1 / 55

Chapter 12

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.

brent
Download Presentation

Chapter 12

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Chapter 12 Room Acoustics II: The Listener and the Room

  2. 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)

  3. Anechoic Chamber

  4. Real Room • Human ears are much better in discriminating small changes in pitch, loudness, and tone color in a real room.

  5. 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

  6. 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

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. 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

  14. 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.

  15. 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.

  16. 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

  17. 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!

  18. 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.

  19. 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

  20. 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.

  21. 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.

  22. 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.

  23. 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.

  24. Head Experiment • Set two microphones separated by the size of your head. • We take measurement with and without the head present.

  25. Left Right Without Head • Source slightly closer to right.

  26. 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

  27. A M L B Position Cues

  28. 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.

  29. 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

  30. 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

  31. 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.

  32. 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.

  33. 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.

  34. 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

  35. 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

  36. Resonant Frequency High Frequency Cut-off Loudspeaker Response

  37. 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

  38. 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

  39. 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

  40. 12-inch speaker at 250 Hz

  41. 12-inch speaker at 1000 Hz

  42. 12-inch speaker at 4000 Hz

  43. Tweeter Highs Mid-range From Amplifier Middles D Woofer Lows Typical Speaker Arrangement

  44. 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

  45. 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.

  46. 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

  47. 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.

  48. 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

  49. 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.

  50. Impulsive Sound Generator • Clapper below works well for high frequencies • Clap hands or bang bucket for low frequencies

More Related