Acoustics 101: Why Are Restaurant Acoustics Different?

A perfect example for this practice is Kate Mantelini’s in Beverly Hills, Calif. It was a well-loved and frequented restaurant, but also infamous for its incredibly poor acoustics. If there were four people dining alone in the center of the main floor it sounded full and busy. Consequently, when it was a full house such as at Film Academy events with every seat taken, standing room only, and wall-to-wall people in the aisles, it was painfully loud, completely unintelligible, and exhausting. Every person leaving these events would complain of having to shout just to be understood, some losing their voices by the end of the night and everyone exhausted by the sheer volume of the room.

Everything starts from the top down or the bottom up. Either the owners must want the space to sound organic, comfortable, and natural (not loud), or all the patrons leave due to the poor acoustics. Either one generally creates enough motivation to take action.

The caveat is that restaurants have organically generated sound (created by people), unlike electro-acoustic environments where electronics and speakers create the sound. Restaurant sound is comprised of people talking, laughing, glasses clinking in celebration, eating, and perhaps some live music. This means you cannot turn down the volume to help solve the issue.

This all means that we have to design for the worst-case scenario: the loudest possible volume. Using the same traditional acoustical math as with Kate Mantelini’s, an acoustically controlled room that sounds great at the loudest volume will also sound great at the softest volume but exceedingly quiet. Twenty-five hundred people will sound like 100 but four people will sound like a deserted library. While this will most definitely solve the problem, it may not sit well with a restaurateur’s need to drive business using loud sound. 

There needs to be a middle ground.

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Traditional Acoustic Solutions

As previously addressed in this series, traditional acoustics is based on absorption and diffusion in addition to classical mechanical theory. In nearly all restaurants the main issues are intelligibility, volume, and reverberation; in everyday terms that means a loud, cave-like sound where it’s impossible to understand what anyone says.

Traditionally this is solved by using a specific mix of diffusion and absorption tailored to the acoustic signature of the room. Absorption is widely used as it’s inexpensive, easily obtainable, simple to install, and can be covered with aesthetically pleasing textiles. The trick is to use enough of it to make a space pleasant and then add some diffusion in specific areas with acoustic anomalies that fall outside the realm of reverberation issues. If you simply covered every surface of a room with 1-inch and 2-inch thick absorption and a few diffusers, it could actually make the issue worse by deadening the high frequency content of the room too much and accentuating all the low frequency, making it sound dark and muddy.  If you use too little, it often feels like nothing has been done.

For best results, the room should be analyzed to determine specifically where absorption or diffusion should be used.

Figuring Out Ideal Absorption

It should be noted that all absorption is not equal. Absorption works like a cross between a sponge and a speed bump; it must be porous enough to allow air molecules and sound energy to permeate it yet have enough density to create a blockage of the same air and energy from getting out. This means a narrow window of density. More than 5 pounds/cubic foot and it reflects mid and low-frequency sound, defeating the purpose entirely and possibly not categorized as absorption anymore. Less than 3 pounds/cubic foot and it literally falls apart under its own weight.

The magic number is 3 or 4 pounds/cubic foot to be effective. When applied correctly, it will have enough performance to control all frequencies (thickness dependent) while physically strong enough for easy installation and surface mounting.

It is also important to note that material thickness affects frequency. There is a direct relationship between the wavelength of sound (frequency) and the depth of material.

Shallow materials affect only high frequencies and deeper materials affect low frequencies. 

A one-inch-thick piece of 3 pounds/cubic foot absorption can only control roughly 13 khz and up. If you were to double the absorption to two inches it would still only control roughly 6.5 khz and higher. In order to control a 100 hz low frequency wave of sound you would need a 3 pounds/cubic foot chunk of absorption over 10-feet thick.

The limitations of traditional absorption are offset by the costs, which are not significant except for the aesthetic portion. The finish layer of acoustically transparent cloth, snaptrack, and installation labor far outweigh the cost of the basic absorptive material.

Using Diffussors

Diffussors randomly and/or specifically spread sound energy across an area in a fashion sometimes known as “scattering.” They do not absorb much energy, only diffuse sound energy, and therefore they are best used in areas where there is space around the diffusor to allow the sound energy to spread out and drop in amplitude/volume by the nature of air friction/impedance. They work best in large open spaces where there is distance between the diffusor and people.

Bass traps fall under the category of absorption and are best used where there is a buildup of low frequency due to poor room acoustics and no other way to mitigate it. Bass traps are a traditional solution to control low-frequency sound in circumstances where a proper acoustical design was not possible or affordable. It is an after-the-fact solution.

How to Combine Absorption and Diffusion Depends on the Room

Again, the combination of absorption and diffusion is dependent on the room’s acoustic signature.  A more reflective room with long reverberation and decay times needs more absorption and some diffusion. A space that is “dead” can use more diffusion and less absorption. Again, to create a working solution, the room should be analyzed and a specific design applied for best results. A talented acoustician will create a design for traditional acoustic treatment that will take into account the constraints of the room and of the acoustic treatment.

As a general rule in restaurants, areas prime for acoustical treatment are any exposed hard surfaces, except architectural details such as glass or textured walls. They can either be near or far from the guests and dining areas as the long reverberation is often due to large open spaces.

Using Guerilla Acoustics to Determine Where Products Should Be Used

For the daring, willing, and able, a Guerilla Acoustics method would be to listen in the room and have a group of people available to talk and move around the space. To do:

• Clap your hands, talk loudly, and have the group talk and laugh loudly then softly, listening all the while to how natural or unnatural it sounds. 
• Then have them move to another location in the room and do it again.
• Have a separate group of people with packing blankets move from place to place directed by the listener.
• Wherever there is an area that sounds strange or unnatural, move the people with blankets to the closest wall or hard surface and see if it makes a difference.
• If it does, make a note, stick a Post-It on the wall, and move on to the next area. 

Remember the ceiling counts as a hard surface though it may be difficult to reach.

Once you’re done, you have a rudimentary map of where acoustic treatments need to be placed.

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It’s not rocket science, but it works more often than you’d expect as a method for acoustical triage. It may stop the bleeding but is no substitute for a proper solution. You’ll need a good ear, willingness to experiment, and a logical mind. And a lot of time for experimentation. But like all things, practice makes perfect. Or you may just hire an acoustician.

Quantum Acoustics™ Solutions

The first two parts of this series discussed how these acoustic devices are based in quantum physics and therefore don’t follow the same rules. Acoustic anomalies cease to exist as it controls the air the sound rides on, thereby controlling all sound, all frequencies, and all amplitudes just by placing quantum devices on the wall or even freestanding in critical areas of the room. 

Unlike traditional solutions, quantum acoustic devices are effective at all frequencies and volumes regardless of thickness or size. Their effectiveness is measured in acoustic resolution or NPS/square foot.  A higher number means higher resolution, which means greater performance. However, if you were to randomly cover every square inch of the room with quantum acoustical devices it would not achieve maximum performance.

For ultimate performance, the room should be measured and analyzed with a well-thought-out design in place. 

As always, the general rule of thumb is more surface area coverage means greater performance. This assumes that every hard surface in a room creates a negative acoustical anomaly.  They are easily solved or quantized but in reality, the architect and interior designers will have a great deal of input on where devices can be placed and, more importantly, where they cannot.

This constraint more than any other will drive the overall performance of the room. It is rare that any designer will use one type of material throughout an entire space. Therefore, the starting point is really a discussion on where cloth-wrapped and exposed wood quantum devices will work within the overall design. 

Also, like all cutting-edge technology, quantum devices operate differently. They are much easier to use in a food service environment. The complex requirements of studio and audio production suites don’t apply here, just a need for clarity and intelligibility. Even a pair of 2-foot x 4-foot devices on a booth divider make a conversation easy and clear. It’s simpler, easier to use, thinner, and lighter. The only caveat is the costs which are in line with the performance and therefore more expensive than traditional solutions.

General areas prime for quantum acoustical treatment are any exposed hard surfaces. Those closer to the people making the sound works extremely well. Areas recommended for quantum device locations are on booth seating dividers at ear level, fastened to walls as close as possible to booth seating or tables, or hanging devices as visual and acoustical banners. Distant areas recommended are in direct line of sight from people talking toward ceilings, HVAC ducting, hood ventilation, and painted drywall. Any glass or textured walls are most likely architectural details and probably won’t be allowed to have coverage with quantum devices.

Summary

Restaurants have become very loud places with a great deal of reverberation and echo. Both traditional and quantum acoustics have solutions for creating quieter, clearer spaces with the ability to have a decent conversation at a reasonable volume. Large, spacious rooms with hard surfaces are a recipe for acoustic disaster. More surface area coverage is better for acoustics than less. New quantum acoustic devices deliver far greater performance and aesthetics while traditional acoustical treatments can often suffice for limited budgets.

ACOUSTICS 101 SERIES: Part 1 / Part 2 / Part 3 / Part 4 / Part 5 / Part 6 / Part 7

Hanson Hsu is the principal acoustician and founder of Delta H Design Inc. (DHDI), a research, design, and build firm providing design and consulting services for architecture and acoustics since 1998. Clients include Universal Music Group, Yahoo Music, Microsoft Studios, LinkedIn, Kanye West, U2, Cher, and more. DHDI specializes in facility design, studio design, transportation, products, and technology innovation using the revolutionary ZR Acoustics design paradigm based on Quantum Acoustics.

For more on Hanson Hsu and DHDI, please visit deltahdesign.com