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Laboratory Evaluation of Annoyance of Low Frequency Noise

2. Description of the Listening Tests

The listening tests were made in a standardised listening room at the Department of Acoustic Technology, Technical University of Denmark, where the noise examples were presented for the test persons. The dimensions of the listening room are 7.52 x 4.75 x 2.76 m, and the room fulfils the recommendations in IEC 268-13 [6]. A total of eight different noise examples were used, presented at three different levels. All presentations were made twice and the sequence of the presentations was randomised. Prior to the listening tests, the test persons were trained using four noise examples. Thus each test person has assessed more than fifty presentations. After each presentation the test person gave evaluations of the noise example on a paper form.

2.1 Noise Examples

Eight different noise examples were chosen for the listening tests. Example No. 1 was the noise from a densely trafficked six-lane highway. The traffic noise served as a reference noise, because there is a well-described relation between the level of road traffic noise and the annoyance of this type of noise. The other noise examples had all a strong low frequency content. The noise examples were:

Table 1.
Description of the noise examples used for the listening tests

Noise example 1 has a broadband character and it is almost continuous. Since it was filtered to simulate an indoor measurement (see next section), the tonal character of the engine noise from passing heavy vehicles is clearly audible though the tire noise is also obvious. Noise example 2 consists of a series of very deep, rumbling single blows from a drop forge. The noise examples 3, 4, 5, and 6 each have one tonal component. Noise example 7 has three tones but two of them are at a low level, and noise example 8 has a characteristic rhythmical pulsating appearance due to the drums.

2.2 Noise duration, levels and presentation

The duration of all the noises was 2 minutes. The noise examples were either recorded indoor or filtered to simulate indoor noise. They were taken from DAT recordings to the hard disk of a PC where they were edited digitally. The examples are presented to the test persons at nominal levels of 20 dB, 27.5 dB, and 35 dB (A-weighted levels). In the listening room the sound was measured at the listening position and subsequently analysed to obtain the objective measurements of the noises. The noise examples were played directly from the PC via a D/A converter to a crossover filter and via four amplifiers to two broadband loudspeakers (KEF 105) and two subwoofers (Amadeus Sub). A detailed description of the set-up can be found in [7].

2.3 Filtering of the Noise Signals

The noise examples were filtered with two types of filtering prior to the presentation. One type of filter compensates for resonance modes in the listening room. The listening position was chosen so that only one mode at 45.5 Hz influenced the sound level. At this mode the room mode increases the sound pressure level by about 18 dB, which is compensated for by a narrow band notch filter as illustrated in Figure 1. This filter was used in all presentations.

Look here!

Figure 1.
Illustration of the filter that compensates for the resonance modes in the listening room.
  

Figure 2.
Illustration of the ‘outdoor-to-indoor’ filter used with the outdoor sound recordings.

The other type of filter was used only with those noise examples that were recorded outdoors. Based on the information in [8], [9] and [10] an ‘outdoor-to-indoor’ filter was created that represents the reduction index of ordinary building materials and construction principles. The filter was defined in the range 16 Hz – 4000 Hz and is illustrated in Figure 2. The level was further rolled off below 15 Hz to suppress wind noise and other ‘errors’ in the recordings that would otherwise be enhanced. Also the level above 4000 Hz was attenuated, as this frequency range was not considered in the project.

From a subjective evaluation, the noise examples sounded ‘natural’ in the listening room. All the examples – including the traffic noise – had a pronounced low frequency characteristic.

The outdoor-to-indoor filtering of the noise examples recorded outdoors changed also the characteristic of the traffic noise. Although the noise was easily recognised as traffic noise, the idea of using this noise as a kind of reference became less obvious. It seems that the test subjects regarded the traffic noise as one of the low frequency noises.

2.4 Test Subjects

Eighteen young persons (9 males and 9 females) with normal hearing were chosen as reference group for the listening tests. The age of the test persons was between 19 and 25 years. This group of listeners is called ‘Reference group’. In addition, listening tests were made with a small group of people that have reported annoyance due to low frequency noise in their homes. This group (called ‘Special group’) consisted of four persons aged between 41 and 57 years, two of each gender. The inclusion of the special group was regarded as a pilot experiment only because of the small number of test persons and the significantly higher age range.

The main results of the project will be based on the listening tests with the reference group. The results from the pilot experiment with the special group are described in Chapter 7.

Pure tone audiometry was made in the frequency range 125 Hz to 8000 Hz with a Madsen Midimate 602 audiometer, equipped with Sennheiser HDA 200 earphones. The calibration of the audiometer was made with the values from [11] which are practically identical to ISO 389-8 [12]. The results of the audiometry are given directly as hearing threshold level in dB HL. Hearing threshold levels at or below 15 dB HL was accepted in the frequency range 125 Hz to 4000 Hz, and a hearing threshold level at 20 dB at a single frequency (including 8 kHz) was also accepted. The average hearing threshold is shown in Figure 3.

In addition to the conventional audiometry, the hearing threshold in the low frequency range was determined. The tests were made with pure tones at 31 Hz, 50 Hz, 80 Hz, and 125 Hz, and the signal length was 800 ms. A computer controlled Tucker Davis system with Sennheiser HDA 200 earphones was used to measure the thresholds according to the Two Alternative Forced Choice method. The present implementation of the procedure determines the 79.4 % point of the psychometric function. A detailed description of the procedure can be found in [13]. The results of these hearing threshold measurements were given as sound pressure level (SPL). No reference values are available at present for the HDA 200 earphone at frequencies below 125 Hz and therefore the measured SPL-thresholds were compared to the standard hearing thresholds given in ISO 389-7 [14]. In this way the deviation from the standardised ISO threshold could express approximate, artificial dB HL values. The average of these artificial hearing levels are also shown in Figure 3. It can be seen in Figure 3 that the two sets of curves differ by 5 dB at 125 Hz. This is due to the artificial reference for the low frequency curves and also because the definition of ‘threshold’ differs in the two methods.

Look here!

Figure 3.
Average pure tone hearing thresholds for the reference group (normal hearing, young subjects) and for the special group. Curves are given separately for the conventional audiometry (125 Hz – 8 kHz) and for the low frequency thresholds (31 Hz - 125 Hz).

From Figure 3 it is seen that the threshold of the reference group has a slight decrease at the highest frequencies (6 kHz and 8 kHz). The special group show a hearing loss at the highest frequencies above 2 kHz. It should be noted though, that the data for the special group are average values over just four persons. Some of the persons had a considerable hearing loss, partly due to a higher age. It is assumed that the high frequency hearing loss does not influence the subjective evaluations of the low frequency noises used in the present investigation.

The low frequency part of the hearing thresholds (Figure 3, 31 Hz – 125 Hz) shows that the special group is about 5 dB less sensitive than the reference group. At 31 Hz the (average) thresholds are almost the same. The threshold data show that the special group of people – who have reported annoyance due to low frequency noise in their homes – do not have higher (better) hearing sensitivity at the low frequencies. The hypothesis that the special group should be able to hear the low frequency sounds more easily than the reference group is not supported by these hearing threshold measurements.

2.5 Subject’s Task

The test persons were given a written introduction to the tests, and they could ask about the procedure throughout the tests. A full training session was made prior to the listening tests. Information about the sound examples was given after all the tests were finalized.

The tests persons were asked to answer four questions after each presentation:

	‘How loud is the sound?’ (on a scale labelled "not audible" in one end and "very loud" in the other end)
	‘How annoying do you find the sound if it was heard in your home during the day and the evening?’ (on a scale labelled "not annoying" in one end and "very annoying" in the other)
	‘How annoying do you find the sound if it was heard in your home during the night?’ (on a scale labelled "not annoying" in one end and "very annoying" in the other)
	‘Is the noise annoying?’ (answer yes or no).

All the scales were 10 cm long, and the response was measured in cm with a ruler and thus given as a figure between 0 and 10.

An example of one of the annoyance scales is given here:

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