Acoustics: Decibels and Vibrating Air

Contents

! Wind Instruments

! Loudness

! Decibels

! Studio Construction and Design

! References

Wind Instruments

! In order to generate a musical note an instrument has to vibrate, whether its a string (on a guitar perhaps), a strip of metal (glockenspiel), or a pipe filled with air (flute).

! Wind instruments produce sound vibrations in a number of ways: by vibrating reeds by the mouthpiece; vibrating lips of the musicians; by circulating the musicians breath within the tube. The inside of the tubes need to be as smooth as possible so as to get the best tone from the instrument.

The length of the tube determines the pitch.

When you blow inside a glass bottle filled with water (ensuring that there is equal air pressure applied on each side of the rim) it will resonate the bottle causing it to vibrate and a frequency will be produced.

With less or more water the bottle will change frequency. The less water the lower the frequency. This is because the compressions and rarefactions are happening further apart, creating longer wavelengths. It is these wavelengths which determine the frequency.

(But this can be adjusted by finger holes, slides, valves...or adding water!)

Flutes The same is happening within wind instruments.

The longer and larger the instrument, the lower its frequency will be. A flute has a mouthpiece on which the musician applies air pressure.

The reeds in the mouthpiece vibrate and this creates a ripple effect and after the air has sussed out the shape and size of the instrument it will produce a frequency.

If no holes are covered up it will produce a higher frequency, as the holes are covered the frequency will get lower and lower: the longer the tube, the wider the wavelength, the lower the frequency.

A Trumpet blown without any valves pressed will produce a high frequency. When the buttons on a trumpet are pressed the amount of air within the tube increases the longer the tube, the wider the wavelength etc.

Trombones have sliders, which when are pulled towards the mouthpiece produce higher frequencies, and when extended create extends the tube distance.

BRASS INSTRUMENTS

Mouthpieces on brass instruments do not have reeds that vibrate - musicians vibrate their lips to push vibrating air into the instrument. Higher pressure from the lips can increase the frequency, however only a few notes can be produced this way. The addition tubing on brass instruments helps solve this problem.

Loudness: 10 x 1= about 2?

! If a solo violinist is suddenly followed by 10 violinists, it doesnt sound 10 times louder. It sounds about twice as loud.

! Similarly a 100 violins isnt perceived by us to be 100 times louder than the solo, more like four times as loud.

! Why is this?

Doubling Signals Does Not Double Perceived Sound Level

! The difference between silence and one flute playing is very noticeable. Like a lamp being switched on in a dark room.

! When another flute plays the same note along with the first, the change in volume is not as great as when the first flute played after silence.

! The third flute will make even less of an impact.

! If there were 68 flutes playing the same note all at the same time we probably wouldnt be able to tell when the 69th flute starts playing. Just as we wouldnt notice a 69th lamp being switched on.

Why we dont hear the 69th flute

! When two signals arrive at the ear it is unlikely that they will arrive in phase and with the same intensity.

! Only two perfect sine waves with exactly the same intensity and in phase when they arrive at the ear would result in perceived doubling (this is also the theory of standing waves).

! The crests of one flutes soundwaves will get pulled back by another flutes soundwaves dips. Waves are usually complex, and the result will average out resulting with only a slightly higher perceived loudness.

Two sine waves in phase with the same intensity.

Two sine waves slightly out of phase. The result is not a doubling of

Another reason why we dont hear the 69th flute

! From the times of earliest cavemen to the present day, our ears are used to prevent us from danger.

! Our ears must to be able to hear quiet noises (like predators creeping up on you), but must not be damaged by other naturally loud noises such as thunder.

Our ears are designed in such a way that any gradual increase in volume has as less and less impact.

This is true of other senses: Five smelly cheeses dont smell 5 times worse than 1 smelly cheese, much like 10 salty peanuts do not taste 5 times saltier than 2 peanuts.

The human ear has a complex way of perceiving sound. We are most sensitive to high mid (squeaky) frequencies, and so we hear them louder than lower frequencies with the same intensity (see Fletcher-Munson curves later).

This is important to know because it means that if there are two instrumentalist playing the same instrument, but one playing the higher notes, and one playing the lower notes, the one playing the lower notes would have to play harder in order to sound as loud as the higher one.

Please turn that up by 17,624.

! The smallest audible sound (almost total silence) is a reference point, and we call that the threshold of hearing.

! The loudest thing we can hear, without instantly tearing the eardrum, is called the threshold of pain.

! The threshold of pain is 1,000,000,000,000 times greater in intensity than the threshold of hearing.

As this table shows: Ten times the intensity is perceived as twice as loud. E.g. a busy restaurant has ten times the intensity levels of a solo violin, but its relative loudness is only double.

There are two main types of measurement: the absolute type; and the comparative one. An example of the absolute type: Sally has two apples, Paul has one. When interpreted as a comparative measurement it would read: Sally has twice as many apples as Paul.

Decibels are a form of comparative measurement, as it is only really possible to measure sound with reference to a starting point: the threshold of hearing.

Decibels ! Decibels were first used to measure

the intensity of energy of power along telephone lines.

! The Bel scale (named after telephone pioneer Alexander Graham-Bell) would interpret an intensity level of 1000 to be 3 Bels, and 1,000,000 was 6 Bels calculated by just counting the zeros.

! This logarithm allows us to reduce very large numerical values into smaller manageable ones.

! But this meant there would be only 12 numbers of loudness (since the threshold of pain has a comparative intensity of 1,000,000,000,000). So it was decided 120 levels would be more appropriate, by multiplying Bels by 10, to give us deci-Bels.

dB SPL

! So far we have been talking about a specific type of decibel dB SPL.

! SPL means sound pressure level and is about the actual intensity caused by sound in air. All decibels are referenced, and in this case its to the quietist audible sound (which some say is a mosquito 3 metres away, or a person sighing 10 metres away).

! With SPL distance is important to know. When a sound source is moves twice its distance away it loses 6dB (-6dB), and rather conveniently when a sound source is twice as close it becomes 6dB louder (+6dB).

VU Meters ! Decibel was first used to measure voltage

(which is the pressure which forces electrons), whereby 0 dBm is equivalent to 1milliwatt of power. dBm was often used to indicate a signal level of 0.775 Volts, but this was only if there was impedance of 600 ohms (as there were in old telephone lines).

! So dBu was introduced to signify 0.775 Volts, regardless of impedance (impedance is opposition to the flow of current).

dBV was introduced to mean 1V (this is less clumsy than 0.775 Volts). 0dB on audio equipment refers to its sweet spot or optimum level.

dBFS means Full Scale and this relates to digital audio systems incidentally the sweet spot for recording and mixing on DAWs is -18dBFS.

Remember Decibels are merely a reference between two signals.

As music producers we are most likely to see dB on audio equipment, particularly the faders and VU meters (Volume Unit).

Fletcher Munson Curves

Loudness, being a psychological quantity, is difficult to measure, so Fletcher Munson averaged their results over many test subjects to derive reasonable averages.

The lowest equal-loudness contour represents the quietest audible tone and is also known as the absolute threshold of hearing. The highest contour is the threshold of pain.

Equal-loudness contours were first measured by Fletcher and Munson using headphones (1933).

In their study, listeners were presented with pure tones at various frequencies and over 10 dB increments in stimulus intensity.

For each frequency and intensity, the listener was also presented with a reference tone at 1000 Hz. The reference tone was adjusted until it was perceived to be of the same loudness as the test tone.

Room Reflections Reverberation is caused by the direct signal reflecting off surfaces.

Sound travels in an omnidirectional manner, so a very reflective surface would means less energy gets absorbed and therefore the Reverb Time would last longer.

Just like light, when soundwaves reflect off a surface the angle of incidence is equal to the angle of reflection.

Some methods are required to limit the amount of reflections.

Sound has the ability to diffract around or through a physical acoustic barrier in other words it can bend around an object and reconstruct its original frequency and amplitude.

A Direct Sound is quickly followed by early reflections and then a gradual decay of reverb. The time it takes in milliseconds for the reverb to diminish by 60dB (so by 1,000,000 times) is called the RT60. The larger the room the more likely the reverb time will be longer, but this will also be determined by how reflective the surfaces are.

REVERBERATION

Studio Design

! A number of factors must be taken into consideration if you are building a recording studio from scratch, converting an existing building into one, or simply using a bedroom for recording.

! Too much absorption in a room can actually be bad for your mixing purposes, and probably wont combat the problem of leakage (when sound escapesand annoys a neighbour). But rooms that are too live will make it nearly impossible to create a credible mix.

! Building a studio from the floor up requires a lot of consideration.

! The floor itself should be floating on a multitude of layers which each in turn help to stop the spread of resonance (especially low frequencies).

! Studios themselves should have at least an outer wall and an inner wall, with preferably a vacuum or some absorptive material in between. Any glass panels should be doubles, with one pane slightly at an angle this helps diffuse the sound as it travels through (parallel sides can create standing waves see later!)

! Ideally doors should be very thick or all the money spent on the floor and walls would be wasted. Many studios have specially designed air conditioning that doesnt allow sound to enter.

! The building is just one factor. The ambience and reflective nature of the room is an important factor not only for the instruments being recorded (recording drums in a very reflective environment like a church probably wouldnt sound very distinct on playback but a choir in a very absorptive room but sound to dry and raspy). Different venues are used for different applications.

Absorption coefficients are numbers between 0 and 1 which indicate how well a specific type of material can absorb sound energy.

Different frequencies have different absorption coefficients and this should be factored when designing a studio.

Shiny, flat materials (glass, marble etc) are highly reflective. Fibreglass and wool are highly absorptive.

Standing Waves Room Modes Opposing parallel walls in studios can give rise to to a phenomenon called standing waves (also called room modes). These occur when sound is reflected back along its own path causing phase differences to interfere with a rooms amplitude response.

Severe problems will cause increasing and decreasing volume levels at various frequencies. The effected frequencies will be those whos wavelength is mathematically related to the distance between the opposing surfaces (wall to wall, floor to ceiling, any flat surface parallel another flat surface).

Managing problems in studio-design

Reflections with long delay times may produce flutter-echoes (or slap-echoes). Absorbent material could be affixed on the wall, or in various places, to reduce this.

Studios tend to have a live end/dead end design absorptive features by the engineer, but more ambience for the clients at the rear. Too much liveliness in a studio control room will not allow you to mix effectively.

References ! Modern Recording Techniques

7th Edition

! How Music Works by John Powell

! Sound on Sound (magazine/website)

! Music Tech magazine

! www.TeachMeAudio.com

! www.recordproduction.com

http://www.askaudiomag.com/articles/studio-acoustics-part-1-an-overview-of-room-issues

http://method-behind-the-music.com/mechanics/brass/

http://hyperphysics.phy-astr.gsu.edu/hbase/ph4060/p406ex7.html

http://www.soundonsound.com/sos/1994_articles/feb94/decibels.html

https://www.youtube.com/watch?v=N5Ch2NThFvY#!