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BRAINBOX MEASURING SOUND (advanced)

Noise pollution is as much a threat to health and psychological well being as any other kind of environmental pollutant. Just as “prevention is the best cure” for illness, elimination or reduction of traffic and other noises is the best cure for the stress, sleeplessness and other problems associated with environmental noise pollution. 1)

The measurement and language of 'sound' is complicated and can be extremely confusing - check out this detailed explanation of sound measurement language and reference tables

An excellent on-line wave and frequency calculator. This is the best single-site source for technical information about sound.

Automate your project using Raspberry Pi voice recognition and control software and Google speech API - or create music or noise fingerprint recognition system.

Distance, Power & Level Measurement

See sound pressure (SPL) versus power (SWL). See SPL to SWL calculator.

In the case of ambient environmental measurements of “background” noise, distance need not be quoted as no single source is present, but when measuring the noise level of a specific piece of equipment the distance should always be stated. A distance of one metre (1 m) from the source is a frequently used standard distance. 2)

In a direct field or free field, the sound pressure level (SPL - measured in Decibels) of a spherical wave decreases with doubling of the distance by (−)6dB.

The Sound Pressure P decreases in inverse proportion to the distance, so that doubling of the distance decreases the sound pressure P to a half (!) of its initial value - not a quarter.

The sound intensity p decreases inversely proportional to the squared distance, so that doubling of the distance deceases the sound intensity/power p to a quarter of its initial value.

Cause & Effect

A sound source (the cause) radiates sound power P and this result in a sound pressure p (the effect) - see 'Decibel Scale' image below.

Decibel Scale

Decibel Scale

The logarithmic decibel scale goes up in powers of ten: every increase of 10dB on the scale is equivalent to a 10-fold increase in sound intensity broadly corresponds with a doubling in loudness. That means a sound of 20dB is 10 times more intense than a sound of 10dB and a 30dB sound is 100 times more intense.

How To Measure Sound

Recording Codecs

NOTE: The mp3 codec uses perceptual coding to achieve data compression, this means that it throws away any signals that are not able to be heard (LF for example)! 3)

NIOSH Free Sound Level Meter (SLM) App.

The NIOSH mobile app automatically provides a readout of the instantaneous sound level in A, C, or Z weighted decibels using a tablet/smartphone’s built-in microphone (or preferably using an external, calibrated, microphone).

The weighting is user-selectable and can be accessed from the ‘Settings’ screen. The app also reports the main metrics that are of importance for proper occupational and general purpose noise measurements.

A, B & C Contours

Standard sound filter contours are used to make the instrument more nearly approximate the normal human ear. The different contours were intended to match the ear at different sound intensities.

Decibels Explained

NOTE: The decibel describes a ratio between two sources: Not what power either of the sources radiates, only the ratio of powers. (Note also the factor 10 in the definition, which puts the 'deci' in decibel 10bb=x10, 20db=x100, 30db=x1000).

How can you have negative decibels? Doesn't zero decibels mean no sound?

Zero decibels is a reference level, not zero sound pressure. So one can have negative decibels. Zero dB is usually set at about the limit of human hearing (in the most sensitive frequency range). The sound level in dB is a measure (on a logarthmic scale) of the ratio of the sound pressure or sound intensity to this reference level. The logarithm of one is zero, so zero dB corresponds to the reference level. Numbers greater than one have positive logarithms, so positive decibels means sound levels greater than that of the reference. Numbers smaller than one have negative logarithms, so negative decibels mean sound levels below the reference level. Source - http://newt.phys.unsw.edu.au/jw/musFAQ.html#z

The Relationship Between Loudness & Decibels

Comparison chartLoudness & Intensity Graph

Is 80 dB twice as loud as 40 dB? How do you translate from decibels to loudness?

Sound level in dB is a physical quantity and may be measured objectively.

Loudness is a perceived quantity and one can only obtain measurements of it by asking people questions about loudness or relative loudness. (Of course different people will give at least slightly different answers.) Relating the two is called psychophysics.

Psychophysics experiments show that subjects report a doubling of loudness for each increase in sound level of approximately 10 dB, all else equal. So, roughly speaking, 50 dB is twice as loud as 40 dB, 60 dB is twice as loud as 50 dB, etc. The units of loudness determined in this way are sones.

This relation implies that loudness and intensity are related by a power law: loudness in sones is proportional to (intensity)log 2 = (intensity)0.3.

For more details, see the page devoted to the decibel scale in which the relationships among sound intensity, sound pressure, dB, dBA, sones and phons are explained. We also have a web service with which you can measure your own hearing response curve.

See nomogram of phon:sone conversion:


Example Hearing Test Graph

ISO 226 Test Filter Graph

Filter Reference dBA and dBc


Sound Spectrum Analysis

This page is a quick index to the Effects, Generators and Analyzers shipped with Audacity. You may also add new plug-ins in various popular formats.

  • An Effect changes the audio in some way.
  • A Generator creates new audio, either in an existing track or in a new track.
  • An Analyze tool performs analysis on a selection of audio - it does not change the audio and does not create new audio.

Audacity - Index of effect generators and analysers

Audacity

“Decibel” is a ratio. It is not an absolute measure in the way that kilograms, dollars or seconds are. A Decibel is always relative to some other value. In the case of audio signals (as you see in audio tracks in Audacity), decibels are relative to the “full scale” reading. In Audacity, that is the height of a track, indicated as +/- 1.0. A full scale signal has a peak amplitude of 0 dB.

If you regularly need to convert dB ↔ linear, there is a plug-in available here: which is a “Quick Conversion Calculator. Simply download the qcalc.ny file, put it in the Audacity plug-ins folder and restart Audacity. The Quick Calculator will then be listed in the “Generate” menu.

Audacity Digital Audio Fundamentals.

The Audacity Analyze Menu contains tools for finding out about the characteristics of your audio, or labeling key features.

Plot Spectrum takes the selected audio (which is a set of sound pressure values at points in time) and converts it to a graph of frequencies (the horizontal scale in Hz) against amplitudes (the vertical scale in dB).

Regular Interval Labels places labels in a long track so as to divide it into smaller, equally sized segments. For example, this can be useful for distributing a large file on the internet. You can either choose the number of labels to be created, or the interval between them. Each label produced contains the chosen label text.

Sound Finder divides a track up by placing region labels for areas of sound that are separated by silence. Use this to make the labels show the exact region of each track to be exported. This lets you remove some or all of the silence between the tracks.

Sample Data Export reads the values of successive samples from the selected audio and prints this data to a plain text, CSV or HTML file.

Spectrogram

The Spectrogram View of an audio track provides a visual indication of how the energy in different frequency bands changes over time. The Spectrogram can show sudden onset of a sound, so it can often be easier to see clicks and other glitches or to line up beats in this view rather than in one of the waveform views. To select Spectrogram view, click on the track name (or the black triangle) in the Track Control Panel which opens the Track Dropdown Menu.

Example Spectral Settings Example LF Spectrum


Sound Barriers

Active Noise Reduction

In an enclosed space such as a residential space, noise transmitted from outside is amplified at the resonant frequency of the indoor space. The resonant frequency of a residential space is generally close to the low frequency sound of 200 Hz or less. In this study, a single channel active noise control system that has only one secondary source was evaluated experimentally for mitigating low frequency resonance. A standing wave that causes resonance was canceled out by the one secondary source. The similar results were also obtained by numerical simulation utilizing the boundary element method. The simulation is used for arranging an optimal position of active noise control system. From this study, it was confirmed that the single channel active noise control system can mitigate indoor low frequency noise on loops of standing wave efficiently. 4) and 5)

Fencing

Basically it's a waste of money. Think of sound as light. It can be red. Blue. Green. Yellow. Orange. You can install filters to stop different wavelengths but none of it's a substitute for a sold limestone wall 3 m high. If the problem is low frequency sound or vibration, you can't stop it without spending a lot of money: Source - 6) and 7)

Ear Plugs

Low frequency sound waves travel more easily through walls, floors, and even the ground. It follows, therefore, that low frequency sounds also travel more easily through ear muffs and ear plugs. That does not mean disposable foam ear plugs will not work, rather, they will not work as well for low frequency sounds as they do against higher frequency sounds.

For example, a foam ear plug might provide a noise reduction of 49 dB at 8000 Hz, but at 125 Hz, the noise is only reduced by 33 dB. Here is what that means in practice: Rap music is often played as high as 90 dB. Assuming your foam ear plugs would reduce the vocals by around 49 dB, your ear plugs would let you hear just 41 dB (90-49=41,) a level just above a whisper. When you consider that another 20 dB will be lost as the sound travels through the wall of your apartment, you will usually be unable to hear the remaining 21 dB of vocals that remain by the time the sound waves get to your ears. Contrast that to the low bass line. Here your ear plugs will just reduce the volume from 90 down to 57 dB (90-33=57) and your walls will barely slow the travel of the bass sound waves, so you will hear the thumping almost as if you had nothing in your ears. The same sort of analysis applies to compressors (e.g. refrigeration), slamming doors and similar sources of low frequency vibration.

As bad as they are at blocking low frequency sounds, though, disposable foam ear plugs are much better than any other kind of ear plugs when used for this purpose, so give them a try after reading the following important clarifications:

Disposable foam ear plugs offer the highest level of protection you can get in a single hearing protective device, but you must get them properly inserted, and you must select the right plug to fit your ears. Here is a short video tutorial on how to properly fit disposable foam ear plugs. Source - 8)

Green Glue

For low frequency sources like subwoofers, it is common that more than 80% of the sound energy is generated at frequencies below 100Hz. Most suppliers of soundproofing products use Rw to rate the performance of their products. However, because Rw only measures sound above 100Hz, we suggest you don't rely on it. The only reliable way to compare the soundproofing performance of different products is to compare results from certified tests that show the acoustic performance at low frequencies; i.e. down to 32Hz.

* Australian Standards for Acoustic Performance: AS 1191-2002, AS 1276-1979, AS/NZS ISO 717.1:2004

At less than Au$30 per square metre Green Glue Noiseproofing Compound represents excellent value for your money. It is suitable for soundproofing both residential and commercial projects and is just as easy to use in new construction as it is in modifying existing rooms or renovating.

Green Glue Noiseproofing Compound is low VOC, has almost no odor, does not contribute to mold growth, can be easily cleaned up with soap and water, and has been certified under the Australian Standard for Fire Hazard AS 1530.4-2005 not to alter the fire rating of the walls or ceilings it is used in. Source 9)

Audio Recording & Data Logging

The PeakTech 8005 seems to have identical specs to the Digitech QM1592 Sound Level Meter - See Data logging with FHEM, the installtion instructions and download module from github + specs

 
 
brainbox/young-sound-engineers/measuring-sound/home.txt · Last modified: 21/08/2018/ 10:43 by 127.0.0.1