Understanding Room Acoustics Diagrams: A Comprehensive Guide
Setting up a listening environment involves more than just placing speakers in a room. The acoustic properties of the room itself play a crucial role in the sound quality. To optimize your listening space, it's essential to understand and interpret room acoustics diagrams. These diagrams provide valuable insights into how sound behaves within a room, allowing you to make informed decisions about acoustic treatment and speaker placement.
So we’ve established that most of us set up and play in conditions that, acoustically speaking, would just about be acceptable for a livestock auction.
Let's explore the key types of measurements and diagrams used in room acoustics analysis.
Frequency Response
The frequency response is a fundamental measurement in acoustics. It is a plot of energy in decibels (dB) against frequency in Hz. Most of us should be familiar with these as they are often displayed in equipment reviews.
It’s useful to look at the frequency response at different levels of resolution. Smoothing, in the form of a moving average filter, is applied to the measurements. Typical filters are 1/3rd octave and 1/12th octave.
Response variations of +/-10db through the bass region below 300hz are clearly visible. An example of a frequency response measurement taken with XTZ Room Analyzer II Pro is shown below.
Example of a frequency response measurement taken with XTZ Room Analyzer II Pro.
Time, Energy, and Frequency (TEF) Measurements
The second is a combination measurement, which shows time, energy and frequency (TEF) all together. There are many ways to look at these measurements, one is the waterfall diagram, another is a cumulative spectral decay (CSD) diagram.
Structural resonances such as a vibrating floor and slowly decaying room modes can be very quickly identified from this measurement. The following is a CSD plot in the form of a sonogram or 2D Waterfall taken with XTZ Room Analyzer II Pro.
The two axes are time and frequency. The data plotted is the measured level of the sound in dB. Bass frequencies take longer to decay than other parts of the frequency range and that the decay of sound is often uneven due to room modes.
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CSD plot in the form of a sonogram or 2D Waterfall taken with XTZ Room Analyzer II Pro.
Decay Time Measurements
The third measurement looks at how long it takes for mid and high frequency energy to decay in the room. One is a single figure measurement, often called T60.
Because it is quite difficult in real world situations to record the time taken for a signal to decay the full 60dB (due to high noise floor or low measurement level) T30 is often used as a proxy. T30 is the 60dB decay time calculated by a line fit to the portion of the decay curve between -5 and -35dB.
Note that we drop the ‘R’ from the front of the time decay measurements in small rooms since there is no reverberant field (R stands for reverberation). The second, and much more useful for our purposes, is a measurement that shows the decay time over different octave or one third octave frequency bands. It is important for decay time to be consistent across the spectrum to preserve balanced reproduction.
It is also possible to look at the same information as a line graph. The following graphic shows T20, T30 and T60 on the same graph.
T20, T30 and T60 on the same graph.
Energy Time Curve (ETC)
The final major measurement type is the energy time curve (ETC), which is a plot of amplitude in dB against time (typically measured in milliseconds). An energy time curve shows how sound energy decays in a room.
Zero on the time axis, and the highest peak on the magnitude axis, represents the direct sound from the speaker. The plot below shows how level changes over time, with each peak being due to a reflection from a nearby boundary such as the floor, ceiling or side walls.
A very important point to remember is that ETCs are spectrally blind (i.e. they contain no information as to the spectral content of the reflected sound). There are unfortunately a lot of standards around that mandate such things as “no peak above -10dB”. We strongly recommend to not use such standards; in fact much modern psychoacoustic work finds that people enjoy listening environments that have high levels of reflections. In our opinion the most critical thing is that the spectral content of the reflected sound be similar to the direct sound.
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Example of Energy Time Curve (ETC).
Equalization (EQ) as a Tool
One of the tools in our sonic arsenal is equalisation, or EQ to use the short term. It’s a very useful tool but often misunderstood, so let’s start right back at the beginning.
Back in the days when the world was only available in Black & White and radio was staffed by men with clipped moustaches, and even more clipped accents, the technical types noticed that the longer a cable run got, the less high frequency there was at the other end. The point of this tale is that EQ was first developed as a corrective tool, not a creative one.
And it quickly became a much more versatile system than a simple compensator for signal loss. Until fairly recently, any decent Sound Reinforcement (PA) rig was bound to have a couple of 31-band graphic equalisers, set to optimise the performance of the Front of House system.
You may wonder, why 31 bands? Well, the more bands an equaliser has across the 20Hz-20kHz audio spectrum, the narrower each one can be. Narrow bands are particularly useful for ‘notching out’ problem frequencies, as caused by standing waves in the room. (Not only does this make for a better-sounding system, it helps to reduce feedback problems from the mics, which predominantly occur at just a few frequencies in any room.)
Also, 31 bands is an agreed standard, with the centre of each band defined by the ISO (International Standards Organisation). The reason a stereo rig needs two 31-band EQs is that they are mono; there is no guarantee that that the left and right speakers will need the same settings.
As the name ‘graphic’ implies, the setting of the sliders provides a visual indication of the total frequency contour. Diagram H shows the ISO frequencies along with some indication of the general parts of the sound spectrum they affect.
ISO frequencies along with some indication of the general parts of the sound spectrum they affect.