When it comes to mixing, we often hear references made to rooms sounding good, or not sounding good, having resonances or “standing waves”. And we seem to be always chasing the right form of acoustic treatment, whether it involves putting up panels, or bass traps, or something more DIY. The implication is that the more neutral, the better. Of course, no room is perfectly flat. That’s why we have room correction software like Sonarworks Reference 4 or IK Multimedia ARC ll room measurement system. They correct for less than neutral frequency ranges that result from the natural reflections in the listening environment.
But how neutral, or flat, is useful? This got me to thinking about how our monitors are gauged, measured, and tested. After all, what good is making your room neutral, if the speakers aren’t putting out properly calibrated sound?
All monitor companies, at least the ones worth their salt, use an anechoic chamber for this sort of research and development to ensure accurate speaker design. The term “anechoic” comes from “an-echoic”, meaning it is non-echoing, or echo-free. An anechoic chamber is a room designed to completely absorb all reflections and waves. They can range in size from a small box to a huge auditorium; depending on the size of the objects and the frequency ranges being tested.
Anechoic chambers minimize the reflection of sound waves on the walls usually by using a series of foam wedges with varying degrees of height. After the collision of the sound with the wedges, the resulting waves bounce up and down in the gap of air between the wedges. The remaining acoustic energy is dissipated and absorbed by the air. Because the wedges are made of foam, further dissipation of the waves that escape the gap happens, minimizing the energy that travels back to the source of the sound.
So What Is The Value Of This?
We don’t listen back in anechoic chambers; so how useful are they in monitor measurements? When sound is generated in this environment, the majority of it is absorbed. The reflections we normally perceive that cue our brains to understand the size and shape of the room are absent. So only the sound going from the speaker to the microphone (for the measurements) is picked up, the room is not changing the sound. This provides extremely detailed analysis and information. There are limitations to this process though, mainly in the low-frequency range. The amount of energy in a single cycle that needs to be absorbed in this environment is inversely related to the frequency. For example, a cycle generated at 50 Hz has 200 times the amount of energy as a cycle generated at 10 kHz; making it 200 times more difficult to absorb. Therefore, measurements in the lowest area of the spectrum are either approximations or a combination of other measuring methods and simulations. Eve Audio, for example, has the means to execute highly accurate measurements down to 70Hz in their anechoic chamber, which ensures clean and balanced calibrations.
By moving the speaker around the chamber and testing the response from different directions, the developer can zero in on the dispersion of the speaker. In the real world, with floors and ceilings, the dispersion has a huge impact on the accuracy of the monitors, since, with the inherent reflections generated, the sound arrives at our ears slightly later than the direct sound from the speaker.
Sound travels at about 1 foot per millisecond. So some of the sound from the tweeters and woofers is reflected off of the surfaces in the room before arriving at our ears. The ratio of the delay and the wavelength determines if certain frequencies get attenuated (or even disappear) or boosted by the time we hear them. Different frequencies getting absorbed in this way is what leads to “bad” sounding rooms. The ideal is that the reverb decay time or RT60 is the same at all frequencies. This is a key target in what the acoustic treatment for a room needs to do. If it is too absorbent at higher frequencies the lower frequencies will decay slower than the high frequencies and that bad news in a studio design.
Anechoic chambers are useful in helping understand how reflections affect our perception of the sound. Multiple speakers can be used, one for the main signal, the others to reproduce the “reflections”. Doing this, in the anechoic chamber, the signal with and without reflections can be compared. If the early reflection is very different from the source signal, our brain perceives it as a separate sound. In an anechoic chamber, the output can be verified and measured not only directly in front of the speaker, but also at increasing angles to each side, as well as above and below. The speaker developers will then consider (and manipulate, through the speaker design) all of these curves so that the listener will perceive as accurate a reproduction of the direct sound as possible.
Speaker Placement In Our Studios
Understanding all of this, we can see why speaker placement in our studios is important. Off-axis, responses are sometimes very different than the direct signal from the monitor; which can be confusing when trying to listen for detail in your mixes. It is important, for this reason, to choose monitors with good horizontal dispersion. Particularly if you have clients in the studio with you. Not everyone can be in the “sweet spot”. So proper R&D in an anechoic chamber is essential for speaker designers to ensure that once reflections are added into the listening environment, the dispersion will be as accurate as possible from as wide an angle as possible.
At about 01:48 into this video, you can see what Eve Audio’s anechoic chamber looks like and how they use it. Here, at about 12:30 into this video, we get a fascinating first hand view and tour of their anechoic chamber with all the functions explained in detail . We don’t need or want our studios to be completely devoid of any reflections. It would feel very unnatural. There would be no aural cues as to the size of the space we are in. I expect walking on the moon might feel like that! But we do want speakers that have been calibrated in this environment so that they are as accurate as possible within as large a radius as is possible.