g.tec medical engineering

g.PAH - gtec's programmable attenuator and headphone buffer for experimental audiology

g.PAH enables you to run EEG experiments with auditory stimulation for objective audiometry. The device is used to accurately attenuate a sound signal in steps of 1 dB. If the input signal is e.g. attenuated by 20 dB then the output signal amplitude is 1/10th of the input signal amplitude. Such an accurate control of damping is necessary in the field of objective audiometry.

Sound signals generated e.g. by a CD player or PC/notebook soundcard are sent to g.PAH input. The device - controlled via a serial interface - attenuates the signals by the specified damping factors. The output sound signal is then sent to a standard high-end head phone or speakers for audiometry and presented to the subject. The device is equipped with 2 separately controlled channels. The included software lets you adjust the attenuation for each channel per mouse-click. The protocol for the serial interface is described and lets you integrate the device into almost any experimental or routine setup.

The advanced design of g.PAH guarantees that the switching times between the gauges are in conformity with the standard EN 60645-1 for audiometers.

Product Highlights

accurate control of damping
2 separately controlled channels
integrate the device into almost any setup
conforms to the standard EN 60645-1 for audiometers
run EEG experiments with accurate auditory stimulation

Example: ASSR stimulated by an amplitude modulated sinosoidal wave

This example displays the ASSR which can be evoked and quantified using g.BSamp, g.Recorder and g.PAH. The upper panel in the figure to the left displays one example of an amplitude modulated sound signal typically used in ASSR: A sinusoidal carrier of 1000 Hz is modulated by 88 Hz. The lower panel displays the power spectrum of the stimulus including the carrier and the 2 spectral lines at the modulation frequencies (lower panel, blue graph).

This stimulation sequence is sent to the programmable attenuator and headbuffer g.PAH via the soundcard of the PC/notebook. The EEG is then recorded via g.Recorder. A trigger signal allows for online averaging of 1 second EEG sweeps in synchrony to the stimulus. Thereafter, the spectrum of the averaged EEG signal is computed yielding the ASSR.

The result of the computation is given by the red graph in the lower panel. The ASSR can clearly be identified at the modulating frequency of 88 Hz.
Depending on the stimulation signal ASSR at different frequencies can be identified in the frequency domain. Typically, Multiple Auditory Steady-State Responses (MASTERs) are evoked using 4 different carrier and 4 different modulating frequencies. The lower panel displays ASSR for a normal hearing subject. The 4 frequency components at the modulating frequencies of 80, 90, 100, and 110 Hz can clearly be identified in the spectrum.

For a comprehensive overview on ASSR see:
Picton TW, John MS, Dimitrijevic A, Purcell D (2003) Human auditory steady-state responses. Int J Audiol. 42: 177-219.