The Importance of Acoustics
January 16, 2014 Sam Burden
Recording studios typically have two rooms, a live room where the instruments are recorded, and a control room where the engineer and producer monitor the sounds they are tracking, mixing, or mastering. In both spaces the acoustics, the way sound behaves inside the rooms, is critical if you want to produce recordings of a commercially releasable quality. Here we’ll briefly explain how the acoustics of a purpose built, properly treated studio such as Empire allow the engineers to make the music shine the way it’s supposed to on record.
This is where it all starts, you can forget thousands of £££’s worth of microphones if the sound you’re trying to record is not right at the source; if it sounds bad in the room, its going to sound bad on your record! Assuming you have a good musician with high quality equipment to record, the room is the next most important thing in achieving a high quality sound. In an ideal situation the sound propagating from a drum, guitar amp, or cello would arrive at a microphone with the same tonal balance as that instrument would produce in the absence of any reflecting surfaces, and that sound would have an even decay time throughout the frequency spectrum, with a suitable length of decay or reverberation for the particular piece of music being played. However unless the room is very well treated, this can be far from the case. The behaviour of sound inside a room is mainly dependent on the effect of ‘room modes’ at low frequencies, and dependent on the effect of direct reflections and diffusion at mid and high frequencies; the point at which these two regions meet is called the Schroeder frequency, which for a small/medium sized room will be somewhere around 200Hz.
Below the Schroeder frequency a room acts as a giant resonator, resonating at frequencies that depend on its dimensions, much in the same way as a bottle will resonate if you blow over the top. These resonances produce standing waves, which cause the sound pressure at each modal frequency to vary with location in the room. Have you ever heard a sustained note played on a bass guitar, whilst walking through a room and noticed that in some areas it’s very loud and in some areas it’s nearly inaudible? This is due to the standing waves caused by room modes, and can skew the recorded frequency balance of an instrument by as much as 20dB if no low frequency absorption or bass trapping is present. At Empire, the live room has purpose built floor to ceiling bass traps built into each corner. These employ a combination of porous and resonant absorption to provide excellent low frequency performance. By absorbing low frequency sound energy, the effect of the standing waves is much reduced and the frequency response is much more even throughout the low frequency range and much more consistent throughout the room. The live room at Empire is therefore perfect for recording any bass instrument, from roaring distorted bass cabs to double bass or cello. Without sufficient bass trapping in a live room it is impossible to record solid sounding bass instruments, since some notes will be greatly emphasised whilst others may be partially cancelled out. The bass traps also bring the low frequency decay time down to a suitable and even level.
Above the Schroeder frequency, sound reflects from the walls like light from a mirror and bounces around until all the energy is absorbed. Here at Empire, broadband absorption is built into the entire ceiling of the live room. This absorbs sound energy from the majority of the audible frequency spectrum to reduce the decay time in the room to a suitable level for recording. There are also broadband absorptive panels at certain locations on the walls. The absorption has been carefully selected so that the decay time in the Empire live room is even throughout the frequency spectrum, producing a balanced sound. If the decay time in a room is not even then all recordings made in that room will be coloured by it. For example if the high frequency decay time is too high, the recordings can sound harsh, and if it is too low, then recordings will sound dull, if the lower midrange decay time is too high then recordings will have the familiar boxy sound often found in bedroom demos, where at Empire, recordings sound full and clear. It is important that some surfaces remain either reflective or diffusing so that the room retains a suitable ‘liveness’ and doesn’t become too dead sounding; at Empire the wooden floor and some of the wall surfaces are reflective. This helps provide the sort of reflections that reinforce the natural sound of acoustic instruments like a drum kit or acoustic guitar. The floor reflections can also be controlled using the large rug. For example acoustic guitars benefit from being recorded over a hard reflective floor, where vocals need to be drier and are recorded on the rug; this is why live rooms often have hard floors and absorbent ceilings.
The live room of Empire has been designed from the ground up with acoustics in mind and as a result has a natural sound that suits virtually anything you might want to record in it. The short decay time can be augmented if necessary by the use of carefully selected digital or convolution reverbs at the mixing stage to create the impression of a much larger space. In this way the live room at Empire is very flexible; if it had a more obvious reverberation character then it would only suit certain instruments or styles of music, where at Empire the room is neutral and is the perfect canvas on which to build your music!
In the control room the sound behaves in a similar way but the goal of the acoustic treatment is slightly different. The ideal control room allows the engineer to hear exactly what they are recording or mixing. The sound at the listening position from the studio monitors must be the most faithful reproduction of the recording possible, thereby allowing the engineer to make decisions based on correct information. If there is not enough low frequency absorption, then room modes will cause peaks and troughs in the frequency response causing some areas to be under represented and vice versa. The engineer would then try to correct the unbalanced sound by modifying the recording to achieve a good balance. When played on another set of speakers in a different room, the recording would no longer sound balanced. This is often why a mix made in your bedroom might sound terrible in your car for example. Consider that the majority of studio monitors available today are capable of a ±3dB frequency response across their frequency range, with really excellent systems being less than this, but what is less often realised is that an untreated room can impart a further ±10dB to the frequency response heard at low frequencies! Without proper acoustic treatment in a control room, you could spend £100,000 on a monitoring system and still have mixes that sound terrible elsewhere. Mixes created at Empire sound right wherever you listen, because once again purpose built bass traps have been employed to control the low frequencies.
To illustrate just how bad the frequency response at the listening position can be without proper acoustic treatment, the following graph shows a mixing room before and after treatment. Before treatment there was a massive hole in the bass response in the 60-90Hz region, and a boomy peak centred at 135Hz. Mixes created in this room before treatment tended to sound boomy in the low bass and thin in the upper bass because the room had been equalised out without realising. Now, mixes sound far better as the bass response and general monitoring conditions are more accurate. This room has had acoustic treatment retrofitted where as Empire has been designed to sound great from the start.
The decay time at the listening position must also be even throughout the frequency range, and short enough so that the engineer can accurately perceive the level of time based effects such as reverberation and delay that they add to the recording during the mix; if the room is too reverberant then the recordings may end up sounding too dry elsewhere or over headphones. At Empire, broadband absorption has been used to control the overall decay time from the listening position as well as to control the stereo image perceived.
The points made here highlight one of the many reasons why professional studios like Empire are still needed in a world where we can all record on our laptops; there is no substitute for the clear balanced sound recorded in a properly treated live room, and the knowledge that the mix will sound great even after you leave the mix room.
Luke Rendell (AMIOA)