12.8 More revision questions
(a) If two tones are broadcast through headphones at an intensity of 100 dB SPL, which will sound louder, a 100 Hz tone or a 1000 Hz tone? Why?
(b) How loud must a 100 Hz tone and a 1000 Hz tone be (in dB SPL) in order to have a loudness level of 50 phons?
(a) How are beats generated?
(b) How are they perceived?
(a) Beats occur when two tones of slightly different frequency are broadcast simultaneously. As the relative phase of two simultaneously applied tones changes continuously, so the tones alternately reinforce and cancel one another.
(b) They are perceived as a single tone with a pitch midway between the two tones but periodically varying in loudness: waxing and waning of loudness. The intensity varies at a rate equal to the frequency difference of the two tones.
Why is it that when you play music softly that has been recorded at a fairly high level, you cannot hear the very high and very low frequencies?
When music is played loudly, above about 80 dB SPL, all tones from about 30 Hz to about 5000 Hz have about the same loudness (they fall on the same loudness contour – see Figure 35). However, when you turn the intensity of the music down, all frequencies do not sound equally loud. At 10 dB for example, frequencies below about 400 Hz (the bass notes) and those above about 8000 Hz (the treble notes) are inaudible. So, if you play the music softly, you won't hear the very low and very high frequencies.
How does the use of masking experiments support the place code hypothesis for pitch perception?
When a test tone is played in the presence of a masking tone or in the presence of masking noise, our ability to hear the test tone is impaired. This is because according to the place code hypothesis there is overlap in the place of vibration on the basilar membrane of the test tone and the masker.
Why are interaural time differences not very useful for localising high-frequency sounds?
For high frequencies, the wavelength of the sound is less than the distance between our ears. This means that the delay in the arrival of the sound at the two ears can create phase ambiguities.
What properties of a sound determine its pitch and its timbre?
Pitch is mainly determined by the frequencies of the low numbered harmonics in a sound, whereas timbre is determined by which frequency regions have more energy – the relative intensities of different harmonics.
At this point you should read Hearing impairments: causes, effects and rehabilitation by David Baguley and Don McFerran attached below. This chapter shows how dysfuction at any level of the auditory system can lead to hearing impairment and discusses some possible treatments for hearing loss.
Click View Document to open Hearing impairments: causes, effects and rehibilitation by David Baguley and Don McFerran