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Room EQ: Myth or Legend? (Part 4)

In the last part of Ben Duncan's series, he examines the acoustic properties on Live End/Dead End studios.


Ben Duncan rounds off with a survey of LEDE studio techniques.

The detailed requirements for an LEDE monitoring environment revolve around just three simple rules. Firstly, the only sound to reach our ears within the first 10mS should be via the direct path from the speaker, (sometimes aptly called the anechoic path). Secondly, reverberation should begin to reach our ears not less than 10mS after the sound's wavefront first emerges from the speakers. Thirdly, the reverb thereafter should be immediate and diffuse, so that no particular frequency pattern predominates.

In turn, these indicate that the room treatment should be considered. To begin with, we have to abolish sound reflections in the front part of our control room: in the pathway between the speakers and our ears. If the direct soundfield glances off adjacent objects en route, the reflected portion will reach our ears only milliseconds later via a shallow angle. The audible ballsup, called 'time smearing', is what we're seeking to eliminate. In the remainder of the studio, anything that's more than 1mS behind the listening positioning, is out of the critical path.

Disposing of reflections up-front begins with making this section of the walls and ceiling as absorbent and dead as possible. It means positioning the monitors so they're focused exactly at the listing position, and generally moving clutter out of the way. If the ceiling's fairly high, try suspending the monitors high up, pointing downwards, so that the direct sound passes over objects on the ground. It's the same basic idea as airborne radar, except that 24-track recorders and equipment racks replace buildings and trees as causes of blurred signals.

In many installations, the mixing console is an arch offender: the sort with wild expanses of hard metal blanking plates is good at producing shallow, glancing reflections about 1mS behind the direct soundfield. Hacking up bits of foam 'anechoic' tiles and attaching them to the console's dead surfaces is a pragmatic way to cut reflections, but take care. The foam could end up looking unsightly, not to mention producing lethal fumes when the first muso sees it as a convenient ashtray.

Figure 1. The speed of sound.
Figure 2. At Eastcote, Phillip Benegal uses a trio of bamboo offcuts to provide multiple splayed reflections.


EES and the Speed of Sound



Imagine a sci-fi studio. Sound reaches your ears from the monitors along a gravitationally placed cone of air, while all around you is a vacuum. Without the pathway provided by the air, room reflections are impossible. Nevertheless, you're not satisfied with the 'thwack' of the kick drum; its leading edge sounds wrong, almost as if it has pre-echo, the sort heard on vinyl disks. What's more the electric bass still sounds muffled.

Before putting an axe through your monitors, cast your eyes at the floor, and then at Figure 1, which shows the speed of sound in some common building materials. Sound travels about ten times faster through solid materials like concrete or wood, than it does through air accordingly, the sound transmitted through the building can reach your ears before the airborne wavefront! When first uncovered ten years ago, SYN-AUD-CON dubbed this premature arrival 'Early Early Sound', commonly abbreviated to EES.

The acid question is, 'Are your monitors, like most, connected in any way to the building?' Presuming that 100% mechanical decoupling is rarely achieved by accident, then as long as we're seated or standing in the same, vibrating superstructure as the monitors, some part of their sound energy is bound to reach us before the direct, airborne soundfield arrives. With sound energy being allowed to wander through the random, resonant pathways of sundry building materials, it's small wonder the EES turns out to be booming and lumpy. Structure-borne sound is at its worst through thin floorboards, the sort which vibrate like reeds, while a thin concrete ceiling raft has its own set of problems at low bass frequencies. Luckily, the human frame is quite absorbent above 250Hz, so EES is principally restricted to fogging up our bass end perceptions; unless the structural vibration itself is loud enough to be audible.

Overcoming EES involves taking steps to isolate both the speakers and yourself from the control room's (inner) shell. In genuine LEDE studios, much design effort goes into decoupling the monitors from the control room, using multiple absorbent layers and rubber block shockmounts. Could there be a simpler way?

For some time, domestic hi-fi listeners have been sold the benefits of 'spiking' speakers. The spikes are sharply pointed cones of solid metal (such as brass), screwed into the base of the speaker stand. When installed correctly, so the whole weight of the cabinet is equally distributed across the tiny area of each cone's apex, they can certainly clean up the bass sound. Considering their inflated price, it's unlikely that so many would be sold if their benefits were purely psychological. Even so, the explanations offered by hi-fi reviewers and journalists have so far missed the point. Typically these revolve around Newton's law 'Every action has an equal, opposite reaction.' That's to say that the spikes make the sound clearer because they prevent the speaker cabinet leaping backwards and forwards (however minutely) in opposition to the cone movements. But the full story must include EES; by greatly reducing the area of the cabinet that's in contact with the room's structure, the transmission of EES (at bass frequencies in particular) is simply much less!

At the human end meanwhile, an Italian or French 'designer' swivel chair with an integral foot rest and gas floatation all goes to help isolate your ears from the EES vibrations coming up from below. More importantly, any high chair takes our ears away from floor reflections and closer to the room's vertical meridian, where the sound is cleaner. In essence, the further away we can get from reflecting surfaces, the better.

Planning for a Good Reverb Sound



Having disposed of reflections and unwanted vibrations in one part of the room (the dead end), it's time to simulate as many little echoes and reflections as possible in the back end of the room (the live end).

First we need smooth, bare, hard surfaces, such as plasterboard, scratch stucco, glass and even mirrors. Wood has its place too; an area laid with parallel hardwood strips in the manner of ship's decking makes an excellent HF diffuser. The heroic Phillip Begenal at Eastcote uses giant bamboo offcuts: a hard shiny surface which bounces sound off in all directions. (See Figure 2.) Hitherto, kosher studios have been windowless caverns, but in LEDE studios, there's room for daylight (gasp!) and provided there are no drum kits, suitably rigid window panes can play their part in simulating reflections. It's difficult to achieve a very dense, even reverberation spectra, something that's arrived at empirically, placing the chosen reflective materials to develop a series of splays and angles between the walls, floor and ceiling so no two surfaces are parallel. If properly done, the reverberation attains qualities akin to pink noise, which even masks the tell-tale reflections off incurable reflective surfaces such as multitrack cabinets.

Heavy Engineering



The official SYN-AUD-CON LEDE specification requires 'a low frequency asymmetrical outer shell, free of pronounced resonances at low frequencies, this shell to be large enough to allow the development of bass frequencies.' In practice, it means making the control room bigger than expected, because bass frequencies below 100Hz ideally need distances of 20ft (7m) or more to develop fully. Meanwhile, the inner shell has to be symmetrical (at least at the dead end) for good stereo imaging, and ideally isolated from the outer shell.

Equally, it's beneficial to have ample headroom, with a 15ft (4.5m) ceiling. There's plenty of space to build in a false ceiling incorporating a series of angles, and to accommodate bass traps and lots of Rockwool.

Summing up, the creation of a genuine LEDE environment invariably means starting from scratch; there's no substitute for a big enough shell and a certain amount of heavy building work. At the opposite extreme, passable results are possible in a bedroom studio simply through the benefits of nearfield monitoring, especially if the speakers are placed on stands (or hung from the ceiling) well away from the walls. Why? Well, if the monitors are placed just 24" from our ears, while the nearest walls are for instance, 6ft away, the direct sound has precedence, because there's some kind of decent interval between direct and reflected energy, even if the reflections begin arriving somewhat earlier than the legitimate 15mS. Moreover, when monitoring close up, the volume doesn't need to be wound up so far, so there's less energy being pumped into the room, which all goes to lessen chaotic reflections.

Further reading: Monitoring Pt2 (HSR Nov 83) - Ben Duncan (photocopies available from HSR).



Previous Article in this issue

TOA 310D

Next article in this issue

Simmons SPM8:2


Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.

 

Home & Studio Recording - May 1987

Donated by: Rob Hodder

Topic:

Acoustics


Series:

Room EQ

This is the only part of this series active so far.


Feature by Ben Duncan

Previous article in this issue:

> TOA 310D

Next article in this issue:

> Simmons SPM8:2


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