Acoustics and vibrations equivalente lei de ohm

[jonas ribeiro]

http://www.sengpielaudio.com/calculator-ak-ohm.htm

• Formulas and calculation •
 
Sound pressure p, particle velocity v, specific acoustic impedance of air Z, sound intensity I or J
 
Acoustics and vibrations
 
• Acoustic equivalent for Ohm's law •
or ohm's law as equivalent in the acoustics for
plane progressive waves − Specific acoustic impedance

The particle velocity v is not the speed of sound c = velocity of sound. Unless otherwise stated, the sound pressure is always meant as RMS value.    eff = RMS Enter any two of the following values and click the calculation button. The missing values will be calculated. One of these values could be the specific acoustic impedance or the characteristic impedance of air: Z0 = 413 N·s/m3 at 20 °C = 68 °F or Z0 = 410 N·s/m3 at 25 °C = 77 °F.

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Sound pressure p   N/m² = Pa ≡ V or E voltage  Particle velocity v   m/s            ≡ I  current Acoustic impedance Z   N·s/m³       ≡ R  resistance  Sound intensity J or I   W/m²          ≡ P  power

 

Fundamentals: Acoustic Laws and Sound Equations in Analogy (Relationship) to the Electric Laws − Formulary  Formula wheel ♥  Important formulas Acoustics   Sound + noise

In acoustics and audio engineering sound pressure, air particle velocity, and acoustic impedance are linear sound field strength quantities. The sound intensity is a quadratic sound energy strength quantity.

Sound pressure p in pascals (newtons per square meter) is not the  same physical quantity as intensity J or I in watts per square meter.  ... and the sound power (acoustic power) does not decrease with  distance r  from the sound source - neither with 1 / r nor as 1 / r2.

Often the sound pressure as a sound field quantity is mixed incorrect with the sound intensity as a sound energy quantity. But I ≈ p2.

Note: The radiated sound power (sound intensity) is the cause - and the sound pressure is the effect. The effect is of particular interest to the sound engineer. The effect of temperature and sound pressure.

Acousticians and sound protectors (noise fighters) need the sound intensity (acoustic intensity). As a sound designer you don't need that. Look out more for the sound pressure that makes an effect to your ears and to the microphones.

Sound pressure and Sound power – Effect and Cause

The well known law V = I × R means accordingly (equivalent) in the acoustics p = v × Z.

Conversion of acoustic values to sound level L in dBSPL Conversions and Calculations of Sound Quantities and their Levels Relationships of acoustic quantities associated with acoustic sound waves How many decibels (dB) is twice (double, half) or three times as loud? Ohm's Law of the electronics V = I × R

Hearing is directly sensitive to sound pressure (ear drums). In stereo the level differences have been called "intensity" differences, but sound intensity is a specifically defined quantity and cannot be sensed by a simple microphone, nor would it be of value in music recordings if it could be. "Intensity" stereophony is better termed as level difference stereophony, because our eardrums and microphone diaphragms are moved by the differences of the sound pressure level.

Since our ears do not respond to the air particle velocity (sound velocity), but only to the sound pressure changes, the particle velocity is irrelevant on the loudness perception.

Acoustic impedance of a medium = Sound pressure / Particle velocity
Characteristic acoustic impedance Z₀ = p / v

Please enter two values, the third will be calculated.

Acoustic impedance Z0  N·s/m³     Sound pressure p  Pa = N/m²  Particle velocity v  m/s

Acoustic impedance of air is Z₀ = 413 N·s/m³ at 20 °C.

Acoustic impedance = Density of medium × Speed of sound
Z₀ = ρ × c

Please enter two values, the third will be calculated.

Density of medium ρ  kg/m³     Acoustic impedance Z0  N·s/m³  Speed of sound c  m/s

Density of air is ρ = 1.204 kg/m³ at 20 °C.

Medium	Density ρ in kg/m³	Speed of sound c in m/s at 20 °C	Acoustic impedance Z0 in N·s/m³
Air	1.204	343	413.5
Water	1 000	1 440	1 440 000
Brick	1 700	4 300	7 310 000
Glass quartz	2 200	5 500	12 100 000
Aluminium	2 700	6 100	16 500 000
Steel	7 500	6 000	45 000 000

The decrease of sound with distance

How does the volume (loudness) decrease with distance from a sound source? How does the sound pressure (voltage) decrease with distance from a sound source? How does the sound intensity (not the sound power) decrease with distance from a sound source? The beginners question is quite simple: How does the sound decrease with distance?

For a spherical wave we get: The sound pressure level (SPL) decreases with doubling of distance by (−)6 dB. It falls to the 1/2 fold (50%) of the initial value of the sound pressure. The sound pressure decreases with the ratio 1/r to the distance.   The sound intensity level decreases with doubling of distance also by (−)6 dB. It falls to the 1/4 fold (25%) of the initial value of the sound intensity. The sound intensity decreases with the ratio 1/r2 to the distance.   The loudness level decreases with doubling of distance also by (−)6 dB. It falls to the 0.66 fold (66%) of the initial value of the sensed loudness. The loudness decreases with the ratio 1/(20.6r) = 1/1.516 r to the distance.

The sound level depends on the distance between the sound source and the   place of measurement, possibly one ear of a subject.  The sound pressure level Lp in dB without the given distance r to the sound  source is really useless. Unfortunately this error (unknown distance) is quite often.

What is sound? Sound is a pressure fluctuation in the air. It has the character of a wave motion which is triggered when an air-particle triggers the next. This domino effect is propagated in air at 343 m / s at 20°C as a longitudinal wave. In liquids and solids the speed is significantly higher (water: 1440 m/s, steel: 6000 m/s). Audible sound pressure fluctuations in Pa = pascal are in the range of about 20 µPa = 0 dB (threshold of hearing) up to 150 Pa = 137.5 dB (threshold of pain). The average air pressure of atmosphere at sea level is 101325 Pa = 1013.25 hPa. The number of pressure variations (cycles) per second is called "frequency of sound". The unit of sound frequency is Hertz (Hz). The audible range for humans is between 20 Hz and 20 000 Hz (20 kHz). High frequencies are perceived louder than low frequencies. Therefore, in measurements of noise, an A weighting filter is often used (human sense of hearing). When there are very loud or very low frequencies, better use a C-weighted filter.

Sound power and sound pressure. Correlation between sound energy quantity, and sound field quantity. A sound source emits sound power and generates a certain sound. That means: The sound power is the cause and the sound pressure is the effect. A comparison of the theory of heat makes the connection clear: The head radiated by an electric heater makes a certain temperature in the room. How high the temperature will be, depends on room size, type of isolation, the presence of other heat sources and so on. The power output of the electric heater is always the same, practically independent of the room in which it is located. The situation is similar with the sound: The sound that we perceive, or a microphone perceives, depends on the distance to the sound source and the acoustic characteristics of the area in which sound waves propagate. In a large, sound-absorbent room a sound source sounds much softer than in a small room with bare concrete walls. But the sound power of the sound source is always the same. Look at: "Sound pressure and Sound power – Effect and Cause" http://www.sengpielaudio.com/SoundPressureAndSoundPower.pdf

The sensitivity (transfer factor) in mV / Pa shows clearly that  microphones change sound pressure (Pa) to audio voltage (mV).  Energy and power plays no role for this microphone transducers.  Also our eardrums are only moved by the sound pressure.  Sound pressure as a sound field quantity cannot be the same  as acoustic intensity as a sound energy quantity.

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