Why do many trained Indian classical vocalists prefer not to use a harmonium as an accompanying instrument during performance?

Introduction / Context:
Indian classical music relies heavily on subtle pitch variations, microtones, and flexible intonation. Many serious vocalists are careful about the instruments that accompany them. The harmonium, although popular and convenient, is sometimes disliked by purists. This question explores the main musical reason behind that preference, relating it to scales and tuning systems rather than loudness or shrillness.

Given Data / Assumptions:

• The context is Indian classical vocal music.
• The instrument in question is the harmonium.
• The concern is about musical accompaniment and tuning, not simply loudness.
• We assume knowledge of tempered and just intonation scales.

Concept / Approach:
Harmoniums used in India are generally tuned to the Western equal tempered scale. In this system, the octave is divided into twelve equal semitones. Indian classical music, however, traditionally uses a system of shrutis and just intonation where intervals between notes are based on simple frequency ratios. Equal temperament slightly compromises these pure ratios to make all keys usable. This mismatch can disturb the exact intonation that an expert vocalist wants to maintain, especially in slow, elaborated ragas.

Step-by-Step Solution:
Step 1: Recall that in equal tempered tuning, all semitone gaps are identical in terms of frequency ratio, which is convenient for Western harmony and key changes. Step 2: Remember that Indian classical ragas are built on specific scales where certain intervals are meant to be slightly higher or lower than the equal tempered versions, to match natural harmonic ratios and desired emotional colour. Step 3: A harmonium with fixed reeds cannot continuously adjust its pitch to match these subtle intonation demands. It will always play the equal tempered notes. Step 4: As a result, a vocalist who wants to use pure intonation may feel constrained or slightly pulled off key by the fixed tempered tuning of the harmonium.

Verification / Alternative check:
In many musicological discussions, scholars and senior artists mention that the harmonium is convenient for practice and teaching but is not ideal for capturing the microtonal nuances of certain ragas. String instruments like tanpura, sarangi, or even the human voice can continuously slide and adjust pitch, but harmonium reeds cannot. This supports the view that temperament, not shrillness or loudness, is the core issue.

Why Other Options Are Wrong:
Because a diatonic scale is used in the harmonium: While diatonic scales do exist, the actual problem is equal tempered tuning rather than the general idea of a diatonic scale. Because the notes of the harmonium are too shrill: Shrillness can be managed by playing softly or adjusting the instrument; it is not the main theoretical objection. Because the intensity of the harmonium notes is too large: Volume can be controlled by the player and is not a fundamental reason for rejection by classical purists.

Common Pitfalls:
Listeners sometimes think artists dislike the harmonium simply because it sounds loud or metallic. The deeper reason is intonation: the inability of fixed reeds to deliver microtonal shades with precision. Confusing tonal quality (timbre) and tuning systems is another common misunderstanding. When preparing for exams, focus on conceptual reasons like equal tempered scale rather than superficial issues like loudness.

Final Answer:
The main reason is that the harmonium uses the equal tempered scale, which does not match the subtle pitch requirements of Indian classical music. Therefore, the correct option is because a tempered scale is used in the harmonium.