All About Sound, Vol. 1: The Science of Sound Quiz

Answer: It depends on your definition of sound

Sound can be defined as either the production of acoustic waves, in which case a tree falling in the woods does make sound. It can also be defined as the human reception and perception of acoustic waves, in which case the tree does not make a sound if no one is there to hear it.

Answer: Reduced density

A vibrating object in a conductive medium, such as a tuning fork in air, pushes and pulls the air molecules. Sound is considered a pressure wave because what pushes and pulls the medium is pressure. When the fork pushes, it compresses the air causing increased density. It then moves the opposite way and pulls on the air reducing density which is rarefaction.

Answer: They travel at a constant speed

Unlike electromagnetic waves, such as radio and light, sound waves require a medium to travel in. Sound waves can travel in many types of media including gasses, liquids and solids. If the medium is completely uniform, sound waves will travel at the same speed. However, many variables, especially temperature, density and elasticity, affect the speed of sound.

As is the case with all waves, sound waves can be reflected which is bouncing off a barrier, such as sound traveling through air hitting a wall.

All waves can also refract which is bending of the direction due to a change in the conducting medium. The change in the medium causes a change in the speed of the wave and this alters the direction. With sound waves, the change in direction is often gradual such as sound traveling through air or water at changing temperatures or density. Conversely with light, refraction is often an abrupt bending such as when light traveling through the air enters water or glass.

Answer: Waves bounce out opposite the direction they come in

Waves are governed by the laws of reflection which state that the angle at which wave hits a barrier equals the outgoing angle but on the other side of perpendicular. Therefore, only a wave coming in perfectly perpendicular to the barrier will bounce back in the opposite direction. For example, a wave coming in at any angle heading northeast and hitting a north-south barrier will bounce back at that same angle but heading northwest.

Echoes are delayed repetitions of a sound which are caused by reflections of sound waves and can be distinctly heard.

Reverberation occurs when a sound is repeatedly reflected, often from multiple barriers, such that it is maintained for a period after the original sound stops. Unlike echoes, reverberations are indistinct sound.

Interference is the scientific term for the interaction of similar waves. If the waves are in phase (the crests and troughs coincide), the crests and troughs add creating higher crests and troughs. This is called constructive interference. When the waves are out of phase, they subtract from each other which is called destructive interference. Since a reflection creates a situation in which the same wave is going in and bouncing out, interference usually occurs.

Answer: Frequency

A frequency of one Hertz (Hz) is defined as one cycle per second of any waveform. One cycle is the time it takes for the wave to go from crest to crest or trough to trough. It is named after Heinrich Hertz (1857-1894) who is credited with providing conclusive proof of the existence of electromagnetic waves. Human hearing is considered, at best, to be in the range of 20-20,000 Hz of sound waves.

Answer: Defined with human hearing considered in the formula

The definition of the decibel originated in the measurement of transmission loss and power in the telephone industry. The bel was named in honor of Alexander Graham Bell (decibel is one tenth of a bel).

The decibel is commonly used as a measure of sound power level, pressure level or intensity level. It is related to the logarithm of a ratio of a quantity of power, pressure or intensity to a reference level of the same variable. For acoustical applications, the reference quantity for sound in air is set at the typical threshold of perception for an average human. And since the human ear is not equally sensitive to all sound frequencies, the acoustic power spectrum is modified by frequency weighting (A-weighting being the most common standard, hence dBA). Furthermore, the human perception of the intensity of sound approximates the logarithm of intensity rather than it being a linear relationship. This makes the dBA scale a useful measure in acoustical applications to human hearing. A typical conversation would be measured at around 60 dBA.

Answer: 2 times the loudness

Because a dB is defined as a ratio of a quantity of sound intensity, power or pressure to a reference level, it is a unitless quantity. The dB is defined to be the logarithm of this ratio. Therefore, when the dB quantity increases by 10, it means that the intensity independent of human hearing has increased by 10 times.

However, human hearing is approximately logarithmic regarding loudness so if the intensity increases by 10 dB, it turns out we perceive the loudness to be about twice as much.

Answer: Pitch

Timbre includes the characteristics of sound which enable us to distinguish sounds which have the same pitch and loudness. Playing the same note means the pitch is the same. The main factors contributing to the timbre of the sound of an instrument are overtone characteristics, the intensity profile of the sound, and vibrato.

Tones have a fundamental or lowest pitch. Occasionally we hear a single pure fundamental tone but usually there are numerous overtones which are higher frequencies that are also present in the sound. Timbre is greatly affected by the number of overtones that are present, their frequencies and relative intensities.

The intensity pattern, or envelope of a sound wave, begins with the attack and decay. The attack is how quickly the sound rises to its peak. In decay, the sound stabilizes. This is followed by sustain and release phases of the sound. For example, a string can be plucked, bowed, strummed or hit, each of which creates a different pattern of sound intensity.

Vibrato is a musical effect consisting of a oscillating small variation of pitch.

Answer: Resonant frequencies

The explanation for the Doppler Effect was discovered in 1842 and is named after the physicist Christian Doppler who mathematically described the phenomenon. His equation calculates how much the pitch (frequency) of a sound changes when the source, observer or both are moving relative to the medium they are in. If the source or observer are moving toward each other, the pitch rises. If they are moving away from each other, the pitch lowers.

Sonic booms are predicted by the Doppler Effect. When a source moves toward you, it produces each succeeding wave from a position that is closer to you compared to the previous wave. This shortens the distance between crests of the sound waves decreasing the wavelength. As the source approaches the speed of sound, the crests (which are areas of higher pressure because sound is a pressure wave) begin to pile up. An airplane traveling faster than the speed of sound creates constant shock waves of piled up pressure, which are heard as booms along its flight path. (This effect can be difficult to understand just by reading text so I would encourage the curious to look at description which includes pictures. Also, for the mathematically inclined, you can look up the Doppler Effect equation and see how it predicts this phenomenon).

The resonant frequency is the naturally occurring frequency of an object at which it vibrates at the highest amplitude.

Answer: False

It is counterintuitive, but the situations of a moving source compared to a moving observer create different results which are mathematically included in the Doppler Effect equation. The Doppler Effect equation calculates the change in pitch an observer hears whether the source, observer or both are moving relative to the medium they are in. The difference between the source or observer moving comes about due to what is changing when each moves.

Consider a source emitting the sound at a particular frequency. When the source moves toward you, it produces each succeeding wave from a position that is closer to you compared to the previous wave it produced. This causes the waves to get clumped together. The result is that the wavelength (the distance from crest to crest of the wave) of the sound waves gets smaller. Because frequency is inversely proportional to wavelength, the observer hears a higher pitch. The opposite happens for a source moving away.

When the observer is moving, the waves emitted by the source are unchanged (the wavelength stays the same). However, the observer moving toward the sound encounters the crests of the waves sooner meaning the frequency increases. Again, the opposite is true for an observer moving away from the source.

Because one situation affects the wavelength (moving source) and the other affects the frequency (moving observer), the effect on pitch is different depending upon which is moving relative to the medium. Mathematically, in the Doppler Effect equation, the observer velocity is in the numerator while the source velocity is in the denominator.