur.
Time to align
Alignment of the monitoring system includes bass management and overall level control. To monitor in surround successfully, you need a surround monitor controller, such as those from Blue Sky, Tascam, Martinsound and Audient. Some consoles, including those from Yamaha and SSL, have built-in surround monitor controls. Added features of some of the more elaborate controllers include the ability to monitor several 5.1 sources at once and to switch between stereo and surround. Use a device that can raise and lower the gain of the entire monitoring system universally. A decent controller includes bass management controls, separate LFE control and independent monitor level adjuestments.
Note: LFE and bass management are not the same. This is confusing because they both affect the subwoofer. The subwoofer channel or low frequency effects (LFE) channel � the key word being effects � is a dedicated playback channel that is part of the playback functions of recorded media such as SACD or DVD. Be careful when using the channel in production or surround encoding. The LFE control is often mistakenly used for bass management.
Bass management is a separate function in monitor controllers and even some consumer systems. Bass management is the playback feature where bass frequencies are taken from the five main audio channels and redirected to the subwoofer. This is used because some main monitors are unable to produce low frequencies. If bass management is used in a professional system, set the crossover frequency at 80Hz, as this is the most common crossover used in consumer systems and reduces localization of the bass frequencies.
It is time to align the levels of the surround monitors. The goal is to have all the monitors playing back at the same level. First, determine if you need bass management. With smaller two-way monitors, bass management should be used (or at least consult the monitor manufacturer).
Once that choice is made, the easiest way to calibrate the system is with an SPL meter. Choose a reference playback level. Most meters calibrate at 80dB or 85dB (I'll use 80dB for this example). Set the SPL meter for Cweighting, slow response, and position it at the central listening position. Face the meter forward when measuring the front monitors, and face the meter toward the rear for the rear monitors. Turn the subwoofer channel off for now. Play pink noise (at house reference level) and direct that signal to each speaker, one at a time. Adjust each monitor until the SPL meter averages 80dB.
Adjusting the subwoofer can get tricky. If you are not using bass management, direct low-pass-filtered pink noise (80Hz is recommended) to the LFE channel only. Set the gain on the subwoofer so that the SPL meter registers about 4dB above the level of the main monitors (84dB). Then add full frequency response pink noise to the front left and right speakers. The SPL meter should read no higher than 5dB above reference. If it rises more than that, the subwoofer may need to be turned down.
If bass management is used, a simplistic solution is available, although it is best to follow the guidelines for bass management calibration that came with the monitor controller. Send the pink noise to one of the front monitors. Then turn off that monitor off (you may need to disconnect it). You should hear the low frequencies coming from the subwoofer if the bass management system is properly engaged. Set the level of the subwoofer so that the SPL meter averages about 6dB lower than the main speakers. After that is complete, listen to some familiar commercial recordings and adjust the subwoofer accordingly.
For some in-depth resources, I suggest the Recording Academy's Producers and Engineers Wings Surround Sound Production Guidelines (grammy.com/pe_wing/5_1_Rec.pdf). Another resource of information is www.5dot1.com. Next time I will address some surround recording and mixing basics, the art and aspects of down-mixing and other general surround practices. I will also provide a detailed comparison of all of the different surround encode/decode systems proposed for HD Radio, including detailed test results of each system.
Kosiorek is the audio recording and mastering engineer at the Corbett Studio at Cincinnati Public Radio.