Sound Pressure Level Correction

The phone(s) used in any given experiment will, in general, maintain different sound pressure levels at different frequencies. Normally, a calibration curve of this behavior is generated (for each DSS) before the SER program is run. An example of such a curve is given in Figure 1. If this curve were perfectly flat, then the stimulus SPL would be constant over all frequencies. Otherwise, a constant SPL can only be achieved if some kind of correction is applied to the waveform at each frequency that is generated.

Each DSS is capable of doing just that. To achieve this, the program loads into the DSS a table of correction values that are associated with the calibration curve of the phone connected to the DSS in question. This is called the Constant-SPL table. If uniform SPL is desired across all frequencies, then the lowest SPL seen on the calibration curve, labeled maxspl, is the maximum possible SPL that the user can choose from. The highest SPL on the curve corresponds to the frequency at which the largest correction will have to be made to produce this uniform SPL.

As the DSS changes the frequency of the waveform, it applies the SPL corrections (from the Constant-SPL table) to the waveform. This corrected wave is passed to a 16-bit DAC. The SPL correction portion of the amplitude will consume 1 bit of resolution for every 6 dB of correction that is done. If, for example, a correction of 54 dB must be made, then 9 bits are used by the correction, leaving only 7 bits of resolution (or, 1 part in 128) for the resulting wave. So, large correction values can result in waveform resolution that is unacceptable to the user.

The parameter SPLCLIP is used to limit how much SPL correction will be done by the "Constant-SPL" hardware in the DSS. In conjunction with this, the parameter DACBITS shows the user how many bits of the 16-bit D-to-A converter are left for defining the waveform after this hardware SPL correction is done.

Note that SPLCLIP is a user-specified parameter, and that its value will be used by both DSS's in the generation of the stimulus. However, because there is a unique phone calibration curve associated with each DSS, the SPL correction done on each DSS will necessarily be unique. Thus, each DSS will have it's own value for the parameters DACBITS, MAXSPL, and ATTEN. The naming convention of DACBITS1, MAXSPL1, and ATTEN1 will refer to the parameters associated with DSS 1, and DACBITS2, MAXSPL2, and ATTEN2, will refer to DSS 2. Remember that these six parameters are displayed in the menu, but they are not specified by the user. Their values are determined by the current settings for frequency range, spl level, and the spl clip value. They are displayed in the menu for information only. In the discussion that follows, references to these parameters are without the '1' or '2' at the end of each name -- that is, the descriptions apply to either DSS 1 or DSS 2, whichever is being used in stimulus generation.

If, for example, the parameter SPLCLIP is set to 20 dB, then SPL corrections greater than 20 dB will not be allowed. If the lowest SPL on the calibration curve is less than 20 dB below the highest SPL, then the SPL over the entire curve will be properly corrected. If not, then a new value of MAXSPL is defined, and SPL values that lie beneath this will not be corrected. This condition is indicated by an asterisk next to the new MAXSPL value. If the clipping option is used for a calibration curve that has a low but narrow notch in it, it allows the overall constant SPL for the majority of the frequencies to be somewhat higher, and it also improves the resolution of the waveform. (That is, DACBITS will be somewhat larger than if greater SPL corrections were allowed.) It still generates uncorrected, and therefore incorrect, SPL's at certain frequencies.

A value of 0 for SPLCLIP means that all SPL corrections will be done by the DSS, according to the calibration stored in the Constant-SPL table (that is, NO clipping will be done). Generally, a value of 10 or 12 dB for SPLCLIP can be used. The ultimate choice depends on the shape of the calibration curve, and whether or not it has any sharp notches.

The following is a summary of the parameters effecting the intensity of the stimulus:

SPL	sound pressure level of the stimulus, in dB
SPLCLIP	maximum allowed SPL correction to be employed, in dB
MAXSPL	a display of the maximum possible SPL, in dB (MAXSPL is an upper limit, and is not specified by the user)
DACBITS	a display of how many bits in the D-to-A converter are available for the waveform, after SPL correction (DACBITS is not specified by the user)
ATTEN	a display of the attenuation setting, which equals MAXSPL - SPL (ATTEN is not specified by the user)

In the FM Sweep stimulus routine, the user can choose to ignore the Constant-SPL table, by setting the parameter 'CORRECT SPL?' to n (no), rather than y (yes). In this case, the maximum possible SPL will be determined from the SPL at the mid-frequency of the range over which the sinusoid is swept.

See also the general description of the SER program.
See also the descriptions of the SER stimulus options:
FM Sweep, Tone Pips, Clicks, Noise, Dual Tones