Friday, March 13, 2009

FFW07: Lessons in coupling & isolation

Setting up studio monitors is never as simple as it sounds; not if you want to get the best performance from them. After finding the right positions within the room, you've got to make sure they're properly mounted and isolated. This First Word, written sometime in 1999, discusses simple things you can do to improve the sound of your monitors by paying attention to mounting, coupling and isolation.

Lessons in coupling and isolation
“Yikes, those Questeds are making my ATCs sound bad!” I cried. Brad was somewhere behind the console, fossicking for test CDs in a dirty blue milk crate. “Ha!” he laughed, “I dare you to print that!” “No, no,” I explained, “my ATCs don’t sound right when those Questeds are sitting next to them... the low mids have gone all lumpy!”

Brad and I had just unpacked the Quested VS2108s and, noting their size, decided to sit them flush beside my ATCs for a quick and easy A/B comparison. But at 34cm wide, the Questeds’ imposing baffles were playing havoc with the dispersion of the considerably slimmer ATCs, coupling the lower frequencies and making them sound dull and ordinary.

Interesting? Not really. It’s simple physics, and it’s something that PA operators deal with whenever they stack multiple boxes side by side. But it’s been about 15 years since I stacked a PA system, and acoustic coupling was the last thing on my mind as we lined up the Questeds beside the ATCs.

Switching between the ATCs and Questeds showed that neither pair were sounding worthy of their respective price tags. Taking the Questeds off the bench returned the familiar ATC sound. Likewise, removing the ATCs and listening to the Questeds in isolation revealed a much higher level of sound quality, certainly more in line with their reputation.

Lesson #1: Never compare monitors in any kind of side-by-side situation. It may seem like a good way to make a direct A/B comparison, but all you’ll actually be testing is how badly each monitor affects the other one. Retailers, take note…

Later that week, during an AudioTechnology ‘Spontaneous Human Consumption’ event at Brad’s place, Michael Stavrou spent a critical moment listening to the Questeds, rubbed his chin for another critical moment, then said, “You got any marbles?” “Dunno,” said Brad, “take a look around.” “How about washers?” Stav asked. “Ditto…”

You can dig up all kinds of interesting stuff while fossicking around Brad’s place but his marbles and washers were too well hidden, so Stav returned with half a dozen beers. He carefully removed the top from each bottle, and placed three tops under each Quested in a triangular shape (one under each front corner, one half way across the back). The performance increase was obvious to all, and became a hot topic for the next half hour or so. Just long enough for the beers to go flat…

Lesson #2: Lifting a monitor’s bottom off the surface it rests on minimises physical contact, thereby reducing the amount of sound energy being drained out of the monitor and into the surface. This ‘draining’ of energy out of the monitor causes a decrease in performance, but it gets worse: if the surface is not sufficiently well-damped, it will re-radiate that energy back into the room, causing an even further decrease in performance.

Brad’s monitor bench is about six feet long, reasonably rigid, and supported at each end. But the Questeds have a large and squarish footprint that provides a good contact area with the bench, and they generate a lot of low frequency energy for their size. Combine these factors with their 22kg weight per box, and you’ve got a powerful source of low frequency energy with a large footprint and considerable mass pressing down onto the bench, allowing an even better draining of energy.

Interestingly, my ATCs are designed with three feet fitted in place for this very reason – and never suffered this problem when mounted on Brad’s monitor bench. But why three feet? Why not four or more?

Lesson #3: Proper monitor performance requires stability. Powerful small monitors, such as the ATCs and Questeds, really need to be held stable. If the box wobbles or rocks in any way, it causes loss of output, blurring of the stereo image and smearing of high frequency detail. JBL’s Doug Button discussed this concept, which he calls ‘inertial grounding’, in my review of JBL’s LSR32s [Vol. 1, Iss. 4.]

So why three feet? Three points defines a single plane, and therefore offers maximum stability - that’s why microphone stands, camera tripods and my favourite ‘non-rocking’ café tables are all designed to stand on three feet. (Of course, ‘tripod’ literally translates to ‘three feet’. Duh!). Increasing the number of feet beyond three increases the possibility of instability and wobbling - not a good thing for microphones, cameras, steaming hot cappuccinos or studio monitors.

While Stav’s beer bottle tops demonstrated the benefits of isolation, they were only a temporary solution. Brad has since replaced them with height-adjustable brass cones designed specifically for decoupling speakers, which are available from your local hi-fi shop. Due to the squarish footprint and weight distribution of the Questeds, he’s using four cones - one under each corner. Being height adjustable, he’s able to fine-tune them for maximum stability. His Questeds are now sounding better than ever.

So if your monitors are sitting flush on their bottoms, get some cones under them ASAP! You won’t regret it. But make sure you put the cones the right way around – which is upside down. Their large flat end connects with the bottom of your monitor, while their small pointy end connects with the surface your monitor sits on. When done correctly, your monitors will look like they’re standing on tip toes. (In fact, the first commercially available cones were called ‘Tip Toes’.)

So how do the cones work? Physically, their pointed ends provide a solid connection between the monitor and the bench, which keeps the monitor from wobbling. But their small contact area with the bench creates a very high acoustic impedance, the sort of thing that sound energy prefers not to travel through. With the weight of the monitors pressing down on them, the cones are able to firmly anchor the monitors to the bench while simultaneously providing acoustic isolation. Amazing, huh?

I first met Brad Watts during the days of AudioTechnology's miserable predecessor, Sound Australasia. I was looking for a Mac-savvy guy to write a regular Macintosh audio column, and Brad came highly recommended from the people at AudioMedia magazine. We made an appointment to meet in my office at Pacific Publications. While waiting for this 'Brad Watts' to arrive, a feral-looking bicycle courier walks through the Pacific Publications cubicle farm, dreadlocks flowing and talking into the air with some kind of assertive certainty, just like a crazy guy. He stops at the door of my office. "Simmo? I'm Brad Watts, hang on a minute mate". He then proceeds to pace in and out of my office door while finishing the phone call taking place on his hands-free kit; obviously helping someone get their Mac working again. It was the first time I'd seen someone using a hands-free kit in such a brazen and open manner. It was also the first time I ever saw Brad Watts.

Two years later Philip Spencer and I saunter out of an important meeting that secured the future of our yet-to-be-published magazine. It’s a beautiful Sydney day and we’re feeling as good as the weather, so we stop at Mo’s, the outdoor café belonging to the Museum of Sydney, for a celebratory drink. The attentive waitress is buzzing around wiping tables and keeping us well stocked with refreshments.
“By the way, my name’s Jackie. What are you guys celebrating?”
“We’ve just launched our new magazine, called AudioTechnology. It’s about sound recording equipment.”
“Really? My boyfriend writes about that kind of stuff. Maybe he could write for you…”
“What’s his name?”
is his name! Brad Watts, actually.”
“He’s in our first issue…”

Fast forward a few more years and I find myself sharing a warehouse conversion at the top of Hibernian House, Surry Hills, with newly-weds Brad and Jackie Watts. Crazy and intense days, in retrospect, littered with marine aquariums, astroturf, huge televisions and way too many fried chicken wings from the Thai takeaway down stairs. But we had a lot of fun. We both had our own studio rooms; Brad with the Quested VS2108s mentioned above (which he promptly bought after the review) and me with my ATCs. Between us, there was probably not a single piece of audio gear on the planet that we could not review the heck out of.

Mac Notes column has been a regular fixture in AudioTechnology since the first issue, along with his prolific product reviews. He's probably the smartest Mac audio guy on the planet. A year or two ago he became a full-time employee of AudioTechnology, a position he well deserved. To be honest, I don't know where the magazine would be without him...

No comments:

Post a Comment