Reliable audio mixes: Analysing and improving your sound room’s acoustics

The audio room inside an NEP truck

By Howard Jones, communications director, Genelec

Producing reliable audio mixes relies on the careful choice, placement and calibration of studio monitors. Since what we hear from a studio monitor is a combination of the performance of the monitor itself, its interaction with the room, and our listening abilities, if we don’t pay close attention to how the monitoring system is performing within the room then it’s likely that our mixes just won’t translate well to other playback systems.

With the advances in DSP and software, we now have systems that can automatically optimise the monitors in the room, and help compensate for some of the room’s negative acoustic effects. Performing this auto-calibration process generates a huge amount of information about the monitors and the room, and here we look at the common issues that rooms can cause.

Frequency domain

Many users will be familiar with graphs that show the frequency response of their monitors, and how flat (or not) the response is across the whole frequency spectrum, such as Figure 1. A neutral and uncoloured monitoring system will have a reasonably flat and smooth response, but the room’s acoustic influence can significantly affect this.

Calibration systems are great at identifying peaks and dips in this response, and this can often be simply improved by moving the monitor’s location. For instance, a significant LF dip can be caused by reflection from a back wall, and moving the monitor closer to the wall will improve the performance as well as extending the system’s LF response.

Frequency notches can also be caused by reflections from hard surfaces, where again re-positioning of monitors can help.

Figure 1: Frequency domain analysis of a studio monitor

Time domain

The best monitoring performance is obtained when the distance from each monitor to the listening position is identical, causing the sound to arrive simultaneously at the listener from each monitor. Additionally, the playback level from each monitor should be identical too. Failure to ensure this can result in very unreliable imaging.

Sometimes the shape of the room will prevent this ideal positioning, particularly in multi-channel set ups where it’s just not possible to mount every monitor equidistant from the listener.

In this case, calibration systems can provide a time delay and playback level correction to ensure optimal listening. They can also highlight early reflections, caused by sound bouncing off reflective surfaces and walls, which combines with the direct sound from the monitors to affect imaging, and add unwanted colouration. These reflections can then be minimised by reducing the listening distance and/or adding absorbent acoustic material to the room.

Time-frequency domain

The reverberation times of a room can have a big impact on monitoring quality. A key figure that can be identified from the calibration process is the RT60, which describes the length of time taken for a sound to decay by 60dB from its original level. Figure 2 shows how decay times can also vary for different frequencies (known as T60), and a good sounding room will have very similar T60s across the entire frequency spectrum, resulting in good detail and balance.

Studying the reverberation times after calibration can give important pointers on the addition of bass traps for instance, which are used to reduce low frequency reverberation times and control the room’s bass response.

Figure 2: Analysis of room reverberation times versus frequency

How good is your room?

When it comes to judging a room’s performance, broadcasters have used the ITU-R BS1116 framework to assess the quality of listening spaces. This standard assesses the frequency response at the listening position, early reflections and reverberation times, and a space that complies with the recommendations is highly suitable for professional audio work.

To measure how good your room is, calibration systems can now provide a service where a detailed room report can be generated automatically, and advice given on simple remedies, plus guidance for more serious issues that might require the help of a qualified acoustician.

Users of our own Smart Active Monitors can now access this Room Acoustic Reporting via the new Grade feature on our GLM calibration software, for instance.

The more you know about the way your monitors and room are performing, the easier it’ll be for you to make adjustments and create a more reliable mixing environment. Knowledge is power, so go ahead and use it!