The Battle For Good Acoustics - Infographic

SOURCE: Based on the first chapter of Mike Senior's excellent book Mixing Secrets For The Small Studio.
For more resources and links on acoustics, monitors, and products, check out Mike's resource page for chapter 1 by clicking here.

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1. Know your enemy

acoustic enemies

Reflection

Sound doesn’t just travel from the speakers to your ears, it radiates out in all directions. It bounces off the walls, ceiling, floor, and other solid objects in the room. These reflections can then, you guessed it, reflect off a surface for a second, third, fourth etc. time. Each reflection will lose a bit of power and they will eventually decay naturally. This pattern of reflections are what give a room a characteristic reverberant sound. Be mindful of this characteristic during your battle! The worst reflective offenders being surfaces within 3 metres of the listening position. Be mindful of the surfaces on your workstation. A solid flat surface like the top of a desk can cause serious reflections that lead to comb filtering - which we will touch on later.

acoustic enemies

Flutter echo

High frequency reflections caused by sounds bouncing between two parallel surfaces. Check for this distinctive metallic buzz by clapping your hands and listening for the brief echo. This is one of the easiest problems to fix, so read on to find out more about the weapons at your disposal!

acoustic enemies

Standing waves

Also known as room modes. More prominent in low frequencies, and with parallel surfaces, this phenomenon cause frequencies volumes to both increase and decrease in specific spots within the room. This is a complex physical process, and it has to do with the particular length of a frequency soundwave - this is, astonishingly enough, called wavelength.

The waves are comprised of high and low pressure areas, so when we combine these regular undulations of pressure with reflections, we can get big problems! Reflected waves layer up making the pressure difference between high and low even more pronounced.

The interaction between wavelength, and room dimensions means that these pressure zones appear at very specific spots in a room. Depending on where you are listening from, the frequency of the standing wave can be boosted or cut, so understanding where these points are is absolutely essential!

You want to listen from a point of neutral pressure, and you want to target these frequencies with acoustic treatment - more on this later.

acoustic enemies

Resonance

The transfer of sound from one object to another. This can add an unwanted frequency boost. Be wary of anything directly connected to a speaker, and the things connected to that thing, and that thing etc. etc. Resonance takes time to dissipate, so sounds continue in what is knows as the 'time domain'. This leads to frequencies 'ringing out' way after the speaker has stopped producing this frequency. This can really upset the balance of frequencies, as well as the 'tightness' of the sound.

acoustic enemies

Comb filtering

When the same sound arrives at the listening position at different times, frequencies become out of phase, and can lead to comb filtering. The high and low parts of the sound interact destructively to change the sound. This is a simple physical effect that occurs at regular points along a frequency and can lead to the sound becoming less 'full', or even produce an unnatural sweeping resonance. This is actually used to produce effects in music production such as chorus and flanging.

 

2. Your weapons

acoustic weapons

Absorbers

Usually made of foam or fibreglass, these can prevent reflections of high frequency sound. They are usually not too deep, and can be easily hung on your walls like a picture. They are probably one of the most affordable weapons in this list! Don't bother with silliness such as egg boxes - they obviously don't work and look awful.

acoustic weapons

Bass traps

Much bigger versions of absorbers. These have real mass, which absorbs the more powerful bass frequencies and helps to address standing waves. They're also much more expensive and more difficult to mount. But if you have any problems with room modes you need to invest in them!

acoustic weapons

Diffusers

These mathematically produced shapes reflect sound, but scatter the frequencies in different directions. This helps to prevent comb filtering. Bookshelves and CD racks can act as diffusers in a pinch. They can really liven up a 'dead' sounding room without causing issues.

acoustic weapons

Isolation/decoupling

This stops vibrations from transferring to other objects, reducing resonance. Use sturdy speaker stands, isolation foam, and add mass beneath the speaker to increase inertia. There are many products available that can do this, ranging from simple foam, to tiny shock absorbers. Make sure that your speakers are thoroughly isolated as this leads to greater performance and fidelity!

acoustic weapons

Your ears

Try playing a sweeping sine wave, and note any frequency that sounds unusually loud or quiet. You want this to sound as flat as possible from your listening position. And huge dips or peaks could be an indication of a standing wave in your room.

 

3. Deployment

acoustic deployment

Listening position

Try to find a spot unaffected by a room mode. Avoid being perfectly between surfaces - being off centre is better. Remember that being off centre means in all three dimensions. Take a stroll or wheel your chair around your room while playing something very familiar. Try to find the spot which sounds the most neutral.

acoustic deployment

Monitor position

Ensure monitors are not too close to a boundary and are decoupled on a sturdy stand to prevent resonance. Make sure they are an equal distance from the listening position, at ear height, and angled toward the listener. It may help to visualise this as an equilateral triangle with your head and the two speakers occupying each of the three corners.

acoustic deployment

Early reflection points

These are where you should place your absorbers. Use a mirror against the wall - if you can see the speaker this is an early reflection point. Don’t forget the ceiling! If there are any close secondary reflection points try to address these too.

acoustic deployment

Bass trap position

Room modes build up where boundaries converge, so the corners are the most effective place to set these up. Bass wavelengths are very long, so leave at least a foot of space behind the bass traps to increase their low-frequency absorption. You can get products that mount on ceilings and the floor where three surfaces meet - these then cover three different potential room modes.

acoustic deployment

Diffuser position

Set up behind the listening position as this is usually a more distant reflection point, and can prevent things from sounding too dead, but not introduce too much reflection close to your listening position where these can be more problematic. Many small studios employ a 'live end, dead end' system where you focus on making the speaker and listening position more dead sounding, but keeping the opposite wall sounding lively. This means the room sounds well treated, but not so dead that it becomes unnatural and unpleasant.

 

4. Advanced tactics

acoustic tactics

Room shape

If you can change the room shape to something without perpendicular and parallel walls, floor, and ceiling, you can eliminate reflections, and room modes. Avoid a cube-shaped room! If you're handy with a saw, you can create your own walls and stuff them with dense acoustic fibreglass. You can create little storage cubbies behind them!

acoustic tactics

Beware bass ports

Bass ports cause any spectral energy to ring on for a short time. This leads to resonance, which we know is bad news! Try getting unported monitors, or blocking the ports. The port is centred around a specific frequency, so this can often lead to a spike. To see what this frequency is, try playing a sine wave through the ported speakers. Watch the speaker cone move - you will see a point in which the cone moves less, but the volume is still high. This is the port's frequency. You can also place your hand in front of the port and feel it displacing the air. Be mindful of this frequency as it may be a problem area.

acoustic tactics

Beware over deadening

If you cover every surface in absorbers, you will kill the high end frequencies in the room. This will lead to an unnatural sound. An anechoic chamber is not a pleasant place to be, it's very disconcerting.

acoustic tactics

Listen in mono

If you’re mixing music, ensuring your mix sounds good in mono is essential. A true mono source eliminates stereo imbalance, and a lot of comb filtering problems. A true mono speaker has only one sound source - as in just one voice coil actuator, and not a woofer/tweeter combo as this can case issues with the frequency crossover.

acoustic tactics

Find problem frequencies

Work out the resonant frequency by dividing 172 by the distance in metres between opposing surfaces. After the sweeping sine wave exercise, divide 172 by any of the problematic frequencies for a measurement in metres. Identify room dimension that work out as a simple multiple of those measurements.

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