Chapter 12. Room Acoustics II: The Listener and the Room. Acoustic Extremes. Anechoic Chamber rock-wool wedges to prevent echoes from walls, ceilings, and floors. No one allowed in room and no furniture either to prevent scattering.
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Chapter 12 Room Acoustics II: The Listener and the Room
Acoustic Extremes • Anechoic Chamber • rock-wool wedges to prevent echoes from walls, ceilings, and floors. • No one allowed in room and no furniture either to prevent scattering. • Even better is to deliver the sound electronically through headphones (acoustically sterile)
Real Room • Human ears are much better in discriminating small changes in pitch, loudness, and tone color in a real room.
Source size Trumpet Experiment • Trumpeter plays a steady A3 (220 Hz). • Harmonics at 440, 660, 880, 1100, 1320, etc • Below 1200 HZ the source is small compared to the wavelength
Human Hearing Response • The human nervous system makes a running average amplitude (loudness) of the partials based on information received from the two ears. • Useful for partials below 1000 Hz. • Pressure fluctuations can be correlated over one-half wavelength. • Over distances more than half wavelength there is no correlation
Correlation length • At 1000 Hz the wavelength is 34.5 cm. • Your two ears are more than ½-wavelength apart and they pick up independent views of the room. • At the trumpeter’s 220 Hz (wavelength = 1.57 m) the ears are less than ½-wavelength apart and the sounds are well correlated. • They are too close together to be useful in getting extra information by the averaging process.
Head Movement • If you move your head a little with the music, you pick up enough variations in the relative partial amplitudes to improve the averaging process. • Low frequency information is limited again because the distances are still small compared to the wavelength and the information at the two ears is essentially the same. • Swaying of the musician has the same effect.
Short Period/Higher Frequency • Our nervous systems can also accumulate the averages it is forming over short periodsof time, in order to take advantage of moving objects in the room • Small motions can be exploited for high frequencies only (above 500 Hz) - this is because of the size of the body in comparison to the wavelengths of the sounds.
Attack and Decay • Comprised of two parts - what the instrument is doing and what the room is doing. • We are interested in the room here.
Low Frequency Attack and Decay • Source acts as a point and sound emanates in all directions with equal strength (homogeneously). • We get the direct sound and a few milliseconds later, six reflections from the walls, etc. • The reflections off of large, flat surfaces do not alter the waves. • The decay is the attack in reverse.
High Frequency Attack and Decay • Source is now directional (the direction the bell is pointed). • The direct sound may have much higher amplitude now than the reflections, if the bell is pointed at us. • Someone away from the line-of-sight gets a different mix of amplitudes.
Joseph Henry • First secretary of the Smithsonian • Acoustics Applied to Public Buildings • He wanted to measure the shortest time in which a reflected sound would be heard as distinct from the direct sound. • At greater times echoes would be distracting
Henry’s Findings • If the echo traveled an extra 20 m, it can be heard as distinct. • 20 m at 345 m/s is a time delay of 58 ms - call it 60 ms • Look at sound arriving well within that time period – say 35 ms.
Precedence Effect • Consider two clicks delivered to two speakers separated by a few feet. Track the time delay Dt between clicks. • I will describe the effect first and then there is a sound file for you to hear it. • The sound file has clicks delivered to the speakers and Dt is varied.
Precedence Effect • When Dt is between about 5 and 20 ms, sound seems to emanate from the speaker emitting the leading click. • When Dt is very short, there is complete fusion, and a "phantom" image occurs between the two loudspeakers • When Dt is very long, fusion no longer occurs and each click is perceived as a separate sound source. Listen
Clifton Effect • Several click pairs (12 ms separation) left speaker first • Reverse the order of the speaker clicks (right speaker first) • At first the two clicks seem to be separated • They then merge together and appears to come from the lead speaker Try it!
Summary • The ear will combine a set of reduplicated sounds (echoes) and hear them as one provided… • that they arrive within about 35 ms of each other, and • that the waveforms are sufficiently similar. • The one tone is heard without any delay.
Summary – cont’d • The perceived time of arrival is that of the first sound. • The loudness may be greater than the first sound alone. • The apparent position of the sound is the position of the first sound. • The effect is present even when the later arrivals have more amplitude. • but less than about three times the amplitude of the first
Design of a Speaker System • Imagine a church with a long nave, so long that the preacher’s voice is not loud enough to carry to the back. • The speaker's mouth acts as a point source and the sound spreads out uniformly from there. The amplitude will be very small in the back. • Some of the echoes will arrive after the 35 ms cutoff for the Precedence Effect to work. These echoes then become annoying distractions.
Handling the Front • Place a speaker above and behind the preacher's head. • Project sound down the length of the hall. • Speaker's output has only a slight delay compared to the direct sound. • Precedence Effect will make the sound seem to originate from the voice.
Handling the Back • Place a few non-directional speakers toward the back • Electronic delay so that the sound from the front speaker arrives slightly before the sound from the back speakers. • Precedence effect we hear the sound arriving from the front. • Back speakers cannot deliver more than three times the original amplitude • Non-directional so as not to announce their position.
Auditory System • The ear/brain has the ability to focus on particular sounds in a room filled with sound • Easier in a room than outside, since the room provides reflections and scattering to help fill in the information.
Head Experiment • Set two microphones separated by the size of your head. • We take measurement with and without the head present.
Left Right Without Head • Source slightly closer to right.
Left Right With Head • Shadowed (left) ear has less amplitude • Right has much higher signal • The two are quite different • Clearly the listener’s head has an influence on the sound
A M L B Position Cues
Listener Clues • Here only one ear is used to simplify • We start with the path lengths equal (MAL = MBL) • If either listener or musician moves, then the ear detects differences in the amplitude of the paths and can detect the change in position.
Additional information • With two ears there is more information to help in locating sound • Caution: we are now considering distant listeners • If the listener is close (< 1 wavelength) then scattering is different
Aural Perception • A person who can move his head processes the information better than one who cannot • Binaural hearing is better than monaural hearing • Headphone disadvantage • we are then deprived of the cross-correlated clues coming from the room • Used in perception studies to limit sounds
More Aural Perception • We can take advantage of distinctive features in a sound pattern to recognize the sound • We learn the scattering pattern of nearby objects very quickly and use these to help distinguish the effect of the objects from the original sound • We can take advantage of several types of auditory information simultaneously • We can detect short time period changes in the sound source.
Aural Processing • On first arrival of a sound, we make a quick preliminary judgment as to the position and nature of the source. • We then use the precedence effect to fill in information in the first 35 ms.
Aural Processing (cont’d) • Other processors are at work to help us sort frequency and time of arrival information. There is evidence that we can sometimes distinguish sound separated by 30 ms. • Sounds separated by more than 60 ms are heard as distinct.
Flutter Echoes • Clap your handsin a large room • Rapidly repeating series of echoes • Period equal to the round trip travel time between walls or floor and ceiling • Usual frequency is several a second, it may sound like something fluttering • Frequency may even be high enough to assign a pitch
Perception of Repeated Notes • Trumpet player plays a quick series of notes (2 – 9/sec) • Listener can hear each note • Oscilloscope gives good agreement at low repetition rate • Irregular jumble at high repetition rate • Irregularities come from the room • The ear can deal with these
Resonant Frequency High Frequency Cut-off Loudspeaker Response
Speaker Response Regions • At frequencies below speaker resonance, response falls rapidly • This is the frequency that the cone oscillates at if displaced from rest • Mid-range is approximately constant • High frequency cut-off • Cone is larger than a few wavelengths of the sound
Rule of Thumb • If wavelength of sound is shorter than half the circumference of the speaker cone, then the response is poor • Ex. Consider a 12-inch diameter speaker • C = 2pr = (2)(3.1416)(6 in.) = 37.7 in. • l = ½ C = 18.85 in. = 1.57 ft. • f = v/l = (1133 ft/s)/(1.57 ft.) = 721 Hz
High Frequency Cut Off • In our example frequencies above 700 Hz are not well reproduced. • At 1400 Hz the response is ¼th the 700 Hz response • At 2800 Hz it is 1/16th as much as at 700 Hz • The beam pattern is less homogeneous above the high frequency cut off
Tweeter Highs Mid-range From Amplifier Middles D Woofer Lows Typical Speaker Arrangement
Problems at Crossover Frequency • Consider frequencies near where one speaker hands off to another • We can have a situation of two sources of almost equal strength • Speaker separations by D = ½ l, 3/2 l, 5/2 l, etc. will lead to total destructive interference for most room modes
Crossover Example • For a frequency of 2000 Hz (crossover between mid-range and tweeter) • Wavelength of the sound is… • l = v/f = (345 m/s)/(2000 Hz) = 0.17 m = 17 cm • The affected spacings would be 8.6 cm, 25.9 cm, 43.1 cm, etc.
Other Problems of Crossover • Electrical circuits controlling the crossovers assume that the electrical responses of the speakers do not change with frequency. • But they usually do, resulting in irregular behavior far from the crossover frequencies
Getting Too Fancy in Testing • Problems can be overlooked if speaker tests are performed in anechoic chambers. • The aim of these rooms is to record the sound from the source before the room modes have a chance to affect it. • Multiple speaker problems will not show up.
Cheaper Speaker Systems • Sometimes you can adjust the cone shape to give ok response over a wider frequency range • only one source, no multi-speaker cancellations exist • no crossover electrical circuit is required, no electrical problems exist
Other Solutions • Two speaker system without electrical circuits • Woofer will work best on the lows, tweeter on the highs • A simple circuit allows more power to the tweeter at higher frequency • In more sophisticated versions, the tweeter and woofer are about ¼ l out of phase at the crossover frequencies, avoiding the cancellation • Your hearing is good at rejecting unwanted sounds – so a lot of this is overkill.
Impulsive Sound Generator • Clapper below works well for high frequencies • Clap hands or bang bucket for low frequencies