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The Importance of Studio Acoustics

Broadcast Engineering, April 1, 2005 by Ken C. Pohlmann

The frequency response of a power amplifier will be the same in your audio control room as it was on the test bench. Grounding and shielding issues aside, an amplifier's performance does not depend on its environment or location. On the other hand, the frequency response and many other performance criteria of microphones and loudspeakers are affected by the room they are in. Indeed, specifications of transducers have limited value because they cannot account for the real-world conditions of their use.

The acoustics of any room can profoundly affect the quality of any audio signal that is acoustically conveyed in that room. Whether you are recording voice or music into an open microphone, or monitoring or mixing over loudspeakers, the room can dictate what is recorded and what you think is recorded. For that reason, any serious studio needs serious acoustics.

Be quiet

First and foremost, any studio must be quiet. Ambient noise will ruin an acoustic recording, or potentially mask detail in a mixdown. Acousticians use a Noise Criterion (NC) rating to determine the noise floor in a room. The NC uses Sound Pressure Level measurements at nine frequencies and fits the resulting curve against standard curves. For example, an NC-15 rating allows an SPL level of 36dB at 125Hz and 17dB at 1kHz. Ratings above NC-20 will be problematic for most audio work. If your NC is too high, you need to take measures to reduce it.

Generally, source treatment is the most effective. Find out where the noise is coming from, and attack it there. For example, turbulence noise from air vents might be reduced by removing the vent's louvers. If footsteps in the room above yours are audible, generously offer to buy padded carpet for them. Low-frequency rumble from an air conditioner can be reduced by isolating the unit from the structural slab it sits on. If source treatment is impossible or not effective enough, you can try isolation techniques to block the noise. For example, use solid-core doors instead of hollow-core doors, replace single-pane glass with double-pane glass, and make door seals airtight to prevent sound from leaking around the edges.
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In a worst-case (biggest budget) scenario, you may have to build new partitions to increase isolation. A partition's acoustical stopping power is measured by its Transmission Loss (TL). For example, if a partition has a TL of 50dB, then an incident sound of 80dB will be reduced to 30dB as it passes through the partition. It is true that heavy, massive partitions (such as concrete) provide good sound isolation. But, lightweight partitions (such as staggered stud walls) can also be effective. Also, consider what frequencies you need to stop. Low frequencies are hard to isolate, and a music studio might require a concrete wall. But a voice-over studio might only need a stud wall and a microphone with a low-cut filter.

Room treatments

Isolation is critical, but room treatment is just as important. Even a quiet room will be unsuitable if its acoustics are wrong. For example, speech intelligibility is poor in a reverberant locker room. A room's reverberation time, measured as RT60, is the time it takes for an initial sound to decrease by 60dB. There is no "correct" reverberation time. A too-long reverberation time will mask detail, while a too-short reverberation time will sound unnatural. Very generally, a ballpark time of one-half to one second is about right for many music studios, while a shorter time of less than one-quarter second is better for voice-over studios.

Reverberation time is governed by absorption. The more absorption, the lower the reverberation time. The easiest way to add absorption is with porous absorbers. Sheets of acoustical foam placed on otherwise reflective surfaces can be added until reverberation time is suitably reduced. Conversely, removing absorbing materials lengthens reverberation time. This might be the ideal excuse to get rid of a ratty old sofa in your control room.

Sound quality is also affected by the way sound reflects from surfaces. A flat reflective wall will give a specular reflection, potentially yielding an echo, or perhaps a kind of "acoustic glare." Placing absorption on the surface would address the problem, but it would also decrease reverberation time.

In small rooms, low frequencies are hardest to tame. Room modes, areas of relatively high and low energy in the room due to reflections from the room surfaces, will yield an uneven frequency response that is inconsistent throughout the room. One solution is the addition of bass traps, which are large hollow tubes filled with absorption, large blocks of acoustic foam or wall-mounted panel absorbers. These will help smooth bass response.

It's also worth noting that sound absorbers are generally poor isolators. If you are bothered by mechanical noise from an adjoining machine room, putting acoustical foam on your wall won't help. Alternatively, putting foam in the machine room might lower its ambient levels a bit, and thus lower the intrusion level into your room.

Outside expertise

If an existing workplace has minor acoustical problems, some common-sense solutions are worth trying. Simple noise source treatment and absorbers and diffusers can do wonders. For major problems, it is more cost-effective to hire an acoustician to advise you. When it comes to walls and floors, wrong decisions can cost you dearly. Get it done right the first time. If you are lucky enough to be planning construction of a new facility, then an acoustician is an absolute must. Many architects are geniuses, but many are not skilled in acoustics; you'll need both an architect and an acoustician. For the latter, talk to the acoustician's past clients and even tour their facilities to make sure the sound quality of the design is solid.

The quality of your acoustical audio work is only as good as the room's acoustics. Frankly, in today's digital era, anyone with a modest budget can set up a recording and mixing studio with electronics that are as good or better than your professional gear. The important difference between spare-bedroom audio and professional audio is the acoustical quality of a professional room.

You can do things a bedroom studio can't. You can make an acoustic recording or a mix and ensure that the recording is clean and accurate. If your studio is not up to acoustical code, then you might offer no technical advantage over the bedroom studio. Good studio acoustics is thus supremely important in keeping your work professional.

About the Author

Ken Pohlmann is director of the Music Engineering Technology program at the Frost School of Music at the University of Miami.

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