If, how and which room correction shall be used for monitor rooms
There are just a few questions in our branch, which are answered in such different ways as the ask for sense or nonsense of correction filters for speakers in listening rooms. While the industry – especially manufacturers of speakers with integrated and automatically adapting filter systems – suggests an easy solution, purists condemn every electronic intervention as an forgery which needs to be avoided.
Why use filters in the first place? Why not simply build better rooms?
In signal theory the listening chain containing signal source, preamp, power amp, crossover filter, speaker and room can be regarded as a serial connection of complex transfer functions.
A quite authentic acoustic reproduction of audio material is given when
1. the overall transfer function offers a linear frequency response
2. the overall delay doesn’t depend on frequency (constant group delay)
3. this group delay is as small as possible (in some cases is this is negligible – for instant for mastering without moving picture)
4. and the transmission is minimal distorted.
With a technical setting of reasonably good quality, the speakers and most of all the listening room exert a major influence. Of course the transfer function of the room depends on the speaker and listening position. The highest demands on a good listening room are:
1. a reflection free zone around the listening position,
2. a low and evenly distributed reverberation time,
3. sufficiently damped room resonances, which otherwise cause increases and decreases of the frequency response and lead to long decay at this resonance frequencies.
The first requirement can be achieved with minimal effort (using porous absorbers for instance). But it is more to it to reduce the reverberation time in lower frequencies (the use of broadband absorbers for instance). And to gain an acceptable treatment of room resonances (if it is possible at all) you depend on broadband and / or tuned bass traps.
We all know there are no acoustically perfect listening rooms. And under real life conditions the third demand often cannot be satisfied sufficiently. In some cases the room is too small, sometimes there is not enough space for bass traps, or doors and windows are where bass traps need to be installed. Sometimes the useful life is too short and a high invest is not worthwhile or not in line with the budget.
In these cases, after all room acoustic preparations are utilised, it is reasonable to examine whether the use of room correction filters is wise at least.
Why then spending so much on construction, why not filter right at the beginning?
Even an ideal correction filter can’t replace room acoustic preparations. This applies for the creation of a reflection free zone as well as for the even reduction of the reverberation time. Furthermore the dependency of location of the sound field in respect of the wave knots can’t be reduced by filters as these change the level of the speakers for all positions. As well as the long decay of the modal frequencies can’t be corrected by filters. Therefore it is not only reasonable but even necessary to deal with the root cause of the problem and optimise the room acoustic itself.
Moreover filters are real components which can have certain „side effects“. With analogue and digital IIR filters there is no change of level without an inevitable (and unwanted) phase change. Here we have: the higher the slew rate, the more extreme and audible the phase change. Especially in the mid and high frequency range, this phase distortion can quickly limit the reasonable use of filters.
But this must not be true for FIR filters as these can be used linear phase and can therefore change the level irrespectively of the phase. However, the filter needs to „look ahead“ and therefore delay the signal; this type of filter holds a significant latency. While this is no problem for some studio use (mixing, mastering), there are problematic applications like working with audio for video or listening while recording. In this very common situation even small delays are extremely disturbing for the artist. Many modern digital filters therefore combine linear phase FIR filters with minimal phase FIR and common IIR filters; successful products convince through a successful compromise between effective level change with minimal phase distortion and minimal latency.
Beside these unavoidable side effects, an analogue signal must first pass an AD converter and later a DA converter to be processed by a digital filter. We’d like to avoid a general discussion about the influence of ADDA conversion at this point; a discussion that appears almost dogmatic sometimes. But what is indisputable is that the quality of conversion (sampling frequency, resolution, dithering, anti-aliasing filter, clocking) is generally audible. Many superior studios decided in favour of analouge summing and monitor controllers, in mastering studios digital technology is consciously used with highly selected ADDA converters. Frequently, some of our customers willingly decide against digital room equalisation. This was the signal for us to develop our room correction filter Cosinus because there was no alternative to digital filters on the market.
What now? Constructional measures or filters? Analogue or digital?
Both! Constructional measures – broadband absorbers, diffuser modules or bass traps, whatever – cannot be replaced by any filter and must be installed consistently and in accordance with budget and space. If there’s still the need to correct the frequency response (mostly this applies to the bass range due to insufficiently damped room resonances), analogue as well as digital filters can be of use. Digital filters are hereby more flexible, reproducible and more easy to operate; we are frequently using good digital room correction filters like the Trinnov or ConEQ. Thankfully we now have the Cosinus as analogue alternative for superior listening rooms, which is limited compared to digital systems (only three band pass filters, frequency range 20 – 240Hz) but does without ADDA conversion. To see which system is the right one for your project, we recommend a non-binding on-site inspection. Here one should not only rely on measurements; without ample listening sessions a serious decision cannot be made. And when the suitable room correction filter is set right in the acoustically treated room, the audio world is back in order.