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Don’t Knock the Knock: Acoustics and the Pursuit of the Perfect Watermelon

After being outside on a hot summer’s day, nothing quite hits the spot like a slice of a cool, crisp, sweet watermelon. Unfortunately, not every watermelon is the same, and everyone has different methods for selecting the best ones.

There are number of visual clues that one can rely on to help identify a good watermelon. A large, yellow field spot on the bottom indicates that the watermelon was on the vine longer and is probably sweeter. Also, the coloring of a “ripe” melon will have strong, consistent stripe pattern; dull dark green stripes alternating with light yellow/pale stripes.

However, even when I followed these visual tells, I would often cut into my purchase only to be dismayed by a Styrofoam-like, flavorless inside or an over-ripened, mushy mess.

This melon melancholy haunted me until a few years back when I saw a middle-aged woman kneeling on the concrete floor of the supermarket with 5 watermelons circling her. I watched as she bent over and carefully knocked on each one, listening and nodding her head like she was holding a séance with the “other side” of the produce aisle. She repeated this process, rearranging the melons in front of her, until she picked up “the one” and put it in her cart, returning the other watermelons to the display bin.

I greeted the woman, trying not to startle her, and admitted that I often struggled to pick out watermelons. Clearly, she knew what she was doing and I was curious if she might share her system with me.

She kindly told me that she was listening for a “hollow” sound, that was full, but not too deep in pitch. I told her that I had heard that this “knock” test is a good way to judge the water content, but I never had much luck. She said that people will make the mistake of holding onto the melon when knocking, which quickly dampens the sound, so you can’t hear much of a tonal difference between melons (like palming the string of a guitar will change its sound to a shorter, more percussive, note).

Her strategy is to pick out 4-6 similar sized melons that have a large, yellowish sugar spot and strong striping, then sets each down so the only point of contact is with the hard floor. Without external dampeners, the melon can really “sing” when knocked, telling her how far along the fruit is. Too deep of a sound and the fruit is over-ripe/mushy, too high-pitch (or not hollow at all) and the watermelon is not ripe enough. She organizes the melons from lowest to highest in pitch and she simply selects from the middle melons to find one or two that are “just right”.

She said that this ritual, as ridiculous as it might appear to other shoppers, is the best way to guarantee a good watermelon and it’s worked for me ever since; so, don’t knock the knock!

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Eight very different 2′ x 2′ sound diffusers.

Acoustics First® has maximized the idea of adaptable designs. One of the most common modular architectural elements is the 2′ x 2′ ceiling grid. While standard, fiber ceiling tiles have their uses, specialized acoustic environments require higher-performing materials – for both absorption and diffusion. While Acoustics First® excels with its Sonora® and Cloudscape® Ceiling tiles, today we are going to focus on the wide range of 2’x 2′ diffusers that have been developed over the several decades.

Sound diffusers in a 2′ x 2′ format have several advantages, other than just being placed in a ceiling grid to help diffuse the ceiling. They integrate well on walls and in arrays, where they can help break up large flat surfaces and help minimize flutter and standing waves from parallel surfaces. While they provide many different aesthetic options, there are also many different functional types of diffusers available in this form-factor to address different acoustic issues, from flutter, bass issues, targeted frequency absorption, and geometric scattering. Let’s look at some of these devices and their uses.

Geometric Diffusers.

Geometric diffusers have been around a long time. These devices break up large flat surfaces and redirect or “scatter” those reflections in different directions. They work great in environments where you need to redirect acoustic energy in a predictable way, and redistribute a specular reflection over a wider area. In a 2′ x 2′ size, you can also get a fair amount of bass absorption, due to the large cavity behind the geometric shapes creating a space that can be stuffed with absorbent material to tune it.

The Pyramidal Diffuser is a classic geometric shape that provides 4 surfaces to break up the acoustic energy, these facets are also asymmetric, which prevents lobing, and reflects energy in different directions.
The QuadraPyramid™ offers the same benefits as the pyramidal, but in a lower profile, and 4 times the number of faces in the same footprint. These offer more control over high frequencies, that respond well to the multitude of smaller faces, due to their shorter wavelengths. These work well in spaces with lower ceilings to help control flutter echoes.
The Double-Duty Diffuser™ is a polycylindrical barrel diffuser, which redirects the energy in a hemispheric pattern. This pattern is easy to predict when trying to help distribute energy to many people – either in an audience, or players in an orchestra. This has the added benefit of a large cavity which works well as a bass trap.

Quadratic/Mathematic Diffusers

Mathematic diffusers are devices that use specific calculations to design their size, shape, and structures to effect their performance. A common type is called the Quadratic Residue Diffuser (sometimes called a Schroeder Diffuser, after its pioneering inventor, Manfred Schroeder). This type uses a Quadratic Residue Sequence that optimizes uniform sound diffusion at specific design frequencies. There are different ways to implement these designs, but two common designations are based on their diffusion patters – 1D or 2D. A 1D Quadratic diffuser mostly spreads energy in one plane, and a 2D provides a hemispheric pattern.

The Quadratic Diffuser is a 1D Quadratic Residue Diffuser that diffuses across the wells. This pattern looks similar to that of a Double-Duty diffuser, but is more disruptive to the phase relations of the reflections and is tuned to a narrower frequency band. These diffusers are often installed with the wells up in the ceiling or wall, which means they do not protrude from the surface, providing a clean flat look.
The ArtDiffusor® Model C is a high-performance 2D Quadratic Diffuser. It uses blocks instead of wells, and has angled caps to further assist the development of the hemispheric diffusion pattern. The Model C is a hybrid Quadratic and Binary design. It is quadratic in the heights and binary in the distribution. This unique combination of mathematic functions and the inclusion of angled reflections adds a new dimension when installing arrays of diffusers to reduce lobing and improve performance.
The ArtDiffusor® Model F is an interesting spinoff of the Model C. The Model F is a specialized low-profile version of the Model C, which uses smaller block structures to tune it to higher frequencies. Like the QuadraPyramid above, the larger number of smaller surfaces tunes this device to frequencies with a shorter wavelengths, in a low-profile design. The Model F is ideal for controlling flutter, ringing and high frequency comb filtering artifacts in spaces with limited space, or specific constraints.

Organic Diffusers.

Organic diffusers are a variation on the classic mathematic diffusers which use different mathematic functions to optimize the diffusion further by creating a smooth transition. Once such method is called Bicubic Interpolation. Instead of having the math restricted to having blocks at certain heights, the interpolation bridges these heights using a function that provides a smooth transition to the next target height. This transition creates unlimited resolution in the frequencies within it’s functional range, providing expanded uniformity throughout its range, and increasing its capabilities. As different frequencies are affected differently depending on their wavelength – the organic diffusers have no hard edges to define their pattern and look differently to different frequencies and energy from varied sources.

The Aeolian™ Diffuser is, at its core, a classic quadratic diffuser. The main difference is that, instead of limiting itself to the standard blocks and wells of a single size, the surface has been modified using bicubic interpolation. So instead of every frequency interacting with 2″x2″ blocks, they hit a rolling organic surface, which will redirect different frequencies in different ways. The different wavelengths interact and reflect off the surface differently – short wavelengths are more specular and longer wavelengths find a larger section of surface to redirect them.
The ArtDiffusor® Model D is another organic diffuser design, but it uses several different modifications to the standard quadratic design. Most quadratic diffusers start with square or rectangular blocks and wells. The Model D started life as rings of different sizes and spacing based on Maximum Length Sequences that were different thicknesses. Those rings were then raised to different heights based on the Quadratic Residue calculations. Randomness was introduced with Boolean logic, which would add height to some rings while subtracting from others when they crossed each others path. The whole geometry finally went through the bicubic interpolation to smooth the transitions. This completely original design is optimized for mid-to-high frequency diffusion and maintains a highly asymmetric diffusion pattern from any angle, at any frequency within the design range.

These diffusers all have the ability to be used in different types of installations for different reasons. Many of these diffusers are mixed and matched in the same room. You will see these on the walls or ceiling, and placed in different locations. There are rooms with Double-Duty diffusers for low frequency control, Model C for Mids, and Model F for flutter, while other rooms may have Aeolians™ on the rear wall and Model C’s and Model F’s to control the ceiling.

Big 3 Studio has Aeolians on the rear wall and black Model C’s and Model F’s on the ceiling above the console.

Keep in mind, these aren’t even all the diffusers we have available, these are just the ones specific to the 2′ x 2′ format. The Aeolian™ has a 1′ x 1′ version called the Aeolian™ Mini. There are flat panel diffusers that are hybrid absorbers and diffuser like the HiPer Panel® and the HiPer Panel® Impact. There are even large format versions of the Double Duty™ diffuser, Pyramidal, and even the Quadratic Diffuser.

For more info about these diffusers, read some of our, “Similar, Yet Different Series,” where we go into more detail about our products… and how some of these are similar, yet different!”


If you have any questions as to which products you need to optimize your space, reach out to Acoustics First® and we can help you find which products will be best for your application. Remember that Acoustics First’s® full line of sound diffusers are all made in the USA, with many available in stock for quick shipping.

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Throwback Thursday: Diffusion Vs. Absorption

On this Throwback Thursday, we look back at a popular Sound and Communications article about why sound diffusion is so much more complex a phenomenon than sound absorption. In this article, we discuss some of the different variables associated with the quantification of diffusion – magnitude, phase, frequency, direction, etc. When comparing its properties against absorption, which is simply the reduction of energy at specific frequencies, the argument is cemented as to why a single-parameter coefficient is insufficient to fully express the phenomenon of acoustic diffusion.

Read the article here:
PDF Version of Sound and Communications Article.

Sound & Communications February 2020

Revisit the original post here:
https://acousticsfirst.info/2020/03/24/acoustics-first-talks-diffusion-in-sound-communications/

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Eat with your Eyes (and Ears): Acoustic Treatment for Restaurants

Silent Picture Panels provide highly attractive and customizable sound absorption for restaurants

When evaluating a restaurant, guests will often look at four factors: food quality, service, price point and atmosphere.  The first three are fairly obvious in terms of how they influence the customers satisfaction, but the link between atmosphere and guest satisfaction is a bit murky.  

Atmosphere is a sort of “catch-all” term for the various room and design elements that contribute to the overall experience of the patrons. “Atmosphere” is usually associated with visual elements, like lighting, table setting and decorations, but “atmosphere” can literally refer to the air in the room (is the restaurant properly ventilated, are there distracting smells from the kitchen?) or more functional/operating elements like the layout of the tables and, our focus in this article, sound management.

Sound Management – How many times have you been to a busy restaurant that is so loud you can’t hold a conversation with those at your table? It’s difficult to understand speech when background noise and reflections from other sources cover up new information. This causes patrons to elevate their voices to be heard, further exacerbating noise level issues.

Studies show that patrons spend more time and money at restaurants that properly address sound management, ensuring their guest don’t feel overwhelmed at peak hours. Key considerations include:

  • Background music: Choose music that complements your restaurant’s concept. Adjust the playlist and volume based on the time of day and desired energy level—soft jazz for a relaxed dinner or upbeat tracks for a lively lunch.
  • Comfortable conversation levels: Keep music and ambient noise at a volume that allows guests to speak without straining to hear each other. The right balance creates a welcoming buzz without becoming disruptive.
  • Consider layout impacts: Open kitchens, high ceilings, or closely packed tables can amplify noise. Think about incorporating design features like partitions and high booths to help “break up” sound that is traveling from table to table.
  • Acoustic design elements: Sound will build up most in spaces that have a lot of hard/reflective surfaces. Use sound-absorbing materials like acoustic panels to reduce reverberation/echoes and create a more intimate atmosphere.
Unpainted/White Tone Tiles are an excellent solution in large restaurants and event spaces.

Tone Tile Panels – In restaurants, it’s especially important to minimize visually obtrusive acoustic treatment so it does detract or conflict with the carefully constructed aesthetic of the dining room. Tone Tiles are a perfect solution for restaurants that require “invisible” sound absorption as they can be field or factory painted to match the wall or ceiling color precisely. They also have a white, lightly textured surface that resembles drywall (Tone Tiles can be used as projector screens). Keep in mind, the more you paint an acoustic panel, the less sound reaches the absorptive substrate. We recommend light passes with water based paint to ensure the surface of the panel remains as acoustically “transparent” as possible

Factory painted Tone Tiles “disappear” into a coffered ceiling.

Silent Picture Panels – Another popular treatment option is our Silent Picture panels. Silent Picture panels can be wrapped with customer supplied artwork, images or branding. Restaurants love the double utility of full-color images and premium sound absorption.

A family-owned restaurant used Silent Image panels to display photos that celebrate their family, past and present.

Can’t I just put foam under the tables? We do not typically recommend acoustic material underneath restaurant tables.  Treating the underside of tables will only “take the edge” off overall sound buildup, primarily attenuating sounds produced below the table (shuffling feet, chair slides, etc.). Sound absorptive treatment is much more effective when in the direct “line of sight” of primary sound sources. Also, installing acoustic foam or felt under tables, where it is likely to be picked at or possibly soiled, present durability and sanitary concerns.

We’ve all heard the expression that we “eat with our eyes”; if the meal is not appealing to look at… it is less pleasing to eat. However, we also “eat with our ears”; if the environment is not conducive to comfortable conversation, then the customer will leave with a bad taste in their mouth, even if the food quality and service is exceptional.  

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Throwback Thursday – Diffusion and Binaural Bob from 10 years ago!

In June of 2015, Acoustics First® posted about our experiment which allowed people to hear the effects of sound diffusers in a small space. The experiment consisted of treating a small office with varying numbers of our new (at the time) Art Diffusor® Model D sound diffusers, then providing various sonic stimuli and capturing and measuring the results with a custom binaural head (called “Binaural Bob.”) This experiment received great feedback and with that input, future experiments were conducted using a similar method, mixing absorbers and diffusers, and having a wider range of stimuli.

One configuration used in the test with 27 Model D’s, Binaural Bob, and a single balloon pop.

But here we look back to the first… This is the video we produced during that experiment way back in 2015 with Binaural Bob, a bunch of Model D’s, a small office, some balloons, bang snaps, and one very heavy book.

If you would like to revisit the original post in it’s entirety, here is the link!
https://acousticsfirst.info/2015/06/09/want-to-hear-acoustic-diffusion-audio-demo/

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