Personal Music Players & Hearing

7. What are the characteristics of personal music players

The SCENIHR opinion states:

3.6. Technical aspects of personal music players

3.6.1.General characteristics

Personal music players have a very wide field of application ranging from professional tools at the workplace to the leisurely consumer and to children who use these devices as toys. They are portable digital music players that play music as audio files, such as MP3. In addition, most of these devices allow to store video, pictures, and to receive radio and TV programs (podcasting). Earphones and external speakers are the typical output devices delivering sound to the listener. Personal music players (PMPs) are widely used in conjunction with several headphones of different styles (insert, supra-aural, vertical, and circumaural).

To identify the risk levels of PMPs one has to realize that the chain of music reproduction is organised in stages which are more or less independent of each other but together affect the output signal level. Personal music players reproduce music from a recording. The sound of a signal has usually been recorded through a microphone and the oscillations resulting from the pressure changes are stored as a sampled wave form. Different procedures and algorithms are used for storing of analogue or digital representations of the wave forms which are offering low or high data compression and provide the means for reconstruction of the original waveform during play-back. The sound level is not significantly affected by the compression algorithm (if any) and by the degree of compression used in a particular recording. Often recordings are made by mixing samples from a number of microphones.

The dynamic range of a recording is generally chosen to fit the technical characteristics of the storage device (e.g. music cassette tape) and depending on the tone engineer and the style of the music. Thus, the full dynamic range of a ‘life music’ situation is typically compressed to a variable degree into the dynamic range available for recording and play back.

Using the appropriate algorithm, the recorded signal is re-synthesized into a waveform, typically, in the form of an analogue electronic signal, by the player. Manufacturers of players usually specify the electronic characteristics of the output (e.g. maximum voltage, impedance) of their equipment.

To produce sound the player is connected to a transducer: headphones, earphones or earbuds. Important for our purposes is how effective the transducer is in transforming voltage into sound energy.

As said above, two matters are important. The electronic coupling of the transducer to the player may affect the output of the player. This is technically a matter of input and output impedances and can be described as a coupling factor. The transducer produces a certain amount of sound energy for a given electronic energy (voltage times current drain) delivered to the input. This is the sensitivity of the transducer.

The earphones or earbuds are inserted in the ear. The place in the ear canal also determines the effectiveness of the transfer from transducer to the tympanic membrane. It seems evident that a circum-aural headphone has to produce more energy than an earbud inserted in the ear canal, simply because of the larger volume of air that has to be excited.

3.6.2. Sound output

The volume of the sound emitted by PMPs varies from manufacturer to manufacturer, and is difficult to estimate.

With the currently available digital formats (e.g. MP3) of sound recording and reproduction, it is possible to reach high levels of sound output without distortion. The personal music players now play not only music, but provide podcasts of various broadcasts or lecture material, which is delivered largely through ear-bud type insert ear phones producing a range of maximum levels around 80-115 dB(A) across different devices. Sound pressure levels change with the insertion depth of the ear-bud in the ear canal, the maximum output provided by the particular device and ear-bud combination and the type of music.

Fligor and Cox (2004) tested some devices by different manufacturers and style of headphones. They found that free-field equivalent sound pressure levels measured at maximum volume control setting ranged from 91 dB(A) to 121 dB(A). Moreover, they estimated an influence of 7-9 dB with an ear-bud type producing the highest levels in the ear canal.

3.6.3. Conclusions

With the currently available digital formats (e.g. MP3) of sound recording and reproduction, it is possible to reach high levels of sound output without distortion (around 80-115 dB(A) of maximum levels across different devices) and the difference in ear-phone type may increase that level. These levels change with the insertion depth of the ear-bud in the ear canal, the volume setting of the device, the maximum output provided by the particular device and ear-bud combination and lastly the type of music. In the worst case scenario, it is possible to estimate maximum levels of about 120 dB(A).

Source & ©: SCENIHR,  Potential health risks of exposure to noise from personal music players and mobile phones including a music playing function (2008), Sections 3.6.Technical aspects of personal music players