CSD 492 Section 1 Test
Define Complex sound
(multiple tones) - Contain more than one frequency component and may be either periodic or aperiodic (EX: noise, human voice, etc.)
Define a pure tone?
(single tone) (sinusoid) (single frequency) - The frequency is the rate of the sinusoidal waveform - simplest form of sound
Know the speed of sound in feet and meters
- 1125 feet per sec (ft/sec) or 343 m/sec
What is reverberation?
- After the offset of a sound it continues (depending upon the surfaces) to be reflected. If, there is enough time delay (distance) and a single reflection you will hear an echo.
What does it take for an object to vibrate?
- An object must have inertia and elasticity to be able to vibrate. - Vibration is the movement of an object from one point in space to another point, then back again to the first point. - A force is exerted onto the object (the object resists change due to inertia—so a force is required). Then its ability to return to the starting position = elasticity
What happens to the period of a pure tone signal if you increase or decrease the frequency?
- As frequency decreases the period increases - As frequency increases the period decreases
What is compression and rarefaction
- Condensation/Compression: A state of increased pressure and density relative to static pressure - Rarefaction: A state of decreased pressure and density relative to static pressure - sound waves are made up of alternating condensations/compressions and rarefactions - the fluctuation in the density of air particles is caused by an object vibrating
Identify the cut-off frequency or knee point on a given filter diagram.
- Cut off frequency- frequency above or below where the filter passes the signal without reducing the amplitude. - To prescribe a filter you need to know the cut off frequency and the roll off rate EX: in the picture above, 2000 Hz is the cut off or knee point for (a), (b), and (d). - For the band pass, there will be 2 cut off frequencies (high and low - LOOK AT NOTES FOR PICTURE
EX test question: what is the highest frequency sound?
- The highest number, has the shortest wavelength
EX test question: what is the lowest frequency sound?
- The lowest number, has the longest wavelength
Know about resonant frequency
- The natural vibratory frequency of the receiving object is called its: Resonant Frequency. Every object has one - Influences the way we perceive sound
What are the 4 factors that define a decibel?
a.) A ratio b.) Non-linear c.) A log function d.) A relative unit of measure (needs a reference)
The speed of sound is ________. It is not affected by _____________.
constant; frequency - Sound travels the at the same rate if it is a pure tone or a car horn or a dog barking.
How do we calculate dB change with distance in the Inverse Square Law?
- Every time the distance is doubled the sound decreases by 6 dB SPL. Likewise, every time the distance is halved the sound increases by 6 dB SPL - EX: If the intensity 100 feet from the source is 45 dB SPL, what is the dB SPL at the following: - 50 feet = 51 dB SPL - 200 feet = 39 dB SPL - 25 feet = 57 dB SPL - LOOK AT NOTES FOR PICTURE
What is the angle of incidence?
- If a sound is reflected off a smooth surface it can bounce at a different angle depending on which way the sound source is being directed. - compare to passing a basketball; sound will bounce off objects - the angel at which it bounces off
Identify the roll off rate of the filter slope from a diagram.
- Roll off rate- rate of attenuation past the cut off frequency (dB per octave). You need at least 2 octaves to calculate this rate. - How many dB per every octave you drop off - To prescribe a filter you need to know the cut off frequency and the roll off rate EX: 6 dB/octave is the roll off rate for (a) in the picture above. For (b), 10 dB/octave is the roll off rate. May have to actually draw dots and figure out on the test. - The steeper the line, the higher the roll off rate/ vise-versa - LOOK AT NOTES FOR PICTURE
What is the "Inverse Square Law"?
- Sound intensity is inversely proportional to the distance from the sound source, as distance increases, sound intensity decreases. - As distance is doubled, sound intensity is reduced by 6dB SPL or 3dB power. EX: - If we measure a sound at say, 10 feet from the sound source and it is 50 dB SPL. Then we measure the same sound at 20 feet the intensity of the sound will be decreased by 6 dB SPL. The sound will now be 44 dB SPL.
How do you find the wavelength (distance) of a wave form?
- Speed of Sound/Frequency - Remember: The Speed of Sound in Air is approx. 1125 feet per second OR if measuring in meters it would be 343 meters per second EX: What is the wavelength of 725 Hz tone? Speed of sound/frequency = 1125ft per sec/725 Hz = 1.55 ft EX: Feet Per Second 6000 Hz --> SS/Freq = 1125/6000 = .1875 feet 1500 Hz ---> 1125/1500 = .75 ft 50 Hz ---> 1125/50 = 22.5 ft Meters Per Second 3455 Hz ---> 343/3455 = .099 m 75 Hz ---> 343/75 = 4.6 m 8055 Hz ---> 343/8055 = .042 m
What determines an object's resonant frequency?
- Stiffness (elasticity- restoring force, EX: rubber band) - As stiffness increases, resonant frequency increases - EX: if something is long and thin, it will have more stiffness - High frequencies are produced by stiff, light objects, E.g., wine glass - Mass (inertia- what is acting upon another object) - As mass increases, resonant frequency decreases - EX: if something is thicker, it will have more mass - Low frequencies are produced by compliant, heavy objects, E.g., diving board
How do you find the frequency of a wave form?
- To find the Frequency =1/time or period - Frequency (f) if measured in cycles per second (cps) or now given the name Hertz (Hz) EX: - If the period of a frequency is found to be .001 sec, its frequency could be found by: f=1/time = 1/10-3 OR 1/.001 = 103 or 1000 Hz
How do you find the period of a wave form?
- To find the Time or Period =1/frequency EX: - If the frequency is known the period can be obtained by: - t= 1/frequency = 1/1000 = .001 or 10-3 seconds or 1millisecond So, 1000 Hz moves at 1000 cycles per second - LOOK UP MORE EXAMPLES
What is free vibration?
- When the force is removed and the object continues in vibration, it is said to be in: Free Vibration - When an object is in free vibration, it vibrates at its resonant or natural frequency
Define Standing wave
- When wavelengths are reflected and hit at perfect opposite phase, sound will cancel. If, complex sounds occur, will have dead spots. - waves that appear to be vibrating vertically without traveling horizontally. They are created from waves with identical frequencies and amplitude interfering with one another while traveling in opposite directions.
What is the head shadow effect and how does it help us identify sounds?
- a low frequency sound wave envelopes around your head because it's a larger wave, whereas a high frequency sound wave goes straight to your ear because it's a quicker sound wave. - low frequency envelops objects so it is harder to locate - high frequencies are easy to locate - some sounds are blocked- high frequency (hit the side of your head, don't pass) - some wrap around the head so they do pass- low frequency
Define Phase
- an object that vibrates in simple harmonic motion represents that portion of a cycle that has elapsed at any moment in time, relative to some arbitrary starting point. - Expressed in degrees: 360 degrees for 1 cycle - indicates the current position of the wave relative to a reference position
Define dBHL
- decibel hearing level, this measure is used when testing someone's hearing - dB HL "equalizes" intensity. So, the softest level we hear at each frequency is equal to 0 dB HL. This makes it much easier to determine whether an individual has normal hearing or not! - (averaged normed level)
Define dBIL
- decibel intensity level (power) measured in watts, used when actually measuring the intensity of sound. - If double the power ratio increase 3 dB
Define dBSL
- decibel sensation level, the intensity above a specific person's threshold of hearing - (amount of dB presented above or below a given threshold level of dB HL)
Define Wavelength
- may be between any point 0 and 360- makes a sinusoidal wave. - The starting point (point A) is always 0 degrees no matter where it starts. It always goes from 0 to 90 to 180 to 270 to 360 (5 points on the sound wave) -the distance of one point on a wave to an identical point on the next wave - 360 degrees and 0 degrees are the same in a sense
Identify the tones amplitude either peak amplitude or peak to peak amplitude from a diagram of a pure tone.
- on the test it will a picture of a sound wave - REMEMBER: one cycle- 0, 90, 180, 270, 360 - the peak is the very top of the sound wave, but peak to peak is the very top and the very bottom point of the sound wave - EX: peak is 7, but peak to peak is 14 (you will add the numbers together) - LOOK AT PICTURE IN NOTES
If given a picture of a pure tone sinusoid be able to find the period, frequency, and phase of the wave.
- phase: find the degrees from point A to point B (is it 90, 180, 270, 360? Count it out). Start with 90 when counting because point A is already 0. - frequency: 1/time = frequency - period: To find the Time or Period =1/frequency
Define dBSPL
- sound pressure level (pressure) measured in dynes or micro-Pascals, used when actually measuring the intensity of sound. - If double the pressure ratio increase 6dB
What are the dB references for the following? a. Pressure (SPL) b. Intensity (IL)
- sound pressure level, sound intensity level - You would use either of these two labels when actually measuring the intensity of sound.
Define Period
- the time required for the traverse of the mass from one point and back again to the same point, while moving it the same direction. In other words: How long does it take to complete a cycle
Define Aperiodic
- there is no repetition of a pattern
Define Cycle
- when a pendular object goes equilibrium (resting at the bottom) to maximum displacement (halfway up/moving up), through equilibrium (at the very top/straight up and down) back to maximum displacement (halfway down/moving down) in the opposite direction and the back to equilibrium (very bottom, the initial resting position) it has completed one cycle. (Imagine its going in a circular motion) - measured in seconds
Be able to identify different filters from a diagram (i.e., high pass, low pass, band pass)
1.) Low Pass: filter lets low frequency pass through unaffected and attenuates the high frequencies- low passes get through but high passes do not. 2.) High Pass: filter lets high frequency pass through unaffected and attenuates the low frequencies- high passes get through but low passes do not. 3.) Band Pass: filter attenuates some of the low and some of the high frequencies, but allows some frequencies in middle to pass (determined by cut off frequencies)- a little of high pass and low pass gets through - LOOK AT NOTES FOR PICTURE
How do we find the resonant frequency of a tube closed at one end?
EX: - Closed tube 1.5 inches long; Column of air in that tube will "resonate" to a sound frequency which has a wavelength of 4 x 1.5 = 6 inches long. EX: w Remember: wavelength = 1125fps/ frequency - w Therefore, 6 inches (.5ft) = w .5= 1125/f w = 1125/.5 w = 2250Hz is the resonant frequency of a tube 1.5 inches long. w Must convert inches to feet EX: w What is the resonant frequency of a tube closed at one end that is 6 feet long? w Take length and multiply by 4 w 6 x 4 = 24 ft (wavelength of that tube) w 1125/24 = 46.87 Hz EX: w What is the resonant frequency of a tube closed at one end that is 5 inches long? w ALWAYS MULTIPLY BY 4 WHEN THE TUBE IS CLOSED AT ONE END w 5 x 4 = 20 inches or 1.667 ft. (remember 12 inches in a foot) so, 20/12 = 1.667 ft is wavelength of tube w 1125/1.667 = 675 Hz - LOOK AT NOTES FOR MORE EXAMPLES
How do you find harmonics?
Harmonics are numbered 1 thru ___, going from the low to high - The first harmonic = fundamental frequency - Harmonics are integer multiples of the fundamental frequency - Harmonic 1 is 1x's the fundamental - Harmonic 2 is 2x's the fundamental - Harmonic 3 is 3x's the fundamental n Etc. EX: The first Harmonic of a sound (fundamental frequency) is 25Hz. This is 1st Harmonic - 50 Hz is 2nd Harmonic - 75 Hz is 3rd Harmonic - 100 Hz is 4th Harmonic NOTE: - The fundamental frequency doesn't have to be present! - Think back to math. The harmonics are multiples of some "unknown" integer. Find the integer (the largest # that will divide into the harmonic frequencies) Ex. 550 Hz, 600 Hz, 700 Hz, 750 Hz - What is the fundamental? (50 Hz)
Define harmonics
pure tone frequency components
Define fundamental frequency
the rate of repetition
