acoustical concepts

Acoustics is both an art and a science. Below we've included some terminology to help those interested in a better understanding how sound is measured and how it behaves. 

the decibel
Sound is measured using the value term decibel notated as “dB”. The dB value is a logarithmic unit of measurement used to describe the power or intensity of sound. This means, as the level of sound increases, the dB value increases exponentially. When we measure the sound pressure level, notated as “SPL”, of a sound system, or the background noise of a restaurant for example, we are measuring it in dB using an SPL meter. Due to its logarithmic nature, every 10 dB increase in SPL equals a doubling of the perceived loudness.

audio frequency
Frequency is the tone or note value of the sound, measured in in Hz. A young, healthy human ear is able to perceive sound from about 20 Hz up to 20,000 Hz - also noted as 20 kHz. Frequencies between 40-150 Hz is the typical range of a consumer subwoofer found in a home theater or within a music system. These “low frequency” sounds have long wavelengths; they vibrate building partitions easier than the higher frequency notes or tones. The human voice has a range of approximately 100 Hz up to 6 kHz. Most of the sound energy for voice is concentrated below the 500 Hz range. Consonants, which occur above 1 kHz, help add intelligibility to what a person is saying. Therefore the loss of high frequency tones can drastically affect speech intelligibility.

reverberation
Every room sounds just a little bit different. The size, shape, and surfaces of each room gives them a unique sonic signature. If you walk into a cathedral in Rome and try to have a conversation with a friend you will hear their voice echo off of the surfaces around you. However, if you walk into a furnished bedroom to have the same conversation, it will sound completely different. You will hear your friend's voice coming directly from their mouth, and not much from anywhere else. The difference between these spaces is the reverberation time. Reverberation time is the persistence of sound after the original sound is made. If you clap in a Roman cathedral, you'll hear that sound cause a large number of echoes that build up and then slowly decay over a few seconds. Unlike the cathedral, a clap in a bedroom might create a few slight echoes, but they dissipate in less than a second.
Ever notice that your choir sounds better in a cathedral then your bedroom? This is because different forms of communication sound better in different spaces. Classrooms, which are good for speech, are designed to have reverb times between .7 sections to 1 second. Rooms with reverb times less than 1 second are considered "dead" spaces. "Live" spaces include churches and auditoriums that are good for classical music, opera, and chamber music. Symphonies and orchestras often sound best in spaces with more than 2 seconds of reverb time.

background noise
Background noise is described as the sum of the environment around us. In our day and age background sounds are a combination of “environmental” sounds generated in nature such as wind, rain, birds chirping, dogs barking, as well as man made sounds such as those from mechanical devices like a HVAC systems, a refrigerator compressor, or a hard drive spinning. Cars, trucks, motorcycles and airplanes at a distance count too. Background noise does not include specific sounds generated by local equipment such as stereo systems, or direct speech within a space. Such sounds are called “foreground noise”. Background noise is important in acoustic testing because it may “mask” or “bury” more prominent foreground sounds. For instance, if the sum of all the background sounds is relatively loud, it could prove more difficult to clearly distinguish a soft spoken person relating a story. The inverse being, if the background noise level is very low, it becomes easier to hear the more subtle noises you may wish not to hear - like the the sound of the TV from an adjacent hotel room or a private conversation between doctor and patient.

pink noise
Unlike background noise, pink noise is a very specific and intentional sound. Pink noise is an equal amplitude sound, meaning, pink noise contains an equal amount (dB level) of sound energy representing all the frequencies between the range of 20Hz to 20,000Hz (20 kHz). Pink noise sounds much like heavy, hard hitting rain beating down on a thin metal roof or a “Shhhhhsshhh” sound. Pink noise is the most common sound used for testing various acoustic properties of rooms, speakers, and entire sound systems. It is a constant and consistent sound that we know is not missing or emphasizing any specific frequencies. We tune sound systems "flat" by playing pink noise and then measuring the frequency response of the sound system in the space with microphones in various parts of the room. We then adjust the equalizer (EQ) on the system for optimal performance. That way, what comes in will be faithfully reproduced back out.

how sound travels
Sound travels through air at approximately 1126 feet per second depending upon the frequency, temperature, and humidity conditions. As it travels, sound, be it noise or beautiful music, may encounter an object, perhaps a wall like that of an open handball court like that found on a school playground. When sound hits this wall some combination of four things happen;

  1. Sound energy is reflected away from the surface. In this example, our handball wall re-directs the sound back in another direction; no sound breaches the wall.
  2. Sound energy is absorbed by the wall material and is converted into a tiny amount of heat - so faint it’s difficult to measure. The surface of the handball wall in this case would need to be made of a porous, soft, fuzzy material like carpet, upholstery, fiberglass, wool, cotton, perhaps ivy?
  3. Sound is transferred into the the wall. The wall becomes excited and begins to shake and move, much like the movements of a speaker cone. Perhaps not enough to see, but if you put your hand on the wall you may be able to feel the vibrations it’s receiving from the sound source. As the wall moves the some form of the original sound is re-broadcast allowing the sound to continue on it’s path. The sound is slowed down a little in time, some of the frequencies were absorbed due to attenuation or reflected back in another direction but some sound is still moving forward.
  4. Sound can go around our wall. Our handball court has no ceiling or side walls so sound is able to “flank” the wall. Similarly, if there was a weak spot in the wall, a small hole for example, some sound would be able to pass through that compromised section of the wall. We expand upon the flanking concept below.

flanking
When sound travels from one room to an adjacent room it doesn’t just excite the wall partition causing it to shake and rebroadcast the sound energy, it also looks for the path of least resistance which is often a “flanking path”. Flanking paths are considered compromised points in a building system or partition. Examples of flanking paths include: doors, windows, power or data outlets & light switches, HVAC vents and ducts etc. When “field testing” a building or room for sound or noise we look for possible flanking paths and make notes of our discoveries and observations.

sound transmission through structures
The measurement of sound through a structure is known as the "sound transmission class" or STC. The STC rating is a single number acquired from controlled laboratory environment. Such testing describes the sound attenuation efficiency of a surface, material or partition such as a window, a wall material like drywall, a piece of glass, most any material could have an STC rating. The higher the number, the better the partition or material is at blocking sound. Real-world conditions can have a significant impact on the performance of a partition, therefore "apparent sound transmission class" or ASTC values are measured. These values are often 5 points lower than the laboratory STC tests. This difference is due to flanking - sound traveling around or through weak points in the partition. An ASTC number is derived by fitting a mathematical “best-fit curve” against the Apparent Transmission Loss, noted as “ATL”. The ATL numbers are calculated using a multitude of variables including: room volume, partition surface area, measured reverb time & calculated absorption within the receiving room.

Each object sound encounters attenuates the volume level of the original sound to some degree. Even as sound travels through the air, it is attenuated. Some building partitions and materials are quite good at attenuating sound. Of course the makeup of the material such as it’s density or property has an effect on how much sound the material may transfer, reflect or absorb. As sound moves between one room to the next, either through a wall or a floor/ceiling partition some amount of sound energy is attenuated or reduced hence the term “noise reduction”. During testing procedures both while conducting STC and ASTC tests, the room from which the sound or noise originates is called the “source room”. The room on the opposite side of the partition, window, wall or material under test is the “receiving room”. Noise reduction is determined by playing a sound source (speaker) in the source room, measuring the SPL within this room, then moving to the receiving room and taking a measurement of the resultant SPL in the receiving room. The values are then subtracted to achieve the noise reduction number across a range of frequencies. This initial number gives us a glimpse into what frequencies the partition is able to block or attenuate.

 

Here at poindexters we understand not everyone needs to know how it all works, but we agree everyone appreciates life a little better when it has less noise, better acoustics, and great music bringing friends together.