Don't miss the advanced techniques at the end of the mastering tutorial :)

Studio Acoustics

Guide to an Improved Acoustic Environment

Studio acoustics is a large and rather complicated subject but you don't need to be an expert to do a good job of your mixing or mastering. I'm going to go through the main points and show you how to easily and quickly achieve an effective acoustic studio environment.

When audio mastering, it is vital that we are able to accurately monitor the sounds that we are working on. A major part of this is the build quality of the speakers and how flat – and therefore accurate – their frequency response is. However, an occasionally overlooked yet vital element is the design of the room within which we’re working. A fantastic monitor speaker response would be practically wasted in a room which unduly coloured the sound they produced. As an extreme example, imagine setting up your speakers at one end of a tunnel, then trying to produce an accurate mix from the other. As you can imagine, the physical manipulations of the tunnel will warp the direct speaker sound, completely changing the way we perceive it. Although less obvious, unsuitable studio acoustics can also be quite harmful to the end result. Depending on its size, shape and reflective characteristics, certain frequencies – or bands of frequencies – can be perceived at different levels from others. If the room happens to reflect more higher frequencies than others, then the sound you are hearing may appear brighter than it really is. If the room exaggerates the low end, then the speaker sound will appear to have more bass than is actually in the mix. Similar to how the speakers have a 'frequency response', the room's response to different frequencies will vary also. The more 'neutral' our room's response, the more accurate the sound reaching our ears will be. So before we discuss factors like monitor placement and acoustic treatment, let's first look at one of the most fundamental aspects which influences the way we perceive sound within the room – the size, shape and raw surface materials of the room.

Size and Shape

The worst case scenario for a studio would be a perfect cube with flat walls, floor and ceiling. Apart from the walls, floor and ceiling allowing the sound waves to reflect around the room, changing the way we perceive the sound (which will be discussed shortly), the major problem is the cube shape itself as it causes a specific set of frequencies to appear to be louder compared to the speaker sound, particularly in the lower frequencies. This is due to a phenomenon known as 'standing waves' – sound waves that literally become trapped between parallel surfaces such as walls, bouncing back and forth, overlapping with each other perfectly, reinforcing themselves every time. Rooms with parallel walls will 'resonate' at certain frequencies. The frequency at which resonance occurs is referred to as the room's 'mode' and is directly related to the distance between the walls (a sound wave's frequency is directly related to its physical length in metres).

No matter what distance the parallel walls are apart, a sound wave of some frequency will be able to fit exactly into that space, then flip over when it hits a wall and go back the other way, perfectly in time with its immediate following sound wave (at that same frequency). Theory states that when you allow two identical sound waves to add together, they double in size.

Interestingly, half that frequency can do the same thing – it hits the wall, flips, then goes back, meaning it fits itself in between the walls after two trips. This adding also occurs when the frequency is doubled, quadrupled, octupled and so on, as they all still fit perfectly into that space.

When this reinforcement happens, a doubling of sound pressure will occur for that particular frequency, which can work out to a 3dB rise in audible level in the main area of the room. Quite a dramatic difference where audio accuracy is concerned.

What you will also find is that depending on where you are sat, you will experience differences in the way the room resonates. Two identical waves overlapping will increase the audible level of that frequency. However when two waves overlap whose wave cycles are opposite (anti-phase), a cancelation will occur giving the perception of a reduction in that particular frequency. The overlapping of identical waves is known as 'constructive interference', whereas the overlapping of anti-phase waves is known as 'destructive interference'. Some areas of the room may cause constructive interference, while other areas may cause destructive interference.

When deciding on a space for your studio, choose a room that deviates from a cube, ideally one that is fairly symmetrical, with slopes, uneven features and details that help to redirect and diffuse reflections and lessen the impact of resonances. If this is not possible, don't worry as I'll be showing you how to lessen the impact of resonances using other methods.

Golden Ratios

Some of you may have heard of the term 'golden ratio' when referring to studio acoustics. This is the ratio between the height, width and length of a room. If you are lucky enough to be in a position to 'choose' the dimensions of your room, then you may consider using the golden ratios to do so. Originating from ancient Greece, these ratios have been applied in many subjects and practices; from architecture to classical music, and even to book design. They also occur frequently in nature which is how they were first discovered. In regards to studio design, the golden ratios are a proven way to obtain a more accurate listening space as they allow for a uniform distribution of resonant frequencies around the room (yes, resonance still occurs). As for how they are calculated, the level of mathematics is probably too advanced for this mastering tutorial. In any case, the purpose of this mastering tutorial is to show you how to achieve a professional finish by utilising what you already have. So rather than have you build an extension on the side of your house, let's move on to how you can transform an existing room into a reliable acoustic studio.

Surface Material

A surface material will have different reflective characteristics depending on the frequency of the sound wave hitting it. A soft surface material such as carpet will absorb much of the higher frequencies preventing them from being reflected back into the room. However, a low frequency will pass straight through the carpet to the reflective solid surface beneath. A heavy drape may provide control of midrange and higher frequencies but again, the lower frequencies find their way through. Controlling the lower frequencies requires more elaborate efforts than the use of things like drapes, or carpet; their long wave lengths require the use of much larger objects.

Just before we reach the discussion on how to control these troublesome reflections, I'd like to point out one more issue you will face – the 'hanging around' effect of the lower frequencies caused by the solid surfaces beneath the soft furnishings reflecting the sound waves back into the room, and the resonating 'trapped' lower frequencies which subsequently take longer to disperse their energy. The result is a lack of definition in the low end as the sonic information literally starts to smear.

So we know that the studio's reflections alter the perceived speaker sound. Should we attempt to 'remove' the reflections from the studio? Fortunately it's okay to have some reflections, we just need to 'neutralise' them the best we can so we can trust the speaker's sound.

Room Contents

Before you begin budgeting for acoustic treatment, there's a lot that can be done using the everyday things around you. As well as audio gear, the room will most likely contain some furnishings which will aid the absorption of reflections, as well as irregular surfaces that serve to break up and scatter incoming sound waves, preventing them from building up and appearing louder in certain areas of the studio. As mentioned, lower frequencies are more of a problem as they can travel straight through thin coverings like curtains and carpets which are effective against high frequencies. Large furniture with soft surfaces such as beds, sofas and padded chairs all soak up a certain amount of the lower frequencies because their material is porous but quite dense, so although the powerful low frequency energy is able to enter the material as vibration, a lot of the energy is dispersed as friction and heat. This is similar to how professional 'bass traps' work – the vibrations cause the fibres inside the dense mineral wool filling to rub together converting kinetic energy into heat.

Half-full bookshelves, angled sofas, a laden coat stand, an open wardrobe, anything that is either absorptive or uneven – and may help to divert and fragment the direct speaker sound – is a worthwhile consideration.

Acoustic Treatment

You may consider the use of acoustic treatment. Wall-mounted tiles made of acoustic foam placed strategically around the monitoring position is a proven way to achieve a professional monitoring environment. It is also worth considering mounting bass traps around the room and in upper room corners where bass frequencies tend to build up. These can be made from mineral wool (as mentioned) or acoustic foam. Another effective form of acoustic treatment can be the use of 'diffusers' – precisely shaped solid objects designed to break up desired frequencies, stopping them from building up. It's likely you might require a combination of all three.

Acoustic tiles:

studio acoustics tile

These absorb the reflections of high mid to high frequencies.

Bass traps:

studio acoustics bass trap

These absorb the low mid and low frequencies.

Diffusers:

studio acoustics diffuser

These are shaped to fragment reflections. The one pictured is known as a 'Quadratic-residue Diffuser', or a 'Schroeder Diffuser' named after Manfred Schroeder who developed the formulae for calculating its dimensions. Their size and dimensions can be tailored to deal with certain frequencies.

Before we look at where is best to place the furniture and/or acoustic treatment around the studio, let's first look at speaker placement with regards to the studio's surfaces.

Speaker Placement

An important factor you need to be aware of is the time between the direct speaker sound and the first reflections from the walls. When this time is too short, the reflections colour the direct speaker sound through a process known as 'comb filtering'. This is when the interaction of the direct sound waves with the slightly 'out of phase' reflected sound waves causes an undesirable filtering effect extending up the frequency spectrum. Increasing the initial delay of the first reflection lessens the impact of comb filtering. So it's okay to have 'some' reflections; the key thing is to try our best to prevent them from reaching our ears too early, and that there isn't an imbalance of the 'tone' of the reflections, such as too much low frequency compared to the rest.

The walls directly behind and beside the speakers are likely to cause issues as the reflections off these will be very short. Also, flat surfaces in front of the speakers like a table or a mixing desk can introduce reflections with very short delay times. Depending on the size of the studio, there's a good chance that the reflections off the walls either side of the listening position will be troublesome too. I will illustrate this shortly.

Putting the discussion on reflections to one side, another important aspect to consider with speaker placement is the distance between the speakers and the distance from your ears. This is essentially what creates the 'sound stage'. You will most likely be dealing with stereo content – some sound will come from the left, some from the right and some from the middle. The balance between these three areas is what you must get right. If the distance between your speakers is much greater than the distance to your ears, the 'fantom centre image' can become weak and the 'stereo width' too wide – as if you're wearing headphones. Drawing the speakers closer together, shorter than the distance to your ears, the fantom centre can become too strong and the stereo width too narrow. A tried and trusted way to arrive at suitable speaker locations, with respect to the listening position, is to create an 'equilateral triangle' between your ears and the speakers. This is known as 'true stereo' or 'true 60 degrees' (the angles within an equilateral triangle are each 60 degrees).

Recap

  • aim to neutralise the acoustic sound of the studio – balance the tone of the reflections;

  • parallel walls cause standing waves – the room will resonate at certain frequencies;

  • different surface materials have different reflective properties;

  • everyday room contents can be used as acoustic treatment when placed correctly;

  • direct speaker sound is coloured by the immediate reflections from the nearby walls;

  • the effectiveness of the perceived sound stage is a result of the distance between the two speakers and your ears.

Putting It All Together

Studio acoustics and design is a large and tricky subject; there are many different ways of looking at how one can be designed. As mentioned, the aim of this mastering tutorial is not to show you how to build a state of the art mastering studio, but how to achieve a professional sound using mostly what you already have. Assuming you only have access to a rectangular room, as most people reading this mastering tutorial will, here's a simple way you can achieve a professional listening environment without breaking the bank. This method is loosely based on a concept by Don Davis of Synergetic Audio Concepts where you have a 'dead end' to the room and a 'live end' to the room.

This first diagram gives a rough idea of how to arrange the furniture/treatment within your room:

first diagram of studio acoustics

Notice the arrangement of the speakers and the space behind the listening position:

second diagram of studio acoustics

Using this arrangement of furniture/treatment will help neutralise the sound of the room, thus allowing you to have more trust in what your monitors are telling you:

third diagram of studio acoustics

Some more tips:

  • Use a mirror to determine which areas may require acoustic tiles – if you can see your speakers in the mirror whilst facing forward, you may need to treat those surfaces.

  • Don't cover all your walls in carpet (it's amazing how many times I have seen this). You have little chance of ever stopping the low frequencies from bouncing around anyway, so you should allow for an amount of reflections across the rest of the spectrum in order to balance the tone of 'all' the reflections.

  • Shock mount monitors to prevent vibrations travelling through the materials of the room, which can reach your ears as sound before the direct speaker sound!

  • Also consider mounting absorption/diffusion 'above and below' the listening position.

  • The larger the room, the better. Low frequency sound waves need space to develop down to their long wavelengths. A tiny room will reduce your accuracy in monitoring the low end.

 

For those who would like to delve a little deeper into studio acoustics, the next page explains how to calculate the frequencies at which a room will resonate - calculating standing waves.

If you're happy with your listening environment, move on to Tonal/Spectral Processing.

David Eley - TGM Audio