Expert acoustician and monitor designer Andy Munro explains why it's so important to get exactly the right monitors…
MEET THE WRITER
Principle owner of Munro Acoustics, Andy co-founded Dynaudio Acoustics in 1990 and still designs custom Dynaudio monitor systems. He is currently designing many of the studios for the new BBC Broadcasting House and he is working with sE Electronics to develop a range of monitors.
If there is one piece of audio equipment that is completely dependent on the way that it is installed, it is the monitor speaker system. Poor speakers will always be poor but many good systems are often ruined by acoustic problems, overloaded amplifiers and unsuitable positioning or mounting. Even the position of the ears can make a big difference to a way a mix turns out.
I LOVE IT WHEN YOU'RE NEAR
Nearly all mixing is done on small speakers in a ‘nearfield’ position. This is a notional zone within a short distance of the drive units where only the direct sound is heard and the balance has been optimised for listening at a reference distance, normally one metre. At that distance, each driver is offset slightly from the ear and so it is very important to find and align one’s head to the acoustic axis; the line along which all the drivers (usually two) are in phase and therefore sum their energy correctly. Moving up or down from that point will cause a small but significant path length difference, which will cause a notch filter. The problem does not occur so much when moving horizontally as long as the speaker is upright. Speakers should not be used on their side unless you can stay put and have very long arms!
MY BEAUTIFUL BALLOON
Even though one metre is close enough to avoid some room issues, it is a sobering fact to note that, at low frequencies, speakers radiate spherically so the ‘wavefront’ will be a balloon of 13m². Our ears can trap considerably less than that, unless your name is Dumbo, so almost all that bass is passing by and filling the room. This is good in that the energy is saved and returned to make the bottom end of your mix audible… but only if the room has been acoustically treated to give an even reverberation and that is not so easy to achieve. That is why outdoor sound systems need so much power and line array directivity. A well designed nearfield monitor will have a tuned bass response that rolls off gently to compensate for the ‘room gain’ at low frequencies. Bass is also enhanced by sitting on consoles as explained in my acoustics article (CLICK HERE).
Very low frequencies can only be reproduced efficiently with a small monitor by adding a bass port that radiates energy at just below the resonance of the bass driver. Carefully designed, front-facing ports can deliver a surprising amount of energy without the boom and poor transient response that is common in many budget systems. Many subwoofers use a tuned box and port that reproduce a tiny bandwidth but with very high efficiency and this approach is strictly for the home movie, cheap thrills fraternity.
IT TAKES TWO
Nearly all speakers are two-way because that is a simple and inexpensive way to cover the entire audio spectrum. The crossover frequency will need to be around 2.5kHz to maintain good directivity control as woofers get very directional at high frequencies and tweeters cannot handle much power below 2kHz. The reason for this is partly because drivers are very inefficient at turning electrical energy into sound. A 100% efficient system would produce a sound pressure level of 109dB at 1m if it were an omnidirectional source (more about that in a minute), but in reality the best you can get is about 90dB. That’s 20dB less, so efficiency is only 1%! The other 99% is turned into heat inside the voice coil of the drivers and that can be scary if your amplifier is pushing 100 Watts into your system for more than a few minutes at a time.
If you think that’s weird then ponder that a whole orchestra playing flat out would not even produce enough energy to run an old-fashioned 100W light bulb. Unfortunately we don’t have the equivalent of LED lamps yet.
So how do we deal with this problem? One way is to increase the directivity so that more sound is funnelled to the ear and this also increases the efficiency of radiation by putting more pressure on the driver surface. The reason drivers are so inefficient, even when using the most powerful magnets and coils, is that air is so much lighter than cones and domes, so there is an impedance mismatch. It can be corrected to some extent by ‘horn loading’. This works for big systems but all horns act like band-pass filters so you end up with three- or four-way systems and a lot of complexity; fine for Glastonbury or U2 stadium gigs but not the answer in small rooms.
Electronics cost a lot less these days and copper is expensive so it makes perfect sense to design active monitors that do away with big inductance coils and heavy-duty crossover boards. Many systems now have built-in amplifiers but this presents compromises with power
supplies and even the acoustic performance of the cabinets.
Another approach is a fully integrated system but with an external control and power amplifier unit. This can be more expensive but offers a no-compromise approach to the speaker cabinet.
This is a last but far from least moment when choosing speakers as nothing has more effect on the final sound balance. The reason is that small boxes do not provide a constant ‘boundary’ for the sound to radiate from. One solution is to flush mount the speakers so that they radiate into a perfect ‘half space’. This is highly recommended for larger systems and is de rigueur for commercial recording studios as it allows loud playback for more people than the guy on the desk. This is a potentially exciting sound that needs careful acoustic design as the nearfield is replaced by a combination of direct and reverberant sound that can be disastrous if not done correctly.
An alternative is to contour the speaker cabinet so that it presents a smooth and continuous surface. This has been the focus of much research over five decades but manufacturing cost has always been an issue. New materials and moulding techniques used in the automotive industry now allow us to do what wouldn’t be possible with folded panels of chipboard.
TAKE YOUR TIME
To put everything together with the best drivers for the job is a very personal business and a full-range, clean, dynamic, analytical sound will appeal to some, while a tight but restricted balance from a very small system will give others the feel of how a mix will work at home or on the train. Personally, I feel the only type worthy of the ‘monitor’ moniker is one with as close to zero distortion of the original sound as possible. Listening to a favourite recording is the best way to judge this, as long as it's representative.
Remember a bad mix will sound much better on a system with reciprocal properties. Boom plus a bass-light speaker equals balance but two wrongs will not make a right when something good is required. Happy listening awaits!
We have a range of monitors to try in our in-store studio. Bring along your own music and have a listen, or call us (01202 597180) for expert monitoring advice.
The technology inside these French-designed beauties includes inverted dome tweeters and extended bass response. Prices start at around £250.
• Adam A7X
These high-quality 100W+50W monitors with 'X-ART' extended frequency response are a favourite with our studio sales team.
If you're looking for as much power-per-pound as possible, this pair is a good bet, supplying 75+45W of bi-amped power for around the £400 mark.
The new Mk2 MR8 now features revoiced high-end drivers, improved A/B amplification and an eight-inch woofer with 100W+50W amplification.
You may also want to check out the Mackie MR5 Mk2.
This bi-amped monitor is perfect for medium to large studios and can be mounted upright or sideways with the included ISO-Pod mount.
This newly hatched monitor was designed by Andy Munro to avoid the sonic colouring that flat surfaces on a regular monitor can cause. Grab a pair now for just shy of £1,500.
At the top of the Rokit ladder is this beast, whose curved frontage is a winner (aren't they always?) as it maximises sound in a small space.