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Room Acoustics and Speaker Placement Explained

June 26, 2015 6 translation missing: en.blogs.article.read_time

When building and designing a music-production work space, the most critical aspect of your design will come down to the acoustic properties of your listening environment and your recording space. Commercial studios invest tens-of-thousands of dollars in the expertise of an acoustician and subsequent materials and construction costs, so that these two spaces perfectly meet their recording and monitoring requirements. Obviously this sort of investment is beyond most project, home and smaller post-production facilities. The construction alone will most likely involve modifications to existing building structures, which is simply not possible under most tenancy lease arrangements.

The aim of this blog is to clarify some common myths that surround acoustic treatment and present some practical information regarding this subject, and point you in the right direction for further reading and research. Most importantly though, is to offer sensible solutions for anyone interested in drastically improving their monitoring and recording spaces, without the huge expense of construction and consultancy.

Near-Field Monitor Placement 

The first area to address is the placement of your monitors - what they will they be mounted on, and where the best location for your speakers regarding both the room, and the proximity of the speakers from your seated position in the room. 

Most near-field reference monitors, will be ideally placed 1-2m in front of you. (depending on the size and power of the speaker). The tweeters should be at the same height as your ears, and angled in towards the centre point between your ears at 60º, forming an equilateral triangle between the loud-speaker boxes and your ears, for an optimum clarity of sound and stereo image. 

The Equilateral Triangle 

monitor placement
Genelec have a detailed Monitor Set Up Guidedownloadable as a PDF document from their website.

Stands

The use of speaker stands are an easy way to achieve the optimum position for your speakers. Ultimate Support’s MS series have some nifty features including decoupling the ground from the stand with rubber feet, and the speaker box itself from the table-top of the stand. They also have separate rails inside the column of the stand to prevent audio and power cables crossing, and potentially creating interference in your signal path.

Ultimate Support MS stand baseUltimate Support MS stand topUltimate Support MS stand Column

Desktop

If your speakers are mounted on a desk, then adjustable speaker support stands will greatly assist in correct speaker placement. They are also have the added advantage of decoupling your speakers from the desk surface itself. 
When placing a loud speaker onto a supporting surface, sound vibrations created by the speaker will in fact create unwanted resonant frequencies. What you should do to avoid this scenario is decouple the speaker box, isolating any direct contact from the desktop or speaker stand.

IsoAcoustics L8R430

What is Decoupling?
Decoupling means physical isolation to prevent further emission of sound. isoACOUSTICS adjustable stands are an excellent and highly effective option for decoupling your speakers. 
They also allow precision adjustment to perfectly tilt the speakers so the tweeters directly align with your ears, helping to clarify the frequency spectrum and stereo image. The Primacoustic RX series of isolations products are also a very good at cleaning up the bottom end whilst decoupling from your desk.You can see our range of speaker isolation products here.

Room Modes and Finding the Sweet Spot
The “sweet spot” is defined as being the area of your control room that has the least interference from resonant frequencies and unwanted sound-wave reflections.
Finding, or actually creating a sweet spot in your control room can be an extremely complicated acoustic problem to solve. There are many factors that will need to be addressed determine a neutral listening environment. 

If your speakers are too close to a hard reflective surface, what will happen is when the wave-form hits this surface, it will bounce and return to your listening position either out of phase; thereby canceling certain frequencies, which may lead you to boost these frequencies unnecessarily whilst mixing, Or In phase. These wave-forms create the opposite effect, boosting certain frequencies. This is where acoustic treatment can significantly make a difference.

Acoustic Room Treatment diagram

Room Treatment 

When creating an acoustically treated space, it is important to strike a balance between a live sounding room; a room that allows early reflections, and a dead space; maximum amount of coverage that eliminates most early reflections. This is best created by addressing where to situate products that are designed to absorb sound, and those that are designed to diffuse sound.

Primacoustic’s YouTube channel, has some very useful videos that demonstrate’s this concept perfectly.


Do all Acoustic Treatment Products Function the Same Way?
When choosing materials to use for absorption, your money will be most wisely invested in products that have a higher Noise Reduction Coefficient (NRC). NRC is a standard that measures the amount of energy that is absorbed when a sound-wave hits a given surface. NRC ratings are between 0 (100% reflective) and 1 (100% absorption). This number is an average of the amount of absorption tested at specific frequencies: Usually 250, 500, 1000 and 2500 Hz.

The NRC of glass for example, is about 0.15 (15% absorption), where as 1 inch thick acoustic foam has an NRC of about 0.50 (50% absorption). Generally any substance with a total absorption of less than 50% will unlikely be an effective absorber of sound.

Primacoustic constructs acoustic panels, mainly made up of glass wool. Their website has some very useful information regarding acoustic treatment in general, and they provide links to the technical specifications detailing each products NRC rating, and fire safety ratings too. All tests were carried out, and certified independently of Primacoustic itself.

A 1 inch thick Broadway Panel™, has an NRC of 0.80 - significantly better when compared to a 1 inch thick piece of acoustic foam. 

Bass Traps

Low Frequencies Will Always Be The Most Problematic - Especially in Small Rooms.
The hardest frequencies to remove are lower frequencies, as the wave forms are larger and contain more energy. This is because the amplitude and loudness reduces the further sound travels. A higher frequency will have clocked up more complete cycles in the same distance that a lower frequency would have in the same amount of time.  

To gauge the distance a particular frequency travels to complete one cycle is determined by dividing the speed that sound travels (1130 ft. per second) by the frequency itself. For example, 100Hz will travel 11.30 ft. in one complete cycle. 1000Hz will only travel 1.13 ft.

Thicker acoustic treatment products will therefore be much more effective in absorbing lower frequencies. Bass Traps are often specially designed products that are designed to be mounted in the corners of your room, as that is where most low frequency resonances, along with flutters and echoes will be most prominent.

Are Sound Absorbing Products All You Need?

When creating an acoustically treated room for mixing, you will generally want to strike a balance between products that absorb sound, and those that will diffuse sound. Too much absorption will end up sounding to dead and unnatural. Diffusers are often made of harder materials such as timber, and are deliberately built as an uneven surface, so that sound-waves will split up and reflect in different directions. This helps to eliminate the likelihood of resonant frequencies building up in the room, and helps to create a more realistic or ‘live’ listening environment. 

Absorption
Acoustic Room Absorption
Diffusion:
Acoustic Room Diffusion

One common configuration for acoustically treating a mix room is have one end Live (diffusion), and the other end Dead (absorption). Diffusers will work well when situated on the back wall of the room, opposite the speakers. As sound hits them, they will reflect frequencies back towards the sweet-spot of the room. Absorption products should be placed behind the speakers on the front wall, on the ceiling above the listener, and diagonally opposite the speakers on the side walls at ear height.

Room Kits

Primacoustic have bundles of their acoustic panels sold as London Kits, that provide you with a range of absorption, diffusion and whole kits to fit out your room. Some also come with Bass Traps. If you are unsure what treatment you may need, these kits provide a great starting place and will dramatically improve you monitoring experience. 

Software Correction

Using a special reference microphone to record a sine wave sweep from you speaker location in the room some software can correct unwanted room frequencies - this can be a great way of treating your room especially if you can't attach panels to a wall. With the software in place a sine wave is swept through the entire frequency band. The microphone is placed in your best monitoring position and records that sweep. The software then determines what frequencies need adjustment and makes the correction. Genelec SAM speakers, like the new outstanding Genelec 8351 studio monitors use their proprietary GLM kit which directly adjusts the speakers via CAT5 cable. IK Multimedia ARC on the other hand works as a plugin on the master bus and provide the correction. Both systems work great. The difference between treated and untreated signal is significant and really helps to improve your mixing environment.


For further reading on acoustics please see the link to Master Handbook of Acoustics, Fifth Edition by F. Alton & Ken C. Pohlmann.

 If you have any questions we are here to help. Just give us a call and we'll be happy to help.

More Info:  Acoustic Treatment | Studio Monitor Stands | Monitor Isolation Products


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